HYDROGEOCHEMICAL AND ISOTOPIC STUDIES OF GROUNDWATER IN COASTAL AQUIFERS OF GHANA: CASE STUDY IN THE CENTRAL REGION THIS THESIS/DISSERTATION IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF DOCTOR OF PHILLOSOPHY IN NUCLEAR EARTH SCIENCE BY SAMUEL YAO GANYAGLO MARCH 2015 University of Ghana http://ugspace.ug.edu.gh i TABLE OF CONTENTS TABLE OF CONTENTS .......................................................................................................... i DECLARATION ..................................................................................................................... v ACKNOWLEDGEMENT ...................................................................................................... vi List of Figures ....................................................................................................................... viii List of Tables ......................................................................................................................... xii ABSTRACT .......................................................................................................................... xiii CHAPTER ONE ...................................................................................................................... 1 INTRODUCTION ................................................................................................................... 1 1.1 BACKGROUND ............................................................................................................... 1 1.2 STATEMENT OF THE PROBLEM ................................................................................. 4 1.3 AIM AND OBJECTIVES.................................................................................................. 6 1.4 WORKING HYPOTHESIS ............................................................................................... 7 1.5 JUSTIFICATION .............................................................................................................. 7 1.6 THE STUDY AREA.......................................................................................................... 8 1.6.1 Location .......................................................................................................................... 8 1.6.2 Relief and Drainage System............................................................................................ 9 1.6.3 Climate .......................................................................................................................... 10 1.6.4 Vegetation ..................................................................................................................... 14 1.6.5 Soil Type ....................................................................................................................... 14 1.6.6 Socio-Economic Activities ........................................................................................... 17 CHAPTER TWO ................................................................................................................... 18 LITERATURE REVIEW ...................................................................................................... 18 2.1 HYDROGEOCHEMISTRY ............................................................................................ 18 2.2 GROUNDWATER SALINISATION ............................................................................. 20 University of Ghana http://ugspace.ug.edu.gh ii 2.3 OVERVIEW OF ENVIRONMENTAL ISOTOPES ....................................................... 29 2.3.1 Stable isotopes ....................................................................................................... 30 2.3.2 Groundwater ‗‗age‘‘ dating and Radioisotopes ..................................................... 34 2.3.3 Theoretical Models For 14C Age Correction ......................................................... 44 2.3.3.1 Statistical Model (Vogel) ................................................................................ 45 2.3.3.2 Alkalinity Model (Tamer’s (1975)) ................................................................. 46 2.3.3.3 Chemical Mass-Balance (CMB) Model .......................................................... 46 2.3.3.4 δ13C mixing (δ13C model) – (Ingerson and Pearson 1964) ............................. 48 2.3.3.5 Matrix exchange (Fontes-Garnier model) ...................................................... 49 2.3.3.6 Complete Transformation of Soil CO2 into (TDIC) (Akiti, 1980) model............ 51 2.4 PREVIOUS STUDIES IN GHANA ................................................................................ 51 2.5 GROUNDWATER RECHARGE MECHANISMS ........................................................ 56 2.6 GEOLOGY ...................................................................................................................... 57 2.7 HYDROGEOLOGY ....................................................................................................... 62 2.7.1 Mode of groundwater occurrence .......................................................................... 62 2.7.2 Aquifer characteristics ........................................................................................... 66 CHAPTER THREE ............................................................................................................... 69 METHODOLOGY ................................................................................................................ 69 3.1 DATA COLLECTION AND ANALYTICAL PROCEDURES ..................................... 69 3.1.1 Desk study ............................................................................................................. 69 3.1.2 Fieldwork and Sample Collection ......................................................................... 70 3.1.2.1 Measurement of Field Parameters.................................................................. 74 3.1.2.2 Radiocarbon Sampling.................................................................................... 78 3.1.2.3 Tritium Sampling ............................................................................................ 81 3.2 LABORATORY ANALYSES ........................................................................................ 81 3.2.1 Major and Minor Ions ............................................................................................ 81 3.2.2 Measurement of Stable Isotopes of Water (δ18O and δ2H) .................................... 82 University of Ghana http://ugspace.ug.edu.gh iii 3.2.3 Tritium (3H) Measurement .................................................................................... 83 3.2.4 Radiocarbon (14C) Measurement ........................................................................... 84 3.3 DATA ANALYSES AND INTERPRETATION ............................................................ 85 3.3.1 Analytical Errors .................................................................................................... 85 3.3.2 Statistical Analysis ................................................................................................ 87 3.3.3 Graphical Methods ................................................................................................ 88 CHAPTER FOUR .................................................................................................................. 90 RESULTS AND DISCUSSION ............................................................................................ 90 4.1 HYDROGEOCHEMISTRY ............................................................................................ 90 4.1.1 Hydrogeochemical characteristics of rainwater, surface water and groundwater . 90 4.1.1.1 Rainwater ........................................................................................................ 90 4.1.1.2 Surface Water.................................................................................................. 94 4.1.1.3 Groundwater ................................................................................................... 98 4.1.1.4 Hydrochemcal Facies ....................................................................................... 125 4.1.1.5 Summary of hydrochemistry of rainwater and surface water and hydrogeochemistry of groundwater .............................................................................. 128 4.2 ISOTOPE STUDIES OF RAINWATER, SURFACE WATER AND GROUNDWATER .............................................................................................................. 130 4.2.1 Stable Isotopes Composition of Rainwater, Surface water and Groundwater ............ 130 4.2.2 Recharge Mechanism in the Study Area ..................................................................... 146 4.2.3 Carbon-13 (δ13C) to Investigate Seawater Intrusion ................................................... 148 4.3 GROUNDWATER AGE DETERMINATION ............................................................. 153 4.3.1 Tritium (3H) content of groundwater ................................................................... 153 4.3.2 Carbon-14 (14C) content of groundwater ............................................................. 158 4.4 ORIGIN OF SALINITY IN THE GROUNDWATERS ............................................... 169 4.4.1 Geochemical considerations ................................................................................ 169 University of Ghana http://ugspace.ug.edu.gh iv 4.4.2 Stable isotope considerations ............................................................................... 183 4.4.3 Chloride (Cl) levels in Rainfall ........................................................................... 185 4.4.4 Chloride (Cl) levels in Soil .................................................................................. 188 CHAPTER FIVE ................................................................................................................. 192 GENERAL CONLUSIONS AND RECOMMENDATIONS ............................................. 192 5.1 CONCLUSIONS............................................................................................................ 192 5.2 RECOMMENDATIONS ............................................................................................... 196 REFERENCES .................................................................................................................... 198 Appendix 1 Available borehole data .................................................................................... 225 Appendix 2 Hydrochemical parameters of the Central Region ........................................... 227 Appendix 3 Theory and Principles of operation of Equipments Employed in this study.... 235 University of Ghana http://ugspace.ug.edu.gh v DECLARATION This dissertation is the result of research work undertaken by SAMUEL YAO GANYAGLO in the Department of Nuclear Sciences and Applications, Graduate School of Nuclear and Allied Sciences, University of Ghana, Legon, under the supervision of Dr. THOMAS TETTEH AKITI, Prof. SHILOH KWABENA OSAE of the Graduate School of Nuclear and Allied Sciences and Dr. THOMAS ARMAH Department of Earth Science, University of Ghana, Legon. Signature………………………………………….. SAMUEL YAO GANYAGLO (Student) Date 18/03/15 . Signature……………………………. Signature………………………………. Dr . THOMAS TETTEH AKITI Prof. SHILOH KWABENA OSAE, PhD (Principal Supervisor) (Co-Supervisor) Date 18/03/15 . Date. 18/03/15 ……………. Signature……………………………… Dr. THOMAS ARMAH (Co-Supervisor) Date 18/03/15 … ………….. University of Ghana http://ugspace.ug.edu.gh vi ACKNOWLEDGEMENT I wish to express my profound gratitude to my supervisors Dr. Thomas Tetteh Akiti, Prof. Shiloh Kwabena Osae and Dr. Thomas Armah for their supervision, useful suggestions and encouragement throughout my studies. The immense contributions and suggestions form Prof. Samuel Boakye Dampre Deputy Director of Graduate school of Nuclear and Allied Sciences and Prof. Dickson Kwaku Adomako former manager of Nuclear Chemistry and Environmental Research Centre are very much appreciated. I would also like to thank my head of Department (Nuclear Science and Applications) Dr. Joseph Richmond Fianko for his words of encouragement throughout my study and for assisting in the collection of the first data set. My colleagues at work (Ghana Atomic Energy Commission) Abass Gibrilla, Elikem Ahialey, and Edward Pappah Bam supported me tremendously in the collection of data from the field. I recall a few times when we had to close late from the field. This would not have been possible if they were not committed and dedicated to the assignment. Their efforts are very much appreciated. I thank the International Atomic Energy Agency (IAEA) for funding this project through the provision of required equipments, materials (chemicals, standards and sampling bottles), analysis of some samples in other laboratories outside Ghana and expert mission. The expert mission actually helped improve the knowledge base of the subject matter. At this juncture I will like to commend Marie Ito formerly of the IAEA and the technical University of Ghana http://ugspace.ug.edu.gh vii officer in charge of this project, Laourence Goucry IAEA expert and Dr. Thomas Vitvar for their in depth technical assistance offered in carrying out this research. I would also like to thank the National Nuclear Research Institute (NNRI) of GAEC for providing the necessary funds and logistical support to undertake the fieldwork. I wish to express my sincere thanks to the various laboratories that supported in the analysis of the samples. They include the inorganic and isotope hydrology labs of the Nuclear Chemistry and Environmental Research Centre/NNRI-GAEC, Water Research Institute (WRI) of Council for Scientific and Industrial Research (CSIR), Isotope Hydrology Lab - Vienna, Laboratory of Radio-Analysis and Environment, National School of Engineers of Sfax, Tunisia, AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland and University of Groningen, Centre for Isotope Research, Netherlands. Finally I thank my father (Mr. E.W.K. Korkor), my mother (Mrs Comfort Korkor) and siblings (Agness, Gabriel, Victoria, Jonathan, Roselyn and Cornelius) for their encouragement and prayer support. Special thanks to Mrs Eunice Ganyaglo and my lovely children (Grace, Caleb, Gilbert and Gilberta). University of Ghana http://ugspace.ug.edu.gh viii List of Figures Fig. 1.1 An abandoned hand-dug well at Nsuekyir ................................................................. 3 Fig. 1.2 An abandoned borehole at Gomoa Abora .................................................................. 3 Fig. 1.3 Drying up of the River Ochi Narkwa during the dry season at Ajumako Abeadze resulting in low flow. ............................................................................................... 5 Fig. 1.4 Location map of the study area ................................................................................. 11 Fig 1.5 Time Series Hydrograph of Ayensu River at Oketsew in the Central Region (After Hydrological Services Department, Ministry of Water Resources, Works and Housing) ......................................................................................................... 12 Fig. 1.6 Total Monthly Rainfall (mm) from 2002 to 2011 from the Saltpond Station (Ghana Meteorological Agency) ........................................................................... 12 Fig. 1.7 Mean Monthly Minimum and Maximum Temperature from 2002 to 2011 at Saltpond Station (Ghana Meteorological Agency) ............................................... 13 Fig. 1.8 Mean Monthly Relative Humidity from 2002 to 2011 recorded at the Saltpond Station of the Ghana Meteorological Agency (GMA) .......................................... 13 Fig. 1.9: Soil map of the study area after Soil Research Institute of the Council for Scientific and Industrial Research (C.S.I.R). ........................................................ 15 Fig. 2.1 Plot of chloride (Cl-) mg/L versus 18O (‰ vs. SMOW) (After Akiti, 1985) ............ 23 Fig. 2.2 A schematic of δ2H-δ18O plot (Fontes and Edmunds, 1989). ................................... 33 Fig. 2.3 Tritium concentration in precipitation since 1950 at two IAEA stations (IAEA/WMO, 2006): Ottawa, Canada (Northern Hemisphere) and kaitoke, New Zealand (Southern Hemisphere). .................................................................. 36 Fig 2.4 The pathway and associated fractionation of 14C and 13C in CO2 during photosynthesis, respiration in soils and dissolution by groundwater (after Gillon et al., 2012, Clark and Fritz, 1997). Carbon-14 production is also shown. .................................................................................................................... 41 Fig 2.5. Schematic representation of the hydrologic cycle showing recharge and discharge zones (Freeze and Cherry, 1979) .......................................................... 56 Fig. 2.6 Geological Map of the Study Area ........................................................................... 60 Fig. 2.7 Water table map of the study area ............................................................................ 64 University of Ghana http://ugspace.ug.edu.gh ix Fig. 2.8a unconfined aquifer in granitic rock ......................................................................... 65 Fig. 2.8b Unconfined aquifer in mica schist rock .................................................................. 65 Fig. 3.1 Rain gauge at the Saltpond station of Ghana Meteorological Agency (GMA) ........ 73 Fig. 3.2 Radiocarbon sampling at Esuehyia in the coastal zone of the Central Region ........ 80 Fig. 4.1 Distribution of EC levels of the groundwaters with distance from the coast. The classification of the waters into fresh, brackish and saline is based on the scheme employed by (Park et al., 2011). ............................................................ 104 Fig 4.2 Spatial distribution of EC with respect to the geology of the area .......................... 105 Fig. 4.3 Spatial distribution of TDS (mg/L) in the study area ............................................. 107 Fig. 4.4 Spatial distribution of Ca2+ in the study area .......................................................... 110 Fig. 4.5 Spatial distribution of Mg2+ in the study area ......................................................... 112 Fig. 4.6 Spatial distribution of Na+ in the study area ........................................................... 114 Fig. 4.7 Spatial distribution of K+ in the study area ............................................................. 116 Fig. 4.8 Spatial distribution of HCO3 - in the study area ...................................................... 120 Fig. 4.9 Spatial distribution of Cl- in the study area ............................................................ 121 Fig. 4.10 Spatial distribution of SO4 2- in the study area ...................................................... 122 Fig. 4.11 Piper trilinear diagram showing hydrochemical facies of surface waters in the study area ............................................................................................................. 126 Fig. 4.12 Piper trilinear diagram showing hydrochemical facies of shallow groundwater in the study area ................................................................................................... 126 Fig. 4.13 Piper trilinear diagram showing hydrochemical facies of deep groundwater in the study area. ...................................................................................................... 127 Fig. 4.14 Piper trilinear diagram showing the hydrochemical facies of surface water, shallow groundwater and deep groundwater in the coastal zone of the Central Region. ................................................................................................................ 128 Fig 4.15 Stable isotope content of Rainfall at the Saltpond Meteorological Station in the coastal zone of the study area from August 2010 to September 2011. ............... 135 University of Ghana http://ugspace.ug.edu.gh x Fig. 4.16 Stable isotope content of Rainfall at the Twifo Praso Meteorological Station in the inland zone of the study area from August 2010 to September 2011. .......... 138 Fig. 4.17 δ2H-δ18O plot for surface water .......................................................................... 140 Fig. 4.18 δ2H-δ18O plot for shallow groundwater .............................................................. 141 Fig. 4.19 δ2H-δ18O plot for deep groundwaters in the study area ...................................... 145 Fig. 4.20 Relationship between δ2H ‰ V-SMOW and δ18O ‰ V-SMOW for rainwater (RW) from Saltpond and Twifo Praso staions, surface water (SW) and groundwater (GW) comprising shallow wells (hand-dug wells (HD)) and Deep wells (boreholes (BH)). ....................................................................................... 147 Fig. 4.21 Ranges for δ13C values in selected natural compound (Clark and Fritz, 1997). .. 150 Fig. 4.22 Plot of δ13C ‰ versus V-PDB against Cl- (mg/L) to deduce the possibility of seawater intrusion in the coastal aquifers of the Central region. ......................... 150 Fig. 4.23 Spatial distribution of tritium in groundwaters showing boreholes with tritium > 1 and those with tritium < 1 with respect to the geology of the area. The arrows show possible flow directions. ............................................................................ 156 Fig. 4.24 Spatial plot of tritium in the study area ................................................................ 157 Fig. 4.25 Relation between HCO3 - (mg/L) and δ13C of DIC (‰) V-PDB. ......................... 161 Fig. 4.26: 14C activities superimposed on the geology of the study area. The arrows show inferred groundwater flow paths which is NW-SE and NNE-SSW towards the sea. ....................................................................................................................... 165 Fig. 4.27 Plot of 14C (pmc) against 13C (PDB) of groundwater in the coastal zone of the Central Region ..................................................................................................... 166 Fig. 4.28 Plot of 3H (TU) versus percent modern carbon (pMC). ....................................... 167 Fig. 4.29 Na-Cl relationship of groundwater in the coastal areas of the Central Region .... 172 Fig. 4.30 Scatter plot between TDS and Na/(Na+Ca) showing rock dominant weathering in the area (after Singh et al 2010) ...................................................................... 172 Fig. 4.31 (a) Na/Cl molar ratio versus Cl (mg/l), (b) Na/Cl molar ratio Versus EC ........... 173 Fig. 4.32 Spatial distribution of Na/Cl molar ratio in the study area with respect to the geology of the area .............................................................................................. 174 Fig 4.33 Br/Cl weight ratio versus Cl (mg/L) ...................................................................... 176 University of Ghana http://ugspace.ug.edu.gh xi Fig. 4.34 (a) SO4 2- (meq/L) versus Cl- (meq/L), (b) SO4 2-/Cl- Versus EC (μS/cm) and (c) SO4 2-/Cl- Versus Cl (mg/L) ................................................................................. 178 Fig. 4.35 Spatial distribution of SO4 2-/Cl- in the study area................................................. 179 Fig. 4.36 Relationship of (a) Ca with SO4, (b) Ca/SO4 with EC, (c) Ca/SO4 with Cl (d) Ca with Na+K and (e ) Ca/Na+K with EC for groundwaters in coastal zone of the Central Region. .................................................................................................... 182 Fig 4.37 (a) δ18O - δ2H plot of the groundwaters in the study area (after Craig, 1961) (b) Relationship between Cl (mg/L) and δ18O‰ V-SMOW (after Akiti, 1985). ..... 184 Fig. 4.38 Cl- concentrations in the various rainfall events between May and August 2010 at the Twifo Praso and Saltpond stations. Higher Cl- concentrations are observed from the Saltpond station. .................................................................... 187 Fig. 4.39 Variation of Cl- (mg/kg) concentration in the unsaturated soil zone with depth (cm) showing zones of salt accumulation. .......................................................... 191 Fig. (A) Absorbance versus concentration of (a) Mg standard and (b) Ca standard ........... 237 Fig. (B) Sample positions in the autosampler tray for a three-standard arrangement. ........ 240 University of Ghana http://ugspace.ug.edu.gh xii List of Tables Table 4.1 Anion chemistry of Rainwater from Saltpond (SP) Meterological Station in the Central Region (May 2010 – August 2010) .......................................................... 92 Table 4.2 Anion chemistry of Rainwater from Twifo Praso (TP) Saltpond Meterological Station in the Central Region (May 2010 – August 2010) .................................... 93 Table 4.3 Hydrochemical data of surface waters in the study area ....................................... 97 Table 4.4 Statistical summary of hydrochemcal parameters of shallow groundwaters (Hand-dug wells) ................................................................................................. 101 Table 4.5 Statistical Summary of hydrochemical parameters of deep groundwaters (Boreholes) .......................................................................................................... 101 Table 4.6 Saturation Indices for calcite and dolomite ......................................................... 123 Table 4.7 Stable isotope composition of rainfall from Saltpond meteorological station in 2010 ..................................................................................................................... 133 Table 4.8 Stable isotope composition of rainfall from Saltpond station in 2011 ................ 134 Table 4.9 Stable isotope composition of rainfall from Twifo Praso (TP) meteorological station in 2010 ..................................................................................................... 136 Table 4.10 Stable isotope composition of rainfall from Twifo Praso (TP) meteorological station in 2011 ..................................................................................................... 137 Table 4.11 Stable isotope composition of surface water ..................................................... 139 Table 4.12 Stable Isotope Compositions of ShallowGroundwaters .................................... 141 Table 4.13 Stable Isotope Composition of Deep Groundwater ........................................... 143 Table 4.14 Tritium content of groundwater in the coastal zone and part of the inland zone of the Central Region .......................................................................................... 155 Table 4.15 14C and 13C content of groundwater in the study area ....................................... 164 Table 4.16 14C ages of groundwater samples in the parts of the Central Region ................ 168 University of Ghana http://ugspace.ug.edu.gh xiii ABSTRACT The major setback in the exploitation of groundwater in the Central Region of Ghana is poor water qualitydue to high salinity. The source of salinity has not been adequately addressed. The focus of this study was, therefore,to understandthe hydrogeochemical processes occurring in the study area in order to determine the origin of salinity and groundwater residence time. The methodology involved desk study, fieldwork, and laboratory work. The desk study comprised review of literature, compilation of existing borehole data, topographical and geological maps. The fieldwork involved collection of rainwater, surface water, and groundwater and soil samples. One hundred and thirty seven (137) rainfall events were obtained from Saltpond and Twifo Prasso Meteorological Stations and Six (6) surface water samples from Ochi Narkwa and Ayensu Rivers. Seventy-eight (78) groundwater samples were collected. Thirty five soil samples from four profiles were collected for measurement of Cl- in the soil zone. Physico-chemical parameters such as pH, temperature, electrical conductivity (EC), total dissolved solids (TDS), salinity, redox potential and alkalinity were measured in the field. In the laboratory,Ca2+, Mg2+, Na+, K+, Cl-, SO4 2-, NO3 -, F-, Br-, PO4 3-, δ18O, δ2H, δ13C,3H and14C were measured.Data obtained were evaluated using bivariate plots, statistical and graphical methods. The rainwater chemistry in the study area showed the dominant anion as Cl- ranging between1.07 mg/L and 22.32 mg/L at the coast and 0.48 mg/L to 8.28 mg/L at 90 km from the coast. Higher Cl- content occurred at the coast suggesting the ocean as a major contributor of Cl- in rainwater.Low TDS between 69.90 mg/L and 93.00 mg/L occurred inthe surface waters showing generally low concentrations of major ions with dominant hydrochemical facies as Na-Cl. In the shallow groundwater, cations University of Ghana http://ugspace.ug.edu.gh xiv occurred in the orderNa+> Ca2+> Mg2+> K+ and the anions in the order Cl-> HCO3 -> SO4 2-. The major hydrochemical facies was Na-Cl. In deep groundwater, the major cations occurred in the order Na+> Ca2+> Mg2+> K+ and the major anions in the order Cl- > HCO3 -> SO4 2-similar to that of shallow groundwater. This suggested that the deep groundwater chemistry were developed from the unsaturated zone. The hydrochemical facies identified were Ca-Mg-HCO3, Na-Cl, Ca-Mg-Cl-SO4 and non-dominant water types. Stable isotope composition of rainwater, shallow groundwater and deep groundwater showed the mechanism of recharge to the aquifers was direct infiltration of local rainfall of mean isotopic composition δ18O = -3.8 ‰ V-SMOW and δ2H = -18 ‰ V-SMOW. It was established through δ13C - Cl- relationship that the groundwaters may not be intruded by seawater water. Tritium in the groundwaters ranged from 0.05 ± 0.07 to 4.75 ± 0.16 TU.Eighty-five percent (85%) of the samplessuggestedmodern recharge or young waters with tritium values ranging between 1.07 ± 0.25 TU and 4.75 ± 0.16 TU. Fifteen percent (15%) of the samples constituted old waters and covered boreholes CR2- 50 at Ekumfi Asokwa, CR4-05 at Sefara Kokodo, CR4-FZ-22 and CR4-FZ-08 at Ayeldu with tritium values ranging between 0.05 ± 0.07 TUand 0.67 ± 0.22 TU. 14C content of the groundwaters ranged between 9.50 pMC and 113.56 pMC. Most of the waters were of modern recharge except borehole CR2-50 at Ekumfi Asokwa which is older.The estimated ‗age‘ or residence time of this older water was 19,459 years before present (BP) based on Akiti‘s model.Spatial distribution of 3H and 14C in the study area showed a localised system of flow suggesting discontinuous aquifer systems in the study area. Groundwater salinization in the coastal zone of the Central Region may be caused largely by halite dissolution and to a minor extent silicate weathering. Study of Cl- profiles in the University of Ghana http://ugspace.ug.edu.gh xv soil zone, revealed occurrence of NaCl in lenses, hence the existence of salt crusts at depths, between 80 and 120 cm which support halite dissolution in the study area.The Na/Cl (0.36–5.18), Br/Cl (0.0054–2.08), SO4 2-/Cl-(0.02–4.09), and Ca/SO4(0.35–10.84) molar ratios suggest that seawater intrusion plays a minimal role in controlling the groundwater chemistry in the study area. In conclusion, the origin of salinity was halite dissolution. Most of the groundwaters were of modern recharge except borehole CR2-50 at Asokwa which is older. The residence time of the older water was 19,459 years BP.Exploitation of groundwater resource of modern recharge is therefore sustainable but susceptible to contamination because it is easily replenished. It is recommended that the recharge areas should be protected by enacting laws that will control anthropogenic activities in these areas. Since the old groundwater encountered in the area is liable to depletion, groundwater abstraction must be regulated to prevent over abstraction that would result in depletion and possible collapse of the aquifer. Future studies into origin of salinity should employ 32S and 34S, 11B and 86Sr/87Sr. Newly-developed dating methods for young waters such as Chlorofluorocarbons (CFC) should be considered to quantitatively determine the ages of the young waters. University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE INTRODUCTION 1.1 BACKGROUND Groundwater is the major source of water supply for domestic, industrial and agricultural sectors of many countries. It is estimated that approximately one third of the world‘s population use groundwater for drinking purposes withdrawing about 20 percent of global water (600-700 km3) annually and much of it from shallow aquifers (Bhattacharya, 2012). Sustainable exploitation of groundwater is of global concern; owing to increase in water demand due to population increase, industrialization and increase in agricultural activities. Groundwater aquifers are being over-pumped especially in the arid and semi- arid areas and increasingly, becoming more polluted, mostly with chemical runoff (fertilizers and pesticides) and animal wastes from agricultural activities. In Ghana, Kortatsi, (1994) reported that there were over 56, 000 groundwater abstraction systems comprising boreholes, hand-dug wells and dug-outs. The yields of aquifers are generally low. Borehole yields hardly exceeded 6 m3/h (100 l/min). Generally, groundwater quality is considered good for domestic and agricultural purposes. However, low pH (3.5-6.0), high iron values (1-64 mg/L) in few cases and high salinity values (5000-14, 584 mg/L) in some coastal aquifers have been reported. In the Central Region of Ghana, groundwater is an important source of potable water supply, especially in the rural communities due to limited surface water supplies, ease of University of Ghana http://ugspace.ug.edu.gh 2 accessibility and high cost involved in the treatment and extension of surface water to rural communities.However, the development of groundwater is mainly for domestic purposes. Nonethelessin order to ensure sustainable utilization of groundwater in the study area there is the need for a thorough understanding of the hydrogeochemistry, an assessment of the general water quality and groundwater renewability. By and large, one of the major setbacks in the exploitation of groundwater in the Central Region is poor water quality in majority of the boreholes due to high salinity. Significant numbers of boreholes have been abandoned as result of high salinity in the coastal areas of the study area (Figs 1.1, 1.2). Electrical conductivities (EC)above 5000µS/cm have been reported both along the coast andinland areas (Armah, 2002).However, the exact source of salinity is not well known. The first attempt at determininig the origin of salinity of groundwater in the coastal zone of the Central Region was carried out by (Armah, 2002). He employed geophysical techniques, hydrochemistry and stable isotopes to trace the source of salinisation.His study attributed salinisation of groundwater to multiplicity of sources including seawater intrusion.Theother sources suggested included processes involving mixing of fresh water with subsurface saline formation. However, this needed to be proved by dating the groundwaters and establish their relationship with salinity. This represents the knowledge gap that this study intends to fill. University of Ghana http://ugspace.ug.edu.gh 3 Generally, the sources of salinity in groundwater could be as a result of: I. Direct seawater intrusion II. Aerosols from sea spray III. Dissolution of soluble salts in the soil zone IV. Slowmovement in the groundwater flow system. Therefore in addressing the sources of saline water in the groundwaters and other associated geochemical problems, a detail hydrogeochemical and environmental stable isotope studies of the groundwaters and ―age‖ (residence time) determination of the various groundwater systems in the study area are required. Environmental stable isotopes (18O, 2H) are commonly used in regional groundwater studies to identify flow regimes and sources of recharge and origin and mixing Fig. 1.1 An abandoned hand-dug well at Nsuekyir Fig. 1.2 An abandoned borehole at Gomoa Abora University of Ghana http://ugspace.ug.edu.gh 4 patterns(Das et al., 1988; Leontiadias et al., 1988).Commonly employed isotope techniques for determining the age of groundwater are tritium and carbon-14. In this study, hydrogeochemical and isotopic techniques have been employed to understand the hydrogeochemistry of the various aquifers in the different geological terranes of the study area and determine the sources of salinity in the groundwaters as well as recharge processes and ―age‖ or residence time of groundwater. 1.2 STATEMENT OF THE PROBLEM The population of the Central Region of Ghana, based on 2010 population census was estimated at 2,107,209, representing an increase of 32.2% over the 2000 population census figure of 1,593,823 (Ghana Statistical Service, 2012). The ever growing population over the years has resulted in growing demand for fresh-water supply for domestic and agricultural purposes. Traditionally, water supply in many rural communities in the central region is from surface water (rivers and streams) and rainwater. The availability of these sources of water supply is dependent on the seasons. Most of the streams and some of the rivers dry up during the dry season as illustrated in (Fig. 1.3). In addition, the surface water resources are polluted giving rise to water borne and water related diseases such as guineaworm, bilharzia and typhoid fever (Dapaah- Siakwan and Gyau-Boakye, 2000). University of Ghana http://ugspace.ug.edu.gh 5 Fig. 1.3 Drying up of the River Ochi Narkwa during the dry season at Ajumako Abeadze resulting in low flow. Groundwater unless polluted is of better quality than surface water and can be available in close proximity to needs without large scale storage, treatment and distribution systems (Dapaah-Siakwan and Gyau-Boakye, 2000). Since groundwater is of a better quality than surface water, the government of Ghana in collaboration with international partners initiated various projects developing groundwater in the form of boreholes as alternative system of water supply to rural communities throughout Ghana, including the Central Region. Notably amongst them is the 3000 well programme which covered most of northern Ghana and parts of southern Ghana including the Central Region. This has resulted in the eradication of guineaworm in the Central Region. Currently the Danish International Development Agency (DANIDA) is supporting the drilling of additional boreholes in the Central Region. Even though efforts have been made to improve rural water supply through groundwater exploitation, this is being done without adequate information on the knowledge of the recharge areas, the general flow direction, ―age‖ or residence time of groundwater and University of Ghana http://ugspace.ug.edu.gh 6 the processes controlling the hydrogeochemical evolution of groundwater. There is lack of adequate knowledge on the source of salinity in the study area.Knowledge on the flow pattern, residence time and hydrogeochemistrywould help to elucidate the possible sources of anthropogenic contamination, source of salinity andhydrogeochemical reactions taking place in an aquifer system. These will help toassess general groundwater quality for different uses and factors governing the chemical composition of groundwater as well as the contribution of each factor. This study applies hydrogeochemistry, stable isotopes (18O,2H and 13C), and radioisotopes (3H and 14C) to understand the hydrogeochemical evolution of groundwater, hence establishing the origin of salinity, recharge processes and residence time of groundwater. 1.3 AIM AND OBJECTIVES The main aimof this study is to establish the hydrogeochemical processes that control the hydrogeochemistry and general groundwater quality in the study area using hydrochemical evaluation, stable and radioisotopes.Specific objectives include: (1) Determination of hydrogeochemical characteristics of the groundwater systems in the study area. (2) Identification of recharge mechanisms in the study area (3)Assessment of the origin of saline waters in the various aquifers University of Ghana http://ugspace.ug.edu.gh 7 (4) Determination of the ―age‖ or residence time of the groundwaters to improve the understanding of recharge process in the study area. 1.4 WORKING HYPOTHESIS The hypotheses being tested are:  Seawater intrusion or geochemical processes in the subsurface account for the evolution of groundwater salinity in the study area. 1.5 JUSTIFICATION Salinization of water resources is one of the most widespread processes that degrades water-quality and endangers future water exploitation. In many areas, particularly in arid and semi-arid zones, ground-water salinization limits the supply of potable fresh water. This problem is intensified in coastal aquifers where human activities result in accelerating water-quality deterioration. It is also observed that in in-land basins ground- water salinization can be one of the most important factors that affect the water quality. Monitoring and identifying the origin of the salinity are crucial for both water management and remediation. However, it is an extremely difficult task to undertake since variety of salinization sources exist.This sources could be anthropogenic or geogenic. Integration of different geochemical and isotopic tools provides a better assessment of the origin and mechanisms of groundwater salinization. University of Ghana http://ugspace.ug.edu.gh 8 In the Central Region of Ghana, rural water supply from groundwater is on the increase. Proper management of this resource is impossible without knowledge of the spatial distribution of fresh and saline groundwater and the processes that determine the evolution of salinity. In addition, for sustainable exploitation of the resource ‗age‘ determination is required. The ‗age‘ of groundwater gives information as to whether groundwater is replenished by the modern hydrologoical cycle or old. Groundwaters replenished by modern hydrological cycle are liable to contamination and needed to be protected at the recharge areas. For older waters there is no replenishment and excessive pumping may lead to groundwater depletion. In older groundwaters pumping is regulated to prevent depletion of groundwater. As the importance of groundwater resources continues to rise in the future; and more ofthe freshwater reserves affected by rising salinity, findings of this study would aid water managers, policy makers, drinking-water suppliers, and scientists to (1) better understand how and why elevated concentrations of dissolved solids occur in groundwater (2) assess and predict vulnerability of groundwater to elevated salinity in unmonitored areas; (3) evaluate current sites and seek new sites for water supply; and (4) evaluate various strategies for resource-development, water management, and salinity control. 1.6 THE STUDY AREA 1.6 .1 Location University of Ghana http://ugspace.ug.edu.gh 9 The study area lies within latitudes 5o 11' 45'' to 5o 28.66' 30''N of the equator and longitudes 0o 33' 41.6'' to 1o 11' 17''W of the Greenwich Meridian covering a total area of 700.09 km2 (Fig. 1.4). It comprises of the Ayensu, Ochi-Narkwa and Pra River Basins. It is made up of 13 administrative districts namely Abura Asebu Kwamankese, Ajumako- Enyan-Essian, Awutu-Efutu-Senya, Agona, Asikuma-Odoben-Brakwa, Assin North, Assin South, Gomoa East, Gomoa West, Komenda-Edina-Eguafo- Abrem, Mfantsiman and Twifo-Hemang Lower Denkyira. It is bounded to the south by the Gulf of Guinea, north by the Ashanti Region, Western Region to the west, Eastern Region to the northeast and Greater Accra Region to the east (Fig. 1.4). This study focuses on the coastal zone covering predominantly the Ochi-Narkwa Basin and part of Ayensu and Ochi Amissah Basins (Fig. 1.4). The choice of the coastal zone is based on the objectives set. 1.6 .2 Relief and Drain age System The project area can be broadly divided into two: the coast, consisting of undulating plains with isolated hills and occasional cliffs characterized by sandy beaches and marsh in certain areas and hinterland, where the land rises between 250 m and 300 m above sea level. Greater part of the study area is drained by the River Ochi-Narkwa and its tributaries. It rises from the northern section of the study area, flowing throughout the year with fluctuations in volume. It flows into the Narkwa Lagoon at the coast and discharges into the sea east of the Narkwa village. One other major river draining a section of the study area is the River Ayensu. The Ayensu River Basin forms part of the Coastal River Basin System of Ghana. The source University of Ghana http://ugspace.ug.edu.gh 10 of this river is the Atewa hills in the East Akim District at an altitude of about 610m above mean sea level. It flows through the Suhum-Kraboa-Coaltar district before entering the sea in the Central Region. It drains an estimated total area of 1709 km2 in the country. The Hydrological Services Department of the Ministry of Water Resources, Works and Housing has been monitoring the flow rate of the River Ayensu at its gauging station in Oketsew. Data obtained from January 2006 to December 2007 show that the peak flows occur in June to July and then October to November (Fig. 1.5). The dry season (December to March) show relatively low flows. 1.6 .3 Climat e The climatic conditions prevailing in the area can be divided into dry equatorial zone and moist semi-equatorial zone. Annual rainfall ranges from 1000 mm along the coast to 2000 mm in the interior. The study area experiences double maximal rainfall pattern. The wettest months are May-June and September-October while the drier periods occur in December-February (Fig. 1.6) based on data obtained from the Ghana Meteorological Agency (GMA) at the Saltpond station from 2002 to 2011. Mean monthly minimum temperature ranges from 22.84oC to 24.87oC whereas mean monthly maximum temperature ranges from 27.20 to 31.95oC. The coolest month occur in August whilst the hottest months occur in March to April (Fig 1.7). The mean monthly relative humidity ranges from 78.80 to 89.10 %. The highest mean monthly relative humidity occurs in the months of June to August corresponding to the peak of the rainy season (Fig. 1.8). Two wind systems are experienced, the South-Western Monsoon(the direction of which influences the rainfall pattern) and the dry Harmattan winds (North-East Trade Winds). University of Ghana http://ugspace.ug.edu.gh 11 Fig. 1.4 Location map of the study area University of Ghana http://ugspace.ug.edu.gh 12 Fig 1.5 Time Series Hydrograph of Ayensu River at Oketsew in the Central Region (After Hydrological Services Department, Ministry of Water Resources, Works and Housing) Fig. 1.6 Total Monthly Rainfall (mm) from 2002 to 2011 from the Saltpond Station (Ghana Meteorological Agency) University of Ghana http://ugspace.ug.edu.gh 13 Fig. 1.7 Mean Monthly Minimum and Maximum Temperature from 2002 to 2011 at Saltpond Station (Ghana Meteorological Agency) Fig. 1.8 Mean Monthly Relative Humidity from 2002 to 2011 recorded at the Saltpond Station of the Ghana Meteorological Agency (GMA) University of Ghana http://ugspace.ug.edu.gh 14 1.6 .4 Vegeta tion The main vegetation zones as described by Benneh and Dickson (1990) are the Coastal Scrub and Grassland, Moist Semi Deciduous Forest and Strand and Mangrove. The coastal scrub and grassland together with the strand and mangrove occur along the coast. The Coastal Scrub and Grassland is also known as Southern Savannah. This makes up one-third of the nation‘s grasslands. It is characterized by grassland and trees with patches of scrub. Closer to the coast is the Strand and Mangrove. The Mangrove trees grow to a height of 12-15 meters and are closely packed and green throughout the year. Ecologically however, this provides a definite habitat, as the zone is of relatively little agricultural importance and does not support livestock. The plants that occur include Ipomoea pes-caprae and Avicenna species. The moist semi deciduous forest dominates the inland areas. It is characterized by tall trees interspersed with grass cover, shrubs and soft woody species. Much of the original dense forest vegetation has been cleared for cultivation of cocoa and oil palm. 1.6 .5 Soil Type The soils in the study area are made of four main groups namely forest ochrosols, savanna ochrosols, tropical black earth, and forest lithosols (Brummer, 1962). Soil map of the study area is presented in (Fig. 1.9). The forest ochrosols are described as deeply weathered soils found in the semi-deciduous forest and parts of the forest savanna transition agro-ecological zones of Ghana (Adjei-Gyapong and Asiamah, 2000).The University of Ghana http://ugspace.ug.edu.gh 15 texture of the subsoil is highly variable. It may be sandy clay loam, silty clay loam, sandy clay or silty clay with (10–40%) quartz gravels and stones and hard iron and manganese dioxide concretions.The top soil generally has a pH range of 5.1- 6.5 showing acidic conditions (Adjei-Gyapong and Asiamah, 2000).The forest ochrosols have a high nutrient value and are suitable for both tree and food crops, such as cocoa, coffee, citrus, maize, cassava, pineapple, and vegetables. Fig. 1.9: Soil map of the study area after Soil Research Institute of the Council for Scientific and Industrial Research (C.S.I.R). University of Ghana http://ugspace.ug.edu.gh 16 Savanna ochrosols occur in areas with semiarid climatic conditions. In the study area they occur mainly along the coast. The solum is relatively thinner than the forest counterparts. Decomposed rock is encountered at about 150 cm depth. Generally the top soils are thin (<20 cm), greyish brown sandy loam, weak granular and friable. The subsoils range from red in summits to brownish yellow middle slope soils. Commonly occurring in some of these soils areironstone concretions and sandstone brashes of about 10–40 percent. These soils support crops such asyams, maize, sorghum, millet, cowpea, groundnuts and cassava. Cashew production is currently on the increase on some of these soils where there is enough moisture to support its growth. The tropical black earth is sticky, and dark in color containing a mixture of high percentage of magnesium, calcium, and lime. During the rainy season, these soils become sticky but compact, hard and crack up during the dry periods. The soils are potentially suitable for rice, cotton, and sugarcane especially when artificial irrigation is applied. The tropical black earth exists along the coastal areas and lagoons. The forest lithosols are found between Nyanyano and Winneba (Fig. 1.9). These soils are also referred to as rocky soils due to underlying hardpan and mostly have poor nutrient value. They can, however, support the cultivation of vegetables. Crops such as sugarcane, maize, and pineapple are grown along the valleys. These soils cover a wide area of the savanna belt of the study area. Major fertilizersemployed in the area include nitrogen, phosphorus, andpotassium. University of Ghana http://ugspace.ug.edu.gh 17 1.6.6 Socio-Econom ic Activit ies The main economic activity of the people in the study area is fishing and farming. Fishing activities are concentrated along the coast whereas farming activities are concentrated inland where the moist semi-deciduous forest occurs. Major crops grown include cassava, maize, plantain, cocoyam, yams, citrus and vegetables (garden eggs and pepper), cocoa and coconut. Non-traditional crops such as cashew and pineapples are also grown especially in the Mando and Abaasa zones. Some communities also grow cocoa. Agro-processing activities have also been spotted in the area. Notably amongst them is gari processing, palm-oil and palm kernel oil extraction often in small groups or by individuals. The exploitation of kaolin for building, ceramic material, talc, granite and silica are common in the area. This has given rise to the building of the Ghana Ceramics Company at Saltpond. Crude oil is being exploited off the coast of Saltpond. University of Ghana http://ugspace.ug.edu.gh 18 CHAPTER TWO LITERATURE REVIEW 2.1 HYDROGEOCHEMISTRY The chemical and biochemical interactions between groundwater and biological materials of soils and rocks provide a wide variety of dissolved inorganic and organic constituents. Other important considerations captured by (Hiscock, 2005) include the varying composition of rainfall and atmospheric dry deposition in groundwater recharge areas, the modification of atmospheric inputs by evapotranspiration, differential uptake by biological processes in the soil zone and mixing with seawater in coastal areas. The principal dissolved components of groundwater are the seven major ions sodium (Na+), calcium (Ca2+), magnesium (Mg2+), potassium (K+), chloride (Cl-), bicarbonate (HCO3 -), and sulphate (SO4 2-) (Freeze and Cherry, 1979, Fetter, 1994, Hiscock, 2005). These cations and anions have been reported to comprise over 90% of total dissolved solids content, regardless of whether the water is dilute rainwater or has salinity greater than seawater (Hiscock, 2005). Minor ions include dissolved iron (Fe2+), strontium (Sr2+), Fluoride (F-), nitrate (NO3 -), bromide (Br-), phosphate (PO4 3-) and Lithium (Li+) while aqueous solutions commonly also contain amounts of trace elements and metal species. The introduction of contaminants into groundwater from human activities can result in some normally minor ions reaching concentrations equivalent to major ions. Typical example includes excessive application of nitrogenous fertilizers which could result in elevated concentrations of NO3 - in soil water and groundwater. University of Ghana http://ugspace.ug.edu.gh 19 Geochemical processes occurring within the groundwaterand reactions with aquifer minerals have aprofound effect on water quality.Hydrogeochemicalcomposition of groundwater can also be indicativeof its origin and history of the passage throughunderground materials with which the water has beenin contact. Waterbodies are continuously subjected to a dynamicstate of change with respect to lithological characteristics and geo-climatic conditions(Prasanna et al 2011). Glyn and Plummer (2005) observed that geochemistry has contributed significantly to the understanding of groundwater systems over the last 50 years. Historic advances include development of the hydrochemical facies concept, application of equilibrium theory, investigation of redox processes, and radiocarbon dating. Chebotarev (1955) put forward the concept that salinity distribution of groundwaters obeys a definite hydrological and geochemical law which can be formulated as the cycle of metamorphism of natural waters in the crust of weathering. Chebotarev (1955) recognized that the distribution of groundwaters with different hydrochemical facies depend on rock-water interaction in relation to hydrogeological environment, with groundwaters evolving form bicarbonate waters at outcrop to saline waters at depth in the earth crust.The Chebotarev‘s sequence of chemical evolution is summarized as: Chemical sequence along flow path HCO3 - →HCO3 - + SO4 2- → SO4 2- + HCO3 - → SO4 2- + Cl- →Cl- + SO4 2- → Cl- (2.1) Increasing age University of Ghana http://ugspace.ug.edu.gh 20 2.2 GROUNDWATER SALINISATION Groundwater salinisation in coastal areas occurs in many aquifers around the world (Barlow, 2003; Beer et al., 1999). Understanding the origin and mechanism of the salinisation process is important in preventing further deterioration of groundwater resources. Theorigin of salinisation is sometimes clearly identified to be modern seawater intrusion into aquifers. However, such a single source is not thecase in many systems. Ancient marine intrusion, winddrivensea spray and marine aerosols deposited at thetopsoil, effects of mobilised salts stored in theunsaturated zone, evaporative enrichment and localpollution have been identified to contribute to the salinization ofgroundwaters (Andreasenand Fleck, 1997; Cruz and Silva, 2000; Cartwright et al., 2004, Faye et al., 2005). Many investigators (Araguas - Araguas, 2003; Beer et al., 1999; Ghabeyen et al., 2006) have mentioned that in addition to these sources of salinity, downward leakage from surficial saline water through improperly constructed well could also contribute to groundwater salinity. Seawater intrusion is frequently observed in the case of unconfined aquifer connected to the sea where strong demand in water resources induces a decrease of thewater table. Most of the ions in groundwater (Na+, K+, Mg2+, Ca2+, HCO3 -, SO4 2-, Cl-, and Br-) have been used to deduce the hydrochemical evidence of seawater intrusion. A good correlation between the various ions with Cl- indicates they may be derived from the same source of saline water. Br/Cl ratio has often been used as a reliable indicator of the origin of salinity due to its specific composition in various saline sources(Capaccioni et al., 2005; De Montety et al., 2008). A contribution of evaporite dissolution leads to a low University of Ghana http://ugspace.ug.edu.gh 21 Br/Cl molar ratio (Cartwright et al., 2006, Ghabayen et al., 2006) whereas anthropogenic sources give a molar ratio around 0.0005 for waste water seepages or around 0.02 for agricultural return flows (Andreasen and Fleck, 1997; Ghabayen et al., 2006; Vengosh et al., 1998). De Montety et al., (2008) employed Na/Cl molar ratio to confirm marine influence of the salt of the groundwaters in the coastal aquifer of Rhone delta (Southern France). They found that most of the samples fall under the dissolution line of halite with a mean slope of 0.81 close to the Na/Cl molar ratio of seawater (0.86) as obtained by (Vengosh et al., 1999). The International Atomic Energy Agency (IAEA, 1983) and Payne (1983) outlined three main mechanisms of groundwater salinity in arid and semi-arid environments. In each case, groundwater is expected to have some specific isotopic and chemical composition. The mechanisms and the expected isotopic signals are: (a) If salinity arises due to mixing with marine water trapped in alluvial sediments the δ2H and δ18O values are together and individually with chloride, linearly correlated along mixing line with seawater as one end-point. (b) Salinity arising due to concentration of salts by evaporation has δ2H and δ18O on a regression line with slope 4 to 6. The EC or Cl is positively correlated with isotopic contents. (c) If the cause of salinity is due to the dissolution of salts from soils and rocks, the stable isotopic concentration is not changed with increase in salinity of groundwater, and there is no correlation between isotopic values and chloride concentrations. A typical example of this mechanism of salinization is illustrated University of Ghana http://ugspace.ug.edu.gh 22 in (Fig. 2.1) for groundwaters in Cape Verde (Akiti, 1985). Majority of the samples fall within the dissolution band indicating major cause of salinity to be dissolution of salts from rocks and soils. A sample, (56-52) located within the mixing band suggests seawater intrusion. The application of isotope tracers to hydrology, the fundamental relationships between δ18O and δ2H and δ18O and salinity has been used to identify different salinisation pathways. Some of these pathways include flushing of airborne salts by precipitation, dissolution of evaporite minerals from the surface, soil, or aquifer components, seawater intrusion or flow induced from pockets of connate brine (Kim et al., 2003; Wang et al., 2012). The chemical composition of groundwater aside from being affected by atmospheric input (i.e. sea spray or aerosols), is also caused by mineral weathering through water-rock interaction, anthropogenic activities and biogeochemical processes. The weathering of minerals generally exert an important control on groundwater chemistry (Kim et al., 2005). This process contributes to the release of major cations (Ca2+, Mg2+, Na+, K+) in groundwater (Kim, 2002; Kim et al., 2003). In crystalline rock terranes most of the cations are derived from the weathering of silicate minerals or are from cyclic salts. The primary source of sodium and potassium is the feldspars. Biotite may also be an additional source of potassium. In felsic gneisses and granites, the primary sources of calcium and magnesium are biotite and amphiboles. University of Ghana http://ugspace.ug.edu.gh 23 Fig. 2.1 Plot of chloride (Cl-) mg/L versus δ18O (‰ vs. VSMOW) (After Akiti, 1985) In the gabbros and amphibolites, amphiboles, pyroxenes and ca-feldspars are observed to be major sources of calcium and magnesium. In crystalline terranes, the source of bicarbonate is attributed to the weathering of silicate minerals since these areas lack carbonate minerals. The weathering of silicate minerals is illustrated in equations 2.2 to University of Ghana http://ugspace.ug.edu.gh 24 2.7. The weathering process is initiated by carbonic acid (H2CO3) produced by reaction of atmospheric carbon dioxide (CO2) and rainwater. As weak H2CO3 in rainwater infiltrates through the soil zone more of it is produced by decomposition of organic matter releasing CO2 which in turn reacts with the infiltrating rainwater. Weathering of silicate minerals as captured by (Rajmohan and Elango 2004) is illustrated in equations 2.3 to 2.7.Plagioclase feldspar (CaAl2Si2O8) weathering is illustrated by equation (2.3), that for albite, Na-rich feldspar (NaAlSi3O8) by 2.4 andK-rich feldspar (KAlSi3O8) orthoclase by 2.5. Equation 2.6 illustrates the weathering of the pyroxenes and Equation 2.7 illustrates the weathering of the amphiboles (hornblende). CO2(g) + H2O ↔ H2CO3 (2.2) CaAl2Si2O8 + 6H2O + 2H2CO3 ↔ Ca 2+ + 2Al(OH)3 + 2H4SiO4 + HCO3 - (2.3) NaAlSi3O8 + 7H2O + H2CO3 ↔ Na + + Al(OH)3 + 3H4SiO4 + HCO3 - (2.4) KAlSi3O8 + 7H2O + H2CO3 ↔ K + + Al(OH)3 + 3H4SiO4 + HCO3 - (2.5) CaMg(Si2O6) + 4CO2 + 6H2O ↔ Ca 2+ + Mg2+ + 4HCO3 - + 2Si(OH)4 (2.6) Ca2MgSi8O22(OH)2 + 14CO2 + 22H2O ↔ 2Ca 2+ + 5Mg2+ + 14HCO3 - + 8Si(OH)4 ( 2.7) Generally, the chemical composition of groundwater is a function of the aquifer‘s lithologic framework (Freeze and Cherry, 1979, Yeko, 1980). This lithologic framework gives rise to various hydrochemical facies in groundwater from which areas of recharge, origin of salinity and flow pattern are assessed. Li et al., (2007) used hydrogeochemical facies in different areas in a complex alluvial fan system, southwest of North China Plain to develop groundwater flow pattern. The hydrogeochemical facies were observed as University of Ghana http://ugspace.ug.edu.gh 25 reflecting the response of chemical processes occurring within lithologic framework and also the pattern of groundwater flow (Back, 1960, 1966). Gomez et al., (2006) emphasized that the nature and distribution of rock formations, structural setting and the hydrogeological system have an important influence in the spatial distribution and geochemistry of groundwater.Gomez et al., (2006) therefore described the chemical composition of groundwater as largely dependent on the geology of the area which in turn determines the different chemical evolution. In the KrishnaRiver Delta, India, the multi aquifer systems were studied through an integrated approach using hydrochemical, hydrogeological and isotopic techniques. The study was undertaken as a result of reported seawater intrusion into the groundwater system of the agriculturally rich region. The results of hydrochemistry and environmental tritium including radiocarbon dates indicated that the origin of salinity in the aquifer system is due to paleo-geographical conditions (Kumar et al., 2011). Further, Cl/Br ratio and Stable isotopes (δ18O and δ2H) were used to study the aquifer-aquifer interconnectivity and to identify perched aquifers within the study area (Kumar et al., 2011). Moussa et al., (2009) realized that majority of Grombalia groundwaters in Tunisia show strong correlation between Na and Cl elements with a correlation coefficient close to 1 which indicates a common origin of Na and Cl. This origin was suggested as dissolution of halite (NaCl) during the transit of water in the unsaturated zone. Moussa et al., (2009) University of Ghana http://ugspace.ug.edu.gh 26 further used the correlation of Ca2+ and SO4 2- to classify groundwater into two distinct groups namely: (1) Those with Ca2+/SO42- ratio close to unity indicating a common origin and located in the central part of the basin (2) The second group is constituted by groundwaters sampled in the southern part and along the limits of the basin. Marie et al., (2001) investigated the sources of salinity in groundwater from Jericho area, Jordan Valley and discovered that Br/Cl and B/Cl ratios decrease with TDS, which suggest that the main mechanism of salinisation in the Draa Basin is derived salt dissolution in the unsaturated zone and salinisation of the underlying shallow groundwater. Ion exchange reactions of Na and Ca often occur when seawater intrudes fresh groundwater. The characteristic cation – exchange process that takes place when seawater intrudes a coastal fresh water aquifer is shownin equation (2.8) after (Appelo and Postma, 2005): 2Na+ (K+) + Ca-X2 ↔ 2Na (K +)-X + Ca2+ (2.8) When fresh groundwaters flush out saline groundwaters, the reverse reactions occur. El- Fiky (2010) investigated the hydrogeochemical evolution and recharge processes of groundwater in Southwestern Sinai, Egypt with the main aim of establishing factors controlling groundwater chemistry and salinity. Study results showed groundwater, University of Ghana http://ugspace.ug.edu.gh 27 evolved from Ca-HCO3recharge water, mixing with the pre-existing groundwater to give a mixed water of Mg-SO4 and Mg-Cl types that eventually reached a final stage of evolutionrepresented by a Na-Cl water type. Different ionic ratios (HCO3/Cl, Ca/Mg, Ca/Na, Ca/SO4 and Br/Cl) revealed the impact of seawater and marine aerosols on the hydrochemical composition of groundwater of the Quaternary aquifer. Dissolution of carbonate and sulphate minerals in the aquifer matrices and recharge as well as cation exchange are said to modify the concentration of ions in the groundwater. The groundwaters were found to be depleted in 2H and 18O anddisplayed an isotopic signature close to that ofmeteoric water with deuterium excessvalues ranging between 10.96 ‰ and 19.4 ‰. These values were found to suggest that the groundwaters resulted from a mixture of recent recharge and an older component recharged under climatic conditions cooler than present. The use of these ionic ratios to efficiently evaluate the degree of seawater intrusion has been reaffirmed by (El Moujabber et al., 2006; Ghabayen et al., 2006; De Montety et al., 2008).Park et al., (2011) investigated evidences of seawater intrusion in the coastal aquifers of Korea and classified groundwater into fresh (< 1500 µS/cm), brackish (1500- 3000µS/cm) and saline (>3000 µS/cm) according to EC levels. The major dissolved components of the brackish and saline waters were Na+ and Cl-. The enrichment of Na+ and Cl- was attributed to seawater intrusion. The Ca/Cl and HCO3/Cl ratios of the groundwaters gradually decreased and approached those of seawater. Mg/Cl, Na/Cl, K/Cl, SO4/Cl and Br/Cl of the groundwaters gradually decreased, and were similar to those of seawater, indicating that Mg2+, Na+, K+, SO4 2- and Br- as well as Cl- in the saline University of Ghana http://ugspace.ug.edu.gh 28 groundwater can be enriched by seawater mixing while Ca2+ and HCO3 - are mainly released by weathering processes. Cl- and Br- ions thus play a very important role in their usage as representative proxies to estimate the influence of seawater on groundwater (Ghabayen et al., 2006). When seawater intrusion (enchroachment) occurs, enrichment of Na+ and Cl- are generally observed (Mondal et al., 2010) indicating that such enrichments can play an important role as indicators of seawater intrusion. However, whereas Cl- is suitable as proxy of seawater intrusion, Na+ is not. Aside seawater mixing, Na+ can also be enriched by water rock interaction processes (silicate mineral weathering processes) (Mondal et al., 2010) and anthropogenic contamination, while Cl- is dominantly supplied from seawater only. Therefore Cl- has been used as proxy to indicate seawater intrusion. A combined hydrogeologic and isotopic investigation using several chemical and isotopic tracers such as Br/Cl, δ18O, δ2H, 3H, 87Sr/86Sr, δ11B, and 14C was carried out in order to determine the sources of recharge, the origin of salinity and the residence time of water in the Souss-Massa basin aquifer in Morrocco. Stable isotope, 3H and 14C data indicated that the high Atlas mountains in the northern margin of the Souss–Massa basin with high rainfall and low δ18O and δ2H values (-6 to -8‰ and -36 to -50‰) constitute the major source of recharge to the Souss–Massa shallow aquifer, particularly along the eastern part of the basin. The 3H and 14C data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years; hence old water is mined, particularly in the coastal areas.The multiple isotope analyses and chemical tracing of University of Ghana http://ugspace.ug.edu.gh 29 groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the Souss–Massa aquifer (Bouchaou et al., 2008). Kim et al., (2003) also identified the origin of salinegroundwater in the eastern part of Jeju volcanic island,Korea by carrying out a hydrogeochemical and isotopicstudy for 18 observation wells. Oxygen (O), Hydrogen (H), Sulphur (S) and Strontium (Sr) isotopicdata clearly showed the mixing of groundwater andseawater; Sr isotopic compositions and Br/Cl and I/Clratios strongly suggested the modern seawater intrusionas the source of salinity. 2.3 OVERVIEW OF ENVIRONMENTAL ISOTOPES Environmental Isotopes may be defined as those isotopes, both stable and radioactive, occurring in the environment in varying concentrations over which the investigator has no direct control (Fontes and Edmunds, 1989). Environmental isotopes commonly used in hydrology are stable isotopes (deuterium, oxygen-18, and carbon-13) and radioisotopes (tritium and carbon-14) (Fontes and Edmunds, 1989). Environmental isotopes are important in tracing groundwater provenance, recharge processes, geochemical reactions and reaction rates (Clark and Fritz, 1997). University of Ghana http://ugspace.ug.edu.gh 30 2.3.1 Stable isotop es Stable isotopes of water, oxygen-18 (18O) and deuterium (2H) are affected by meteorological processes that provide a characteristic finger print of their origin (Fontes, 1980; Gat, 1981). This finger print forms the basis for investigating the provenance of groundwater. It thus follows that waters from different sources or those exposed to different processes such as evaporation and mixing, often acquire identifiable isotopic contents which serve as natural tracers (Dassi et al., 2005). For example isotopic and hydrochemical compositions combined with geological and hydrogeological settings have been used to identify the recharge and flow charactristics and evaluate the continuity of the Lower Cretaceous Nubian sandstone aquifer in the Sinai Peninsula, Egypt. Study results show a considerable depletion in stable isotopic content (18O and 2H) and low deuterium excess (d-excess) reflecting old meteoric groundwater that recharged the aquifer in pluvial times. The continuity of the aquifer in the Northern and Central Sinai is evidenced by the isotopic similarity of samples taken from above and below the central Ragarbert El-Naam fault in the area (El Samie, 2001). Stable isotope composition of waters established that deep groundwater is an ancient water recharged probably during the late Pleistocene and early Holocene periods in an attempt to understand the mechamism that contribute to groundwater mineralistion in Tunisian Chott‘s region (Kamel et al., 2008). The evolution of δ18O and δ2H composition of meteoric waters begins with the evaporation from the oceans (IAEA, 1983, Faure,1986, Fritz and Fontes 1980;Mazor, 1991). Deuterium and oxygen-18 occur in the oceans in concentrations of about 310 ppm University of Ghana http://ugspace.ug.edu.gh 31 and 1990 ppm for molecular species H2HO and H2 18O respectively. The varying concentrations of these isotopes in natural waters can be measured in an isotope ratio mass spectrometer and are expressed in the delta (δ) notation as follows: 𝛿 = ( 𝑅𝑠−𝑅𝑠𝑡𝑑 𝑅𝑠𝑡𝑑 )𝑥 1000 (2.9) Where Rs = the isotope ratio (2H/1H, 18O/16O) of the sample and Rstd = the isotope ratio (2H/1H or 18O/16O) of the standard. Delta values are expressed in parts per thousand (per mil ‰) deviation from the standard. The universally adopted standard for deuterium and oxygen-18 is the SMOW (Standard Mean Ocean Water) which later became the Vienna-SMOW (V-SMOW). The oxygen-18 for V-SMOW is the same as that defined for SMOW but deuterium content is 0.2‰ lower. Thus for all practical purposes V-SMOW and SMOW are considered identical since the accuracy of measurements is usually 1‰ and 0.1‰ for deuterium and oxygen- 18 respectively. When water changes state through condensation or vapourisation, isotopic fractionation occurs due to differences in vapour pressures and diffusion velocities in air of the different isotopic species of water (Sidle, 1998, Mazor, 1991). The degree of isotopic fractionation is inversely related to temperature. There is seasonal change in the stable isotopic compositions of precipitation at a given location, with more depleted values occurring in the colder months.More depleted values are also observed at higher latitudes. Precipitation falling at higher elevations is more depleted than that falling at lower University of Ghana http://ugspace.ug.edu.gh 32 elevations; this latter property is of particular utility in the hydrological applications. Precipitation at continental locations is more depleted than that which falls nearer the coast (Fritz and Fontes 1980, Clark and Fritz, 1997). In contrast to the condensation process, evaporation does not take place under equilibrium conditions. The effective fractionation factors are greater than the equilibrium values. When water undergoes evaporation the lighter isotopic species (1H2 16O) preferentially leave the surface, so the remaining water becomes enriched in the heavier isotopic species (1H2 18Oor 1H2H18O) (Sidle, 1998, Kendal and McDonnell, 1998, Singh and Kumar, 2005). The degree of enrichment depends on the temperature, relative humidity of the atmosphere and the hydrological balance of the surface water body. The enrichment of oxygen-18 is about one order of magnitude less than deuterium. An important process which controls the enrichment of the surface water is molecular exchange, which occurs between surface water and the atmospheric water vapour. The deuterium and oxygen-18 values of natural waters obey the following general relation after (Criag, 1961): 𝛿2𝐻 = 𝑎𝛿18𝑂 + 𝑑 (2.10) For waters which have not been subjected to evaporation the value of ‗a‘ is 8 and the average global value of d for precipitation is 10. The deuterium excess (d),the intercept of the GMWL (Dansgaard, 1964) is defined as: 𝑑 = 𝛿2𝐻 − 8𝛿18𝑂 (2.11) University of Ghana http://ugspace.ug.edu.gh 33 The stable isotopic composition of natural waters is often plotted on a δ2H/δ18O diagram (Fig. 2.2). The figure illustrates the average global precipitation meteoric water line AB. Water samples which have not undergone significant evaporation will plot on such a line. However, water samples which have undergone appreciable evaporation will plot on a line CD, the slope of which is usually in the range 4 to 6. Apello and Postma (2005) quoted a figure of 4 to7. The intercept C of the meteoric water line AB represent the stable isotope composition of surface water prior to enrichment by evaporation. Geothermal waters will plot on the line EF, where there is no significant change in deuterium with change in oxygen-18 content. Fig. 2.2 A schematic of δ2H-δ18O plot (Fontes and Edmunds, 1989). University of Ghana http://ugspace.ug.edu.gh 34 2.3.2 Ground w ater ‘‘age’’ datin g an d Radioisotop es A simple definition of age, or residence time, is the interval of time that has elapsed since groundwater at a location in a flow regime entered the subsurface (Berthke and Johnson, 2008). Groundwater age or residence time is one of the important information necessary for sustainable management of groundwater resources. Important information such as recharge, flow rates and paths, sustainable yield and assessment of groundwater vulnerability to contamination are obtained from groundwater age. In this study, groundwater ages have been determined to establish the recharge mechanism and assess if the aquifers have been over exploited or under exploited through the application of radioisotopes. The most widely used radioisotopes for dating groundwater is tritium (3H) for dating young waters and carbon-14 (14C) for dating old waters. However, for this study 3H and 14C were used to date both young and old waters following flow paths inferred from hydrochemistry of the groundwaters. Radioisotopes are isotopes of elements in which the nucleus is unstable and disintegrates spontaneously, resulting in the emission of radiation and formation of another isotope. The radioisotopes routinely employed in hydrogeology include tritium and Carbon-14 (14C) (Clark and Fritz, 1997). Tritium (T) or 3H is a radioactive isotope of hydrogen (having two neutrons and one proton) with a half-life of 12.32 years (Lucas and Unteweger, 2000). Tritium is introduced into the hydrologic cycle by both natural and human sources. Natural University of Ghana http://ugspace.ug.edu.gh 35 production of tritium involves the bombardment of neutrons (n) from cosmic rays with nitrogen (N) in the atmosphere according to equation (2.9) (Solomon and Cook, 2000). 14N + n →12C + 3H (2.12) Ferronsky (1982) stated that about 3 to 5% of all neutrons in the upperatmosphere react with nitrogen to form 3H. The tritium formed, combines with oxygen to form water which falls as precipitation (rain, hail and snow) and eventually becomes concentrated in levels detectable in groundwater. Human sources of tritium production includeatmospheric weapon testing. Other sources of tritium are weapons production industries, nuclear industries and digital watch manufacturers. These industries release tritium into thelower atmosphere and directly into the hydrologic cycle. During the atmospheric nuclear weapon testing, large quantities of 3H were introduced into the atmosphere mostly in 1954-55, 1958, and 1961-62 (Fig 2.3). Prior to 1957, most of the 3H released during testing only reached the lower atmosphere referred to astroposphere. The nuclear explosions during this time were relatively small and were carried out at relatively low altitudes. Precipitation quickly removed this pulse of tritium. After 1957, the nuclear explosions were greater in magnitude and carried out at higher altitudes. This large pulse of tritium, which reached the stratosphere, has had a long term effect on the concentration of tritium in precipitation (Mayo and Klauk, 1991, Plummer, 2005). Tritium reached its peak in the atmosphere in 1963 in the Northern Hemisphere (Fig. 2.3). After 1963, the concentration of tritium in the Northern Hemisphere began todecrease dramatically as a result of cessation in the atmospheric weapon testing while University of Ghana http://ugspace.ug.edu.gh 36 the tritium concentration in the Southern Hemisphere increased due to stratospheric transfer of the bomb-produced tritium (Mayo, 1991) (Fig.2.3). The derease of tritium in precipitation from 1963 has also been observed in the tropics notably Sao Tome and Entebbe (Blay, 2012). Tritium concentrations are measured in tritium units (TU) where 1 TU is defined as the presence of one tritium in 1018 atoms of hydrogen (H) (Nimmo et al., 2005). Fig. 2.3Tritium concentration in precipitation since 1950 at two IAEA stations (IAEA/WMO, 2006): Ottawa, Canada (Northern Hemisphere)and kaitoke, New Zealand(Southern Hemisphere). University of Ghana http://ugspace.ug.edu.gh 37 The spiked nature of the tritium input function results in considerable ambiguity in age interpretation; nevertheless, the presence of tritium in water samples is a reliable indicator of samples that contain at least a fraction of post-1950s water (Plummer 2005).Moussa et al., (2010) stated that due to the radioactive decay, groundwater derived from precipitation that fell before the onset of atmospheric testing of nuclear weapons would have contained less than 0.75 TU. Dating of groundwater by decay of tritium is based on the assumption that the tritium input into a groundwater is known and that the ―residual‖ tritium measured in groundwater is the result of decay alone, as expressed in the equation: 𝐴𝑡 = Aoe −λt (2.13) Ao - is the initial tritium activity or concentration (expressed in TU) At – is the residual activity (measured in a sample) remaining after decay over time t. The decay term λ is given by: 𝜆 = 𝑙𝑛 2 𝑡1/2 (2.14) Using tritium half – life t1/2 = 12.32 years, the age or residence time (t) of groundwater can be obtained from the equation: t = −17.78ln 𝐴𝑡 𝐴𝑜 (2.15) University of Ghana http://ugspace.ug.edu.gh 38 Thermonuclear bomb tritium has been observed over the years to be attenuated by the oceans and tritium production in the atmosphere is back to natural background level. This renders the estimation of initial tritium input into groundwater system problematic, hence difficulty in the quantative interpretation of groundwater residence time. Only qualitative interpretation can thus be made (Rowe et al., 1999, Anders and Schroeder, 2003). Measurable 3H in groundwater signifies modern recharge. Levels greater than approximately 30 TU suggest implication of thermo nuclear bomb tritium indicating recharge during the 1960s. Groundwaters containing levels of tritium that are close to detection (approximately 1TU) are considered submodern or paleogroundwaters that have mixed with shallow modern groundwaters in discharge groundwaters in discharge zone (eg. springs) or within boreholes (Moore, 2011). It has been established that tritium concentration alone generally cannot be used to quantitatively date ground water, but can be used to qualitatively determine if ground water is modern (less than about 50 years in age) or pre-modern (older than about 50 years in age) (Clark and Fritz, 1997). Mckenzie et al., (2012) also stated that low tritium values in modern precipitation and missing data during critical spike period from July 1964 to January 1965 mean it is not possible to precisely state ages based on measured tritium values. It thus possible to bracket age samples as being modern and pre-modern. The radiocarbon age dating of groundwater is described as one of the most accessible and widely used technique to age date groundwater resources (Meredith, 2009). Radiocarbon University of Ghana http://ugspace.ug.edu.gh 39 (14C) is the radioactive isotope of carbon and has a half-life of 5,730 years (Hiscock, 2005; Appelo and Postma, 2005; Meridith, 2009). Carbon-14 (14C) is produced in the atmosphere by bombardment of nitrogen (14N) with a flux of neutrons (n) from cosmic radiation leading to the release of a proton (p) as illustrated in (Fig. 2.4) . The 14C produced, is oxidised by oxygen to 14CO2 and incoporated into vegetation by photosynthesis and later released in the soil by decay and root respiration. This leads to a build up of a huge reservior of 14C in the soil zone (Geyh, 2000). The decay of vegetation and root respiration return much of the 14C to the atmosphere through diffusion (Fig 2.4). There is therefore a continous exchange of CO2 between the soil zone and the atmosphere. Geyh (2000) described such a system in which there is a continous exchange of CO2 between the soil zone and the atmosphere as an open system. When water infiltrates into the soil zone, dissolution of soil CO2occurs and results in the production of four major species which are CO2 (aq), carbonic acid (H2CO3), bicarbonate (HCO3 -) and carbonate (CO3 2-). These species constitute the dissolved inorganic carbon (DIC).Their distribution or relative concentration is a function of pH. At low pH, H2CO3 dominates the DIC, whereas HCO3 - is the major species between 6.4 and 10.3. CO3 2- dominates only under very alkaline conditions (Clark and Fritz, 1997). University of Ghana http://ugspace.ug.edu.gh 40 The amount of CO2 that dissolves in an infiltrating water depend on the geochemistry of the recharge environment: pH of the water, the partial pressure of CO2, and the weathering reactions that take place in the soil. Consequently as groundwater recharge through soils, they gain levels of14C-active DIC (CO2 + HCO3 - + CO3 2- + H2CO3) that are much higher than that provided for direct dissolution of atmospheric CO2. Once the infiltrated water gets to the saturated zone, the groundwater can be subjected to secondary hydrochemical reactions. Any resultant isotopic alterations that take place during thisperiod of groundwater storage and aging do so within the constraints of a closed geochemicalsystem. 14 C ages are expressed in terms of years before present (B.P.) with the 14 C concentrations expressed as percent modern carbon (pMC).The modern activity of 14 C is by convention set as 13.56 decays-per-minute per gram (dpm/g c) of carbon, with the ‗zero year‘ for this activity as 1950 A.D. (Kalin, 2000). The 1950 value is considered to have an activity of 100 pMC.After 1950, 14 C in the atmosphere increased due to atmospheric testing of nuclear weapons, and 14 C concentrations greater than 100 pMC often result, indicating the presence of bomb carbon. By international convention, the modern reference standard for radiocarbon dating is 95 percent of the 14 C content of the National Bureau of Standards (NBS) oxalic acid (= 0.95 * 13.56 dpm/g C in the year 1950 A.D.). The measured activity of a sample A is then given as a percentage of this standard activity, or pMC (Mook, 1980). University of Ghana http://ugspace.ug.edu.gh 41 n 14N 14C 14CO2 P Fig 2.4 The pathway and associated fractionation of 14C and 13C in CO2 during photosynthesis, respiration in soils and dissolution by groundwater (after Gillon et al., 2012, Clark and Fritz, 1997). Carbon-14 production is also shown. University of Ghana http://ugspace.ug.edu.gh 42 The ubiquity of carbon in groundwater makes it an ideal isotope to use for age dating groundwater (Meridith, 2009). The cabon-14 age of groundwater that is no longer in contact with the atmosphere is obtained from the decay equation expressed in equation (2.13). For 14C, of half-life 5730 years, the equation (2.13) becomes: 𝑡 = −8267ln⁡ 𝐴𝑡 𝐴𝑜 (2.16) Where: t is the time since the sample was isolated from the atmosphere, λ is the radioactive decay constant, A t is the measured 14 C activity in the sample at time t, and A 0 is initial 14 C activity at time zero. In order to determine the age of a groundwater sample, with this equation, it is necessary to determine AO (Meridith, 2009). However, the initial carbon-14 (Ao) tends to be diluted by chemical reactions and evolution of carbonate systems. The result is an artificial ‗aging‘ of groundwaters. Unraveling the relevant processes and distinguishing 14C decay from 14C dilution is an engaging geochemical problem (Douglas et al., 2007). The most typical reactions that tend to dilute AOas outlined by (Mayo, 1991) include: (1) calcite (limestone) dissolution, beginning from the recharge area (2) dolomite dissolution (3) exchange with aquifer matrix (4) oxidation of ―old‖ organics found within the aquifer and other biochemical reactions (5) diffusion of 14C into the aquifer matrix. The dilution of 14C in groundwater along its flow path is accounted for in the decay equation by the dilution factor, q. The 14C-activity of dissolved inorganic carbon (DIC) in the University of Ghana http://ugspace.ug.edu.gh 43 groundwater recharge environment following calcite dissolution (AOCorrected) is equal to the modern 14C in the soil (AO) times the dilution factor. Thus 𝐴𝑂𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = q.𝐴𝑂 (2.17) The decay equation corrected for dilution becomes: 𝐴𝑡 = 𝑞𝐴𝑜𝑒 −𝜆𝑡 (2.18) The corrected age then becomes: 𝑡 = 8267𝑙𝑛 𝑞 .𝐴𝑜 𝐴𝑡 (2.19) 𝑡 = 8267𝑙𝑛 𝐴𝑜 𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝐴𝑡 (2.20) Various models have been published for determining the dilution factor q hence correction for AO. The choice of a model depends on the kind of system one is dealing with whether it is an open or closed system. Under open system conditions, the DIC is continuously exchanging with the infinite reservoir of 14C-active soil CO2. In this case, the initial 14C activity of DIC (Ao) remains unchanged at 100 pMC hence no need for correction. Under closed system conditions, groundwater is isolated from atmospheric interaction and chemical reactions involving carbonate minerals along flow path and within the aquifer tend to affect the initial 14C (Ao) of the DIC. Ao is therefore corrected for using the theoretical models outlined.Fontes (1979) emphasized that estimation of initial 14C activity of total dissolved inorganic carbon (DIC) requires an investigation of the water University of Ghana http://ugspace.ug.edu.gh 44 chemistry, soil and aquifer mineralogy. This investigation helps to unravel the kind of system one is dealing with. Carbon isotopes of dissolved inorganic carbon (DIC) in groundwater have been used to provide additional information on the age as well as to support evidence of the mixing process obtained from the stable isotopes and tritium data. 14C is a reliable age dating tool of moderately old water (5-35 kyear), but corrections must be made to account for water rock interactions (Dassi et al., 2005). Mahlknecht (2006) stated that a trend of decreasing 14C activity associated with a change toward heavier δ13C values indicates that geochemical reactions may be affecting 14C concentration along groundwater flow system. In this study the geology, hydrogeochemical processes and aquifer mineralogy are being considered in the determination of initial activity of 14C. Currell et al., (2010) investigated the recharge history and controls on groundwater quality in the Yuncheng Basin, north China and realized that shallow groundwater has higher radiocarbon activities (>70 pmC), indicating a significant component of modern (post-1950s) recharge. 2.3.3 Theoretical Model s For 14 C Age Corre ctio n Various models have been proposed for determination of the dilution factor q for correction of the initial activity Ao. The application of the models is dependent on the geology of the area, geochemistry and chemical reactions involved and whether reactions are taking place in an open or closed system. The proposed models are: University of Ghana http://ugspace.ug.edu.gh 45 1. Statistical (Vogel 1970) model 2. Alkalinity (Tamers (1975) ) model 3. Chemical Mass Balance model 4. δ13C mixing (Ingerson- Pearson) model 5. Chemical and δ13C balance (Fontes-Garnier (F&G)) model 6. Complete Transformation of Soil CO2 into Total Dissolve Inorganic Carbon (TDIC) (Akiti, 1980) model 2.3.3.1 Statistical Model (Vogel) The statistical model assume that after the initial carbon uptake in the soil zone by infiltrating water, some 14C dilution will occur through addition of 14C-free carbon. Statistical evaluations are possible if geochemical evolution can be averaged over the recharge area to estimate an initial value for the 14C activity of the aqueous carbonate. This initial value represents the fraction of 14C activity remaining after secondary carbon addition. The value often quoted is q = 0.85, implying 85% of the initial 14C concentration remains after dilution. Vogel (1970) reported characteristic q values in various geological systems as: 1. 0.65 – 0.75 for karst systems 2. 0.75 – 0.90 for sediments with fine grained carbonates such as loes 3. 0.900 -1.00 for crystalline rocks University of Ghana http://ugspace.ug.edu.gh 46 2.3.3.2 Alkalinity Model (Tamer’s (1975)) In Tamer‘s model half of the bicarbonate comes from CO2 gas and the other half from the carbonate minerals (Tamer, 1975). This means that half of the carbon introduced into the groundwater is live carbon and half is dead carbon. It is also assumed that all of the CO2 comes from the soil zone and that all the bicarbonate from the dissolution of carbonate minerals. The dilution factor qAKL is formulated as: 𝑞𝐴𝐿𝐾 = 𝑚𝐻2𝐶𝑂3+1/2𝑚𝐻𝐶𝑂3 − 𝑚𝐻2𝐶𝑂3+𝑚𝐻𝐶𝑂3 − (2.21) Where m is the molality of the various carbonate species involved in the carbonate dissolution. This model assumes fully closed system conditions, where no exchange with soil CO2 during calcite dissolution occurs. 2.3.3.3 Chemical Mass-Balance (CMB) Model The chemical mass balance model is a closed system model where carbonate dissolution takes place below the water table and the DIC does not exchange with the soil CO2. The DIC gained from dissolving soil CO2 is compared to that measured in the groundwater sample. The correction factor ―q‖ is obtained from: 𝑞 = 𝑚𝐷𝐼𝐶𝑟𝑒𝑐 𝑕 𝑚𝐷𝐼𝐶𝑓𝑖𝑛𝑎𝑙 (2.22) Where mDICrech is the 14C-active DIC gained by dissolution of soil CO2 during recharge, and mDICfinal is the total carbonate content at the time of sampling ( 14C-active/14C-dead). University of Ghana http://ugspace.ug.edu.gh 47 The model requires that mDICrech be calculated from estimated PCO2-pH conditions for the recharge environment. If the concentration of DIC has been measured in the recharge area groundwaters (where A14CDIC = A 14Catm), then this can be used. This approach does not account for open system carbonate dissolution. The initial carbonate content during recharge (mDICrech) is obtained by assuming pH and PCO2 values and the application of carbonate equilibria to determine [H2CO3] and [HCO3 -]. The final carbonate content (DICfinal) is best represented by the measured (titrated) alkalinity (designated as CMB-Alk model). When titrated alkalinities are unreliable the DICfinal value can be calculated from chemical data (designated CMB-chem mode): mDICfinal = mDICrech + [mCa 2+ + mMg2+ - mSO4 2- +1/2(mNa+ + mK+ - mCl-)] (2.23) where chemical constituents are measured concentrations (moles/L). This form of the model was proposed by Fontes and Garnier (1979) to account for Ca2+ and Mg2+ added by carbonate dissolution, with a correction for Ca2+ added through the dissolution of gypsum (mSO4 2-), and Ca2+ lost through ion exchange processes (mNa+ + mK+ - mCl-). Cl- is included in this term to account for Na+ gained through incorporation of salinity from seawater or halite dissolution. The CMB and Alk models assume that the system is closed with respect to soil CO2 during carbonate dissolution. University of Ghana http://ugspace.ug.edu.gh 48 2.3.3.4 δ13C mixing (δ13C model) – (Ingerson and Pearson 1964) Carbon-13 (13C) is said to be a good tracer of open and closed system evolution of DIC in groundwaters. The large difference in δ13C between the soil-derived DIC and carbonate minerals in the aquifer provides a reliable measure of 14C dilution by carbonate dissolution. The δ13C mixing model allows for incorporation of 14C-active DIC during carbonate dissolution under open system conditions and subsequent 14C dilution under closed system conditions. Ingerson andPearson (1964) introduced a δ13C correction based on variation in 13C abundances. A process that adds removes or exchanges carbon from the DIC pool and thereby alters the 14C concentrations also affects the 13C concentrations. The q factor was obtained from a carbon isotope –mass balance as: 𝑞 = 𝛿13𝐶𝐷𝐼𝐶 −𝛿 13𝐶𝐶𝑎𝑟𝑏 𝛿13𝐶𝑆𝑜𝑖𝑙 −𝛿 13𝐶𝐶𝑎𝑟𝑏 (2.24) Where δ13CDIC = measured 13C in groundwater δ13CSoil = δ 13C of the soil CO2 (usually close to -23‰) δ13CCarb = δ 13C of calcite being dissolved (usually close to 0‰) The δ13C model assumed that carbonate dissolution takes place under closed system conditions, precluding exchange with the soil gas reservoir. The above equation is only correct if the CO2(Soil) is taken up by water without significant fractionation effects. This is the case in low pH environments where only about 1‰ depletion accompanies dissolution of CO2(Soil). At higher pH values (pH 7.5 to 10) the DIC in equilibrium with the CO2(Soil) is enriched in 13C. The δ13CSoil is therefore replaced by an initial δ 13C value for DIC in the infiltrating groundwaters (δ13Crech) defined as: University of Ghana http://ugspace.ug.edu.gh 49 𝛿13𝐶𝑟𝑒𝑐𝑕 = 𝛿 13𝐶𝑆𝑜𝑖𝑙 + 𝜀 13𝐶𝐷𝐼𝐶−𝐶𝑂2 𝑆𝑜𝑖𝑙 (2.25) 𝜀13𝐶𝐷𝐼𝐶−𝐶𝑂2(𝑆𝑜𝑖𝑙 ) is the pH dependent enrichment between soil CO2 and the aqueous carbon. The modified dilution model is: 𝑞𝛿13𝐶 = 𝛿13𝐶𝐷𝐼𝐶 −𝛿 13𝐶𝐶𝑎𝑟𝑏 𝛿13𝐶𝑟𝑒𝑐 𝑕−𝛿13𝐶𝐶𝑎𝑟𝑏 (2.26) This equation allows for carbonate dissolution under open and closed system conditions. The enrichment factor 𝜀 is based on the pH of the groundwater during recharge. 𝜀13𝐶𝐷𝐼𝐶−𝐶𝑂2(𝑆𝑜𝑖𝑙 ) = 𝑚𝐶𝑂2(𝑎𝑞 ). 𝜀 13𝐶𝐶𝑂2 𝑎𝑞 −𝐶𝑂2(𝑔) +𝑚𝐻𝐶𝑂3. (𝜀 13𝐶𝐻𝐶𝑂3−𝐶𝑂2(𝑔)) (2.27) Where m is the mole fraction of the two carbonate species and enrichment factors, 𝜀13C are from fractionation equations. 2.3.3.5 Matrix exchange (Fontes-Garnier model) The exchange of carbon isotopes between the DIC and carbonate minerals in the aquifer matrix is often considered as a cause for δ13CDICenrichment and 14C dilution. Groundwaters that are essentially at equilibrium with calcite will exchange carbonate across the mineral solution interface where CO3 2- and Ca2+ are in a continual process of crystallization (Clark and Fritz, 1997). Fontes and Garnier (1979) developed a correction model (F-G model) that uses cation concentrations to determine the contribution of University of Ghana http://ugspace.ug.edu.gh 50 14Cfree matrix carbonate and isotope mass balance to apportion 14CDIC into that exchanged with: (i) CO2 gas in the soil (open system exchange) and (ii) The carbonate matrix (matrix exchange) In their model, the total of matrix derived carbonate is calculated as: 𝑚𝐷𝐼𝐶𝐶𝑎𝑟𝑏 = 𝑚𝐶𝑎 2+ +𝑚𝑀𝑔2+ −𝑚𝑆𝑂4 2− + 1/2 𝑚𝑁𝑎+ +𝑚𝐾+ −𝑚𝐶𝑙− (2.28) This equation accounts for carbonate dissolution based on Ca and Mg with a correction for evaporate dissolution (mSO4 2-) and cation exchange (mNa+ + mK+) with a correction for Na+ from salt, (mCl-). This DIC is then apportioned into two components - that which has exchanged with soil CO2 ( 14C active) in an open system and that which has exchanged with carbonate matrix (considered 14C-dead) under closed system conditions. The fraction of this DIC that has exchanged with soil CO2 in an open system (mDICCO2-exch) is calculated from the mass- balance relationship: 𝑚𝐷𝐼𝐶𝐶𝑂2−𝑒𝑥𝑐𝑕 = 𝛿13𝐶𝑚𝑒𝑎𝑠 .𝑚𝐷𝐼𝐶𝑚𝑒𝑎𝑠 −𝛿 13𝐶𝐶𝑎𝑟𝑏 .𝑚𝐷𝐼𝐶𝐶𝑎𝑟𝑏 −𝛿 13𝐶𝑆𝑜𝑖𝑙 .(𝑚𝐷𝐼𝐶𝑚𝑒𝑎𝑠 −𝑚𝐷𝐼𝐶𝐶𝑎𝑟𝑏 ) 𝛿13𝐶𝑆𝑜𝑖𝑙 −𝜀 13𝐶𝐶𝑂2−𝐶𝑎𝐶𝑂 3−𝛿13𝐶𝐶𝑎𝑟𝑏 (2.29) The dilution factor then becomes: 𝑞𝐹−𝐺 = 𝑚𝐷𝐼𝐶𝑚𝑒𝑎𝑠 −𝑚𝐷𝐼𝐶𝐶𝑎𝑟𝑏 +𝑚𝐷𝐼𝐶𝐶𝑂2−𝑒𝑥𝑐 𝑕 𝑚𝐷𝐼𝐶𝑚𝑒𝑎𝑠 (2.30) University of Ghana http://ugspace.ug.edu.gh 51 2.3.3.6 Complete Transformation of Soil CO2 into (TDIC) (Akiti, 1980) model The model considers total dissolved inorganic carbon (TDIC) in groundwater under open and closed system conditions by using the equation: 𝛿𝐶𝑇 = 𝑥 𝛿𝐶𝑂2 − 𝜀(𝐶𝑂2−𝐻2𝐶𝑂3) + 1− 𝑥 [𝛿𝐶𝑂2 − 𝜀(𝐶𝑂2−𝐻𝐶𝑂3)](2.31) Where x is the proportion of mH2CO3 in the solution (1-x) is the proportion of mHCO3 - in solution δCT is the value of carbon-13 of total dissolved inorganic carbon. 𝜀 – enrichment factor Under open system conditions, δ13CT of (TDIC) must approach the maximum enrichment as in the case of HCO3 - exchanging with CO2 in the soil zone. If the system is closed with respect to the atmosphere without dissolution of any solid carbon in the aquifer, then the TDIC is considered as that of the gaseous CO2 of the soil before recharge. This is the case of formation of bicarbonate (HCO3 -) from carbonic (H2CO3) to counter balance the rising level of pH due to the dissolution of cations from silicates. 2.4 P REVIOUS STUDIES IN GHANA In Ghana, very few people have employed hydrogeochemistry and environmental isotopes to evaluate recharge processes, understand groundwater flow systems, origin of salinity, and ‗age‘ determination of groundwater. Armah (2000) studied the salinisation of groundwater in parts of southern Ghana including the Central Region and realized that University of Ghana http://ugspace.ug.edu.gh 52 stable isotope analyses showed no relationship between the groundwater system and possible seawater intrusion. Preliminary physico-chemical parameters according to him indicated that groundwater quality varies even within the individual rock units and therefore suggested different causes for the salinisation. Kortatsi (2006) employed chemical and isotopic tools to characterize and identify the relevant water-rock interactions, which are responsible for the poor groundwater quality in the Accra Plains of Ghana. He identified the processes contributing considerably to the concentration of major ions in the groundwater as halite dissolution, carbonate dissolution and precipitation, seawater intrusion, cation exchange, evaporation concentration of solutes and aluminosilicates dissolution. Cl-/Br- ratio values were determined and samples closed to the coast (< 15km) had values in the range 304-345.5 suggesting marine input or in the degree of seawater intrusion. Further evidence of seawater intrusion according to Kortatsi (2006) is provided by the existence of CaCl2 or MgCl2 in some boreholes a few kilometers from the coast. This deduction is based on Appelo and Postma (1999) which stated that when seawater intrudes into fresh coastal aquifers CaCl2 or MgCl2 waters result. Stable isotope content of the groundwater suggests mainly direct integrative recharge. Few samples plot along the meteoric- seawater mix line. Akiti (1980) also investigated the geochemical and isotope hydrology of the Accra Plains of Ghana and identified three main hydrochemical facies namely NaCl, NaHCO3 and MgCl2 waters. The NaCl waters are the dominant type. He indicated that the high sodium University of Ghana http://ugspace.ug.edu.gh 53 chloride (NaCl) nature of the waters led to a search of the origin of this salt. Since the area is close to the sea three hypotheses were advanced: 1. The groundwater contain seawater due to seawater encroachment 2. The ions chloride and sodium could be carried through the atmosphere as aerosols. 3. The presence of sodium chloride would be due to washing of salts accumulated in the soils. Akiti (1980) stated however, that the first hypothesis is less probable since the groundwaters containing high salt contents are found mostly in the interior and not at the coast. This implies that hard rocks are not all that permeable to allow for salt water encroachment. He also measured the stable isotopic composition of rainwater and established a local meteoric water line for Ghana given by the following equation: δ2H‰ = 7.86 δ18O‰ +13.61 (2.32) A similar eqation is obtained by Nkotagu (1996) for rains at Dodoma in Tanzania. This is given as: δ2H‰ = 7.9δ18O‰ + 13.83 (2.33) This suggests a tropical meteoric water line. Helstrup et al., (2007) found out that the most relevant controls on the water quality within the Cretaceous-Eocene limestone aquifer of the Keta Basin, Ghana and the coastal University of Ghana http://ugspace.ug.edu.gh 54 sedimentary basin of Togo using Q-mode hierarchical cluster analysis (HCA) and mass- balance modeling are: (1) carbonate equilibria, (2) silicate weathering reactions, (3) limited mixing with saline water and (4) ion exchange. The combined use of HCA and mass-balance modeling was emphasized as a useful approach in interpreting groundwater hydrochemistry in an area where large uncertainties exist in understanding of the groundwater flow system. Acheampong and Hess (2000) used stable and radioactive isotope to investigate the source of recharge and the age of the shallow groundwater system within the Southern Voltaian Sedimentary Basin of Ghana. Generally, they found very low tritium concentrations in the groundwater samples in the range less than 1-7.2 TU and useful in the identification of modern recharge. Radiocarbon ages were found to range from about 3200 ± 350 years B.P. to modern and indicate young recharge to the shallow groundwater system. Stable isotopic data of the groundwater samples were found to lie or close to the global meteoric water line (GMWL) on the δD-δ18O plot, an indication that the shallow groundwater in the area is derived from meteoric water that has undergone no significant degree of kinetic evaporation during recharge ruling out possibility of paleowaters in the area. JØrgensen and Banoeng-Yakubo (2001) applied environmental isotopes (18O, 2H, and 87Sr/86Sr) to groundwater with emphasis on saline groundwater aquifers in the Keta Basin of Ghana and realized that groundwaters from deep and dug wells in near coastal aquifers are characterized by relatively high chloride contents, and marine influence is evidenced University of Ghana http://ugspace.ug.edu.gh 55 by well-defined mixing lines for strontium isotopes, and hydrogen and oxygen stable isotopes with isotopic compositions of seawater as one end member. Banoeng-Yakubo et al., (2009) observed that groundwater hydrochemistry in parts of the Volta Region of Ghana is controlled by weathering of silicate and carbonate minerals as well as the chemistry of infiltrating water. Yidana et al., (2008) also investigated the hydrogeological and hydrochemical characterization of the Voltaian Basin in the Afram Plains area and realized that PHREEQC modeling and mineral stability diagrams indicate that groundwater quality is controlled by incongruent weathering of silicate minerals in the aquifers. Hierarchical cluster analysis performed on the raw chemical data revealed two main facies: the calcium-sodium-chloride-bicarbonate facies and magnesium- potassium-sulphate-nitrate facies for the southern and northern sections of the Afram Plains. Analysis of water quality using multivariate and spatial analysis in the Keta basin of Ghana revealed that salinity of groundwater is attributed largely to mineral weathering and seawater intrusion. Groundwater is observed to be stable with kaolinite and Na- smectite field suggesting Na-rich silicate minerals over the high temperature Ca-rich minerals. University of Ghana http://ugspace.ug.edu.gh 56 2.5 GROUNDWATER RECHARGE MECHANISMS Groundwater rechargeis the entry to the saturated zone of water made available at the water table (upper limit of saturarted zone).Conversely, in a discharge area there is a component to the direction of groundwater flow that is upward (Fig. 2.5). Groundwater discharge is the removal of water from the saturated zone across the water table surface. The patterns of groundwater flow from the recharge to the discharge areas form groundwater flow systems, which constitute the framework for understanding recharge processes. Fig 2.5. Schematic representation of the hydrologic cycle showing recharge and discharge zones (Freeze and Cherry, 1979) University of Ghana http://ugspace.ug.edu.gh 57 Recharge can be classified as direct recharge from percolation ofprecipitation and indirect recharge from runoff ponding. Other classifications includelocalized or focused recharge, preferential recharge, induced recharge andmountain frontrecharge. Groundwater in arid and semi-arid regions canexhibit a wide range of isotopic compositions becausethe factors that lead to recharge are usually governedby specific conditions being met at the time of recharge(Allison, 1982).Such conditions could include flashfloods, which may recharge through localized features(preferred pathways) in the vadose zone, as well asdiffuse (matrix pathway) recharge (Nkotagu, 1996). The mean values of δ2H and δ18O of the shallow and deep groundwaters of fractured crystalline basement area of Dodoma, Tanzania have been used to define the recharge mechanism as re-dissolving and leaching of accumulated surficial and soil salts into the groundwater system by surface runoff formed from rain events that are highly depleted in δ2H and δ18O.The leaching takes place predominantly through macropores such asfaults, fractures, joints and highly permeable soils with consequently little evaporation near the ground surface or unsaturated zone (Nkotagu, 1996). 2.6 GEOLOGY The Geological Survey Department (GSD) of Ghana, in collaboration with Bundesanstalt fur Geowissenchaften and Rohstoffe, Hannover, Federal Republic of Germany have come out with a new Geological Map of Ghana that spells out four major rock complexes namely: University of Ghana http://ugspace.ug.edu.gh 58  Paleoproterozoic Supracrustal and intrusive rocks made up of Birimian Supergroup, Tarkwaian Group, Tamnean Plutonic Suite and the Eburnean Plutonic Suite. These groups of rocks are reported to be formed between 2195 Ma and 2072 Ma.  Neoproterozoic to Early Cambrian comprising the Voltaian Supergroup, Bombouaka Group, Oti-Pendjari Group and the Obusum Group at the top.  Panafrican Dahomeyide orogenic belt consisting of the Buem Structural Unit, the Togo Structural Unit and a variety of gneisses of the Dahomeyan Supergroup.  Coastal Sedimentary Basins of Ordovician to Cretaceous age comprising Sekondian Group, Accraian Group, Amisian Group and Apollonian Group. The study area falls within the Paleoproterozoic Supracrustals and intrusive rocks made up of the Birimian Supergroup, a small section of the Tarkwaian Group and the Eburnean Plutonic Suite (Fig. 2.6). A smaller section of the study area also falls within Amisian group of the Mesozoic comprising conglomerate, mudstone, micaceous sandstone and arkose. This group forms part of the Coastal Sedimentary Basin. The Birimian Supergroup is characterized by a North East (NE) trending, parallel, evenly-spaced ‗volcanic belts‘ and intervening ‗sedimentary basins‘. The ‗volcanic belts‘ are termed the Volcano – Plutonic Group and the ‗sedimentary basins‘ are called the Sedimentary – Volcano – Sedimentary Group. Rocks of the Volcano Group are composed of low grade metamorphic tholeiitic basalts with intercalated volcaniclastics as University of Ghana http://ugspace.ug.edu.gh 59 well as minor andesitic and felsic flow rocks and locally chemical sediments. Volcanic rocks in most of these belts are intruded by coeval, comagmatic and synvolcanic granitoid plutons mainly tonalite and granodiorite. Rocks of Sedimentary – Volcano – Sedimentary Group are low - grade metamorphosed, tightly to isoclinally folded sediments consisting of volcaniclastics, volcaniclastic wackes, wackes and argillites (Fig. 2.6) which, on basis of relative abundance, form different but broadly contemporaneous lithological facies representing different depositional paleo-environments. These sediments were believed to be probably derived from the adjacent Volcano-Plutonic belts, either as detritus or as proximal and distal volcanic ejecta from belt of volcanism. The Volcano – Plutonic Group and the Sedimentary – Volcano – Sedimentary Group are observed to represent partly contemporaneous lateral facies equivalent, with sedimentation probably outlasting the bulk of belt volcanism and plutonism. Most of the Birimian belt volcanic, volcaniclastics and associated synvolcanic belt plutons were formed between 2195 and 2150 Ma. However, belt of volcanism and plutonism is reported to have extended down to 2135 Ma. Uplift and erosion of the Birimian rocks and deposition of the erosional products as immature, clastic, shallow water sediments constitute the Tarkwaian Group. It consists of quartz – pebble conglomerates, sandstones and minor argillites, siltstone and tuffs. University of Ghana http://ugspace.ug.edu.gh 60 Fig. 2.6 Geological Map of the Study Area University of Ghana http://ugspace.ug.edu.gh 61 Extensional tectonic regime between 2120 Ma and 2115 Ma followed by crustal shortening and associated regional metamorphism defines the Eburnean tectono – thermal event, which folded and metamorphosed the previously formed Paleoproterozoic rocks and is responsible for, inter alia, the formation of high – strain zones at the Birimian belt/basin boundaries. The Birimian Basins and locally some Birimian Volcano – Plutonic belts were intruded by extensive, syn- and late – kinematics frequently peraluminous granitoid intrusions of the Eburnean Plutonic Suite displaying crystallization ages between 2116 Ma and 2088 Ma. In the study area, undifferentiated Biotite (± hornblende, ± muscovite) granitoid, Biotite gneiss and Hornblende – biotite granitoid (Fig. 2.6) form part of the Eburnean Plutonic Suite. Typical rock types in this group have been reported by (Leube et al., 1990, Hirdes et al., 1992 and Taylor et al., 1992) as quartz diorites, tonalities and trondhjemites, granodiorites, adamellites and granites which are intrusives into the ‗sedimentary basins‘. The main ferromagnesian mineral is biotite which is commonly accompanied by muscovite.The belt of granitoids comprise quartz diorite, tonalite and trondhjemite, granodiorite, admellite and to a minor extent granite. The characteristic mafic mineral is hornblende accompanied by increasing amount offelsic components include the plagioclase feldspars, which commonly alter to saussurite and sericite. University of Ghana http://ugspace.ug.edu.gh 62 2.7 HYDROGEOLOGY 2.7 .1 Mode of grou n dwater oc cu rr en ce Groundwater occurs in many different geological formations. Nearly all rocks in the upper part of the earth‘s crust, whatever their type, origin, age, possess openings called pores or voids. In unconsolidated, granular materials the voids are the spaces between the grains, which may become reduced by compaction, and cementation. Unconsolidated materials ranging in texture from fine sand to coarse gravel are being developed into water supply wells (Fetter, 1994). Materials that are free from silt and clay are best for groundwater occurrence. The hydraulic conductivities of some deposits of unconsolidated sands and gravels are among the highest of any earth materials (Fetter, 1994). Unconsolidated materials are most often close to a source of recharge, such as a stream or lake. Shallow unconsolidated aquifers are in the region of rapid circulation of water usually in local flow systems. The geology of the study area as discussed in section 2.6 lies within the Crystalline Basement terrain with very little or no primary porosity. The mode of groundwater occurrence is therefore through the development of secondary porosities or permeabilities as a result of fracturing, jointing, shearing and deep weathering (Kortatsi and Dogoli, 1993; Banoeng-Yakubo, 1989, 2000). The fractures developed from tectonic movement, pressure relief due to erosion of overburden rock, crackin during heating and cooling of the rock mass and the compression and tensional forces caused by regional tectonic stresses (Davis and Turk, 1964; Banoeng-Yakubo, 2000). Research has shown that the frequency of occurrence of fractures in crystalline rocks has generally been found to University of Ghana http://ugspace.ug.edu.gh 63 decrease with depth (Davis and Turk 1964; Davis and Dewiest, 1966; LeGrand, 1967 and Landers and Turk, 1973). However, underground mines have encountered heavy flows of groundwater hundreds of meters beneath the surface, indicating that some fractures extend to great depths (Hurr and Richards, 1966). Banoeng-Yakubo, (1989, 2000) identified three aquifer types in the crystalline terrain in the Upper West Region of Ghana as weathered rock aquifer, fractured quartz vein and fractured un-weathered rock aquifer. It is worth noting that these aquifer types are related and can occur together. In the study area, information on borehole records is scanty. The existing information includes borehole yield, static water level (SWL), weathered depth, final borehole depth, lithology and aquifer type. Detail pumping test records are not available. Out of 84 boreholes sampled only 41 have records on yield, SWL, weathered depth, final drilling depth, lithology and aquifer type (Appendix 1). The borehole yield varies from 7.5 l/min to 179.75 l/min. The mean yield is 32.14 l/min. The lowest yield of 7.5 l/min occurs in the schist aquifer. The highest value of 179.75 occurs in the biotite gneiss/biotite granite contact in Katakyiase in the hinterland (Fig. 2.6). Generally, the water levels vary from 4 m above means sea level (a.m.s.l) to 106 m (a.m.s.l) with a mean of 47.99 m (a.m.s.l).The water levels were contoured using krigging method in ArcGIS to determine the general flow direction. A three diamensional representation of the flow direction is is illustrated in (Fig. 2.7). The general groundwater flow direction is from Northwest (NW) to Southeast (SE).Groundwater thus flows from a higher topography (inland areas) to the coast (low lying areas). University of Ghana http://ugspace.ug.edu.gh 64 Fig. 2.7 Water table map of the study area The weathered depth varies from 0 to 30 m with a mean of 14.05 m. The total depth of the boreholes ranges from 18 to 94.5 m with a mean of 34.48 m. Adequate hydrogeologic information could not be obtained from the lithologic logs to enable the aquifer types in the area to be determined.However, the few lithologic logs evaluated in granitic and schist aquifers showed the occurrence of unconfined condictions in the study area (Fig. 2.8a and Fig. 2.8b). In Fig. 2.8a groundwater occurred in highly weathered granite which was fractured with quartz veins depicting an unconfined aquifer. A similar observation was made in Fig. 2.8b in mica schist rock. University of Ghana http://ugspace.ug.edu.gh 65 Fig. 2.8a unconfined aquifer in granitic rock Fig. 2.8b Unconfined aquifer in mica schist rock University of Ghana http://ugspace.ug.edu.gh 66 Community Water and Sanitation Agency (CWSA) captures some of the aquifer types as confined and semi-confined in its database. Fifty-two percent (52%) of the boreholes are classified as confined and forty-eight percent (48%) as semi-confined. These aquifer types are not peculiar to any specific lithology. It occurs in almost all the lithologies in the area namely sediment/volcaniclastic sediment, wacke sediment, and biotite gneiss and biotite granitoid. 2.7 .2 Aquif er characteri stics The parameters that describe the characteristics of an aquifer are coefficient of transmissivity, coefficient of storage or storativity and specific capacity. The coefficient of transmissivity of an aquifer is defined as the rate at which water flows through a vertical strip of the aquifer 1m wide and extending through the full saturated thickness under a hydraulic gradient of 1 (100 percent) (Driscoll, 1989, Freeze and Cherry, 1979). Generally, Driscoll (1989) mentioned that values of transmissivity (T) range from less than 12.4 m2/day to more than 12,400 m2/day. An aquifer of transmissivity less than 12.4 m2/day can yield enough water for only domestic purposes. When transmissivity is 124 m2/day or more, borehole yields can be adequate for industrial, municipal and irrigation purposes.The higher the transmissivity, the more prolific theaquifer and the less drawdown observed in the well. The transmissivity and storativity are important because they describe the hydraulic characteristics of a water bearing formation. Since most of the boreholes closer to the coast and partly inland are old boreholes and therefore lack detail pumping test records, records of newly drilled boreholes in similar geological terrain and in close vicinity to the study area were obtained and studied to have a generally fair idea University of Ghana http://ugspace.ug.edu.gh 67 regarding the aquifer characteristics in the area. Forty-six of such boreholes located in thesediment/volcaniclastic sediment, wacke sediment, biotite gneiss and biotite granitoid aquifers were studied. Parameters obtained included borehole yields, depths, static water levels, dynamic water level (pumping water level), drawdown and specific capacity. The transmissivity was estimated from the specific capacity. The Specific Capacity of a borehole is simply the pumping rate(yield) divided by the drawdown (Johnson, 2005). It is an important parameter that can be used to determine the maximum yield of a borehole, identify potential borehole, the pump and determine aquifer problems. This will accordingly help develop a proper well maintenance schedule. It can also be used to estimate the transmissivity of the aquifer(s) tapped by the borehole‘s perforations. Driscoll (1986) stated that typically, a well should run continuously for at least 24 hoursat a constant yield before recording the drawdown. The same time frame should be used for each subsequenttest for equal comparisons to the initial test. Johnson (2005) observed shorter time frames are sometimes used from electric company pumpefficiency tests or Step-Drawdown tests, but these shortertimes may not sufficiently allow the water levels to stabilizefor a reliable specific capacity calculation. Cooper-Jacob (1946) established an equation for estimating specific capacity, Q/Sw of a borehole: 𝑄 𝑆𝑤 = 𝑇 0.183log⁡[ 2.25𝑇𝑡 𝑟2𝑆 ] (2.34) University of Ghana http://ugspace.ug.edu.gh 68 Where Q is constant discharge rate (pumping rate), r is the pumped well radius, S is the storativity, Sw is the drawdown in the well, T is the transmissivity and t is the time. Using equation (2.34), Batu (1998) expressed Transmissivity in m2/day according to the equations (2.35 and 2.36) using S = 0.001for confined aquifer and S = 0.075 for unconfined aquifer, r = 0.15 m and t = 1day. 𝑇 = 1.385 𝑄 𝑆𝑤 For confined aquifer (2.35) 𝑇 = 1.042 𝑄 𝑆𝑤 For unconfined aquifer (2.36) Where T is transmissivity in [m2/day], Q is constant discharge rate [m3/h] and Sw is the drawdown in the pumped borehole after 1 day [m]. Equations 2.35 and 2.36 were therefore used in estimating the transmissivity of the boreholes in the study area. In the unconfined aquifers transmissivity ranges from 0.28 to 26.89 m2/day with a mean of 4.06 m2/day. In confined aquifers transmissivity ranges between 0.31 m2/day to 35.74 m2/day. University of Ghana http://ugspace.ug.edu.gh 69 CHAPTER THREE METHODOLOGY This chapter discusses the mode of data collection and the methods used in the analyses of samples and interpretation of data. The mode of data collection involved desk study, fieldwork and laboratory work. The samples collected were first measured for field parameters and then sent to the laboratory for analyses of major ions, stable isotopes (δ18O, δ2H and13C) and radioisotopes (3H and 14C). Piper, and Compositional diagrams were used to interpret the results from which various hydrochemical facies in the groundwaters were identified. ArcGIS version 9.3, Geographic Information System (GIS) software, was used in generating spatial distribution maps of the various hydrochemical parameters measured in the study area. The statistical analyses carried out included measures of central tendency and correlation. 3 .1 DATA COLLECTION AND ANALYTICAL PROCEDURES 3 .1.1 Desk stud y In order to achieve the objectives of the study, a desk study was carried out to assess the general hydrogeological and hydrochemical conditions prevailing in the various lithologies in the coastal zone and partly inland of the Central Region. It involved a review of available literature, collection of topographical, hydrogeolocal and geological maps from the University of Ghana, Community Water and Sanitation Agency (CWSA- Central Region), Water Research Institute (WRI), Survey Department and Geological Survey Department (GSD). University of Ghana http://ugspace.ug.edu.gh 70 Some available borehole records such as lithologic logs, and water quality data obtained from CWSA and WRI were compiled and studied. Based on the available borehole records, topographical, hydrogeological and geological maps, sampling sites were selected before embarking on the fieldwork. 3 .1.2 Fieldw ork and Sample Colle ction The fieldwork comprises water sampling mainly from boreholes, hand-dug wells and surface water (rivers) and measurement of field parameters. Aside sampling groundwater and surface water, rainwater was also sampled from two meteorological stations at Saltpond and Twifo Praso to determine the stable isotope and chloride content. Soil samples were also taken between Akwakrom and Asokwa to study the chloride (Cl-) content of the soil. A total of eighty-four (84) water samples comprising seventy (70) boreholes, eight (8) hand-dug wellsand six (6) surface water samples were obtained. One hundred and thirty- seven (137) rainwater samples were collected at the Saltpond and Twifo Praso Metorological stations from 2010 to 2011 on event basis. Thirty-five soil samples from four (4) piezometers at various depths were obtained and studied for contribution of Cl- in the soil to salinity of groundwater. All water samples were collected in pre conditioned high density polyethylene (HDPE) bottles. They were conditioned by washing initially with detergent, then with ten (10%) percent nitric acid, and finally rinsing several times University of Ghana http://ugspace.ug.edu.gh 71 with distilled water. This was carried out to ensure that the sample bottles were free from contaminants. At the sampling point, the borehole was first purged until, the pH, electrical conductivity (EC) and temperature (T OC) were stable.The purpose of purging the borehole was to evacuate the stagnant water in the borehole casing prior to sampling so as to obtain a representative sample of in-situ groundwater. In the course of purging the aquifer, the water discharged was measured for pH, EC and T for three successive measurements until the pH, EC and T were stable. Other parameters such as TDS, Eh and alkalinity were measured after which samples were taken. Samples were initially collected in a sterilised bucket immediately after purging the aquifer and filtered through 0.45-micron meter cellulose filters with the aid of hand operated vacuum pump. Part of the filtered water was used to rinse the sample bottles thrice before sampling as suggested by (Boghici, 2003). The remaining filtered water was then transferred into two 250 ml bottles for cation and anion analyses. The samples for cation analysis were acidified with 0.2 M HNO3 to keep the ions in solution. The sampled waters were tightly capped. Similar procedures were employed for the sampling of the hand-dug wells.Each sample bottle was provided with an identification label on which the following information was legibly and indelibly written:  Sample station identification number  Date and Time of sampling  Record of any stability preservative treatment  Prevailing weather condition at the time of sampling. University of Ghana http://ugspace.ug.edu.gh 72 Samples for stable isotopes (δ18O and δ2H) analyses were not filtered. The sample bottles were directly filled with the borehole water and then capped as outlined by IAEA(www.iaea.org/water and Boghici (2003)). The samples were collected in 50 ml HDPE bottles. Surface water samples were collected by means of a dip sampler and the water obtained transferred into HDPE bottles for measurement of 18O, 2H and major ions. Field parameters were measured prior to this activity. In an attempt to avoid the incidental inclusion of disturbed sediment in the sample, surface water samples were collected from a downstream to upstream direction. Rainwater samples were also collected at two meteorological stations namely Saltpond and Twifo Praso using the rain gauge (Fig. 3.1). The rain gauge consists of an outer cylinder, a measuring tube, and a funnel. The measuring tube measures to a hundredth inch. When it is full, it contains one inch of rain. By carefully pouring the rain from the outer cylinder back into the measuring tube, a total rainfall amount was accurately measured. The rain gauge was mounted 30 to 50 cm above the ground to avoid splashes. After measuring the rainfall amount for a specific event it was poured into 50 ml HDPE for 18O and 2H analyses. Some of the collected samples were then transferred into 250 ml HDPE bottle for analyses of some major ions notably Cl-. All water samples were preserved at 4oC until laboratory analyses were performed, in order to limit bacterial activity and degradation of nutrient species (NO3 -, SO4 2- and PO4 3-). University of Ghana http://ugspace.ug.edu.gh 73 Fig. 3.1 Rain gauge at the Saltpond station of Ghana Meteorological Agency (GMA) Thirty-five soil samples from four profiles were collected by means of a hand auger. The samples were collected at 20 cm intervals to maximum depths ranging between 60 and 200 cm. The soil samples were collected into clean polyethylene zipped lock bags with a plastic trowel as described by IAEA, 2008. The bags were immediately sealed, well labeled and kept in ice-chests at a relatively low temperature (4OC). Preserving the samples at low temperature prevents it from undergoing physical and chemical changes such as drying, evaporation and oxidation.The soil samples obtained at various depths were carefully identified and described. University of Ghana http://ugspace.ug.edu.gh 74 3.1.2.1 Measurement of Field Parameters Geochemical studies are based on measurement of inorganic constituents or species in groundwater and a series of physico-chemical parameters that control the interaction of these species. Parameters that control interaction of these species which are usually measured in the field are Temperature, pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Alkalinity and Oxidation-Reduction Potential (Eh).Each of these parameters except alkalinity was measured in the field by immersing probes in samples of groundwater and surface water. The alkalinity was measured by titrimetric method. The temperature of groundwater is a fundamental measurement and is required for all thermodynamic calculations and modeling. It is also required for correcting electrical conductivity measurements and for pH measurement. It is measured as close to in situ conditions as possible using a temperature probe that is a component of the pH meter. Electrical Conductivity (EC) is proportional to the quantity of dissolved ions present in solution and provides a rough idea of the total dissolved solids. EC is measured in millisiemens per meter (1mSm-1 = 10μScm-1 = 10μmhoscm-1). For this study all EC values were recorded in μScm-1. The pH is one of the most important field parameters measured because it controls most of the organic and inorganic constituents in groundwater. It is an expression of the negative logarithm of hydrogen ion (H+) activity (pH = -log (H+)). It is fundamental to thermodynamic calculations and to the interpretation of δ13C data. In natural waters pH is University of Ghana http://ugspace.ug.edu.gh 75 generally between 6.5 and 8 (Fisher, 2002). There are three different methods of pH measurement: pH indicator paper, liquid calorimetric indicators and electronic meters. The use of pH indicator paper is simple and inexpensive, but the method is not very accurate and employs a subjective assessment of colour by the user. Liquid colorimetric indicators change colour in accordance with the pH of the water with which they are mixed. The colour that develops can then be compared with a standard colour chart, colour glass standards, or with a set of prepared liquid standards. Calorimetric methods are reasonably simple and accurate to about 0.2 pH units (APHA 1992). Their main disadvantage is that standards for comparison or a comparator instrument must be transported to the sampling station. Moreover, physical or chemical characteristics of the water may interfere with the colour developed by the indicator and may lead to an incorrect measurement. The third measurement, electrometric pH measurement, is accurate and free from interferences. Pocket - sized, battery powered, portable meters that give readings with an accuracy of plus or minus five (±5) percent are suitable for field use. However, a more complex one designed for measurement of conductivity, temperature and salinity as well as pH was used for this work. This equipment is capable of attaining an accuracy of plus or minus (±0.002) pH units. Prior to sampling the probes were calibratedwith standard solutions. The pH was calibrated with a reference buffer solution of pH 4 and 7. The conductivity probe was calibrated against a standard solution made of KCl of value 1480 µS/cm. The probes were calibrated each morning before measurement. University of Ghana http://ugspace.ug.edu.gh 76 The pH of groundwater is important because it dictates the primary aggressive nature of water in chemical weathering of rocks and minerals. Hem (2002) identified the reaction of dissolved carbon dioxide with water as one of the most important processes in establishing the pH of natural water systems. It is represented by three step reactions as follows: CO2(g) + H2O(l) ↔ H2CO3(aq) (3.1) H2CO3(aq) ↔ H +(aq) + HCO3 -(aq) (3.2) HCO3 -(aq) ↔ H+(aq) + CO3 2-(aq) (3.3) Both the second and third steps produce H+ which influences the pH of the solution. . The concentration of total dissolved solids (TDS) in groundwater is usually determined by weighing the solid residue obtained by evaporating a measured volume of filtered sample to dryness. However, in this study, TDS was measured by means of the conductivity probe. Oxidation-reduction potential (Eh) isthe transfer of electrons from one ion to another in an aqueous solution. This redox reaction takes place in twohalf reactions which are always coupled in nature. The oxidation reaction results in an ion losing or donating its electron(s) to another ion, and the reduction reaction results in an ion gaining or accepting electron(s).The oxidation potential of an aqueous solution is called the Eh, University of Ghana http://ugspace.ug.edu.gh 77 which issynonymous with oxidation reduction potential (Boghici, 2003). The sign of the potential ispositive if the reaction is oxidizing and negative if it is reducing. The Eh was measured using the pH probe constructed to measure both pH and Eh in milli volts (mV) at 25 OC. Alkalinity (as HCO3 -)was also determined in the field by titrating aliquots of the sample with 0.02M hydrochloric acid (HCl) using methyl orange as indicator. A change in colour from yellow to orange indicated the end point and volume of acid used recorded as titre value. The procedure was repeated and the average titre calculated. The equation of the reaction occurring is: HCl(aq) + HCO3 - (aq) ↔ CO2(g) + H2O(l) + Cl -(aq) (3.4) From the equation of the reaction, 1mole of HCl reacts with 1mole of HCO3 - . The mole ratio is therefore 1:1. Hence the concentration of HCO3 - in millimol/l is obtained from the equation: Where  HCLTV = total volume of HCl needed to titrate to pH 4.4   HCLM Molarity of HCl a = aliquot of extract in ml       10003  aMaVHCO HCLHCLT University of Ghana http://ugspace.ug.edu.gh 78 The Molar Mass of HCO3 - = 61 Hence       6110003  aMaVHCO HCLHCLT =     a MV HCLHCLT 61000 (3.5) 3.1.2.2 Radiocarbon Sampling In 13C and 14C determinations, about 2.5 g of carbon is required. In order to obtain this amount of carbon in groundwater, a sample volume of 40 – 60 litres is required. Hence sample processing in the field has been recommended by various laboratories to avoid shipment of large samples. In this study the International Atomic Energy Agency (IAEA) procedures for processing water samples earmarked for 13C and 14C analyses were used. The application of this procedure depended on the alkalinity of water. The higher the alkalinity the lower the volume of water required to precipitate all the inorganic carbonate species in the water at high pH and vise versa. It is the precipitates that were shipped to an appropriate laboratory for analyses. The apparatus for carrying out 14C precipitation in the field included a 60 litre specially fabricated plastic container to which a wide mouthed plastic bottle of 1 litre capacity was screwed. The amount of water required was calculated using the alkalinity measurement from each borehole. The container was mounted on a stand and filled up to the top with groundwater (Fig 3.2). A stirrer was inserted and the lid closed. This was followed by the University of Ghana http://ugspace.ug.edu.gh 79 addition of a 50 ml carbonate free concentrated sodium hydroxide (NaOH) to raise the pH of the sample to about 11. The raised pH was confirmed with a pH testing paper. The exposure of the content of the container to the atmosphere was minimized after adding the sodium hydroxide, to prevent contamination by atmospheric carbon dioxide. This was done by quickly capping the container with the lid. The sample was stirred thoroughly after adding the NaOH. This was followed by the addition of 5gof iron sulphate (FeSO4) to facilitate the formation of carbonate precipitate.Strontium chloride (SrCl2)powder weighing about 150 g was then added to form a fine cloud ofcarbonates after which the lid was closed and the sample stirred. Finally 40 ml of Praestol solution was added to the sample to speed up the precipitation process. Having added all these reagents, the precipitate formed and settled to the bottomof the apparatus, filling the 1 litre bottle attached to the bottom of the container (Fig 3.2). After allthe precipitate (SrCO3)had settled at the bottom of the funnel portion of the container filling the attachedbottle (Fig. 3.2), a rubber stopper fitted at the end of a steel rod was inserted and the container closedat the bottom, enabling the filled 1 litre bottle to be removed. The bottles were then tightly capped and properly labeled, indicating the sample number, date, country code and Global Positioning System (GPS) location. University of Ghana http://ugspace.ug.edu.gh 80 Fig. 3.2 Radiocarbon sampling at Esuehyia in the coastal zone of the Central Region University of Ghana http://ugspace.ug.edu.gh 81 3.1.2.3 Tritium Sampling Sampling groundwater for tritium analysis does not require any special procedure. One litre HDPE bottles from the IAEA were first rinsed thoroughly with the groundwater samples and then filled to capacity after which they were tightly sealed to avoid contact with air. They were properly labeled and the dates of sampling clearly stated. They were then sent to the IAEA laboratory in Vienna – Austria for analysis. 3 .2 LABORATORY ANALYSES 3 .2.1 Major and Minor Ions Various analytical methods have been employed in the determination of chemical composition of water. Some of the routinely employed methods include Atomic Absorption Spectrometer (AAS), Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES), Flame Emission Spectrometry (FES), Potentiometric and Calorimetric titration and Ion Chromatogaphy (IC). In this research, the AAS, FES and IC were used for measurement of major and minor ions as well as trace metals. The AAS was used to measure Ca, Mg and trace metals, FES was used to measure Na and K and IC was used to measure some of the anions namely Cl-, SO4 2-, Br-, NO3 -, PO4 3-, F-. Calcium (Ca), Magnesium (Mg), Manganese (Mn), Copper (Cu), Zinc (Zn), Lead (Pb), Iron (Fe), Chromum (Cr), Cadmium (Cd) and Cobalt (Co) were determined by AAS. Prior to their determination, 5 ml of the water samples were measured in triplicates into TFM Teflon vessels of microwave digester (ETHOS 900). This was followed by the University of Ghana http://ugspace.ug.edu.gh 82 addition of 6 ml of 65% concentrated HNO3, 3 ml of 35% HCl and 0.25% H2O2 to each vessel containing the sample. The vessels were swirled gently to allow for mixing and fitted vertically into the microwave digester and digested for 20 minutes. When the digestion process was completed, the solution containing the samples was cooled in a water bath for twenty (20) minutes to reduce high temperature and pressure build-up within the vessels. The mixture was then transferred quantitatively into a volumetric flask and diluted to 20 ml using deionised water. All the samples were analysed using VARIAN AA 240FS Flame Atomic Absorption Spectrometer. During the digestion and sample preparation, blank samples were also digested along with each set of samples and subsequently analyzed for some of the elements outlined earlier. This was to ensure quality assurance and quality control (QA/QC). All the major and minor ions were measured at the National Nuclear Research Institute (NNRI) of The Ghana Atomic Energy Commission (GAEC). Samples for major and minor ions were also measured at Water Research Institute (WRI), Accra, Ghana and School of Engineers, Sfax, Tunisia. Even though there were differences in the results they established the same trend as obtained from the inorganic lab of NNRI. 3 .2.2 Measure men t of Stable Isotop e s of Water (δ18 O and δ2H) The Los Gatos Research (LGR) instrument, LGR DLT-100 (model 908-0008) was used to determine δ18O and δ2H composition of the various water samples at the Isotope Hydrology Laboratory of the International Atomic Energy Agency (IAEA). This University of Ghana http://ugspace.ug.edu.gh 83 equipment also called the Liquid Water Isotope Analyser (LWIA) measures absorption around a wavelength of 1390 nm to calculate molecular concentrations of 2HHO, HH18O and HHO. Molecular concentrations are converted into atomic ratios, 2H/1H and 18O/16O and a post-processing procedure was used to calculate delta-scale (δ) values with respect to Vienna Standard Mean Ocean Water (VSMOW) (Coplen, 1996) as stated in equation 2.9. The instrument contains the laser analysis system and an internal computer, a CTC LC- PAL liquid autosampler, a small membrane vacuum pump, and a room air intake line that passes air through a Drierite column for moisture removal. The autosampler and theDLT- 100 are connected by a ~1 m long polytetrafluoroethylene(PTFE) transfer line. A Hamilton microlitre syringe(model 7701.2N) was used to inject 0.75 litre of sample through a PTFE septum in theautosampler. The injection port of the autosampler was heated to 80°C to help vaporize thesample under vacuum immediately upon injection. The vapor then travels down the transferline into the pre-evacuated mirrored chamber for analysis.The instrument has a precision of approximately 1‰ for δ2H and 0.2‰ for δ18O. 3 .2.3 Tritiu m ( 3 H) Measure men t The measurement of tritium activity involved two major parts. The first part consisted of prec-concentration of tritium in the sampled groundwater using electrolytic enrichment. The first step in pre-concentration is distillation. The water samples were distilled to a conductivity of 25µS/cm to remove impurities which may cause corrosion of the electrodes for the electrolytic process. Two grams (2g) of peroxide was added to the University of Ghana http://ugspace.ug.edu.gh 84 water samples in 500 ml standard glass flask. This was then transfered into an electrolytic cell and weighed. The initial mass of water (Wi) was then obtained.The weighed samples in the cells were subjected to electrolysis resulting in the release of O2 and H2 at the anode and cathode respectively. After electrolysis the cells were weighed and the final mass of water obatained as Wf. The samples were then transfered from the cell into a distillation flask and neutralised with 8g of PbCl2 and subjected to final distillation. The final distillates were mixed with a cocktail and poured into polyethylene vials. The vials together with two standards and two dead waters were placed in the Liquid Scintililation Counter (LSC) for tritium activity measurement. The first batch of tritium samples, numbering 10 were analsed at the Isotope Hydrology Lab of the IAEA in Vienna, Austria between 2010 and 2011. The second batch of 17 samples were analysed at School of Engineers, Sfax, Tunisia in June 2013. 3 .2.4 Radiocarb on ( 14 C) Measure men t The SrCO3 precipitates obtained in the field as discussed in section 3.1.2.3 were decomposed to CO2 using HCl (water solution 1:1) in a glass flaskas illustrated in equation (3.6). The CO2 produced was removed by nitrogen, dried and frozen. SrCO3 + 2HCl → SrCl2 + CO2 (g)↑ + H2O (3.6) University of Ghana http://ugspace.ug.edu.gh 85 A small portion of cleaned CO2 was separated for 13C determination, and the rest used for benzene synthesis. Benzene (C6H6)was synthesized as follows: Benzene was mixed with scintillation cocktail and measured in Liquid Scintillation Counter. The uncertainty of radiocarbon determination was 0.7 pMC (one sigma) and that of 13C determination was 0.08 ‰ (one sigma). Twenty four (24) samples were analysed for 14C. The first batch of eight (8) samples was analysed at AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland in September, 2011. The second batch of sixteen (16) samples was analysed at University of Groningen, Centre for Isotope Research, in the Netherlands in January 2013. 3 .3 DATA ANALYSES AND INTERPRETATION This section discusses the analytical error and highlights statistical analysis and various graphical forms used for interpretation. The main analytical error determined is the charge balance error. The statistical analyses comprise mean, standard deviation and coefficient of correlation. The major graphical forms used in the interpretation of results in this study were Piper, and Compositional diagrams (bivariate plots). 3 .3.1 Analytical Errors Analytical error is defined as a cumulative outcome of all errors involved in a measurement. One of the commonest errors determined in the evaluation of hydrogeochemistry of groundwaters is the charge balance error (CBE). The charge University of Ghana http://ugspace.ug.edu.gh 86 balance error was evaluated based on the assumption that water is a neutral substance and, therefore, the sum of cation milliequivalents should equal the sum of anion milliequivalents. However, this is not always the case in the measured values for major cation and anions. Significant deviation from equality can occur (Wigley, 1977 and Freeze and Cherry, 1979) and this may be attributed to; (1) analytical errors in the concentration determinations or (2) ionic species at significant concentration levels that were not included in the analysis. The deviation from equality is expressed in the form: 100          ac ac zmzm zmzmE (3.7) E = charge-balance error expressed in percent z = ionic valence mc = molality of the cation species ma = molality of the anion species Freeze and Cherry (1979), reports that water analytical laboratories consider a charge balance error of less than 5% to be acceptable. Guler et al., (2002) considered a charge balance of ±10.4% for South Lahontan Hydrologic databases (SLHDATA). Values with errors greater than ±10.4% were not used in their study. Booth and Bertsch (1999) set University of Ghana http://ugspace.ug.edu.gh 87 charge balance error at 5-10% in their studies. In the present study the charge balance error falls within the range ±2 to ±10% and considered for analysis and interpretation. 3 .3.2 Statistical Analysis The statistical analyses carried out included the mean, standard deviation and correlation coefficient. The coefficient of correlation originated by Karl Pearson about 1900, describes the strength of the relationship between two-interval scaled or ratio-scaled variables. Designated by r, it is often referred to as Pearson‘s r and as the Pearson moment correlation coefficient. It assumes values from –1.00 to +1.00 inclusive. If there is no relationship between the two sets of variables, Pearson‘s r will be zero. A coefficient of correlation r close to zero shows that the relationship is quite weak. Coefficients of –0.91 and +0.91 have equal strength; both indicate very strong correlation between two sets of variables. Mathematically the coefficient of correlation (r) is determined as follows:                2222     YYnXXn YXXYnr (3.8) n = the number of paired observations X = dependent variable Y = independent variable University of Ghana http://ugspace.ug.edu.gh 88 In this study r was determined using Microsoft Excel version 2010. The mean was determined by summing the concentrations of individual parameters and dividing it by the total number of samples. 3 .3.3 Graphical Method s Graphical methods are used to aid interpretations. A variety of graphical techniques have been devised since the early 1920s in order to facilitate the classification of groundwaters, with the ultimate goal of grouping samples into similar homogeneous groups each representing a hydrochemical facies (Guler et al., 2002). Guler et al (2002) and Freeze and Cherry (1979) outlined several commonly used graphical methods as: Collins bar, Pie, Stiff pattern, Schoeller Semilogarithmic, Piper and Durov diagrams. Piper and Dourv‘s diagrams are used for large data sets while the rest of the diagrams are most suitable for smaller data sets. In this study the Piper diagram was used for the interpretation of the chemical composition of both surface water and groundwater. The Piper diagram is the most widely used graphical form. The diagram displays the relative concentrations of the major cations and anions on two separate plots, together with a central diamond plot where the points from the two trilinear plots are projected. The central diamond-shaped field (quadrilateral field) is used to show the overall chemical character of the water. Back (1961) and Back and Hanshaw (1965) quoted in Freeze and Cherry (1979) defined subdivisions of the diamond field, which represent water type categories that form the basis for one common classification scheme for University of Ghana http://ugspace.ug.edu.gh 89 natural waters. The mixing of water from different sources or evolution pathways can also be illustrated by this diagram (Freeze and Cherry, 1979). University of Ghana http://ugspace.ug.edu.gh 90 CHAPTER FOUR RESULTS AND DISCUSSION 4 .1 HYDROGEOCHEMISTRY Hydrogeochemistry is the study of the chemical characteristics of groundwater and surface water as related to local and regional geology. A close examination of the hydrological cycle revealed that part of rainwater infiltrates into the ground to recharge groundwater, while the rest of the rainwater runs on the surface as runoff finding its way into surface waters. The interaction between rainwater, surface water and groundwater within the hydrological cycle has prompted the sudy of their hydrogeochemical characteristics leadingto the establishment of the general flow direction and origin of salinity of groundwater. 4 .1.1 Hydrogeoch e mica l characte ristics of rain w ater, surf ace water and grou n dw ater In this section major and some minor anions for rainwater, were discussed. Detailed physico-chemical parameters for both surface water and groundwaters were presented and their hydrochemical facies established. 4.1.1.1 Rainwater Generally, the anions (Cl-, SO4 2-, NO3 -, PO4 3-, F-, Br- and NO3 -) of rainwater, collected from Saltpond and Twifo Praso meteorological stations, were determined to find out if University of Ghana http://ugspace.ug.edu.gh 91 Cl- levels are in concentrations that could be deposited in the soil zone to cause salinization of groundwater over a period. Detail results of rainwater chemistry from Saltpond and Twifo Praso meteorological stations are presented in Tables 4.1 and 4.2. The Saltpond station is located in the coastal zone with elevation of 45 m above mean sea level. The Twifo Praso station is located far inland (90 km from the Saltpond station) at an elevation of 90 m above mean sea level. F- levels at the Saltpond station ranged between 0.22 mg/L and 1.07 mg/L with a mean of 0.74 mg/L. F- levels were therefore low in rainwater. Cl- ranged between 1.07 mg/L and 22.32 mg/L with a mean of 9.71 mg/L. Appreciable amount of Cl- was observed in the rains of the Saltpond station attributable to sea aerosols rich in Cl-. NO2 - ranged from 0.25 mg/L to 3.36 mg/L with a mean of 1.81 mg/L. Br- varied from 0.01 mg/L to 0.11 mg/L with a mean of 0.04 mg/L. NO3 - ranged from 0.15 mg/L to 18.15 mg/L with a mean of 3.90 mg/L. PO4 3- ranged from 0.01 mg/L to 3.80 mg/L with a mean of 0.84 mg/L. SO4 2- ranged from 0.36 mg/L to 18.41 mg/L with a mean of 3.86 mg/L.At the Twifo Praso meteorological station F- ranged from 0.01 mg/L to 3.55 mg/L with a mean of 0.42 mg/L. Cl- levels ranged from 0.48 mg/L to 8.28 mg/L with a mean of 2.25 mg/L. NO2 - ranged between 0.02 mg/L and 0.33 mg/L with a mean of 0.10 mg/L. NO3 - ranged from 0.27 mg/L to 4.42 mg/L with a mean of 1.60 mg/L. PO4 3- ranged between 0.06 mg/L and 1.47 mg/L with a mean of 0.56 mg/L. SO4 2- varied from 0.15 to 6.01 with a mean of 1.14 mg/L. In comparing anion concerntration of rainwater from Saltpond station to that of Twifo Praso, higher concentrations are obtained at Saltpond station than at Twifo Praso station. University of Ghana http://ugspace.ug.edu.gh 92 Table 4.1 Anion chemistry of Rainwater from Saltpond (SP) Meterological Station in the Central Region (May 2010 – August 2010) Sample ID Rainfall Amount (mm) F- (mg/L) Cl- (mg/L) NO2 - (mg/L) Br- (mg/L) NO3 - (mg/L) PO4 3- (mg/L) SO4 2- (mg/L) SP1 5 <0.001 9.53 <0.001 <0.001 <0.001 <0.001 2.26 SP2 <0.001 14.50 <0.001 <0.001 <0.001 1.895 2.84 SP3 1.7 <0.001 10.46 <0.001 <0.001 <0.001 <0.001 9.24 SP4 <0.001 5.97 <0.001 0.045 <0.001 <0.001 1.81 SP5 45.9 1.039 7.21 <0.001 <0.001 <0.001 <0.001 3.57 SP6 <0.001 10.89 <0.001 0.105 <0.001 <0.001 4.59 SP7 28.1 <0.001 7.61 <0.001 <0.001 <0.001 <0.001 1.29 SP8 <0.001 7.45 <0.001 <0.001 1.38 <0.001 6.02 SP9 54.2 <0.001 15.05 <0.001 <0.001 1.00 0.046 1.10 SP10 6.1 <0.001 8.25 <0.001 <0.001 1.28 <0.001 5.12 SP11 19.9 <0.001 7.60 <0.001 <0.001 0.85 <0.001 0.76 SP12 22.6 <0.001 9.24 <0.001 <0.001 0.55 1.405 0.89 SP13 71.5 <0.001 1.91 <0.001 <0.001 0.49 <0.001 0.46 SP14 2.5 <0.001 11.46 <0.001 <0.001 1.78 0.095 1.45 SP15 <0.001 10.52 <0.001 <0.001 4.23 3.795 4.10 SP16 17.4 <0.001 14.33 <0.001 <0.001 4.73 0.007 3.08 SP17 2.8 <0.001 12.16 0.248 0.053 2.12 <0.001 2.67 SP18 3.5 <0.001 14.25 <0.001 0.01 1.38 <0.001 2.01 SP19 31 <0.001 1.07 <0.001 <0.001 0.33 <0.001 1.53 SP20 11.4 <0.001 12.60 <0.001 0.015 6.51 <0.001 5.70 SP21 14.4 1.072 7.33 <0.001 <0.001 2.01 <0.001 1.80 SP22 42.3 0.564 1.55 <0.001 <0.001 0.15 0.028 0.36 SP23 2.9 0.886 7.67 <0.001 <0.001 10.85 <0.001 3.47 SP24 2 0.897 8.11 <0.001 0.016 3.25 0.158 3.48 SP25 8 0.219 14.05 3.362 <0.001 11.33 <0.001 5.31 SP26 24.1 1.045 3.84 <0.001 <0.001 1.52 0.577 1.09 SP28 3.3 0.361 10.93 <0.001 <0.001 1.36 <0.001 2.99 SP29 12.3 <0.001 15.79 <0.001 <0.001 18.15 <0.001 18.39 SP30 3.7 0.857 12.86 <0.001 <0.001 3.14 <0.001 2.42 SP31 7 <0.001 22.32 <0.001 <0.001 13.61 0.371 18.41 SP32 16.4 0.467 4.54 <0.001 <0.001 1.586 <0.001 1.46 Minimum 2.00 0.22 1.07 0.25 0.01 0.15 0.01 0.36 Maximum 42.30 1.07 22.32 3.36 0.11 18.15 3.80 18.41 Mean 12.66 0.74 9.71 1.81 0.04 3.90 0.84 3.86 University of Ghana http://ugspace.ug.edu.gh 93 Table 4.2 Anion chemistry of Rainwater from Twifo Praso (TP) Saltpond Meterological Station in the Central Region (May 2010 – August 2010) Sample ID Rainfall Amount (mm) F- (mg/L) Cl- (mg/L) NO2 - (mg/L) Br- (mg/L) NO3 - (mg/L) PO4 3- (mg/L) SO4 2- (mg/L) TP1 10.7 0.46 4.25 <0.001 <0.001 1.15 <0.001 0.58 TP2 9.5 0.24 0.94 <0.001 <0.001 0.91 <0.001 0.65 TP3 8.5 <0.001 5.44 0.06 <0.001 4.42 <0.001 0.77 TP4 10.7 0.73 0.88 <0.001 <0.001 2.12 0.63 0.60 TP5 10.8 0.43 1.39 <0.001 <0.001 1.11 <0.001 0.68 TP6 14.5 <0.001 1.08 0.33 <0.001 0.92 <0.001 1.29 TP7 46.3 0.44 1.15 <0.001 <0.001 0.86 <0.001 0.15 TP8 8.7 0.27 2.14 0.04 <0.001 1.21 <0.001 0.61 TP9 43.5 0.06 0.78 <0.001 <0.001 0.96 <0.001 0.86 TP10 1.7 0.41 7.45 <0.001 <0.001 1.38 <0.001 6.01 TP11 10.8 0.10 2.92 <0.001 <0.001 2.09 <0.001 0.69 TP12 5.8 0.13 1.94 0.12 <0.001 1.78 <0.001 0.91 TP13 11.6 0.20 2.74 <0.001 <0.001 1.04 0.06 1.18 TP14 9.5 0.12 0.61 0.02 <0.001 0.83 <0.001 0.57 TP15 7.9 <0.001 3.50 0.04 <0.001 4.07 <0.001 1.03 TP16 9.4 0.09 1.86 <0.001 <0.001 1.01 <0.001 1.01 TP17 9.1 0.02 1.04 <0.001 <0.001 1.86 <0.001 1.92 TP18 2.3 0.11 3.76 <0.001 <0.001 3.80 <0.001 2.69 TP19 6.4 0.01 0.60 <0.001 <0.001 1.15 <0.001 1.51 TP20 21 0.10 0.57 <0.001 <0.001 1.14 <0.001 0.67 TP21 3.4 <0.001 1.53 <0.001 <0.001 2.07 1.47 0.88 TP22 12.4 <0.001 0.49 <0.001 <0.001 0.27 <0.001 0.27 TP23 3.9 3.55 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 TP24 4.3 <0.001 8.28 <0.001 <0.001 <0.001 <0.001 <0.001 TP25 9.1 <0.001 0.48 <0.001 <0.001 <0.001 <0.001 <0.001 TP26 19.9 0.29 0.60 <0.001 <0.001 0.66 0.06 0.65 TP27 60.7 0.75 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Minimum 1.70 0.01 0.48 0.02 - 0.27 0.06 0.15 Maximum 60.70 3.55 8.28 0.33 - 4.42 1.47 6.01 Mean 13.36 0.42 2.25 0.10 - 1.60 0.56 1.14 University of Ghana http://ugspace.ug.edu.gh 94 Anion chemistry of rainwater from the Saltpond station show the following sequence of occurrence: Cl-> SO4 2-> NO3 -> PO4 3-> F- > Br-> NO2 whilst that for the Twifo Praso station is of the order Cl- > NO3 -> SO4 2-> PO4 3-> F-> Br-> NO2. Higher SO4 2- observed in Saltpond rains compared to the Twifo Praso rains is as a result of heavy vehicular activities on the major international road that passes through Saltpond to neighbouring countries like Ivory Coast. SO4 2- may come from SO2 gas released from exhause fumes. 4.1.1.2 Surface Water Six (6) surface water samples obtained from River Ochi Narkwa in the Ochi Narkwa Basin and River Ochi Ama Amissa in the Amissa Basin were studied for hydrochemistry. The hydrochemistry was investigated to find out if there is any possible interaction between surface water and groundwater. The detail hydrochemical parameters of the surface waters and their statistical summary are presented in Table 4.3. The temperature in the surface waters vary between 25.90 and 28.60 oC. The mean temperature was 27.05 OC. Temperature is important because of its influence on water chemistry. Generally, the rate of chemical reactions increases at higher temperatures. Water, at higher temperatures dissolve minerals fromthe rocks it is in contact with resulting in higher EC and TDS. The pH varies from 7.09 to 7.60 pH units showing almost neutral to slightly alkaline condition. The pH state of surface waters is important because aquatic organisms have a University of Ghana http://ugspace.ug.edu.gh 95 tolerance for very narrow pH ranges. A pH value higher or lower than 6 to 8 range for stream water can decrease the survival of aquatic organisms and lead to the loss of stream ecosystem diversity. Generally, the pH for natural waters are summarised as:  6.5 to 8.5 for streams and groundwater  5.0 to 8.5 for natural waters  5.5 to 6.0 for fresh rainwater The surface waters in the study area are therefore within acceptable pH range for human consumption and aquatic life. The redox potential (Eh) for the surface waters varies from -17.40 to 5.40 mV indicating areducing to oxidising medium. The mean Eh is -5.72 and the median is -6.25. Generally, low electrical conductivities (EC), total dissolved solids (TDS) and salinities are observed in the surface waters. The EC ranged between 135.50 to 186.10 mg/L with a mean of 154.91 mg/L and a median value of 144.95 mg/L. The total dissolved solids (TDS) ranges from 69.90 to 93.00 mg/L with a mean of 77.55 mg/L and a median value of 72.55 mg/L. The mean salinity in surface waters was 100 mg/L. The low EC, TDS and salinty could result from the fact that the waters are mostly neutral and contact with rocks do not result in increased weatherability of the rocks to release dissolved ions that will cause EC, TDS and salinity to be high. Low pH waters in contact with rocks will increase the weathering of the rocks leading to higher dissolved ions and consequently higher EC and salinity content of the waters. University of Ghana http://ugspace.ug.edu.gh 96 Calcium (Ca2+) varies from 6.40 to 22.40 mg/L with a mean of 14.30 mg/L and a median of 12.70 mg/L. Ca2+ of the surface waters are very low. Sodium (Na+) ranges between 36.40 and 54.90 mg/L. The mean Na+ content was 44.54 and the median was 41.90 mg/L. Low magnesium content was observed in the surface waters. Values ranged between 3.84 to 5.76 mg/L with a mean of 4.67 mg/L and a median value of 4.82 mg/L. Potassium varies from 6.40 to 8.60 mg/L with a mean of 7.32 mg/L and a median of 7.10 mg/L. Potassium is generally, resistant to weathering and therefore low in water bodies. Elevated concentrations may result result from anthropogenic activities such as application of inorganic fertilisers for agricultural activities. The chloride (Cl-) levels in the surface water vary from 16.0 to 53.99 mg/L and a mean of 28.99 mg/L. The median Cl- value was 19.99 mg/L. The low content of Cl- in surface waters may result from atmospheric input and good agricultural practices in the area. Sulphate (SO4 2-) ranged between 26.83 to 89.83 mg/L. The mean SO4 2- content of the surface waters is 64.03 mg/L and the median was 74.92 mg/L. Bicarbonate (HCO3 -) ranges between 48.8 to 67.10 mg/L and has a mean of 58.76 mg/L and a median of 58.87 mg/L. Very low nitrate (NO3 -) levels occur in the surface waters. NO3 - ranges between 0.05 and 0.72 mg/L with a mean of 0.49 mg/L and a median of 0.57 mg/L.The low NO3 - content of the surface waters is an indication that surface waters have not been contaminated with NO3 - due probably to sound agricultural and sanitation practices in the area. University of Ghana http://ugspace.ug.edu.gh 97 Table 4.3 Hydrochemical data of surface waters in the study area Sample ID Temp pH EC TDS Eh Salinity Ca2+ Na+ Mg2+ K+ Cl- SO4 2- HCO3 - NO3 - F- PO4 3- Units (oC) (μS/cm) mg/l mV mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l CR2-16 26.9 7.10 179.3 89.7 -3.4 100 6.40 54.90 5.76 8.60 53.99 26.83 57.95 0.40 0.31 0.01 CR2-29 25.9 7.22 186.1 93 5.4 100 12.60 44.60 3.84 6.50 45.99 29.17 67.10 0.05 0.21 0.04 CR2-37 26.9 7.09 146.7 73.3 2.9 100 22.40 39.20 4.84 6.70 19.99 89.83 59.78 0.72 0.49 0.14 CR2-38 27.8 7.55 143.2 71.8 -12.7 100 20.80 36.40 4.93 6.40 17.99 87.00 61.00 0.56 0.12 0.01 CR2-52 27.2 7.60 138.7 69.6 -17.4 100 12.80 53.70 3.84 7.50 16.00 88.50 57.95 0.59 0.76 0.02 CR2-48 28.6 7.49 135.5 67.9 -9.1 100 11.20 38.60 4.80 8.20 19.99 62.83 48.80 0.62 0.76 0.04 Minimu m 25.90 7.09 135.50 67.90 - 17.40 100.00 6.40 36.40 3.8 7 6.40 16.00 26.83 48.80 0.05 0.12 0.01 Maximu m 28.60 7.60 186.10 93.00 5.40 100. 00 22.40 54.90 5.16 8.60 53.99 89.8 3 67 .10 0.72 0.76 0.14 Mean 27.22 154.92 77.55 - 5.72 100.00 14.37 44.57 4.6 7 4.6 7 28.99 64.03 58.76 0.49 0.44 0.04 University of Ghana http://ugspace.ug.edu.gh 98 Phosphate (PO4 3-) ranges between 0.01 and 0.14 mg/L in the area. The mean PO4 3- content was 0.04 mg/L and a median of 0.03 mg/L. Very low PO4 3- content is realised in the surface waters. The sources of PO4 3- in water include phosphate fertilisers applied in agricultural activities and rocks. PO4 3- bearing rocks do not occur in the study area. The absence of PO4 3- bearing rocks and sound agricultural practices could be the only reasons for low PO4 3- content in the surface waters. Flouride (F-) levels observed in the surface waters were very low. F-values ranged between 0.12 and 0.76 mg/L with a mean of 0.44 mg/L and a median value of 0.40 mg/L. 4.1.1.3 Groundwater The hydrogeochemical characteristics of the groundwaters were studied using eight (8) shallow groundwaters (hand-dug wells) and seventy (70) deep groundwaters (boreholes). The statiscal summaries of the hydrogeochemical parameters for the shallow groundwaters are presented in Table 4.4 while that for deep groundwaters are in Table 4.5. Detailed hydrogeochemical parameters are presented in Appendix 2. The temperatures of the shallow groundwaters vary from 26.9 to 30.8 OC with a mean of 28.45OC (Table 4.4). The highest temperature of 30.8 OC occus in CR2-54 at Anokye. The lowest temperature of 26.9 OC occurs in CR4-FZ-03 at Duadze in the inland area. Inthe deep groundwaters, temperatures fall between 23.20 to 30.90 oC with a mean of 27.98 oC and a median value of 28.1 oC (Table 4.5). The lowest value of 23.20 oC occurs in borehole CR4-06 at Sefara Kokodo whilst the highest value of 30.9 oC occurs in borehole CR2-50 at Ekumfi Swedru. The median temperature is 28.10 oC. Shallow and deep groundwaters exhibit similar temperature variations in the study area. University of Ghana http://ugspace.ug.edu.gh 99 The minimum pH of the shallow groundwaters is 5.20 and occurs in CR4-FZ-03 at Duadze in the biotite granite terrane. Granitoid aquifers usually give acidic waters. The maximum pH is 9.30 and ocurrs in CR4-01 at Ekumfi Asafa in the sediments.The Eh of the shallow groundwaters vary between -104.1 mV and 104.5 mV. The mean Eh is 13.92 mV. The median Eh is 25.4 mV. The negative Eh implies reduction is taking place in the wells and positive Eh means oxidation is taking place. Two of the nine shallow groundwater samples collected show reduction processes in the area. The rest of the samples exhibit oxidation processes. The pH for deep groundwater varies from 5.32 to 7.84 pH units. Most of the groundwater samples exhibit slightly acidic conditions except boreholes CR2-20 at Ekumfi Abor and CR2-50 at Ekumfi Asokwa. These boreholes exhibit slightly basic conditions. In the case of borehole CR2-20 at Ekumfi Abor pH is almost neutral (7.09 pH units). The redox potential for deep groundwater ranges between -36.60 to 107.40 mV with a median of 40.95 mV and a mean of 40.65 mV. The lowest value of -36.60 mV occurs in borehole CR2-50 at Ekumfi Asokwa. The highest value of 107.40 occurs in borehole CR2-28 at Obontser. Only borehole CR2-50 at Ekumfi Asokwa shows reducing medium. The rest of the deep groundwater samples show an oxidising environment. The EC of the shallow groundwaters ranges from 540 to 6050 μS/cm. The mean EC content is 2102 mg/L. The median is 1682 mg/L. Elevated concentrations in CR2-54 (6050 μS/cm) at Anokye located 4 km from the coast and CR2-53 (5050 μS/cm) at Abonko located 16 km from the coast could be as a result of seawater intrusion, sea University of Ghana http://ugspace.ug.edu.gh 100 aerosols or weathering of rocks.It could also be as a result of evaporation, concentrating salts in the unsaturated zone resulting in higher EC since the depth of the hand dug-wells vary between 6 and 12 m. CR4-FZ-22 has higher EC (2001 μS/cm) but located further inland and unlikely to be affected by seawater intrusion. Probably sea aerosols, weathering of rocks and evaporation in the unsaturated zone may be responsible for the elevated concentrations. Similarly in the case of hand-dug well CR2-54 located 16 km from the coast, influence of seawater is unlikely. The reasons advanced for CR4-FZ-22 may apply in this case. Two hand-dug wells CR4-01 and CR4-02 at Ekumfi Asaafa with EC values of 1682 μS/cm and 1695 μS/cm respectively located less than 1 km from the coast is fresher than those quite a distant from the coast. Thus fresher shallow groundwaters occur closer to the coast. Hand-dug wells CR4-FZ-03, CR4-FZ-14 and CR4-FZ-21 have low EC that signify fresh water. These hand-dug wells are located in the inland areas. The TDS in shallow groundwater ranges between 270 mg/L and 3030 mg/L. The mean TDS is 1097.78 mg/L. The median is 841 mg/L. The highest TDS of 3030 mg/L occurs in hand-dug well CR2-54 at Anokye. The probable reasons advanced for elevated EC holds for the Elevated TDS in the area. Salinity of the shallow groundwaters ranges from 300 to 3330 mg/L with a mean of 1144.44 mg/L. The highest salinity occurs in hand-dug well CR2-54 at Anokye and lowest of 300 mg/L occurs in hand-dug wells CR4-FZ-14 at Abora Dunkwa and CR4-FZ-21 at Abora Bando. Salinity, EC and TDS are related, hence possible reseasons accounting for high EC and TDS also hold for the salinity. University of Ghana http://ugspace.ug.edu.gh 101 Table 4.4Statistical summary of hydrochemcal parameters of shallow groundwaters (Hand-dug wells) Parameter Temp(oC) pH Eh (mV) EC (μS/cm) TDS (mg/L) Salinity (mg/L) Ca2+ (mg/L) Mg2+ (mg/L) Na+ (mg/L) K+ (mg/L) HCO3 - (mg/L) Cl- (mg/L) SO4 2- (mg/L) NO3 - (mg/L) Minimm 26.90 5.20 - 104.10 540.00 270.00 300.00 9.34 8.33 26.40 4.45 12.20 39.38 20.50 0.15 Maximun 30.80 9.30 104.50 6050.00 3030.00 3300.00 183.20 168.00 1429.50 468.50 277.53 2749.50 184.67 0.81 Mean 28.54 13.92 2192.00 1097.78 1144.44 68.54 45.14 366.93 83.83 141.31 788.00 88.18 0.51 Table 4.5Statistical Summary of hydrochemical parameters of deep groundwaters (Boreholes) Parameters Temp pH Eh EC TDS Salinity Ca2+ Mg2+ Na+ K+ HCO3 - Cl- F- Br- SO4 2- NO3 - PO4 3- Units (oC) (mV) (uS/cm) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Minimum 23.20 5.32 -36.60 120.60 60.30 100.00 10.87 2.88 22.15 3.70 9.15 30.57 0.00 1.20 7.46 0.02 0.00 Maximum 30.90 7.84 107.40 2900.00 14420.00 17700.00 456.70 416.70 2769.50 101.90 780.80 4798.50 0.79 219.30 751.55 2.26 0.51 Mean 27.91 44.16 2346.92 1183.50 1298.57 104.62 44.76 335.96 18.72 170.10 595.36 0.31 47.92 129.09 0.55 0.07 Median 28.05 41.60 1294.00 648.00 700.00 86.40 21.13 258.00 11.72 140.30 426.95 0.34 34.14 102.17 0.49 0.04 University of Ghana http://ugspace.ug.edu.gh 102 In the case of deep groundwaters EC ranges from120.6µS/cm to 29000 µS/cm. It has a median of 1294 µS/cm and a mean of 2346.92 µS/cm. Elevated EC is observed in the groundwaters. The highest value of 29, 000 µS/cm occurs in borehole CR2-45 at Gomoa Abora which is 5.9 km from the coast located within the amphibolite aquifers. The mineralogical composition as it pertains to the study area is not available from the geology of the area. However, typical minerals found in amphibolites include hornblende + plagioclase feldspars, Garnet, pyroxenes, biotite, titanite, magnetite, epidote, chlorite and quartz. These minerals could breakdown to yield various ions in groundwater that would result in elevated concentrations of EC. Generally, the study area falls within the crystalline terrane. Crystalline rocks are non permeable to groundwater flow except through weathering and development of fractures referred to as secondary permeabilities as captured in section under hydrogeology. Weathering of crystalline rocks mostly yield clay minerals and movement of groundwater in such terranes is slow leading to longer contact with rocks to its destination. This allows more time for the minerals to dissolve resulting in higher EC. Park et al., (2011) evaluated the relationship between EC levels and distance from the coast in South Korean aquifers and observed negative correlation in alluvial coastal aquifers but no correlation in bedrock coastal aquifers. They developed a classification scheme to describe the quality of water in South Korea. Groundwaters with EC < 1500 μS/cm were classified as fresh, those having EC within the range 1500 and 3000 μS/cm were classified as brackish and finally, EC levels > 3000 μS/cm were described as saline. EC levels in the study area were plotted against distance from the coast (Fig 4.1). A weak negative correlation (r = - University of Ghana http://ugspace.ug.edu.gh 103 0.41) was observed implying, salinity does not depend on distance from the coast. Using the classification scheme of Park et al., (2011) it is realised in (Fig. 4.1) that some boreholes within 7 kmfrom the coast have EC between100 and 1000 μS/cm and therefore Fresh.Similarly within the same distance from the coast there were borehoholes and hand-dug wells that had electrical conductivities above 1000 μS/cm and therefore saline. From 7 km to about 15 km some borehole waters have varied EC cutting across fresh to saline water. However, at distances greater than 15 km the boreholes are generally fresh with EC less than 1000 μS/cm. Similar observations are made in (Fig. 4.2) which shows the spatial distribution of EC with respect to the geology of the area. The origin of salinity, hence, high electrical conductivities in the area is complex. It could be as a result of weathering of rocks in the area, seawater intrusion, sea aerosol spray, halite dissolution and slow movement of groundwater in the area. These hydrogeochemical processes would be investigated further in subsequent sections by thoroughly evaluating the major ions, ionic ratio and stable isotopes of water to arrive at the predominant process or proceses accounting for the groundwater salinity in the area. Fifty – nine percent of groundwater samples have EC < 1500 μS/cm and forty – one percent have EC > 1500 μS/cm. The possible groundwater flow direction is indicated in (Fig. 4.1) where groundwater flows from the inland areas where electrical conductivities are quite low towards the coast with increasing electrical conductivities. University of Ghana http://ugspace.ug.edu.gh 104 Fig. 4.1 Distribution of EC levels of the groundwaters with distance from the coast. The classification of the waters into fresh, brackish and saline is based on the scheme employed by (Park et al., 2011). University of Ghana http://ugspace.ug.edu.gh 105 Fig 4.2 Spatial distribution of EC with respect to the geology of the area University of Ghana http://ugspace.ug.edu.gh 106 The total dissolved solids (TDS) for deep groundwaters ranges from 60.30 to 14420.00 mg/L with a median of 648.00 and a mean of 1183.50 mg/L. Higher values in deep groundwater may be attributable to longer residence time. Longer residence time may result in the breakdown of minerals in rocks resulting in increased TDS. Other hydrogeochemical processes such as present day seawater intrusion, past seawater intrusion, sea aerosol spray and halite dissolution may result in elevated TDS in the area. The spatial distribution of TDS in the area is shown in (Fig. 4.3). Since EC and TDS are linearly related, the map of TDS is similar to that of EC. Higher TDS and saline waters occur mostly in the sediment/volcaniclastitic sediment locally referred to as mica schist.In this same geological terrane fresher waters can be observed even closer to the coast (Fig. 4.3). In the extreme north-western section of the TDS distribution map where biotite gneiss aquifers occur deep groundwater is fresh with TDS ranging between 27 and 538 mg/L.The deep groundwaters have salinities ranging between 100 and 17,700 mg/L. The mean salinity is 1298.57 mg/L and the median is 700 mg/L. The salinity of the surface water samples are extremely low compared to the groundwater samples due probably to short contact time of surface water with rocks. Ca2+ in the shallow groundwaters ranges between 9.34 and 183.20 mg/L with a mean of 68.54 mg/L. The median Ca2+ content is 52.64 mg/L. Low Ca2+ content thus occur in the shallow groundwaters. Mg2+ varies from 8.33 to 168 mg/L with a mean of 45.14 mg/L. The median Mg2+ content is 31.40 mg/L. University of Ghana http://ugspace.ug.edu.gh 107 Fig. 4.3 Spatial distribution of TDS (mg/L) in the study area University of Ghana http://ugspace.ug.edu.gh 108 Na+ varies from 26.40 to 1429.50 mg/L. The mean Na+ content of the shallow groundwaters is 366.93 mg/L and the median is 165.56 mg/L. Higher Na+ content of 1429.50 mg/L occurs in CR2-53 at Abonko and the lowest value of 26.40 mg/L occurs in CR4-FZ-14 at Abora Dunkwa. Higher Na+ content may be due to weathering of Na rich feldspars (albite) as illustrated in equation (2.4), seawater intrusion or sea aerosols. The lower Na+ content in CR4-FZ-14 is as a result of the fact that it is further away from the coast and unlikely to be affected by seawater intrusion or sea aerosols. It is located on a higher topographic ground depicting a recharge area. K+ varies from 4.45 mg/L to 468.50 mg/L. The mean K+ content is 83.83 mg/L and the median is 13.50 mg/L. Higher K+ content of 468.50 is observed in hand-dug well CR2-54 at Anokye and attributable to anthropogenic input since this hand-dug is not covered. HCO3 - in the shallow groundwaters ranged from 12.20 to 277.53 mg/L with a mean of 141.31 mg/L and a median of 115.90 mg/L. Higher HCO3 - content occurs in CR2-54 at Anokye in the mica schist aquifers. These aquifers do not contain carbonate minerals that will allow for the presence of HCO3 - in the groundwaters. HCO3 - may therefore come from reaction of CO2and H2O in the soil zone (equation 2.2) captured in section 2.2 under Literature Review. CO2(g) + H2O ↔ H2CO3 (2.2) It could also result from hydrolysis of silicate minerals as illustrated in equations 2.3 to 2.7 in section 2.2 under Literature Review. Lower HCO3 - content occurs in CR4-FZ-03 at Duadze located in the biotite schist aquifers. These aquifers do not contain carbonate minerals hence the low content of HCO3 -. Cl- in the shallow groundwaters ranges University of Ghana http://ugspace.ug.edu.gh 109 between 39.38 and 2749.50 mg/L with a mean of 788.00 mg/L. The median is 315.47 mg/L. Higher Cl- (mg/L) occurs in CR2-54 at Anokye and lower Cl- in CR4-FZ-14 at Abora Dunkwa. SO4 2- varies between 20.50 and 184.67 mg/L with a mean of 88.18 mg/L. The median is 89.62 mg/L. Very low SO4 2- concentrations occur in the study area. This is due to the fact that the geology of the area does not contain SO4 2- bearing minerals like gypsum and anhydrite. NO3 - varies from 0.15 to 0.18 mg/L with a mean of 0.15 mg/L and median of 0.59 mg/L. Low NO3 - concentrations occur in the shallow groundwaters. The Ca2+in deep groundwater varies from 10.87 in borehole CR4-FZ-09 at Ayeldu in the biotite granite terrane to 456.70 mg/L in borehole CR4-06 at Sefara Kokodo closer to the coast in the biotite granite terrane. The median Ca2+ value is 3.80 mg/L and the mean is 109.33 mg/L. One of the commonest mineral in the rocks of the area is Ca-feldspars (CaAl2Si2O8). This may breakdown in the presence of carbonic acid (H2CO3) generated in the soil zone to release Ca2+ as illustrated in equation (2.3). Other possible sources of Ca2+ in the groundwater include break down of the mineral hornblende [Ca2(MgFeAl)5(AlSi)8O22] and pyroxenes. The calcium distribution map of the area with respect to the geology reveals a higher Ca content in boreholes CR4-06 at Sefara Kokodo, CR4-FZ-16 at Abora Obokor, CR2-18 at Ekumfi Eyisam and CR2-45 at Gomoa Abora (Fig. 4.4). These boreholes are located within biotite granite, mica schist and amphibolite aquifers (Fig. 4.4). These aquifers contain Ca-feldspars that break down to release Ca2+ in groundwater. University of Ghana http://ugspace.ug.edu.gh 110 Fig. 4.4 Spatial distribution of Ca2+ in the study area University of Ghana http://ugspace.ug.edu.gh 111 Mg2+ varies from 2.88 in CR2-44 at Enyan Abowinum to 416.70 in borehole CR4-06 at Sefara Kokodo. The median value is 21.13 and the mean Mg2+ content of deep groundwater is 44.76 mg/L (Table 4.5). Higher Mg2+ occurs in the deep groundwaters. Mg2+ in groundwater may come from boitite [K(Mg,Fe)3(AlSi3)O10(OH,F)2] and hornblende (CaMgSi2O22(OH)2). Generally Mg 2+ content of deep groundwater in the area is low even though groundwater is tapped from aquifers bearing Mg2+ minerals such as biotite, muscovite and hornblende (Fig 4.5). The chemical weathering stability series established by Goldich (1938) reveals that some of the magnesium bearing minerals such as muscovite, biotite and some amphiboles are quite resistant to weathering. In the case of muscovite it occupies the second position in the series after quartz the most stable. Considering the series muscovite is more stable than biotite and biotite more stable than the amphiboles. Compared to Ca2+ and Na+ minerals, muscovite and boitite are more resistant to weathering hence Mg2+ less abundant in groundwater.Na+ varies from 22.15 in borehole CR4-FZ-08 at Ayeldu to 2769.50 in borehole CR2-45 at Gomoa Abora with a median value of 258.00 and a mean of 335.96 mg/L (Table 4.5). Na+ is the dominant cation in the study area. The order of occurrence of the cations is Na+ > Ca2+> Mg2+> K+. Higher concentrations of Na+ occur in the deep groundwater. Na+, Ca2+, Mg2+ and K+ are said to be significant constituents of silicate rocks (Freeze and Cherry, 1979).Hence Na+ in groundwater may come from silicate weathering as illustrated in equation (2.4) in section 2.2, seawater intrusion, sea aerosol spray and halites. University of Ghana http://ugspace.ug.edu.gh 112 Fig. 4.5 Spatial distribution of Mg2+ in the study area University of Ghana http://ugspace.ug.edu.gh 113 However from the geology of the area halites do not occur. Halites if present may be concentrated in the soil zone by evaporation and leached into the groundwater by infiltrating rain. The spatial distribution of Na+in the area is shown in Fig. 4.6. Higher Na content occurs in the biotite granite closer to the coast and mica schist aquifers. Extremely low Na+ content can also be found in the mica schist aquifers closer to the coast as well as in the biotite granite and biotite gneiss aquifers further from the coast. NaAlSi3O8 + 7H2O + H2CO3 ↔ Na + + Al(OH)3 + 3H4SiO4 + HCO3 - (2.4) K+ varies from 3.70 to 101.90 mg/L with a median value of 11.72 and mean of 18.72 mg/L. High levels of K+ (101.90 mg/L) occur in borehole CR2-32 at Gomoa Brebiano. This borehole of 30 m depth taps water from the mica schist aquifers in the sediment/volcaniclastic sediments. Generally, micas (muscovite and biotite) are major constituents of the sediment/volcaniclastic sediment in the area and therefore responsible for K+ in the groundwater.This is revealed in Fig. 4.7 where higher K+ content occurs in the mica schist aquifer. However, the high values could also be attributable to contamination by agricultural activities. In the granitoid aquifers, the presence of K- feldspars (orthoclase) may be responsible for occurrence of K+ in the groundwaters. The hydrololysis of K-feldspars in the presence of H2CO3 to yield K + in waters is given in equation (2.5) under section 2.2. Generally, compared to the other cations, K+ has the least occurrence in groundwater due to stability of K-bearing minerals. KAlSi3O8 + 7H2O + H2CO3 ↔ K + + Al(OH)3 + 3H4SiO4 + HCO3 - (2.5) University of Ghana http://ugspace.ug.edu.gh 114 Fig. 4.6 Spatial distribution of Na+ in the study area University of Ghana http://ugspace.ug.edu.gh 115 HCO3 - varies from 9.15 to 780.80 mg/L with a median of 140.30 and a mean of 170.10 mg/L (Table 4.5). The study area is a crystalline terrane comprising of igneous and metamorphic rocks.The major rock types as mentioned in section 2.6 include biotite gneiss, biotite granitoid, hornblende biotite granitoid, wacke sediment (schist), sediment/volcaniclastic sediment undifferentiated, locally referred to as mica schist and amphibolite. Majority of the samples were taken from the sediment/volcaniclastic sediment terrane because it is the terrane that holds most of the saline waters. Few samples were taken from the other terranes. Carbonate minerals such as calcite (CaCO3) and dolomite (CaMgCO3) responsible for production of HCO3 - in groundwaters are not major constituents of these rocks. The only way HCO3 - could occur in the groundwaters is by dissolution of CO2 in the soil zone as illustrated in equation (2.2) in section 2.2 and weathering of silicate minerals (Na-feldspars, Ca-feldspars, hornblende, micas (muscovite and biotite) and pyroxenes) which are major constituents of these rocks). The weathering of these minerals to yield HCO3 - in groundwater is illustrated in equations (2.3) to (2.7) in section 2.2. This explains why HCO3 - is not a dominant anion in the study area. Cl- is the dominant anion in the study area. In the HCO3 - distribution map (Fig. 4.8) the highest HCO3 - content is observed in the gneissic aquifers at Otuam, Sefara Kokodo and Adansemanmu with values ranging between 408.71 and 780.80. Values ranging between 9.15 and 408.70 occur in the mica schist aquifers. Extremely low values occur northwards in the biotite granite and biotite gneiss aquifers further from the coast (Fig 4.8). University of Ghana http://ugspace.ug.edu.gh 116 Fig. 4.7 Spatial distribution of K+ in the study area University of Ghana http://ugspace.ug.edu.gh 117 Even though silicate minerals mentioned earlier may be responsible for the variations in the HCO3 - through weathering the bulk of HCO3 - may come from the soil zone through dissolution of CO2. Cl - varies from 30.57 in borehole CR4-FZ-08 at Ayeldu to 4798.50 mg/L in borehole CR2-49 at Ekumfi Akwakrom with a median of 426.95 and a mean of 595.36 mg/L (Table 4.5). Elevated concentrations occur in the deep groundwaters. High values ranging between 1, 209.75 and 4,798.50 are realised in borehole CR2-23 at Ekumfi Engow, CR2-45 at Gomoa Abora and CR2-49 at AKwakrom, (Fig 4.9). These values also correspond to where there is high Na+. Generally, Cl- is not a significant constituent of silicate rocks. The presence of Cl- in groundwater is normally attributed to atmospheric sources, decomposition of organic matter in the soil and trace impurities in rocks and minerals (Freeze and Cherry, 1979). It may also result from seawater encroachment and intrusion due to nearness to the coast. This would however be discussed further in subsequent sections. SO4 2- ranges between 7.46 and 751.55 mg/L with a median value of 102.17 and a mean of 129.09 mg/L (Table 4.5). SO4 2- is the least dominant major anion in the study area. Just like Cl-, SO4 2- is not a major constituent of silicate rocks hence low content generally in the study area. Elevated concentration (751.55 mg/L) in borehole CR4-04 at Otuam in biotite granite aquifer may be due to oxidation of pyrite obeserved in the rocks of the area during the fieldwork. Figure 4.10 shows the SO4 2- distribution map of the area. High SO4 2- content is found in boreholes at Otuam, Sefara Kokodo, Adansemanmu located in biotite granite aquifers and Gomoa Abora located in the amphibolite aquifers. In the deep groundwaters, the order of occurrence of the major anions are Cl-> HCO3 - > SO4 2-. A similar order was observed for the shallow groundwaters which suggest a hydraulic connection between shallow University of Ghana http://ugspace.ug.edu.gh 118 groundwater and deep groundwater. F-varies from 0 to 0.79 mg/L with a median of 0.20 and a mean of 0.25 mg/L. Elevated F- responsible for dental and skeletal fluorosis is thus absent in the area. This may result from the fact that the mineral fluorite which dissolves in groundwater to cause elevated F- is not in high quantities in the rocks of the area. NO3 - in deep groundwaters varies from 0.02 to 2.26 mg/L with a median value of 0.49 and a mean of 0.54 mg/L. NO3 - values are quite low in the area (Table 4.5). This can be attributed to less vigorous anthropogenic activities in the area. PO4 3- ranges from 0 to 0.51 mg/L with a median value of 0.04 and a mean of 0.07 mg/L. Low PO4 3- content in deep groundwater may be attributed to lack of phosphate minerals in the rocks of the area and less anthropogenic activities as observed for the shallow aquifers. The saturation indces of the groundwaters were determined to identify minerals in solution. The saturation index as defined by (Appelo and Posma (2005), Meridith, 2009) is: 𝑆𝐼 = log 𝑄 𝐾 (4. 1) Where Q is the reaction quotient and K the equilibrium constant. If SI = 0, the mineral is said to be at equilibrium. If SI > 0 the mineral is considered to be supersaturated and will tend to precipitate. If SI < 0, the mineral is considered to be undersaturated and will tend to dissolve. The saturated indices of the various water samples were obtained using phreeqC of Aquachem version 5.1. University of Ghana http://ugspace.ug.edu.gh 119 Calcite and dolomite saturation indices were determined for the deep groundwaters. Results of the saturation indices are given in Table 4.6. Calcite saturation indices for deep groundwaters range between -3.22 and 0.59 with a mean of -1.14 and a median of -0.99. Dolomite saturation indices range between -6.75 and 1.04 with a mean of -2.40 and a median of -2.02. The deep groundwaters are undersaturated with calcite and dolomite except boreholes CR2-50 at Ekumfi Asokwa and CR2-20 at Ekumfi Abor. The waters from these boreholes show super-saturation with respect to calcite and dolomite. The two boreholes are located within the same geological terrane made of mica schist aquifers of the Sediment/Volcaniclastic Sediment. Calcite and dolomite are not known to be associated with mica schist aquifers. Supersatuartion of these minerals in the boreholes may be due to secondary mineralisation arising from hydrolysis of silicate minerals. University of Ghana http://ugspace.ug.edu.gh 120 Fig. 4.8 Spatial distribution of HCO3 - in the study area University of Ghana http://ugspace.ug.edu.gh 121 Fig. 4.9 Spatial distribution of Cl- in the study area University of Ghana http://ugspace.ug.edu.gh 122 Fig. 4.10 Spatial distribution of SO4 2- in the study area University of Ghana http://ugspace.ug.edu.gh 123 Table 4.6 Saturation Indices for calcite and dolomite Community Sample ID SI (Calcite) SI (Dolomite) Kweikrom CR2-01 -0.81 -2.50 Gomoa Abotsia CR2-04 -0.75 -1.86 Gomoa Obiri CR2-06 -1.14 -2.65 Gomoa Asaman CR2-08 -0.75 -1.96 Ekumfi Gyabenkwaa CR2-09 -0.29 -1.37 Ekumfi Eduagyir CR2-12 -1.19 -2.84 Ekumfi Esuehyia CR2-14 -0.35 -0.95 Gomoa Antseadze CR2-15 -0.61 -1.72 Ekumfi Eyisam CR2-17 -0.57 -1.67 Ekumfi Eyisam CR2-18 -0.09 -0.84 Ekumfi Abor CR2-20 0.20 -0.01 Ekumfi Abor CR2-21 -0.14 -0.79 Ekumfi Techiman CR2-22 -0.60 -1.61 Ekumfi Engow CR2-23 -0.38 -0.97 Edwansa Kokodo CR2-24 -0.95 -1.90 Edwansa Kokodo CR2-25 -1.44 -3.34 Onyaadze CR2-26 -0.29 -0.88 Obontser CR2-27 -0.80 -1.75 Obontser CR2-28 -3.09 -6.58 Ata Kwaa CR2-30 -0.65 -1.68 Otabanadze CR2-31 -1.25 -2.31 Gomoa Brebiano CR2-32 -1.88 -4.32 Ajumako Ansa CR2-33 -1.01 -2.25 Gomoa Sampa CR2-35 -1.97 -3.89 Ajumako Kyeibi CR2-40 -1.37 -2.89 Ajumako Afransie CR2-41 -3.01 -6.09 Dwenase CR2-43 -1.99 -4.21 Enyan Abowinum CR2-44 -3.22 -6.75 Gomoa Abora CR2-45 -0.57 -1.32 Kyeibi CR2-46 -2.20 -4.92 Baffikrom CR2-48 -1.14 -2.27 Ekumfi Akwaakrom CR2-49 -0.70 -1.26 Ekumfi Asokwa CR2-50 0.59 1.04 Ekumfi Swedru CR2-51 -0.46 -0.63 Ewoyaa CR2-55 -0.82 -1.56 University of Ghana http://ugspace.ug.edu.gh 124 Table 4.6 Continued Community SampleID SI(Calcite) SI (Dolomite) Afrangua CR2-57 -1.26 -2.68 Goekrom CR2-58 -0.60 -1.11 Edzimber CR2-59 -0.95 -1.99 Buranamoa CR2-60 -1.13 -2.79 Nsanfo CR2-61 -0.77 -1.73 Brofoyedur CR2-62 -0.48 -1.11 Dwendwenbadze CR2-63 -0.88 -1.87 Effutuakwa CR4-FZ-01 -1.20 -2.21 Nkusukumopem CR4-FZ-02 -0.61 -0.77 Abeadze Abbankrom CR4-FZ-04 -2.26 -4.35 Abeadze Abbankrom CR4-FZ-05 -2.34 -4.53 Etsisunkwa CR4-FZ-06 -0.87 -1.50 Kyekyewowere CR4-FZ-07 -0.50 -1.10 Ayeldu CR4-FZ-08 -2.79 -5.62 Katakyiase CR4-FZ-09 -1.57 -3.16 Katakyiase CR4-FZ-10 -1.47 -2.92 Katakyiase CR4-FZ-11 -1.35 -2.65 Abeadze kwakrom CR4-FZ-12 -0.99 -2.20 Etsefawomanye CR4-FZ-13 -1.28 -2.87 Old Odonase CR4-FZ-15 -1.89 -3.76 Abora Obokor CR4-FZ-16 -0.98 -1.65 Abora Kwamankese CR4-FZ-17 -1.33 -2.61 Mpesiaduadze CR4-FZ-18 -1.20 -2.12 Abora CR4-FZ-19 -2.29 -5.11 Akwantia Kokodo CR4-FZ-20 -2.59 -5.01 Pomase CR4-FZ-22 -1.03 -2.02 Obohen CR4-FZ-23 -1.37 -2.83 Aborabuase CR4-FZ-24 -2.40 -4.80 Empirow CR4-FZ-25 -2.33 -4.55 Assin Manso CR4-FZ-26 -1.33 -2.60 Otuam CR4-03 -0.99 -1.81 Otuam CR4-04 -0.02 0.52 Sefara KoKodo CR4-05 -0.97 -1.76 Sefara KoKodo CR4-06 -0.19 -0.08 Adansemanmu CR4-07 -0.18 -0.34 Nanaben CR4-08 -0.69 -1.14 Minimum -3.22 -6.75 Maximum 0.59 1.04 Mean -1.14 -2.40 University of Ghana http://ugspace.ug.edu.gh 125 4.1.1.4 Hydrochemcal Facies The hydrochemical facies of the surface waters as shown by the Piper (1944) Trilinear diagram (Fig. 4.11) suggest NaCl waters even though the surface waters are very fresh. The Cl waters may come from rainfall in the area. The order of occurrence of the major cations in the shallow groundwaters is Na+> Ca2+> Mg2+> K+ and that for the anions is Cl-> HCO3 -> SO4 2-.The Piper (1944) trilinear plot of the shallow groundwaters shows two main water types. These are Na-Cl and non dominant water types (Fig. 4.12). The Na-Cl water types may result from sea aerosols, seawater intrusion and concentration of halites in the soil zone by evaporation. This is discussed further in subsequent sections. The major ion concentrations of the deep groundwaters are plotted on the Piper (1944) trilinear diagram (Fig. 4.13). Four main water types were identified. They included Ca- Mg-HCO3 water type labelled I, NaCl water type labelled II, Ca-Mg-Cl-SO4 water type labelled III and the non-dominant water type labelled IV. The likely processes that may be responsible for this water types include seawater intrusion, halite dissolution, silicate weathering, cation exchange, sea aerosol spray and deposition of salts in the soil zone as outlined in previous sections. Groundwater evolves from a Ca-Mg-HCO3 water type (Fresh water) to a Na-Cl water type (saline). A general flow direction is thus shown in Fig. 4.13 where flow is from inland (fresh water zone) towards the coast (saline zone). University of Ghana http://ugspace.ug.edu.gh 126 Fig. 4.11 Piper trilinear diagram showing hydrochemical facies of surface waters in the study area Fig. 4.12 Piper trilinear diagram showing hydrochemical facies of shallow groundwater in the study area University of Ghana http://ugspace.ug.edu.gh 127 A hand-dug well (shallow groundwater) and two broeholes (deep groundwater) plot at the seawater zone on the Piper trilinear diagram (Fig. 4.14) showing they have been affected by seawater intrusion. Generally, the dominant hydrochemical facies common to the three water points (surface water, shallow groundwater and deep groundwater) is Na-Cl as shown in (Fig. 4.14). Fig. 4.13 Piper trilinear diagram showing hydrochemical facies of deep groundwater in the study area. University of Ghana http://ugspace.ug.edu.gh 128 Fig. 4.14 Piper trilinear diagram showing the hydrochemical facies of surface water, shallow groundwater and deepgroundwater in the coastal zone of the Central Region. 4.1.1.5 Summary of hydrochemistry of rainwater and surface water and hydrogeochemistryof groundwater The rainwater chemistry in the study area shows the dominant anion as Cl-. Cl- ranged between 1.07 mg/L and 22.32 mg/L and a mean of 9.71 mg/L at the Saltpond Meterological Station. At the Twifo Praso Staion Cl- varied from 0.48 mg/L to 8.28 mg/L with a mean of 2.25 mg/L. Higher Cl- content was observed at the Saltpond station closer to the coast suggesting that the ocean is a major contributor of Cl- in rainwater. University of Ghana http://ugspace.ug.edu.gh 129 Generally extremely low physical and chemical parameters were observed for surface waters. This could be attributed to short contact time with rocks and probably less vigorous anthropogenic activities upstream. The major hydrochemical facies was NaCl. The dominant source of this facies in the surface waters is rainfall. The hydrogeochemistry of the shallow groundwaters in the area shows that the dominant cation is Na+ and the dominant anion is Cl-. The cations occur in the orderNa+> Ca2+> Mg2+> K+ and the anions in the order Cl-> HCO3 -> SO4 2-. The major hydrochemical facies is Na-Cl. The deep groundwaters were generally slightly acidic.Fifty – nine percent (59%) of the deep groundwaters have EC <1500 and forty – one percent (41 %) have EC > 1500 μS/cm. On the basis of EC groundwater flow is from the inland areas (northern section of the area) towards the coast (southern section). Two boreholes CR2-50 at Ekumfi Asokwa and CR2-20 at Ekumfi Abor show super-saturation with respect to calcite and dolomite which may be due to secondary mineralisation due probably to hydrolysis of silicate minerals. The occurrence of major cations in the area is in the order Na+> Ca2+> Mg2+> K+ and that for the major anion is Cl-> HCO3 -> SO4 2-. The hdrochemical facies identified are Ca-Mg-HCO3, Na-Cl, Ca-Mg-Cl-SO4 and non-dominant water types. University of Ghana http://ugspace.ug.edu.gh 130 4.2 ISOTOPE STUDIES OF RAINWATER, SURFACE WATER AND GROUNDWATER Rainwater, surface water and groundwater are important components of the hydrological cycle, and the interaction between surface water and groundwater is important in the management of both water resources.As effective tracers in hydrological cycle research, environmental isotope and hydrochemistry can reveal the interrelationships between surface water and groundwater effectively. The goals of the isotope study were to characterize local precipitation, and to constrain the origin(s)of the groundwaters. 4.2.1 S table Isotop es Composition of Rainw ater, Surface water and Groundw ater Rainfall samples collected on event basis, from the Saltpond and Twifo Praso meteorological stations from August 2010 to September 2011 in the study area were analysed for stable isotopes (18O and 2H). The stable isotopes data for rainfall for Saltpond station for 2010 and 2011 are presented in (Tables 4.7 and 4.8). The stable isotope composition of Saltpond rainfall ranges from -5.33 to +2.44 for δ18O ‰ V- SMOW and a mean of -1.47. δ2H ‰ V-SMOW values ranged between -35.21 and +17.8‰ with a mean of -4.10‰. δ2H ‰ versus δ18O ‰ plot (Fig. 4.15) for Saltpond station revealed a slope of 6.49 and an intercept of 5.46. The linear relationship between δ2H and δ18O (Fig. 4.15) with a correlation coefficient (r = 0.95) is defined by the equation: 𝛿2𝐻 = 6.49𝛿18𝑂 + 5.46(4.2) University of Ghana http://ugspace.ug.edu.gh 131 The slope and the intercept are below that of the global meteoric water line (GMWL) developed by (Craig, 1961) and the local meteoric water line for Ghana (Akiti, 1980), explained in sections 2.3.1 and 2.4. The lower values observed in the slope and intercept indicate evaporation of rain drops before it recharges various aquifer systems. Dansgaard (1964) described such an evaporation of rain as the amount effect on the isotopic composition of rain. The deuterium excess (d) ranges from -7.21 to +14.93 ‰ and a mean of +7.68 ‰. The deuterium excess is obtained from the equation: 𝑑 = 𝛿2𝐻 − 8𝛿18𝑂 (Dansgaard, 1964) (4.3) The slope (6.49) and the mean deuterium excess ―d‖ value of 7.68 (δ2H-δ18O) are both less than those of the GMWL and suggest that much of the rainfall occurred at a mean humidity of less than 100% (Gonfiantini, 1986). Evaporation under a relative humidity of less than 100% will result in a slope of 5 ± 2 on a δ2H-δ18O plot (Acheampong and Hess, 2000). The mean monthly relative humidity as captured under section 1.6.3 in the area ranges from 78.80 to 89.10 % and therefore accounts for the lower slope and deuterium excess as suggested by Gonfiantini (1986) and (Acheampong and Hess, 2000) . Lee and Kim (2007) emphasised that generally, deuterium excess value is regarded as the most useful parameter for characterising the vapour origin of water. Low deuterium excess values of precipitation reflect slow evaporation at its source region due to high humidity, whereas the high deuterium excess values reflect fast evaporation at its source region due to low humidity (Clark and Fritz, 1997). The highest deuterium excess of 14.93 is associated with the rainfall event that occurred on 28/4/2011 (Table 4.8) and identified as SP26 and may depict fast evaporation, accompanied by low humidity. The University of Ghana http://ugspace.ug.edu.gh 132 lowest value of -6.12 on Table 4.7, associated with rainfall event that occurred on 13/09/2010 may have undergone slow evaporation and high humidity. δ2H and δ18O composition of rainfall at the Twifo Praso station does not show significant difference from the observation made at the Saltpond station even though it is at a higher altitude than the Saltpond station. Stable isotope compositions of rainfall events from Twifo Praso station are presented in Tables 4.9 and 4.10 respectively.The regression line as shown in (Fig. 4.16) is given as: 𝛿2𝐻 = 6.15𝛿18𝑂 + 3.44 (4.4) The slope and intercept are lower than the slope of the Global Meteoric Water Line as observed for the Saltpond station. The deuterium excess for 2010 rainfall at this station ranges between -11.24 and +15.18 and a mean of +1.64 (Table 4.10) while that for 2011 rainfall ranges between-22.68 to +14.62 and a mean of +5.06 (Table 4.11). University of Ghana http://ugspace.ug.edu.gh 133 Table 4.7Stable isotope composition of rainfall from Saltpond meteorological station in 2010 Sample ID Sampling Date 18O Error 2H Error Deuterium excess SP01 20/08/2010 -1.57 0.05 -9.5 0.58 3.06 SP02 29/08/2010 -0.7 0.05 -3.85 0.64 1.75 SP03 13/09/2010 1.5 0.05 5.88 0.65 -6.12 SP04 17/09/2010 -0.58 0.05 -0.06 0.77 4.58 SP06 20/09/2010 -0.64 0.05 3.59 0.83 8.71 SP07 22/09/2010 -1.04 0.05 -2.69 0.59 5.63 SP08 23/09/2010 -5.03 0.05 -35.21 0.64 5.03 SP09 10/4/2010 -0.7 0.05 -1.02 0.58 4.58 SP10 10/7/2010 -1.62 0.05 -7.02 0.55 5.94 SP11 10/10/2010 -2.85 0.05 -17.18 0.52 5.62 SP12 14/10/2010 -0.15 0.05 0.82 0.62 2.02 SP13 20/10/2010 -5.33 0.05 -33.67 0.5 8.97 SP14 11/5/2010 -3.79 0.04 -23.4 0.57 6.92 SP15 11/12/2010 -1.42 0.05 0.58 0.66 11.94 SP16 13/11/2010 -0.46 0.05 4.17 0.61 7.85 SP17 17/11/2010 0.57 0.05 8.82 0.64 4.26 SP18 18/11/2010 -0.38 0.05 7.28 0.63 10.32 SP19 20/11/2010 -1.88 0.05 -1.75 0.65 13.29 SP20 27/11/2010 -0.25 0.05 8.85 0.64 10.85 SP21 12/5/2010 -0.62 0.05 2.68 0.76 7.64 SP22 12/6/2010 -0.07 0.04 3.54 0.66 4.1 Minimum -5.33 -35.21 -7.21 Maximum 2.44 17.8 14.93 Mean -1.47 -4.10 7.68 SP- Saltpond University of Ghana http://ugspace.ug.edu.gh 134 Table 4.8Stable isotope composition of rainfall from Saltpond station in 2011 Sample ID Sampling Date 18O Error 2H Error Deuterium excess SP23 9/2/11 -0.11 0.05 7.27 0.38 8.13 SP24 22/2/11 0.38 0.05 8.46 0.38 5.45 SP25 10/4/11 -0.85 0.05 3.38 0.36 10.20 SP26 28/4/11 -2.95 0.05 -8.68 0.32 14.93 SP27 28/4/11 -2.16 0.05 -5.82 0.33 11.45 SP28 28/4/11 -2.91 0.05 -9.04 0.32 14.22 SP29 29/4/11 -4.18 0.04 -22.36 0.25 11.07 SP30 10/5/11 0.31 0.05 -4.72 0.33 -7.21 SP31 14/5/11 -4.36 0.05 -23.82 0.28 11.04 SP32 18/5/11 -1.38 0.04 -5.34 0.33 5.70 SP33 19/5/11 -3.67 0.04 -18.46 0.26 10.90 SP34 20/5/11 -0.76 0.05 5.45 0.37 11.49 SP35 20/5/11 -0.44 0.06 5.23 0.60 8.77 SP36 22/5/11 -1.37 0.06 -2.35 0.55 8.60 SP37 23/5/11 -2.02 0.06 -3.63 0.54 12.51 SP38 24/5/11 0.41 0.07 5.69 0.60 2.39 SP39 28/5/11 2.44 0.09 17.83 0.68 -1.65 SP40 30/5/11 -2.00 0.06 -5.91 0.53 10.12 SP41 2/6/11 -1.17 0.06 1.58 0.57 10.97 SP42 4/6/11 -3.67 0.06 -18.88 0.52 10.45 SP43 7/6/11 -2.48 0.05 -9.05 0.45 10.79 SP44 8/6/11 -1.06 0.07 0.35 0.65 8.84 SP45 11/6/11 -1.50 0.06 -2.00 0.55 9.98 SP46 12/6/11 -2.74 0.05 -13.03 0.48 8.88 SP47 15/6/11 -0.47 0.06 4.27 0.59 8.07 SP48 18/6/11 -3.78 0.06 -18.88 0.45 11.40 SP49 18/6/11 -4.49 0.05 -22.34 0.43 13.56 SP50 22/6/11 -0.37 0.06 4.31 0.59 7.24 SP51 12/8/11 -0.21 0.05 3.31 0.58 4.99 SP52 26/8/11 -0.57 0.06 3.36 0.59 7.91 SP53 27/8/11 -0.62 0.06 4.75 0.59 9.73 SP54 31/8/11 -0.78 0.06 1.48 0.57 7.69 SP55 20/9/11 -1.27 0.06 -1.70 0.64 8.49 SP56 21/9/11 -1.85 0.07 -7.23 0.61 7.59 SP57 26/9/11 -2.76 0.07 -13.53 0.56 8.54 Minimum -4.49 -23.80 -7.21 Maximum 2.44 17.80 14.93 Mean -1.58 -4.00 8.66 University of Ghana http://ugspace.ug.edu.gh 135 Fig 4.15 Stable isotope content of Rainfall at the Saltpond Meteorological Station in the coastal zone of the study area from August 2010 to September 2011. University of Ghana http://ugspace.ug.edu.gh 136 Table 4.9 Stable isotope composition of rainfall from Twifo Praso (TP) meteorological station in 2010 Sample ID Sampling Date 18O Error 2H Error Deuterium excess TP01 4/3/2010 -1.77 0.04 -11.87 0.65 2.29 TP02 17/08/2010 -2.73 0.05 -15.26 0.51 6.58 TP03 21/08/2010 1.13 0.04 6.07 1.02 -2.97 TP04 29/08/2010 1.24 0.05 9.91 1.01 -0.01 TP05 31/08/2010 0.68 0.05 12.97 1.03 7.53 TP06 9/1/2010 1.10 0.03 8.87 0.78 0.07 TP07 9/4/2010 0.37 0.05 13.76 0.55 10.80 TP08 9/11/2010 0.78 0.05 6.92 0.80 0.68 TP09 17/09/2010 0.19 0.05 -1.00 0.59 -2.52 TP10 19/09/2010 2.27 0.05 6.92 1.08 -11.24 TP11 21/09/2010 -0.63 0.05 -0.37 0.57 4.67 TP12 24/09/2010 -7.81 0.05 -62.98 0.88 -0.50 TP13 30/09/2010 -1.26 0.05 -3.43 1.22 6.65 TP14 10/1/2010 0.48 0.05 -2.52 0.57 -6.36 TP15 10/3/2010 -2.50 0.05 -19.00 0.50 1.00 TP16 10/4/2010 1.54 0.04 4.51 0.56 -7.81 TP17 10/10/2010 0.78 0.05 -1.66 0.80 -7.90 TP18 10/11/2010 -3.56 0.05 -31.34 0.50 -2.86 TP19 14/10/2010 -0.72 0.05 -3.81 0.57 1.95 TP20 18/10/2010 -0.90 0.05 -2.94 0.74 4.26 TP21 20/10/2010 -0.44 0.05 -6.89 0.70 -3.37 T922 25/10/2010 -0.72 0.05 -1.34 0.59 4.42 TP23 26/10/2010 -0.09 0.05 7.60 0.50 8.32 TP24 27/10/2010 0.15 0.05 -1.58 0.62 -2.78 T925 28/10/2010 -0.56 0.05 -4.23 0.67 0.25 TP26 30/10/2010 -2.01 0.05 -8.79 0.91 7.29 TP27 31/10/2010 -3.07 0.05 -9.38 0.61 15.18 TP28 11/5/2010 0.53 0.05 6.80 0.63 2.56 TP29 11/7/2010 -2.64 0.05 -14.36 0.54 6.76 TP30 11/9/2010 -1.21 0.05 -4.05 0.56 5.63 TP31 13/11/2010 1.47 0.05 17.14 0.69 5.38 TP32 18/11/2010 0.57 0.05 11.48 0.65 6.92 TP33 12/9/2010 4.89 0.05 32.45 0.81 -6.67 Minimum -7.81 -62.98 -11.24 Maximum 4.98 32.45 15.18 Mean -0.44 -1.86 1.64 University of Ghana http://ugspace.ug.edu.gh 137 Table 4.10 Stable isotope composition of rainfall from Twifo Praso (TP) meteorological station in 2011 SampleID Sampling Date 18O Error 2H Error Deuterium excess TP34 26/12/2010 2.76 0.11 23.76 0.84 1.71 TP35 28/12/2010 0.13 0.09 10.80 0.74 9.72 TP36 29/12/2010 1.74 0.10 14.68 0.77 0.74 TP37 31/12/2010 2.99 0.10 20.77 0.73 -3.16 TP38 25/1/2011 0.48 0.09 12.74 0.76 8.93 TP39 27/1/2011 0.02 0.09 7.25 0.72 7.11 TP40 28/1/2011 1.01 0.11 8.08 0.72 0.03 TP41 29/1/2011 4.87 0.12 23.50 0.84 -15.47 TP42 10/2/2011 2.42 0.10 15.83 0.78 -3.53 TP43 12/2/2011 -0.17 0.08 2.20 0.68 3.57 TP44 14/2/2011 -0.08 0.09 0.94 0.67 1.62 TP45 17/2/2011 1.88 0.10 17.10 0.79 2.03 TP46 22/2/2011 0.01 0.09 4.76 0.70 4.69 TP47 25/2/2011 -2.45 0.06 -10.37 0.59 9.25 TP48 6/3/2011 0.88 0.09 10.29 0.85 3.26 TP49 7/3/2011 1.77 0.09 19.18 0.81 5.04 TP50 9/3/2011 0.60 0.09 14.07 0.77 9.26 TP51 10/3/2011 1.81 0.10 19.97 0.81 5.49 TP52 11/3/2011 1.14 0.09 11.35 0.75 2.22 TP53 14/3/2011 0.83 0.09 8.50 0.84 1.87 TP54 26/3/2011 0.35 0.09 12.68 0.76 9.89 TP55 27/3/2011 -0.51 0.10 5.36 0.81 9.45 TP56 28/3/2011 -0.62 0.08 2.72 0.61 7.64 TP57 1/4/2011 -0.69 0.08 7.86 0.52 13.34 TP58 2/4/2011 0.68 0.07 12.31 0.45 6.83 TP59 3/4/2011 -0.11 0.09 0.02 0.66 0.92 TP60 17/4/2011 -3.07 0.07 -17.11 0.54 7.46 TP61 21/4/2011 -0.90 0.09 1.06 0.77 8.29 TP62 22/4/2011 -0.33 0.07 1.67 0.67 4.31 TP63 23/4/2011 -0.68 0.08 2.71 0.68 8.19 TP64 28/4/2011 -1.80 0.07 -7.30 0.61 7.09 TP65 3/5/2011 -1.23 0.08 0.40 0.67 10.24 TP66 9/5/2011 -0.97 0.06 2.91 0.43 10.69 TP67 10/5/2011 -3.86 0.04 -19.12 0.37 11.75 TP68 14/5/2011 -3.29 0.05 -13.95 0.37 12.37 University of Ghana http://ugspace.ug.edu.gh 138 Table 4.10 Continued. Sample ID Sampling Date 18O Error 2H Error Deuterium excess TP71 28/5/2011 -1.64 0.05 -5.57 0.41 7.58 TP72 1/6/2011 -2.55 0.05 -10.00 0.77 10.41 TP73 3/6/2011 -0.75 0.05 5.27 0.72 11.24 TP74 30/6/2011 1.12 0.05 16.33 0.77 7.39 TP76 16/7/2011 0.65 0.04 3.25 0.59 -1.93 TP77 18/7/2011 4.96 0.06 17.01 0.69 -22.68 TP78 25/7/2011 2.03 0.05 18.38 0.63 2.11 TP79 3/8/2011 1.35 0.04 11.72 0.61 0.88 TP80 4/8/2011 0.15 0.04 9.40 0.60 8.16 Maximum -3.86 -19.10 -22.68 Maximum 4.96 23.80 14.62 Mean 0.13 6.10 5.06 Fig. 4.16 Stable isotope content of Rainfall at the Twifo Praso Meteorological Station in the inland zone of the study area from August 2010 to September 2011. University of Ghana http://ugspace.ug.edu.gh 139 Eight surface water samples were analysed for stable isotopes. The stable isotope compositions of surface waters are presented in (Table 4.11). In the surface waters δ18O‰V-SMOW varies from -1.33 to +0.18 with a mean of -0.74 whilst δ2H‰V-SMOW varies from -6.13 to +1.53 with a mean of -1.29. The relationship between δ2H and δ18O for surface water is presented in (Fig 4.17). Surface water samples deviate from the Global Meteoric Water Line (GWML), showing enrichment of isotopic compositions signifying evaporation (Fig 4.17). Deuterium excess ranges between 0.09 to 8.62 and a mean of 4.61. The regression line for the evaporation of the surface waters is defined by the equation (4.5) as: 𝛿2𝐻 = 3.41𝛿18𝑂 + 1.22 (4.5) Table 4.11 Stable isotope composition of surface water Community Sample ID δ18O‰ V-SMOW δ2H‰ V-SMOM deuterium excess (d) Gomoa Brofo (River Ochi Narkwa) CR2-ROB -0.99 -2.19 5.73 Nsuekyi (River Ayensu) CR2-03R1AY -0.99 -6.13 1.79 Ekumfi Ekotsi (River Ochi Narkwa) CR2-16 0.18 1.53 0.09 Esakyir (River Ochi Narkwa) CR2-29 -0.81 0.56 7.04 Ajumako Abeadze (River Ochi Narkwa) CR2-37 -0.9 -1.98 5.22 Ajumako Esikado (River Ochi Narkwa) CR2-38 -0.41 0.12 3.4 Baffikrom (Ama Amissah) CR2-48 -1.33 -2.02 8.62 Mankessim (Ochi Ama Amissah) CR2-52 -0.65 -0.22 4.98 Minimum -1.33 -6.13 0.09 Maximum 0.18 1.53 8.62 Mean -0.74 -1.29 4.61 (d)=δD-8δ18O University of Ghana http://ugspace.ug.edu.gh 140 Fig. 4.17 δ2H-δ18O plot for surface water The stable isotope composition of shallow groundwaters in the study area ranges between -3.09 and -1.60 ‰ V-SMOW for δ18O and a mean of -2.36 ‰ V-SMOW. δ2H ranges between -12.35 and -6.06 ‰ V-SMOW and a mean of -10.01 ‰ V-SMOW. The deuterium excess ranges between +3.98 and +12.80 ‰ V-SMOW with a mean of 8.90 ‰ V-SMOW. The isotopic compositions of the shallow groundwaters are presented in (Table 4.12). The depleted nature of isotopic compositions suggests a meteoric origin. The isotopic compositons of the shallow groundwaters are displaced to the right of both the Local Meteoric Water Line (LMWL) and the Global Meteoric Water Line (GMWL) (Fig. 4.18) signifying evaporation defined by the regression line: 𝛿2𝐻 = 2.42𝛿18𝑂 − 4.31 (4.6) University of Ghana http://ugspace.ug.edu.gh 141 Table 4.12 Stable Isotope Compositions of ShallowGroundwaters Community Sample ID δ18O Error δ2H Error Deuterium Excess (d) Kweikrom CR2-01HD -1.6 0.05 -8.82 1.04 3.98 Nsuekyir CR2-03HD -2.12 0.03 -11.81 0.67 5.15 Kwesigyan No. 1 CR2-47 -1.9 0.05 -10.62 0.77 4.58 Abonko CR2-53 -1.81 0.05 -6.07 0.61 8.41 Anokye CR2-54 -2.39 0.05 -12.33 0.58 6.79 Ekumfi Asaafa CR4-01 2.43 0.08 -9.35 0.57 10.1 Ekumfi Asaafa CR4-02 -2.72 0.05 -11.11 0.44 10.7 Duadze CR4-FZ-03 -2.84 0.06 -10.65 0.44 12.1 Abora Dunkwa CR4-FZ-14 -2.17 0.06 -6.44 0.5 10.9 Abora Bando CR4-FZ-21 -2.92 0.05 -10.59 0.42 12.8 Pomase CR4-FZ-22 -3.09 0.07 -12.35 0.34 12.4 Minimum -3.09 -12.35 3.98 Maximum -1.60 -6.06 12.80 Mean -2.36 -10.01 8.90 Fig. 4.18 δ2H-δ18O plot for shallow groundwater University of Ghana http://ugspace.ug.edu.gh 142 Thirty-nine (39) deep groundwater samples were analysed for stable isotopes (δ18O and δ2H). Theδ18O values range between -3.11 and -1.43 ‰ V-SMOW with a mean of -2.42 ‰V-SMOW. The δ2H values vary between -17.61 and -7.04 ‰ V-SMOW with a mean of -11.97 ‰ V-SMOW. These values are more depleted than those of seawater (δ18O = 0‰, δ2H = 0‰) and suggest meteoric origin for the deep gorundwaters. The stable isotope composition of the deep groundwaters is presented in (Table 4.13). The most depleted deep groundwater samples (δ18O = -3.11 ‰ V-SMOW, δ2H = -17.61 ‰ V- SMOW) occur in boreholes CR2-56 at Afrangua in the southwestern section of the study area and CR2-09 at Ekumfi Gyabenkwaa in the southern section in the mica schist aquifers. The most enriched (δ18O = -1.43 ‰ V-SMOW, δ2H = -7.04 ‰ V-SMOW) occur in boreholes CR2-31 at Otabanadze in the middle of the study area within the mica schist aquifers and CR2-46 at Ajumako Kyeibi in the northern section of the study area within the biotite granite aquifers. There is no significant trend in isotopic composition with geology. The relationship between δ2H and δ18O for the deep groundwaters is presented in (Fig 4.19). The deep groundwaters deviate from the Global Meteoric Water Line (GMWL) and the Local Meteoric Water Line (LMWL) plotting on a regression line defined by equation 4.7. The regression line indicates that the deep groundwaters have been subjected to evaporation before recharge with slope 3.66 and intercept -3.12. Similar observation was made for the shallow groundwaters. University of Ghana http://ugspace.ug.edu.gh 143 The borehole depths in the area vary between 18 and 94.5 m with a mean of 34.48 m. Majority of the boreholes have been constructed at depths closer to the surface and therefore could be affected by evaporation as shown in (Fig. 4.19). It is therefore possible to have salt deposition in the unsaturated zone. Table 4.13 Stable Isotope Composition of Deep Groundwater Community Sample ID δ18O Error in δ18O Error in δ2H Error in δ2H Deuterium Excess Kweikrom CR2-01 -1.72 0.05 -11.48 0.53 2.28 Gomoa Abotsia CR2-04 -2.17 0.05 -14.17 1.06 3.19 Gomoa Obiri CR2-06 -2.32 0.05 -8.98 0.61 9.58 Gomoa Obiri CR2-07 -2.36 0.05 -12.27 0.58 6.61 Gomoa Asaman CR2-08 -2.42 0.05 -14.60 0.91 4.76 Ekumfi Gyabenkwaa CR2-09 -2.94 0.05 -17.61 0.50 5.91 Ekumfi Eduagyir CR2-12 -2.78 0.04 -12.38 0.88 9.86 Ekumfi Esuehyia CR2-14 -2.63 0.05 -11.31 0.82 9.73 Gomoa Antseadze CR2-15 -2.18 0.05 -7.60 0.73 9.84 Ekumfi Eyisam CR2-17 -1.95 0.05 -11.58 0.58 4.02 Ekumfi Eyisam CR2-18 -2.78 0.05 -11.70 0.60 10.54 Ekumfi Abor CR2-20 -1.79 0.05 -9.71 0.66 4.61 Ekumfi Abor CR2-21 -2.51 0.05 -14.06 0.63 6.02 Ekumfi Techiman CR2-22 -1.79 0.05 -12.17 0.62 2.15 Ekumfi Engow CR2-23 -2.89 0.05 -14.13 0.58 8.99 Edwansa Kokodo CR2-24 -3.00 0.05 -14.36 0.62 9.64 Edwansa Kokodo CR2-25 -2.93 0.05 -13.38 0.60 10.06 Onyaadze CR2-26 -2.47 0.05 -11.32 0.58 8.44 Obontser CR2-27 -2.12 0.05 -10.50 0.59 6.46 University of Ghana http://ugspace.ug.edu.gh 144 Table 4.13 Continued Community Sample ID δ18O‰ VSMOW Error in δ18O δ2H‰ VSMOW Error in δ2H Deuterium Excess Obontser CR2-28 -2.38 0.05 -12.62 0.66 6.42 Ata Kwaa CR2-30 -2.39 0.05 -14.81 1.68 4.31 Otabanadze CR2-31 -1.43 0.05 -7.22 0.60 4.22 Gomoa Brebiano CR2-32 -2.08 0.05 -9.61 0.65 7.03 Ajumako Ansa CR2-33 -2.35 0.05 -10.25 0.59 8.55 Gomoa Sampa CR2-35 -2.69 0.05 -12.13 0.58 9.39 Ajumako Ntananta CR2-39 -1.44 0.05 -9.04 0.84 2.48 Ajumako Kyeibi CR2-40 -2.65 0.05 -12.28 0.74 8.92 Ajumako Afransie CR2-41 -2.09 0.05 -9.79 0.77 6.93 Dwenase CR2-43 -2.27 0.05 -9.46 0.73 8.7 Enyan Abowinum CR2-44 -2.86 0.05 -12.34 0.58 10.54 Gomoa Abora CR2-45 -2.61 0.05 -11.69 0.60 9.19 Kyeibi CR2-46 -1.86 0.05 -7.04 0.60 7.84 Ekumfi Akwaakrom CR2-49 -2.73 0.05 -13.59 0.58 8.25 Ekumfi Asokwa CR2-50 -2.61 0.05 -14.45 0.58 6.43 Ekumfi Swedru CR2-51 -2.83 0.04 -14.09 0.55 8.55 Ewoyaa CR2-55 -3.07 0.05 -14.19 0.58 10.37 Afrangua CR2-56 -3.11 0.05 -13.96 0.81 10.92 Afrangua CR2-57 -2.65 0.05 -12.94 1.04 8.26 Goekrom CR2-58 -2.42 0.05 -11.97 1.13 7.39 Minimum -3.11 -17.61 2.15 Maximum -1.43 -7.04 10.92 Mean -2.42 -11.97 7.37 University of Ghana http://ugspace.ug.edu.gh 145 Fig. 4.19 δ2H-δ18O plot for deep groundwaters in the study area In summary, stable isotope composition of saltpond rainfall varies from -5.33 to +2.44 for δ18O ‰ V-SMOW with a mean of -1.47 ‰ V-SMOW. δ2H ‰ V-SMOW varies from -35.21 to +17.80 with a mean of -4.10. δ18O –δ2H plot reveal a slope of 6.49 and deuterium excess of +5.46 signifying evaporation of rain drops before recharge. δ18O and δ2H composition of Twifo Praso rainfall does not show significant difference to that observed from the saltpond station. δ2H – δ18O cross plot defines an equation with slope 6.15 and deuterium excess +3.44.Isotopic compositions suggest significant evaporation of the surface waters in the study area.Stable isotope composition of the shallow groundwaters exhibit evaporation defined by the regression Line: 𝛿2𝐻 = 2.42𝛿18𝑂 − 4.31.The depleted nature of the stable isotope composition of the deep groundwaters University of Ghana http://ugspace.ug.edu.gh 146 suggests a meteoric orgin. The waters are subjected to evaporation defined by the regression line:𝛿2𝐻 = 3.66𝛿18𝑂 − 3.12. 4.2.2 Recharge Mechan ism i n the Study Area Stable isotope compositions (δ18O and δ2H) provide important information on the recharge conditions of groundwater and its relationship with surface water (Acheampong and Hess, 2000). Majority of the shallow and the deep groundwaters plot below the LMWL and the GMWL (Fig. 4.20). A few of the shallow groundwaters plot above the GMWL but below the LMWL implying that local rainfall control the recharge processes in the area. The groundwaters even though cluster around the meteoric water lines are displaced to the right of the two meteoric water lines signifying that rainfall events have been modified by evaporation before recharging the groundwaters. The groundwater regression line, referred to as the evaporation line intercept the LMWL approximately at a point with δ18O = -3.8 ‰ V-SMOW and δ2H = -18 ‰ V-SMOW (Fig. 4.20). The interception of the evaporation line with LMWL identifies the mean isotopic composition of the parent rain water capable of recharge (Fontes, 1980). Thus the groundwaters were recharged by rainfall with mean δ18O and δ2H values of -3.8 and -18 ‰ V-SMOW. University of Ghana http://ugspace.ug.edu.gh 147 Fig. 4.20 Relationship between δ2H ‰ V-SMOW and δ18O ‰ V-SMOW for rainwater (RW) from Saltpond and Twifo Praso staions, surface water (SW) and groundwater (GW) comprising shallow wells (hand-dug wells (HD)) and Deep wells (boreholes (BH)). The rainfall samples from the meterological stations at Saltpond and Twifo Praso have slopes of 6.47 and 6.17 and intercepts of 5.47 and 3.44 respectively (Fig. 4.20). These slopes and intercepts are lower than those of the GMWL and LMWL by Akiti (1980) suggesting that the rain drops have been modified by evaporation before recharge. University of Ghana http://ugspace.ug.edu.gh 148 The evaporation line for the surface waters with equation defined as: 𝛿2𝐻 = 3.41𝛿18𝑂 + 1.22 is parallel to that of the groundwaters suggesting that there is no significant relationship between the groundwaters and surface waters. However, the surface waters have the capability of recharging the aquifers at a later date. The surface waters have isotopic compositions strongly affected by evaporation and therefore displaced further away from the GMWL and LMWL. In summary the mechanism of recharge to the grounwaters is by direct infiltration of past local rainfall of mean isotopic composition δ18O = -3.8 ‰ V-SMOW and δ2H = -18 ‰ V-SMOW. 4.2.3 Carbon - 13 (δ13 C) to Investigate Seaw ater Intru sion This study employs δ13C of groundwater to investigate seawater intrusion. δ13C is a stable isotope and usally influence by geochemical reactions in the carbonate terrane. However, the geology of the area does not include carbonate rocks. Carbonate minerals would only occur by secondary mineralisation. It is therefore assumed that δ13C of seawater is unlikely to be affected by geochemical reactions and thus be conserved for tracing of seawater intrusion in the groundwaters. 13C is expressed in δ – notation (as per mil deviation with respect to international standard Vienna – Pee Dee Belemnite (V-PDB)). The standard is a carbonate marine fossil of Jurassic age; therefore, most carbonate rocks of marine origin in aquifer matrix tend to give δ13C value close to 0‰ (Araguas-Araguas, 2003). Biogenic sources of University of Ghana http://ugspace.ug.edu.gh 149 carbon to total dissolved inorganic carbon (TDIC) give 13C value around -25‰ vs. V- PDB. Carbon-13 (δ13C) content of seawater varies from -2 to +2 ‰ V-PDB. In the Mediteranean Sea, Yechieli (2001) reports a value of 0‰ V-PDB. Ranges for δ13C values in selected natural compounds have been given in (Fig. 4.21). δ13C for the ocean is centred around 0‰ V-PDB. Groundwater ranges from 0‰ V-PDB to -20 ‰ V-PDB. If groundwater is caused by seawater intrusion thenδ13C values should be close to that of the ocean as indicated in (Fig. 4.21) or show δ13C enrichment towards the value for the ocean. Carbon-13 of groundwater in the study area varies from -19.30 to -7.94 ‰ V- PDB. The mean is -14.56 ‰ V-PDB. The groundwaters in the study area are more depleted in δ13C compared to the ocean or seawater. In applying δ13C ‰ V-PDB as an indicator of seawater intrusion to coastal aquifers, δ13C values of saline groundwater and non-saline groundwater were plotted against chloride concentrations in mg/L. If seawater intrusion is the cause of salinity of groundwater, then increase in Cl- concentration of groundwater with a corresponding enrichment of δ13C contents of groundwater would occur. A plot of δ13C against Cl- (mg/L) of 24 groundwater samples of the study area revealed a weak correlation (r = 0.07) indicating they are not of a common source (Fig. 4.22). It can also be found from (Fig. 4.22) that δ13C becomes more depleted with increasing Cl- (mg/L). This indicates that significant component of the dissolved inorganic carbon (DIC) is not derived from marine sources but rather from biogenic sources. The scenario that emerges is that groundwater salinization may not be as a result of seawater intrusion. University of Ghana http://ugspace.ug.edu.gh 150 Fig. 4.21 Ranges for δ13C values in selected natural compound (Clark and Fritz, 1997). Fig. 4.22 Plot of δ13C ‰ versus V-PDB against Cl- (mg/L) to deduce the possibility of seawater intrusion in the coastal aquifers of the Central region. University of Ghana http://ugspace.ug.edu.gh 151 In quantifying the fraction of seawater in groundwater, the δ13C mass balance equation was employed. Borehole CR2-51 at Ekumfi Swedru, one of the saline groundwaters was chosen for the quantification. The quantification is based on the assumption that the chemical composition of groundwater from borehole CR2-51 is as a result of mixing of seawater (more saline water) and a nearby borehole CR2-50 at Ekumfi Asokwa (less saline water) constituting two end members. These water bodies were chosen on the basis of their salinity and hydrogeological considerations including reasonable level of hydraulic connectivity. The hydraulic connectivity of the two boreholes CR2-51 and CR2-50 was established by means of the closeness in value of their stable isotope compositions. The δ18O value for CR2-51 is -2.61 ‰ V-SMOW and δ2H is -14.45 ‰ V- SMOW. The δ18O for CR2-50 is -2.83 ‰ V-SMOW and δ2H is +14.09 ‰ V-SMOW. The δ13C mass balance equation is given by: xδ13C + (1-x) δ13C = δ13CTotal (4.7) where x is the fraction of seawater water in groundwater.Avrahamov et al., (2010) used this equation to determine the various carbon sources in groundwater in the Dead Sea area of Israel. Attempt to use it to determine fraction of seawater in groundwater assuming there is interaction could not be successful as most of the values obtained are negative. May be if δ13C of seawater is measured in the area better results could be obtained. A more conservative tracer, Cl- was therefore used for the quantification of the seawater fraction. University of Ghana http://ugspace.ug.edu.gh 152 Quantification of seawater intrusion using Cl- is given by equation (4.8) after (Avrahamov, 2010; Wang and Jiao, 2012) as: 𝑓𝑠𝑒𝑎 = 𝑀𝑠𝑎𝑚𝑝𝑙𝑒 −𝑀𝑓𝑟𝑒𝑠 𝑕 𝑀𝑆𝑒𝑎 −𝑀𝑓𝑟𝑒𝑠 𝑕 (4.8) Msample is the Cl - concentration of the sample, Msea is the seawater Cl - concentration and Mfresh is the Cl - concentration of the fresh water. This equation was used based on the assumption that seawater intrude the groundwaters even though geochemical evaluation of the groundwaters show that majority of the groundwaters were not intruded by seawater. The determination of the fraction of seawater is to confirm earlier deductions that seawater intrusion may not be a major influence on the hydrogeochemistry of the groundwaters. The freshest water in the study area CR2-52 characterised by low TDS of 69.6 μS/cm and low Cl- of 16.00 mg/L constitute the fresh water end member and seawater as the saline water end member using theoretical Cl- value of 19, 000 mg/L. Computation of seawater fraction (Fsea) revealed ninety-two percent (92%) of the samples have seawater fraction within the range 0-6% and eight percent (8%) show significant seawater intrusion within the range 11-25%. The eight percent cover boreholes CR2-45 at Gomoa Abora (located 5.09 km from the coast), CR2-49 at Ekumfi Akwakrom, CR2-53 at Abonko a hand-dug well (15.75 km from the coast), CR2-54 at Anokye also a hand- dug well (3.05 km from the coast), CR4-FZ-16 at Abora Obokor, CR4-03 at Otuam and CR4-05 at Sefara Kokodo. University of Ghana http://ugspace.ug.edu.gh 153 In summary, it is established through δ13C - Cl- relationship that the groundwaters may not be intruded by seawater water. Estimation of seawater fraction assuming intrusion could not be possible with δ13C mass balance in this study. Probaly if δ13C of seawater is measured in the study area better results could be obtained. Quantification of seawater fraction using Cl- yielded ninety-two percent (92%) of samples within the range 0-6 % considered insignificant to cause salinization of groundwater and eight percent (8%) of samples within the range 11-25 % may be considered significant to cause salinization of groundwater. The eight percent cover boreholes CR2-45 at Gomoa Abora, CR2-49 at Ekumfi Akwakrom, CR2-53 at Abonko a hand-dug well, CR2-54 at Anokye also a hand- dug well, CR4-FZ-16 at Abora Obokor, CR4-03 at Otuam and CR4-05 at Sefara Kokodo. 4.3 GROUNDWATER AGE DETERMINATION 4.3.1 Tritiu m ( 3 H) conte n t of grou nd w ater The useful range for dating groundwater with 3H is less than about 50 years (Clark and Fritz, 1997). Lu et al (2006) stated that groundwater having tritium concentrations greater than 1.0 TU is interpreted as water mixed with modern water or recharged after 1952 when tritium was released into the atmosphere as a result of atmospheric testing of nuclear weapons. 3H is frequently used as an age tracer in groundwater systems and as indicator of groundwater sources (Bouchaou et al., 2008) Tritium (3H) was determined in twenty-seven (27) groundwater samples from the study area. The results obtained ranges from 0.05 ± 0.07 TU in borehole CR2-50 at Ekumfi Asokwa to 4.75 ± 0.16 TU in borehole CR2-51 at Ekumfi Swedru (Table 4.14). The University of Ghana http://ugspace.ug.edu.gh 154 mean tritium value is 1.48 ± 0.25 TU. Boreholes CR4-05 at Sefara Kokodo, CR4-FZ-22 at Pomase, and CR4-FZ-06 at Etsisunkwa and CR2-50 at Ekumfi Asokwa have tritium values < 1 and indicative of old water. This means that groundwater was recharged before thermonuclear bomb testing in 1952. Eighty-five per cent (85%) of the samples analysed have values > 1 suggesting recent or modern recharge (direct infiltration of recent rainfall). Thus recharge took place after thermonuclear bomb test in 1952. Figure 4.23 shows the spatial distribution of tritium superimposed on the geology of the study area. There is no significant influence of geology to tritium concentrations obtained. With the exception of borehole CR2-51 at Ekumfi Swedru which recorded tritium value of 4.75 ± 0.16 TU, most of the boreholes have tritium values just a little over 1 TU. A hand- dug well CR4-01 at Ekumfi Asaafa and borehole CR-FZ-19 at Abora showed tritium values of 2.03 and 2.54 TU respectively. Apart from the four (4) boreholes mentioned earlier, which showed very low tritium values (< 1 TU) in the coastal area located within the sediment and volcaniclastic sediment (locally mica schist), not marked variations in tritium values are realised. This may probably be due to tritium gradually decaying to its natural background levels after the cessation of thermonuclear bomb tests in 1963. The tritium concentrations do not follow any particular trend for definition of the flow path. In Fig. 4.24 tritim distribution show a localised system of flow suggesting a discontinuous aquifer system in the study area. University of Ghana http://ugspace.ug.edu.gh 155 Table 4.14 Tritium content of groundwater in the coastal zone and part of the inland zone of the Central Region Sample ID Community Sampling Date Tritium (TU) Depth (m) Well type CR2-01 Kweikrom 11-12-2010 1.29 ± 0.27 36 BH CR2-07 Gomoa Obiri 12-12-2010 1.57 ± 0.27 31 BH CR2-14 Ekumfi Esuehyia 12-12-2010 1.82 ± 0.30 32 BH CR2-15 Gomoa Antseadze 12-12-2010 1.40 ± 0.27 30 BH CR2-18 Ekumfi Eyisam 13-12-2010 1.96 ± 0.24 33 BH CR2-20 Ekumfi Abor 13-12-2010 1.31 ± 0.28 BH CR3-26 Onyaadze 13-12-2010 1.60 ± 0.29 BH CR2 30 Ata Kwaa 14-12-2010 1.35 ± 0.29 BH CR2 50 Ekumfi Asokwa 01-10-2011 0.05 ± 0.07 BH CR2 51 Ekumfi Swedru 01-10- 2011 4.75 ± 0.16 BH CR4-01 Ekumfi Asaafa 15-09-2012 2.03 ± 0.30 HDW CR4-02 Ekumfi Asaafa 15-09-2012 1.35 ± 0.33 HDW CR4-FZ-01 Effutuakwa 19-09-2012 1.84 ± 0.26 BH CR4-FZ-02 Nkusukumopem 19-09-2012 1.54 ± 0.27 BH CR4-FZ-06 Etsisunkwa 19-09-2012 0.67 ± 0.22 BH CR4-FZ-08 Ayeldu 19-09-2012 1.67 ± 0.27 BH CR4-FZ-13 Etsefawomanye 19-09-2012 1.45 ± 0.24 BH CR4-FZ-16 Abora Obokor 20-09-2012 1.12 ± 0.34 BH CR4-FZ-19 Abora 20-09-2012 2.54 ± 0.23 BH CR4-FZ-22 Pomase 20-09-2012 0.66 ± 0.23 BH CR4-FZ-23 Obohen 20-09-2012 1.00 ± 0.22 BH CR4-03 Otuam (BH1) 10-09-2012 1.08 ± 0.25 BH CR4-04 Otuam (BH2) 10-09-2012 1.25 ± 0.27 BH CR4-05 Sefara KoKodo 10-09-2012 0.64 ± 0.22 BH CR4-06 Sefara KoKodo 10-09-2012 1.10 ± 0.22 BH CR4-07 Adansemanmu 10-09-2012 1.07 ± 0.25 BH CR4-08 Nanaben 10-09-2012 1.84 ± 0.26 BH Minimum 0.05 ± 0.07 Maximum 4.75 ± 0.16 Mean 1.48 ± 0.25 BH: Borehole; HDW: Hand dug well University of Ghana http://ugspace.ug.edu.gh 156 Fig. 4.23 Spatial distribution of tritium in groundwaters showing boreholes with tritium > 1 and those with tritium < 1 with respect to the geology of the area. The arrows show possible flow directions. University of Ghana http://ugspace.ug.edu.gh 157 Fig. 4.24 Spatial plot of tritium in the study area In summary, Tritium in the groundwaters range from 0.05 ± 0.07 to 4.75 ± 0.16 TU.Eighty-five (%) of the samples analysed suggest modern recharge or young waters.Four groundwaters samples from boreholes CR4-05 at Sefara Kokodo, CR4-FZ- 22 at Pomase, and CR4-FZ-06 at Etsisunkwa and CR2-50 at Ekumfi Asokwa showed very low tritium concentrations and considered old waters. Spatial distribution of tritium in the study area shows a localised system of flow suggesting discontinuous aquifer systems in the study area. University of Ghana http://ugspace.ug.edu.gh 158 4.3.2 Carbon -14 ( 14 C) conten t of grou n dw ater Carbon-14 is not part of the water molecule, so its activity is affected by chemical reactions between the aquifer material and the dissolved constituents in the water that occur during and after initial infiltration. Chemical reactions can either add or remove carbon, therefore knowledge of which chemical reactions occurred during recharge and transport through the aquifer is necessary to estimate the initial carbon-14 for dating groundwater. Chemical reactions between groundwater and aquifer material that involve carbon can be evaluated by carbon-13 composition of water. The carbon-13 concentration in groundwater depends upon numerous factors, including whether the system is open or close, the type of vegetation in the recharge area, and whether carbonates are dissolved or precipitated during recharge. Under open system conditions, groundwater is assumed to be in equilibrium with soil CO2 so the calculated log PCO2 (log of partial pressure of CO2 in water) value of a water sample reflects that of the soil. Bacterial oxidation of vegetation in soils and respiration of CO2 in the root zone maintains log PCO2 levels between -2.5 and -1.5 (Clark and Fritz, 1997). In this study, the Partial Pressure of CO2 in both the soil zone and groundwater zone could not be determined due to lack of required equipment for such a purpose. Hence a careful assessment of the hydrogeochemistry discussed in section 4.1, geology of the area, saturated index (SI) and δ13C were used to determine the type of system prevailing University of Ghana http://ugspace.ug.edu.gh 159 in the area. The hydrogeochemistry revealed the predominant water type as NaCl indicating a non-carbonate terrane reflective of the geology of the area. In the case of the saturation index, most of the boreholes show under saturation with respect to calcite and dolomite a further indication that carbonate minerals are rare except boreholes CR2-20 at Ekumfi Abor and CR2-50 at Ekumfi Asokwa, which are supersaturated with respect to calcite and dolomite. This means that secondary carbonate mineralization may be taking place in aquifers yielding water to these boreholes. This may result in the initial 14C activity becoming less than 100 pMC. Super-saturation with respect to calcite and dolomite suggest a reaction under close system. There is therefore the need to correct for the initial 14C activity (Ao) for reliable radiocarbon dating for these boreholes. The δ13C composition of the groundwater (Table 4.15) as discussed in section 4.2.3 reveal values close to that of the soil zone (-23 ‰) indicating that most of the CO2 generated are from the soil zone or unsaturated zone. Akiti (1980) explained that in the tropical regions, δ13C of soil organic matter is very close to that of DIC which is formed without fractionation. Hence 14C activity of (DIC) equals 14C activity of organic matter and requires no correction term. Acheampong and Hess (2000) realized δ13C concentrations between -20.3 and -15.2 ‰ from shallow groundwater system in Southern Voltaian Sedimentary Basin of Ghana. These values were observed to fall closer to the range of values of some common plant material within the range -23 to -3 ‰ than those from carbonate minerals (Mazor, 1991). Wang and Jiao (2012) found that inorganic University of Ghana http://ugspace.ug.edu.gh 160 carbon evolution in a confined carbonate matrix aquifer receiving soil CO2 results in δ 13C values of -15 ‰. HCO3 - concentration of the groundwaters in the study area was plotted against δ13C ‰ to determine the source of HCO3 - in the groundwaters (Fig. 4.25). The plot shows an increase in HCO3 - with corresponding depletion in δ13C ‰ towards theoretical value of - 23 ‰ suggesting most of the HCO3 - may be coming from the soil zone. An increase in HCO3 - with a corresponding enrichment in δ13C ‰ is indicative of significant input of inorganic carbon by methanogenisis (Wang and Jiao 2012). Akiti (1980) proposed that increase in HCO3 - with a corresponding depletion in δ13C of groundwater suggests HCO3 - come from carbonic acid in the soil zone and hydrolysis of silicate minerals depicting a closed system. In the Upper Regions of Ghana, Akiti (1980) obtained δ13C values within the range -19 ‰ to -17 ‰ which compares to what was obtained for this study. Akiti (1980) explains that δ13C values of the upper region of Ghana are too low to be explained by open system conditions. Low δ13C of the groundwaters also implies there are no carbonates in the rocks to result in mixing and isotopic exchange hence no need for 14C correction. The data obtained for this study thus fits Akiti‘s (1980) model. On the basis of this model and considering the geology and hydrogeochmistry of the area the system operating is the closed system. However, since no carbonate minerals ocurr in the rocks of the area to cause isotopic mixing and exchange, carbon-14 dating was done without correction except borehole CR2-50 at Asokwa. In the case of the Asokwa borehole, groundwater is aging as given by the 14C content of 9.5 pMC and a tritium concentration of 0.05 TU (Table 4.16). However, the δ13C concentration is -18.40 ‰ V-PDB. This is University of Ghana http://ugspace.ug.edu.gh 161 very close to that of the soil zone. Since there is no isotopic exchange occurring in the system, δ13C remains unchanged in the system. The hydrogeochemistry is thus dominated by dissolution of silicate minerals resulting in increase in pH of 7.48. The system is thus controlled only by radioactive decay hence age estimation can be attempted for groundwater in this borehole. Fig. 4.25 Relation between HCO3 - (mg/L) and δ13C of DIC (‰) V-PDB. University of Ghana http://ugspace.ug.edu.gh 162 Under closed systems several sources of total dissolved inorganic carbon (TDIC) occur and therefore require correction models. These models are based on different mixing reactions: chemical mixing (Tamers 1975), isotopic mixing (Ingerson and Pearson 1964) and models of chemical mixing and isotopic exchange (Fontes and Garnier 1979). The isotope contents of different carbon sources introduced in the determination of the ages of the groundwaters are: (1) δ13C(soil) (‰ vs. PDB) = -23 (2) A14C(soil) (pMC) =100 (3) δ13C(minerals) = 0 (4) A14C(mineral) = 0 (5) Half-life = 5730 years The groundwater ages were calculated and compared using four correction models: Pearson (Ingerson and pearson 1964), Fontes and Garnier (Fontes and Garnier 1979) and Tamers (Tamers, 1975). The Carbon-14 content of the groundwater ranges from 9.50 percent modern carbon (pMC) in borehole CR2-50 at Ekumfi Asokwa to 113.56 pMC in borehole CR3-26 at onyaadze (Table 4.15). The mean carbon-14 content of groundwater in the area is 99.01 pMC. There is no significant difference in activities of mica schist aquifers and those of granitoid aquifers (Fig. 4.26). The 14C content of the groundwaters in this study also compares to what was obtained by Akiti (1980) in the upper region of Ghana in the granitoid aquifers. Akiti (1980) values ranged between 86 to 111 pMC showing mostly University of Ghana http://ugspace.ug.edu.gh 163 young waters. Inferred flow direction from spatial distribution of 14C agrees with that of tritium showing a localised system of flow (Fig 4.26). With the exception of borehole CR2-50 at Ekumfi Asokwa the rest of the boreholes have 14C activities close to or higher than 100 pMC revealing a modern recharge as shown in the plots of 14C against δ13C (Fig. 4.27) and 3H against 14C (Fig. 4.28) implying 14C activities do not need to be corrected. Trituim would therefore be a more appropriate dating technique. Three groups of water have been identified in the area combining 3H and 14C data (Fig. 4.28). The first group is characterised by high 3H and high 14C signifying modern recharge. The second group is a mixture of young and old water. The third group is characterised by low 3H and low 14C signifying older water and includes borehole CR2-50 at Ekumfi Asokwa. Borehole CR2-50 at Ekumfi Asokwa with lowest carbon-14 activity of 9.5 can therefore be dated withcarbon-14. The saturated Index showed supersaturation with respect to calcite and dolomite indicating a carbonate mineral reaction in the aquifer, hence depicting a closed system. In this instance, all the models Tamers, Pearson and Ingerson and Fontes and Garnier were used to determine the age of groundwater in this borehole. The uncorrected age of groundwater based on Akiti‘s model gave 19,459 years BP for the Asokwa borehole. Corrected ages employing Tamer‘s model gave an age of 13,920 years BP, Pearson and Ingerson revealed age of 17,614 years BP and Fontes and Garnier‘s model gave age of 18,908 years BP. This groundwater is not being renewed and must be abstracted with caution to avoid mining. Even though carbon-14 is most appropriate for dating old waters, it was further used to date the modern or young waters using the same University of Ghana http://ugspace.ug.edu.gh 164 models. Almost all the models gave negative ages (Table 4.16) suggesting they are modern or young waters and therefore need no correction. Table 4.1514C and 13C content of groundwater in the study area ID Community Sampling date C-14/ pMC delta 13C(‰VPDB) Cr2-7 Gomoa Obiri 12/12/2010 108.90 ± 1.73 -17.90 Cr2-14 Ekumfi Esuehyia 12/12/2010 112.80 ± 1.79 -17.70 Cr2-18 Ekumfi Eyisam 13/12/2010 113.50 ± 2.54 -18.00 Cr2-20 Ekumfi Abor 13/12/2010 113.30 ± 1.70 -17.20 Cr2-30 Ata Kwaa 14/12/2010 102.20 ± 0.20 -16.40 Cr2-45 Gomoa Abora 12/12/2010 87.00 ± 0.22 -13.90 Cr2-50 Ekumfi Asokwa 16/12/2010 9.50 ± 0.19 -18.40 Cr2-51 Ekumfi Swedru 16/12/2010 93.50 ± 0.07 -19.30 CR4-01 Ekumfi Asaafa 15-09-12 97.78 ± 0.42 -12.61 CR4-02 Ekumfi Asaafa 15-09-12 79.84 ± 0.34 -15.10 CR4-FZ-01 Effutuakwa 19-09-12 109.42 ± 0.43 -13.95 CR4-FZ-02 Nkusukumopem 19-09-12 108.88 ± 0.46 -14.67 CR4-FZ-06 Etsisunkwa 19-09-12 109.42 ± 0.44 -13.92 CR4-FZ-08 Ayeldu 19-09-12 110.83 ± 0.45 -7.94 CR4-FZ-13 Etsefawomanye 19-09-12 96.36 ± 0.40 -15.55 CR4-FZ-16 Abora Obokor 20-09-12 106.54 ± 0.44 -10.01 CR4-FZ-19 Abora 20-09-12 102.14 ± 0.43 -11.95 CR4-FZ-22 Pomase 20-09-12 92.86 ± 0.40 -16.37 CR4-FZ-23 Obohen 20-09-12 101.01 ± 0.41 -14.60 CR3-26 Onyaadze 20-09-12 113.56 ± 0.45 -11.56 CR4-04 Otuam (BH2) 9/10/2012 85.56 ± 0.37 -15.11 CR4-05 Sefara KoKodo 9/10/2012 110.39 ± 0.44 -10.46 CR4-07 Adansemanmu 9/10/2012 104.09 ± 0.42 -15.47 CR4-08 Nanaben 9/10/2012 106.8 ± 0.43 -11.25 Maximum 9.5 ± 0.19 -19.30 Maximum 113.56 ± 0.45 -7.94 Mean 99.01 ± 0.63 -14.56 University of Ghana http://ugspace.ug.edu.gh 165 Fig. 4.26: 14C activities superimposed on the geology of the study area. The arrows show inferred groundwater flow paths which is more localised. University of Ghana http://ugspace.ug.edu.gh 166 Fig. 4.27 Plot of 14C (pmc) against 13C (PDB) of groundwater in the coastal zone of the Central Region University of Ghana http://ugspace.ug.edu.gh 167 Fig. 4.28 Plot of 3H (TU) versus percent modern carbon (pMC). On the basis of stable isotopes of water, tritium and carbon-14, groundwater in the study area is recharged mainly by rainwater. Exploitation of this resource is therefore sustainable but susceptible to contamination because it is easily replenished. The recharge areas must therefore be protected by enacting laws that will control anthropogenic activities in these areas. The old groundwater encountered in the area is liable to depletion. Groundwater abstraction must therefore be regulated to prevent over abstraction that would result in depletion and possible collapse of the aquifer. University of Ghana http://ugspace.ug.edu.gh 168 Table 4.1614C ages of groundwater samples in the parts of the Central Region ID Commun ity Age Age Age Age Ao=100 Ta mers Pearson F. & G. CR2-7 Gomoa Obiri -705 -5180 -2777 -1810 CR2-14 Ekumfi Esuehyia -996 -3674 -3161 -2926 CR2-18 Ekumfi Eyisam -1047 -4532 -3073 -2456 CR2-20 Ekumfi Abor -1032 -5615 -3434 -2539 CR2-30 Ata Kwaa -180 -3481 -2976 -2735 CR2-45 Gomoa Abora 1151 -1652 -3012 -3087 CR2- 50 Ekumf i Asokw a 19459 13970 17614 18908 CR2-51 Ekumfi Swedru 556 -2789 -894 -140 CR4-01 Ekumfi Asaafa 186 -5536 -4783 -4441 CR4-02 Ekumfi Asaafa 1861 -3790 -1617 -721 CR4-FZ-01 Effutuakwa -744 -3296 -4878 -4960 CR4-FZ-02 Nkusukumopem -703 -4558 -4421 -4353 CR4-FZ-06 Etsisunkwa -744 -3075 -4895 -4992 CR4-FZ-08 Ayeldu -850 -1997 -9642 -10250 CR4-FZ-13 Etsefawomanye 307 -2553 -2929 -2946 CR4-FZ-16 Abora Obokor -524 -2123 -7401 -7756 CR4-FZ-19 Abora -175 -605 -5588 -5908 CR4-FZ-22 Pomase 612 -1601 -2199 -2229 CR4-FZ-23 Obohen -83 -2768 -3840 -3894 CR3-26 Onyaadze -1051 -4159 -6738 -6882 CR4-04 Otuam (BH2) 1289 -2159 -2184 -2185 CR4-05 Sefara KoKodo -817 -3294 -7331 -7527 CR4-07 Adansemanmu -331 -4110 -3610 -3348 CR4-08 Nanaben -544 -3768 -6456 -6574 University of Ghana http://ugspace.ug.edu.gh 169 In summary, 14C content of the groundwaters ranges between 9.50 pMC in borehole CR2-50 at Ekumfi Asokwa to 113.56 pMC in borehole CR3-26 at onyaadze.Most of the waters are modern or young except borehole CR2-50 at Ekumfi Asokwa which has 14C activity of 9.5 pMC and therefore considered old. The lowest tritium value of 0.05 TU occurred in this same borehole thus confirming groundwater in this borehole to be old.Three groups of water have been identified in the area combining 3H and 14C data. The first group is characterised by high 3H and high 14C signifying modern recharge. The second group is a mixture of young and old water. The third group is characterised by low 3H and low 14C signifying older water and includes borehole CR2-50 at Ekumfi Asokwa. The estimated age or residence time of this older water is 19,459 years BP based on Akiti model. Tamers, Pearson and Ingerson, and Fontes and Garnier‘s model gave ages between 13,920 and 21,732 years BP respectively. 4.4 ORIGIN OF SALINITY IN THE GROUNDWATERS 4.4 .1 Geoche mical con si d eration s Compositional diagrams (bivariate plots) have been used by various authors to explain the sources of ions and geochemical processes in groundwater. Mckenzie et al (2012) found that one approach to understanding source contributions to hydrological systems is by looking at bivariate plots comparing major ions or stable isotopes. In this study, bivariate plots (Na-Cl relatioships, Br-Cl weight ratio, SO4 2--Cl- and Ca-SO4 2- relationship) were employed to undertand the hydrogeochemical processes taking place in the study area; hence determine the origin of salinity of groundwater. University of Ghana http://ugspace.ug.edu.gh 170 A plot of Na+ against Cl- showed a strong correlation (Fig 4.29). A correlation of 0.93 was obtained indicating most likely a common source of saline water. In (Fig 4.29), the Na-Cl relationship showed most of the samples lie on the 1:1 line. This means that the sources of Na+ and Cl- in the groundwater may be coming from halite dissolution. Considering the general geology of the area halite deposits do not occur in the area. Halite if present may have been concentrated in the soil zone through evaporation and flushed into the groundwater zone by infiltrated rainwater. A Na/Cl molar ratio was used to determine sources of Na ions and identify geochemical processes that affect Na concentrations.Na/Cl molar ratios have been used to determine whether halite dissolution or silicate weathering is contributing to the hydrogeochemistry of the groundwater. A ratio equal to 1 implies halite dissolution is responsible for contributing Na+ ions and a ratio greater than 1 means silicate weathering may be contributing to Na+ ions in the groundwater (Singh et al., 2010). For this study, the Na/Cl molar ratio ranges from 0.36 to 5.18. About 83.33 % of the samples have Na/Cl molar ratio equal to 1 suggesting that halite dissolution is a major hydrogeochemical process in the area. About 14.33% of the samples have Na/Cl molar ratio greater than 1 and therefore show indications of silicate weathering. This is further confirmed by a plot of TDS against Na/Na+Ca (Fig 4.30) proposed by (Singh et al., 2010) where most of the samples plot in the rock dominance region accounted for by weathering. About 2.38 % of the samples have Na/Cl molar ratio less than 1 and suggest seawater intrusion. University of Ghana http://ugspace.ug.edu.gh 171 In deducing the origin of salinity, various authors (Marie et al., 2001, Helstrup, 2006, Moussa et al., 2009, Rao, 2007, De Montety et al., 2008, El-Fiky, 2010, Al-Charideh, 2011, Cheong et al., 2011, Naseem et al., 2011, Park et al, 2011) have employed various ionic ratios (Na/Cl, Br/Cl, SO4/Cl, Ca/SO4 and Ca/Mg) to discriminate between salinisation caused by seawater intrusion from those caused by other geochemical processes. Generally, these ratios tend to decrease and attain values equal or lower than seawater giving indications of seawater intrusion or sea aerosol spray rich in these elements. The Na/Cl ratio discussed earlier aside distinguishing between halite dissolution has also been used to identify seawater intrusion by plotting the ratio against Cl (mg/L). According to (Cheong et al., 2011) Na/Cl ionic ratio of 0.86 is a criterion for the discrimination of seawater intrusion. In this study, Na/Cl ratios were plotted against Cl (mg/L) and EC (μS/cm) (Fig 4.31). The plot reveals that majority of the samples plot above the 0.86 line suggesting, other geochemical processes may be contributing to the release of Na+ in the groundwater (Fig 4.31a). One of such geochemical process is silicate weathering as revealed in (Fig. 4.30). The various minerals that will undergo silicate weathering to yield Na+, Ca2+, Mg2+ and K+ in groundwater and surface water have already been explained in previous sections and illustrated in equations (2.3) to (2.7). However, few boreholes have values equal to 0.86 and less than 0.86 showing characteristics of seawater intrusion or reverse ion exchange. Areas showing Na/Cl molar ratio equal to 0.86, less than 0.86 and greater than 0.86 is shown in (Fig. 4.32). There is no specific trend of the Na/Cl molar ratio with respect to the geology of the area. University of Ghana http://ugspace.ug.edu.gh 172 Fig. 4.29 Na-Cl relationship of groundwater in the coastal areas of the Central Region Fig. 4.30 Scatter plot between TDS and Na/(Na+Ca) showing rock dominant weathering in the area (after Singh et al 2010) University of Ghana http://ugspace.ug.edu.gh 173 Fig. 4.31 (a) Na/Cl molar ratio versus Cl (mg/l), (b) Na/Cl molar ratio Versus EC Generally, it is expected that evaporation process would cause an increase in concentrations of all species in groundwater. If evaporation process is dominant assuming that no mineral species are precipitated, Na/Cl ratio would be unchanged (Jankowski and Acworth 1997). Hence the plot of Na/Cl versus EC would give a horizontal line which would then be an effective indicator of concentration by evaporation or evapotranspiration. The Na/Cl molar ratio as a function of EC (Fig. 4.31b) decreases with increasing EC suggesting a possible seawater intrusion in the chemical composition of groundwater in the area. Salinisation due to halite dissolution and sea aerosols would give a similar curve. Even though (Fig. 4.30) shows significant process of silicate weathering, few of the boreholes and hand-dug wells showed indications of evaporation as suggested by stable isotope results. University of Ghana http://ugspace.ug.edu.gh 174 Fig. 4.32 Spatial distribution of Na/Cl molar ratio in the study area with respect to the geology of the area University of Ghana http://ugspace.ug.edu.gh 175 The ratios of Br- to Cl- have been extensively used to detect the origin of dissolved salts in groundwater and brines since both form stable anions of Cl- and Br- in water and are usually not affected by water rock interaction (Vengosh and Pankratov, 1998). Boggs and Adams (1992) argued that decomposing organic matter and sorption of Br- on clays and iron oxides in the soil affect Br/Cl ratio in water. However, studies by (Sophocleous et al., 1990 and Star and Glotfelty, 1990) have demonstrated the conservative characteristics of Br/Cl ratios for tracing the origin of salinity in groundwater. Br/Cl ratio has often been used as a reliable indicator of the origin of salinity due to its specific composition in various saline sources (Vengosh et al., 2005, De Montety et al., 2008, El-Fiky et al., 2010). The Br/Cl ratio of seawater as captured by (Helstrup, 2006) who worked in parts of Ghana and Togo close to the Gulf of Guinea is 0.0035 ± 0.0002 (weight ratio). However, various ratios of 0.00155-0.0017 have been reported in the Mediterranean seas (El-Fiky, 2010). In this study, the Br/Cl ratios of the groundwaters range from 0.0054 to 2.075. Four of the boreholes CR2-01 at Kweikrom, CR2-22 at Ekumfi Techiman, CR2-23 at Ekumfi Engow and CR2-45 at Gomoa Abora plot close to the seawater line and therefore show characteristics of seawater transgression into the groundwater, sea aerosol spray or halite dissolution (Fig 4.33). The rest of the samples plot far above the Br/Cl ratio for seawater, thereby, suggesting other sources apart from seawater intrusion. However, Fig 4.33 reveals a general decrease in Br/Cl ratio with increasing Cl (mg/L) as shown by the arrow AB suggesting a tendency to seawater characteristics. This would further be evaluated and confirmed with stable isotopes. University of Ghana http://ugspace.ug.edu.gh 176 Fig 4.33 Br/Cl weight ratio versus Cl (mg/L) A plot of SO4 against Cl showed a moderate positive correlation of 0.62 (Fig 4.34a). SO4/Cl ratio has also been used to investigate issues relating to seawater intrusion. Marie et al., (2001) recorded a seawater ratio for SO4 2-/Cl- as 0.15. A ratio greater than 0.15 means additional SO4 has been input either from fertiliser application, leaching of contaminated landfill and oxidation of sulphides (Marie et al., 2001). SO4 2-/Cl- ratios for samples collected in the study area range between 0.02 and 4.09. Sixty-eight percent (68%) of the samples analysed have SO4 2-/Cl- ratio above seawater value indicating additional sources of SO4 2- into the groundwater system (Fig. 4.34c). Additional sources of SO4 2- in both groundwater and surface water could be attributed to application of fertilisers and oxidation of pyrites in the rocks. Thirty-two percent (32%) of the samples have SO4 2-/Cl- either equal to or less than the seawater value hence showing characteristics of seawater origin. A plot of SO4/Cl ratio against EC (Fig 4.34b) reveals a University of Ghana http://ugspace.ug.edu.gh 177 weak correlation (r = 0.24) implying, SO4 2-/Cl- ratio is not dependent on EC. Generally a decrease in SO4 2-/Cl- ratio results in an increase in EC and salinisation is attributed to seawater intrusion. Even though a weak correlation is realised for the relationship between SO4 2-/Cl- ratio and EC in Fig 4.34b there is an increasing EC with decreasing SO4 2-/Cl- signifying either seawater intrusion or sea aerosol spray. Similarly, a decreasing SO4 2-/Cl- molar ratio with increasing Cl is observed for the plot of SO4 2-/Cl- versus Cl (mg/L) (Fig. 4.34c). The spatial distribution map of SO4 2-/Cl- is shown in (Fig. 4.35). There is no specific trend with respect to geology. Majority of the samples show SO4 2-/Cl-> 0.15. Areas showing SO4 2-/Cl- = 0.15 are quite a distant from the sea. They are found within the mica schist aquifers except one borehole CR4-FZ-02 at Nkusukumopen which is located in the biotite granitoid aquifers. Various other plots involving the major ions (Ca versus SO4, Ca/SO4 versus EC, Ca versus Na+K and Ca/Na+K versus EC) were carried out to determine the sources of Ca2+, SO4 2-, Na+ and K+. It is also to see if these ions are associated with seawater. University of Ghana http://ugspace.ug.edu.gh 178 Fig. 4.34 (a) SO4 2- (meq/L) versus Cl- (meq/L), (b) SO4 2-/Cl- Versus EC (μS/cm) and (c) SO4 2-/Cl- Versus Cl (mg/L) University of Ghana http://ugspace.ug.edu.gh 179 Fig. 4.35 Spatial distribution of SO4 2-/Cl- in the study area University of Ghana http://ugspace.ug.edu.gh 180 The plot of Ca versus SO4 (Fig 4.36a) showed strongly to moderate correlation (r = 0.69). Rao (2007) mentioned that Ca and SO4 ions in groundwater are provided partly by gypsum dissolution. In Fig 4.36a most of the groundwaters deviate from the 1:1 line indicating another source of Ca. Additional Ca in the groundwater may be produced from dissolution of anorthite rich plagioclase feldspars discussed earlier and illustrated in equation (2.3). Ca/SO4 molar ratio plays an important role in determining the sources of Ca and SO4 in groundwater. Ca/SO4 ratio exceeding seawater value and approaching a value of 1.0 is attributed to dissolution of sulphate minerals (gypsum and anhydrite) (El- Fiky, 2010). High Ca/SO4 ratios are also inferred to mean influence of gypsum dissolution (Naseem et al., 2011). The results from this study show that Ca/SO4 ratio ranges from 0.35 to 10.84 compared to 0.40 of seawater. A plot of Ca/SO4ratio with EC (Fig 4.36b) revealed a very weak correlation (r = 0.08) indicating a lesser effect of seawater intrusion. Lee and Song, (2007) indicated that a low Ca/SO4 ratio accompanied by high EC signifies the effect of seawater intrusion. In Fig 4.36b and 4.36c there is a general increase in Ca/SO4 with EC and Ca/SO4 with Cl ruling out a possible seawater effect. A relation between Ca and Na + K (Fig 4.36d) revealed a strong positive correlation (r = 0.68) indicating a common source for Ca, Na and K. This source may be dissolution of silicate minerals such as plagioclase feldspars, micas, hornblende and K-feldspars which are major constituents of the rocks in the area as discussed earlier. Excess Na + K over Ca is realised in (Fig. 4.36d) suggesting cation exchange reaction in the area. In this reaction Na rich clay material in the aquifer replaces the Ca of the groundwater thus enriching the groundwater University of Ghana http://ugspace.ug.edu.gh 181 with Na resulting in salinisation of the groundwater. The plot of Ca/Na + K versus EC (Fig. 4.36e) showed a weak correlation (r =0.29) meaning Ca/Na + K is not dependent on EC implying Ca2+, Na+ and K+ may not be coming from seawater. University of Ghana http://ugspace.ug.edu.gh 182 Fig. 4.36 Relationship of (a) Ca with SO4, (b) Ca/SO4 with EC, (c) Ca/SO4 with Cl (d) Ca with Na+K and (e ) Ca/Na+K with EC for groundwaters in coastal zone of the Central Region. In summary, from geochemical view point, groundwater salinization in the coastal zone of the Central Region may be caused largely by halite dissolution and to a minor extent silicate weathering. Halite may be concentrated in the soil zone by evaporation as result of deposition of sea aerosols on the earth surface. Na/Cl molar ratio (0.36 – 5.18), Br/Cl (0.0054 – 2.08), SO4 2-/Cl- (0.02 – 4.09) and Ca/SO4suggest that seawater intrusion plays a minimal role in controlling the groundwater chemistry in the area. However, these deductions would be discussedfurther employing stable isotopes of water (δ18O and δ2H) with some major ions. Chloride (Cl-) levels in both rainfall and soil zone would also be evaluated to confirm the issue of sea aerosol spray or halite dissolution. University of Ghana http://ugspace.ug.edu.gh 183 4.4 .2 Stable isotop e con sid eration s The Stable isotopes of water (δ18O and δ2H) are the most conservative tracers during their transportation in the hydrological systems. They are most often employed in investigations dealing with seawater intrusion and or groundwater salinisation problems. Yurtsever (1994) stated that in studies dealing with seawater intrusion and salinisation process identification, it is appropriate to consider the isotopic and hydrochemical evolution. Such an approach enables a clear distinction to be made of the salinisation process (or processes) for cases where the fresh water salinity may be caused by leaching of salt formations, mineral dissolution and salt accumulation due to evaporation. Stable isotopes thus provide an effective label for seawater and fresh water to enable tracing of seawater intrusion as well as identifying processes that may be responsible for water salinisation. In this study, δ18O - δ2H plot of the groundwaters (Fig 4.37a) and their relationship with seawater has been considered for discussion. Similarly δ18O – Cl relationship has been studied to identify salinisation process (or processes) in the area. The δ18O - δ2H plot (Fig. 4.37a) shows that there is no relationship of groundwater with seawater. Stable isotope composition of seawater obtained from only one locality Ekumfi Asaafa and plotted on the diagram is (δ18O = 0.54 ‰ V-SMOW, δ2H = 3.66 ‰ V-SMOW).In an attempt to confirm the effect of evaporation in the study area and investigate further, the effect of seawater intrusion, Cl was plotted against δ18O‰ V-SMOW (Fig 4.37b). There is no significant relationship between Cl with δ18O‰ V-SMOW. However, significant evaporation is observed for the surface water whilst very few boreholes exhibit University of Ghana http://ugspace.ug.edu.gh 184 characteristics of Evaporation (Fig 4.37b). The upper curve represents the mean of the fresh groundwaters (chloride content lower than 400 mg/L) plus 2ζ and the lower curve, the mean minus 2ζ. It can be seen from (Fig. 4.37b) that there is no marked change in δ18O‰ V-SMOW with increasing chloride (Cl-) content implying that salinity originates from dissolution of salts from soils and rocks. This explains why majority of the groundwaters plot in the dissolution band. Two boreholes appear to plot in the mixing band suggesting seawater intrusion. One of the hand-dug wells sampled show concentration of chloride by evaporation. Fig 4.37 (a) δ18O - δ2H plot of the groundwaters in the study area (after Craig, 1961)(b) Relationship between Cl (mg/L) and δ18O‰ V-SMOW (after Akiti, 1985). University of Ghana http://ugspace.ug.edu.gh 185 In summary, stable isotope considerations proved that seawater intrusion is not the predominant process contributing to high salinity waters in the study area. Rather, dissolution of salts in the soil zone and minerals in rocks may be responsible for the salinity of groundwaters in the area. The stable isotope considerations thus agree with the conclusion drawn from the geochemical considerations. This study reveals that halite is one of the major salts that may be concentrated in the soil zone by evaporation to cause the salinization of groundwater. In affirming this deduction Cl levels in rain and soil were determined and studied, as presented in the following section. 4 . 4 .3 Chlorid e (Cl) level s in Rainf all Salinity may result from the accumulation of salts over long period of time through two major natural processes in the soil or groundwater. The first process is the weathering of parent materials containing soluble salts. Weathering processes breakdown rocks and release soluble salts of various types, mainly chlorides of sodium, calcium and magnesium. Sodium chloride however, is described as the most soluble salt. This process has been confirmed by TDS versus Na/Na+Ca plot in Fig. 4.30 that weathering contributes to the salinity of groundwater in the area. The second process is the deposition of sea aerosols comprising oceanic salts carried in wind and rain. Sea aerosols comprise ocean salts carried inland by wind and deposited by rainfall in the soil zones and are mainly sodium chloride. Chloride (Cl-) composition of rainwater varies greatly depending on prevailing wind and distance from the coast. Cl- levels in rainfall at Saltpond ranged from 1.07 mg/L to 22.32 mg/L with a mean of 9.71 University of Ghana http://ugspace.ug.edu.gh 186 mg/L. At the Twifo Praso meteorological stations Cl- levels ranged between 0.48 mg/L to 8.28 mg/L with a mean of 2.25 mg/L. Higher Cl- concentrations occur in rainfall at the Saltpond Meteorological station located in the coastal area suggesting sea aerosols may be contributing to Cl- in rainwater. Cl- in rain, over time may accumulate in the soil zone when evaporation takes place. This may lead to the formation of Cl- salts in the soil zone that has the potential of causing salinisation of groundwater.Figure 4.38 is a plot of the rainfall amount and weighted mean Cl- against the various months in which the rainwater was sampled at both stations. At the Saltpond station lower rainfall results in higher Cl- levels while higher rainfall leads to lower Cl- levels. Higher Cl- levels occur in the month of May with a value of 11 mg/L. At the peak of the raining season in June Cl- values reduce to 6 mg/L. As the rainfall amount decline from July to August, Cl- levels increase to 10 mg/L.At the Twifo Praso station weighted mean Cl- values are generally low varying between 2 mg/L in May to almost 0 mg/L in August with a lower amount of rainfall (Fig. 4.38). The lower Cl- levels in the Twifo Praso rains are as result of the fact that Twifo Praso is located further inland in the forest zone where heavier rainfall of about 2000 mm per annum is experienced compared to the coast where rainfall is about 1000 mm per annum. Heavier rainfall thus results in dilution of Cl- concentrations hence lower Cl- values. Rains generated from fresh water bodies do not have large quantities of Cl-. This also explains the lower concentrations of Cl- in Twifo Praso rains compared to that obtained from the coast. University of Ghana http://ugspace.ug.edu.gh 187 Fig. 4.38 Cl- concentrations in the various rainfall events between May and August 2010 at the Twifo Praso and Saltpond stations. Higher Cl- concentrations are observed from the Saltpond station. Other studies have reported, Cl- concentrations correlate with proximity to oceans rich in salt (Blackburn and Macleod, 1983; Hingston and Gailites 1976; Keywood et al., 1997; Root et al., 2004). Considerable Cl- concentrations ocurr in the rains from the coast capable of concentrating salts in the unsaturated soil zone as a result of evaporation to cause salinization of groundwater. University of Ghana http://ugspace.ug.edu.gh 188 4.4 .4 Chlorid e (Cl) level s in Soil Cl- in the unsaturated soil zone is one of the essential elements in plants, however, it is excluded to a large extent by roots during absorption of water and nutrients and its concentration rises as a function of evapotranspiration (Grimaldi et al., 2009). These results in Cl- build up in the soil zone that may be flushed to the groundwater zone to cause salinization of groundwater. Cl- in the soil zone as explained in section 4.4.3 may come from atmospheric deposition in infiltrating rainfall. Atmospheric deposition may include sea aerosol spray that may find its way into the soil zone through infiltrated rainfall. Studies of Cl- levels in the unsaturated soil zone may reveal information on Cl- concentrations that could be used to assess whether soluble salts or halites in the soil zone could contribute to groundwater salinity. Lax and Peterson (2009) explained that the variable Cl- concentration in shallow groundwater, suggests the unsaturated zone serves as a reservoir for Cl- which tend to be a long term source of Cl- to groundwater. Chebotarev (1955) summarized the chemical sequence of groundwater along flow path as: Chemical sequence along flow path HCO3 - →HCO3 - + SO4 2- → SO4 2- + HCO3 - → SO4 2- + Cl- →Cl- + SO4 2- → Cl- Increasing age According to Chebotarev (1955) groundwater tends to evolve chemically toward the composition of seawater as it moves from shallow zones of active flushing through University of Ghana http://ugspace.ug.edu.gh 189 intermediate zones to zones where the flow is very sluggish and the water is old. Cl- levels in the deep groundwaters in the area are high which can also be attributed to long distance of travel along flow path based on Chebotarev‘s sequence. Chemical analysis of the shallow groundwaters shows major water types as Na-Cl and non-dominant type (or mixed water types) suggesting halite (NaCl salt) dissolution in the shallow groundwaters. This prompted the analysis of Cl- in the soils of the unsaturated zone. Cl- concentration data from 4 soil profiles (Fig. 4.39) in the vicinity of Ekumfi Akwakrom towards Ekumfi Asokwa was used to assess the occurrence of Cl- in the unsaturated soil zone (Otoo, 2013). The Cl- profiles displayed large variations in concentration with low Cl- concentrations of 49.99 mg/kg occurring between 20 and 60 cm and higher Cl- concentrations varying between 99.97 to 449.86 occurring between 80 and 120 cm below the surface (Fig 4.39). All measured Cl- profiles are bulge shaped with decreasing Cl- concentrations below the bulge at depth. The upper parts of profiles 1, 3 and 4 reveal a constant Cl- value of 49.99 mg/kg indicating a period of non-deposition of Cl- or Cl- may have been leached to deeper zones of the profiles and eventually to the saturated zone (Fig. 4.39 a, c, d). In the case of profile 2 (Fig. 4.39b) minor peak or bulge occurs at the upper part of the profile with a value of 49.99 mg/kg. The major Cl- peak occurs at 140 cm with a value of 249.22 mg/kg.Cl- may have been accumulated in this zone over a period. The lower Cl- values below 140 cm indicate that Cl- is being dissolved and leached to deeper depths. Minor peaks are observed between 20 cm and 100 cm. Cl- peaks or bulges show periods of maximum Cl- accumulation in the unsaturated soil zone. University of Ghana http://ugspace.ug.edu.gh 190 It can be concluded thatNaClzones occur in lenses, hence the existence of salt crusts at different depths notably between 80 and 120 cm. The deposition is as a result of rainfall and sea aerosol spray. It therefore, follows that dissolution of these salts in the soil zone contributes to the high salinity in the groundwaters as revealed by results from both geochemical and stable isotopic considerations. Considerable differences in shape and concentrations of Cl- in the four (4) profiles in the coastal zone of the Central Region occur. This is a common observation made by various authors for unsaturated zone Cl- profiles in the semi-arid areas (Scanlon et al., 2006; Sharma and Hughes, 1985; Cook et al., 1989; Allison, 1988; Gaye, 1994; Gaye and Edmunds 1996; Edmunds et al., 1999). University of Ghana http://ugspace.ug.edu.gh 191 Fig. 4.39 Variation of Cl- (mg/kg) concentration in the unsaturated soil zone with depth (cm)showing zones of salt accumulation. University of Ghana http://ugspace.ug.edu.gh 192 CHAPTER FIVE GENERAL CONLUSIONS AND RECOMMENDATIONS 5.1 CONCLUSIONS  Anion chemistry of rainwater from the Saltpond station occurs in the order Cl-> SO4 2->NO3 - > PO4 3-> F-> Br-> NO2whilst that for the Twifo Praso meteo station is of the order Cl- > NO3 ->SO4 2-> PO4 3-> F- > Br- > NO2. Mean Cl- content of rainwater at the saltpond station is 9.71 mg/L and that for the Twifo Praso station is 3.12 mg/L. Higher Cl- content is observed at the Saltpond station closer to the coast suggesting that the ocean is a major contributor of Cl- in rainwater. Stable isotope composition of saltpond rainfall varies from -5.33 to 2.44 for δ18O ‰ V- SMOW with a mean of -1.47 ‰ V-SMOW. δ2H ‰ V-SMOW varies from -35.21 to 17.80 with a mean of -4.10. δ18O –δ2H plot reveal a slope of 6.49 and deuterium excess of 5.46 signifying evaporation of rain drops before recharge. δ18O and δ2H composition of Twifo Praso rainfall does not show significant difference to that observed from the Saltpond station. . δ18O –δ2H cross plot defines an equation with slope 6.15 and deuterium excess 3.44.  Generally extremely low physical and chemical parameters are observed for surface waters. This could be attributed to short contact time with rocks and probably less vigorous anthropogenic activities upstream. The major hydrochemical facies is NaCl. The dominant source of this facies in the surface waters is rainfall. Isotopic compositions suggest significant evaporation of the surface waters in the study area. University of Ghana http://ugspace.ug.edu.gh 193  The hydrochemistry of the shallow groundwaters in the area shows that the dominant cation is Na+ and the dominant anion is Cl-. The cations occur in the orderNa+> Ca2+> Mg2+> K+ and the anions in the order Cl-> HCO3 -> SO4 2-. The major hydrochemical facies is Na-Cl. Stable isotope composition of the shallow groundwaters exhibit evaporation defined by the regression Line: 𝛿2𝐻 = 2.42𝛿18𝑂 − 4.31.  Fifty – nine percent (59%) of the deep groundwaters have EC < 1500 and forty – one percent (41 %) have EC > 1500 μS/cm. On the basis of EC groundwater flow is from the inland areas (northern section of the area) towards the coast (southern section). Elevated EC in the study area could be attributed to the impervious nature of the rocks which results in longer contact with the rocksleading to slow groundwater movement thereby allowing more minerals to dissolve. Two boreholes CR2-50 at Ekumfi Asokwa and CR2-20 at Ekumfi Abor show super- saturation with respect to calcite and dolomite which may be due to secondary mineralisation due probably to hydrolysis of silicate minerals.The occurrence of major cations in the deep groundwaters is in the order Na+> Ca2+> Mg2+> K+ and that for the major anion is Cl-> HCO3 -> SO4 2-. The hydrochemical facies identified are Ca-Mg-HCO3, Na-Cl, CaCl2/Ca-Mg-SO4 and non-dominant water types. The depleted nature of the stable isotope composition of the deep groundwaters suggests a meteoric origin. The waters are subjected to evaporation defined by the regression line:𝛿2𝐻 = 3.66𝛿18𝑂 − 3.12. University of Ghana http://ugspace.ug.edu.gh 194  Very low NO3- concentrations ranging from 0.02 to 2.26 mg/L were observed in the area. Similarly F- and PO4 3- are quite low. This signifies good agricultural practices in the area and less abundant of some of these minerals (F- and PO4 3-) in the rocks of the area.  The mechanism of recharge to the groundwaters is by direct infiltration of local rainfall of mean isotopic composition δ18O = -3.8 ‰ V-SMOW and δ2H = -18 ‰ V-SMOW.  It is established through δ13C - Cl- relationship that the groundwaters may not be intruded by seawater. Estimation of seawater fraction assuming intrusion could not be possible with δ13C mass balance in this study. Probaly if δ13C of seawater is measured in the study area better results could be obtained. Quantification of seawater fraction using Cl- yielded ninety-two percent (92%) of samples within the range 0-6 % considered insignificant to cause salinization of groundwater and eight percent (8%) of samples within the range 11-25 % that may be considered significant to cause salinization of groundwater. The eight percent cover boreholes CR2-45 at Gomoa Abora, CR2-49 at Ekumfi Akwakrom, CR2-53 at Abonko a hand-dug well, CR2-54 at Anokye also a hand-dug well, CR4-FZ-16 at Abora Obokor, CR4-03 at Otuam and CR4-05 at Sefara Kokodo. University of Ghana http://ugspace.ug.edu.gh 195  Tritium in the groundwaters range from 0.05 ± 0.07 to 4.75 ± 0.16 TU.Eighty-five (%) of the samples analysed suggest modern recharge or young waters. Four groundwaters samples from boreholes CR4-05 at Sefara Kokodo, CR4-FZ-22 at Pomase, and CR4-FZ-06 at Etsisunkwa and CR2-50 at Ekumfi Asokwa showed very low tritium concentrations and considered old waters.  14C content of the groundwaters range between 9.50 pMC in borehole CR2-50 at Ekumfi Asokwa to 113.56 pMC in borehole CR3-26 at Onyaadze. Most of the waters are modern or young except borehole CR2-50 at Ekumfi Asokwa which has 14C activity of 9.5 pMC and therefore considered old. The lowest tritium value of 0.05 TU occurred in this same borehole thus confirming groundwater in this borehole to be old. The estimated ‗age‘ or residence time of this older water is 19, 459 years BP based on Akit‘s model.  Generally, on the basis of spatial plots of tritium and carbon-14, groundwater flow in the study area is more localised suggesting occurrence of discontinuous aquifer systems.  Groundwater salinization in the coastal zone of the Central Region may be caused by halite dissolution and to a minor extent silicate weathering. Halite may be concentrated in the soil zone by evaporation as a resultof deposition of sea aerosols on the earth surface. Na/Cl molar ratio (0.36 – 5.18), Br/Cl (0.0054 – University of Ghana http://ugspace.ug.edu.gh 196 2.08), SO4 2-/Cl- (0.02 – 4.09) and Ca/SO4 (0.35 – 10.84)suggest that seawater intrusion plays a minimal role in controlling the groundwater chemistry in the area. Stable isotope considerations proved that seawater intrusion is not the predominant process contributing to high salinity waters in the study area. Rather, dissolution of salts in the soil zone and minerals in rocks may be responsible for the salinity of groundwaters in the area.  Halite (NaCl) occurs at certain depths within the soil zone notably between 80 and 120 cm as revealed by Cl- profiles in the soil zone. The deposition is as a result of rainfall and sea aerosol spray. It therefore follows that dissolution of these salts in the soil zone contributes to the high salinity in the groundwaters as established by results from both geochemical and stable isotopic considerations. 5.2 RECOMMENDATIONS 1. Further studies involving the application of 32S and 34S, 11B and 86Sr/87Sr should be carried out to increase the understanding of the origin of salinity in the area and issues relating to water rock interaction. 2. High salinity wells should be selected for monitoring over a period to see if salinity increases or decreases with time and overall assessment of the hydrogeological conditions of the coastal zone. University of Ghana http://ugspace.ug.edu.gh 197 3. In order to establish the absolute ages of the young waters in the study area, tritium should be analysed together with its daughter nuclide helium (3He) to determine the initial tritium activity. This will enable one to be able to calculate the recharge rate of groundwater in the area. Newly developed techniques for age dating young waters such as Chlorofluorocarbons (CFC), Krypton and Argon should be considered in future age dating of groundwaters if the necessary funds are available. 4. The current database for hydrochemistry and groundwater age dating should be expanded to cover larger area of the coastal and inland zones to see if older waters occur. 5. 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University of Ghana http://ugspace.ug.edu.gh 225 Appendix 1 Availab le boreh ole data Community Depth (m) Weathered Depth(m) Yield (l/min) SWL (m) Aquifer Type Lithology Gomoa Abotsia (BH1) 28 16 133.00 11.69 confined Schist Gomoa Obiri (BH1) 30 6 10.00 2.55 confined Schist Ekumfi Gyabenkwaa (BH) 90 27 11.67 20 semi-confined Schist Ekumfi Esuehyia (BH) 32 10 40.00 8.98 confined Granite/Gneiss Gomoa Antseadze (BH) 30 18 11.67 6.78 semi-confined Schist Ekumfi Eyisam (BH2) 30 14 7.50 5.42 semi-confined Schist Ekumfi Engow (BH) 33 17 10.00 12.41 semi-confined Schist Edwansa Kokodo (BH1) 30 12 11.67 5.71 semi-confined Schist Edwansa Kokodo (BH2) 30 21 10.00 7.56 semi-confined Schist Obontser (BH1) 28 20 54.00 12.59 confined Granite/Gneiss Obontser (BH2) 34 24 19.00 9.14 confined Schist Gomoa Brebiano (BH) 30 0 11.67 4.82 semi-cofined Schist Ajumako Kyeibi (BH) 18 6 10.00 1.59 semi-cofined Schist Ajumako Afransie (BH1) 26 8 20.00 6.79 confined Granite/Gneiss Dwenase (BH) 21 9 22.00 0 confined Schist Enyan Abowinum (BH) 32 18 8.00 3.63 confined Schist Ekumfi Akwaakrom (BH) 36 20 10.00 14.89 semi-confined Schist Ekumfi Asokwa (BH) 94.5 0 18.00 15.5 confined Schist Ewoyaa (BH) 21 5 7.50 2.94 semi-confined Schist Afrangua (BH2) 36.6 15 36.00 3 confined Granite/Gneiss Goekrom (BH) 45 30 89.33 3.81 semi-confined Granite/Gneiss Edzimber (BH) 19 19 11.67 8.35 semi-confined Schist Buranamoa (BH) 44 18 40.00 9.1 confined Granite/Gneiss Brofoyedur (BH) 27 15 11.67 4.28 semi-confined Schist Dwendwenbadze 21 18 11.67 5.85 semi-confined Schist Effutuakwa 35 28 20.00 7.63 confined Schist Abeadze Abbankrom (BH1) 25 17 50.00 0 confined Dyke University of Ghana http://ugspace.ug.edu.gh 226 Appendix 1 Availab le boreh ole data ( Continu ed ) Community Depth (m) Weathered Depth(m) Yield (l/min) SWL (m) Aquifer Type Lithology Abeadze Abbankrom (BH2) 24 16 10.00 0 confined Granite/Gneiss Kyekyewowere 42 4 11.67 1.29 semi-cofined Granite/Gneiss Ayeldu 37 19 27.00 12.3 uncofined Granite/Gneiss Katakyiase (BH1) 63 18 96.67 4.94 semi-confined Gneiss/Granite contact Katakyiase (BH2) 42 4 179.17 4.05 semi-confined Granite/Schist contact Abeadze kwakrom 36 2 35.00 0.19 semi-cofined Schist Etsefawomanye 26 10 54.00 1.32 confined Schist Old Odonase 47 28 12.00 0.03 confined Granite/Gneiss Obohen 27 11 23.00 3.2 confined Granite/Gneiss Empirow 30 20 12.00 0.32 confined Granite/Gneiss Assin Manso 32 19 33.00 6.01 confined Granite/Gneiss Otuam (BH1) 31 1 60.50 2.85 confined Granite/Gneiss Otuam (BH2) 31 6 15.00 2.21 confined Granite/Gneiss Nanaben 31 7 10.00 3.39 confined Granite/Gneiss University of Ghana http://ugspace.ug.edu.gh 227 Appendix 2 Hydroch emical paramete rs of the Central Region Community Water Point Altitude (m) Sample Name Sampling Date EC (μS/cm) Temp (oC) pH TDS (mg/L) Eh (mV) Salinity (mg/L) Kweikrom BH 46 CR2-01 12/11/2010 1311 28.8 6.53 660 36.6 700 Kweikrom HD 52 CR2-01HD 12/11/2010 769 28.1 6.83 387 25.4 400 Gomoa Abotsia BH 35 CR2-04 12/12/2010 1277 29.7 6.68 634 33.8 600 Gomoa Obiri BH 28 CR2-06 12/12/2010 627 30.4 6.38 313 50.2 300 Gomoa Asaman BH 47 CR2-08 12/12/2010 2350 29.7 6.18 1179 60.6 1200 Ekumfi Gyabenkwaa BH 24 CR2-09 12/12/2010 3570 28.9 6.54 1795 34.7 1900 Ekumfi Eduagyir BH 25 CR2-12 12/13/2010 2860 28 6.3 1429 50.1 1500 Ekumfi Esuehyia BH 28 CR2-14 12/12/2010 1472 29.6 6.64 737 36.2 700 Gomoa Antseadze BH 24 CR2-15 12/12/2010 1361 30 6.63 683 39.1 700 Ekumfi Ekotsi SW1 8 CR2-16 12/13/2010 179.3 26.9 7.1 89.7 -3.4 100 Ekumfi Eyisam BH 20 CR2-17 12/13/2010 1864 30.3 6.53 930 41.6 900 Ekumfi Eyisam BH 24 CR2-18 12/13/2010 3390 29.6 6.63 1698 34.4 1800 Ekumfi Abor BH 27 CR2-20 12/13/2010 1757 28.2 7.09 883 10.3 900 Ekumfi Abor BH 37 CR2-21 12/15/2010 2140 28.5 6.82 1671 25.5 1100 Ekumfi Techiman BH 24 CR2-22 12/13/2010 3530 28.7 6.25 1783 56.7 1900 Ekumfi Engow BH 34 CR2-23 12/13/2010 4870 28.2 6.25 2460 56.6 2600 Edwansa Kokodo BH 41 CR2-24 12/13/2010 2690 28.7 6.17 1353 61.1 1400 Edwansa Kokodo BH 39 CR2-25 12/13/2010 1025 29.5 5.93 513 73.9 500 Onyaadze BH 38 CR2-26 12/13/2010 1661 29.1 6.66 831 33.6 800 Obontser BH 35 CR2-27 12/13/2010 1203 28 6.39 600 47 600 Obontser BH 45 CR2-28 12/13/2010 1397 28.6 5.32 708 107.4 700 Esakyir SW1 16 CR2-29 12/14/2010 186.1 25.9 7.22 93 5.4 100 Ata Kwaa BH 36 CR2-30 12/14/2010 972 27.8 6.62 486 37.1 500 Otabanadze BH 56 CR2-31 12/14/2010 2530 29 6.29 1267 52.6 1300 Gomoa Brebiano BH 41 CR2-32 12/14/2010 1038 29.1 5.89 522 77.9 500 Ajumako Ansa BH 37 CR2-33 12/14/2010 1255 27.7 6.33 636 52.6 600 University of Ghana http://ugspace.ug.edu.gh 228 Appendix 2 Hydroch emical paramete rs of the Central Region (Contin u ed ) Community Water Point Altitude (m) Sample Name Sampling Date EC (μS/cm) Temp (oC) pH TDS (mg/L) Eh (mV) Salinity (mg/L) Gomoa Sampa BH 33 CR2-35 12/14/2010 434 28 6.27 218 56.5 200 Ajumako Abeadze SW1 30 CR2-37 12/15/2010 146.7 26.9 7.09 73.3 2.9 100 Ajumako Esikado SW1 35 CR2-38 12/15/2010 143.2 27.8 7.55 71.8 -12.7 100 Ajumako Kyeibi* BH 59 CR2-40 12/14/2010 678 26.7 6.47 340 44.7 300 Ajumako Afransie BH 81 CR2-41 12/14/2010 266 28.5 5.74 133.5 85.3 100 Dwenase BH 75 CR2-43 12/14/2010 382 26.6 6.19 192.1 59.6 200 Enyan Abowinum BH 69 CR2-44 12/15/2010 466 28.3 5.71 234 85.2 200 Gomoa Abora BH 32 CR2-45 12/12/2010 29000 29.2 6.46 14420 45.3 17700 Kyeibi* BH 68 CR2-46 12/14/2010 712 26.8 5.9 356 80 300 Baffikrom BH 14 CR2-48 12/15/2010 135.5 28.6 7.49 67.9 -9.1 100 Ekumfi Akwaakrom BH 23 CR2-49 12/16/2010 17000 29.5 6.15 8490 55.3 9900 Ekumfi Asokwa BH 12 CR2-50 12/16/2010 1257 30 7.84 629 -36.6 600 Ekumfi Swedru BH 18 CR2-51 12/16/2010 4230 30.9 6.62 212 29.5 2200 Mankessim SW2 13 CR2-52 12/16/2010 138.7 27.2 7.6 69.6 -17.4 100 Abonko HD 12 CR2-53 12/16/2010 5050 30.5 6.93 2530 12.2 2700 Anokye HD 18 CR2-54 12/16/2010 6050 30.8 6.9 3030 14.7 3300 Ewoyaa BH 30 CR2-55 12/16/2010 1419 28.7 6.69 712 26.2 700 Afrangua BH 73 CR2-57 12/16/2010 402 30.4 6.45 201 39 200 Goekrom BH 47 CR2-58 12/16/2010 2900 28.8 6.47 1452 37.9 1500 Edzimber BH 29 CR2-59 12/17/2010 1915 29.9 6.41 961 41.2 1000 Buranamoa BH 33 CR2-60 12/17/2010 1024 27.9 6.3 514 47.2 500 Nsanfo BH 45 CR2-61 12/17/2010 1800 29 6.36 960 43.9 900 Brofoyedur BH 37 CR2-62 12/17/2010 3300 27.9 6.57 1649 32.8 1700 Dwendwenbadze BH 51 CR2-63 12/17/2010 1481 27.9 6.4 742 41.7 700 Ekumfi Asaafa HD 10 CR4-01 9/15/2012 1682 29 9.3 841 -104.1 800 Ekumfi Asaafa HD 9 CR4-02 9/15/2012 1695 28 8.35 848 -54.7 900 University of Ghana http://ugspace.ug.edu.gh 229 Appendix 2 Hydroch emical paramete rs of the Central Region (Contin u ed ) Community Water Point Altitude (m) Sample Name Sampling Date EC (μS/cm) Temp (oC) pH TDS (mg/L) Eh (mV) Salinity (mg/L) Effutuakwa BH 44 CR4-FZ-01 9/19/2012 1451 27.6 6.39 726 36.2 700 Nkusukumopem BH 55 CR4-FZ-02 9/19/2012 2000 28.1 6.8 1002 11.9 1000 Duadze HD 72 CR4-FZ-03 9/19/2012 1256 26.9 5.2 628 104.5 600 Abeadze Abbankrom BH 47 CR4-FZ-04 9/19/2012 387 28.5 5.95 193.7 62.8 200 Abeadze Abbankrom BH 48 CR4-FZ-05 9/19/2012 436 28.7 5.95 218 62.9 200 Etsisunkwa BH 39 CR4-FZ-06 9/19/2012 2070 27.8 6.32 1033 40.7 1100 Kyekyewowere BH 65 CR4-FZ-07 9/19/2012 633 27.9 6.84 316 9.8 300 Ayeldu BH 91 CR4-FZ-08 9/19/2012 181.7 27.5 5.88 90.9 66.9 100 Katakyiase BH 111 CR4-FZ-09 9/19/2012 599 28 6.25 299 44.6 300 Katakyiase BH 107 CR4-FZ-10 9/19/2012 379 26.8 6.37 189.5 37.2 200 Katakyiase BH 103 CR4-FZ-11 9/19/2012 347 27.2 6.56 173.6 26.4 200 Abeadze kwakrom BH 77 CR4-FZ-12 9/19/2012 427 26.6 6.62 213 22.9 200 Etsefawomanye BH 74 CR4-FZ-13 9/19/2012 330 26.5 6.49 165.1 30.3 200 Abora Dunkwa HD 109 CR4-FZ-14 9/19/2012 540 28.1 6.32 270 40.6 300 Old Odonase BH 103 CR4-FZ-15 9/19/2012 387 26.6 6.2 193.6 47.8 200 Abora Obokor BH 38 CR4-FZ-16 9/20/2012 6250 27.7 6.07 3120 55.6 3400 Abora Kwamankese BH 57 CR4-FZ-17 9/20/2012 882 26.6 6.31 441 41.2 400 Mpesiaduadze BH 39 CR4-FZ-18 9/20/2012 1079 28.7 6.41 538 35.2 500 Abora BH 74 CR4-FZ-19 9/20/2012 455 27.2 5.39 227 96 200 Akwantia Kokodo BH 69 CR4-FZ-20 9/20/2012 1533 27.4 5.46 766 92.1 800 Abora Bando HD 80 CR4-FZ-21 9/20/2012 685 27.2 6.27 342 43.5 300 Pomase BH 63 CR4-FZ-22 9/20/2012 2001 28.3 6.28 1004 43.2 1000 Obohen BH 78 CR4-FZ-23 9/20/2012 483 27.8 6.43 242 33.8 200 Aborabuase BH 80 CR4-FZ-24 9/20/2012 283 27 6.18 141.3 48.9 100 Empirow BH 62 CR4-FZ-25 9/20/2012 473 27.5 6.18 237 49.1 200 Assin Manso BH 106 CR4-FZ-26 9/20/2012 120.6 26.6 6.43 60.3 34.2 100 University of Ghana http://ugspace.ug.edu.gh 230 Appendix 2 Hydroch emical paramete rs of the Central Region (Contin u ed ) Community Water Point Altitude (m) Sample Name Sampling Date EC (μS/cm) Temp (oC) pH TDS (mg/L) Eh (mV) Salinity (mg/L) Otuam BH CR4-03 9/10/2012 913 23.4 6.89 457 12.2 400 Otuam BH CR4-04 9/10/2012 9460 23.4 6.69 4730 23.4 5300 Sefara KoKodo BH 25 CR4-05 9/10/2012 3780 23.3 6.39 1888 40.5 2000 Sefara KoKodo BH 17 CR4-06 9/10/2012 10790 23.2 6.76 5400 19.9 6100 Adansemanmu BH 36 CR4-07 9/10/2012 2140 23.2 6.8 1074 17.6 1100 Nanaben BH 22 CR4-08 9/10/2012 3005 23.3 6.62 1526 23.3 1600 BH: Borehole, HD: Hand dug well, SW1: Surface water (Ochi Nakwa), SW2: Surface water (Ochi Amissah) University of Ghana http://ugspace.ug.edu.gh 231 APPENDIX 2 Hydrochemical parameters of the Central Region (Continued) Community Water Point Ca2+ /mg/L Na+ /mg/L Mg2+/ mg/L K+/ mg/L Cl- /mg/L SO4 2- /mg/L HCO3 - /mg/L NO3 - /mg/L F- /mg/L PO4 3- /mg/L Br/Cl Fsea % CBE Kweikrom BH 126.40 370.00 6.72 6.50 533.96 131.33 128.10 0.32 0.59 0.01 0.006 3 8 Kweikrom HD 83.60 73.90 10.56 74.80 105.00 92.33 268.40 0.81 0.29 0.02 0.012 0 4 Gomoa Abotsia BH 86.40 301.00 15.36 10.20 475.95 116.83 140.30 0.19 0.2 0.15 0.050 2 2 Gomoa Obiri BH 67.20 117.60 11.52 6.90 163.99 85.83 122.00 0.27 0.32 0.00 0.144 1 7 Gomoa Asaman BH 213.20 467.00 28.40 36.00 749.75 145.83 217.95 0.44 0.39 0.08 0.052 4 6 Ekumfi Gyabenkwaa BH 262.60 521.00 17.28 35.50 833.99 198.67 237.90 0.43 0.49 0.03 0.025 4 10 Ekumfi Eduagyir BH 108.80 472.00 15.36 14.20 799.75 157.17 115.90 0.23 0.43 0.01 0.153 4 0 Ekumfi Esuehyia BH 146.40 333.00 32.64 18.30 611.95 102.17 253.15 0.55 0.14 0.05 0.119 3 3 Gomoa Antseadze BH 124.00 350.00 15.36 11.40 513.97 122.83 158.60 0.51 0.34 0.05 0.203 3 8 Ekumfi Ekotsi SW1 6.40 54.90 5.76 8.60 53.99 26.83 57.95 0.40 0.31 0.01 0.228 0 5 Ekumfi Eyisam BH 171.20 391.00 20.16 13.90 639.93 117.50 164.70 0.70 0.54 0.15 0.101 3 9 Ekumfi Eyisam BH 251.20 495.00 21.68 16.50 763.99 214.50 320.10 0.53 0.50 0.10 0.110 4 8 Ekumfi Abor BH 129.00 406.00 20.16 15.40 599.94 122.50 390.40 0.51 0.34 0.05 0.106 3 0 Ekumfi Abor BH 176.00 346.00 22.16 22.00 643.99 131.67 240.30 0.49 0.71 0.02 0.056 3 3 Ekumfi Techiman BH 233.60 519.00 37.92 24.70 821.99 168.83 252.50 0.67 0.61 0.07 0.005 4 7 Ekumfi Engow BH 312.80 676.00 79.12 25.10 1209.75 221.17 364.70 0.49 0.27 0.14 0.010 6 8 Edwansa Kokodo BH 132.20 489.00 53.76 21.20 769.75 143.00 228.10 0.61 0.19 0.03 0.087 4 7 Edwansa Kokodo BH 116.80 266.00 16.32 14.40 475.95 106.33 120.15 0.69 0.34 0.08 0.029 2 4 Onyaadze BH 121.20 423.00 24.56 22.20 478.94 128.67 341.60 0.54 0.37 0.02 0.059 2 8 Obontser BH 108.80 327.00 32.64 20.50 625.96 129.00 219.60 0.80 0.12 0.09 0.281 3 -3 Obontser BH 143.20 265.00 23.04 8.10 459.92 153.67 9.15 0.85 0.07 0.07 0.081 2 9 University of Ghana http://ugspace.ug.edu.gh 232 APPENDIX 2 Hydrochemical parameters of the Central Region (Continued) Community Water Point Ca2+ /mg/L Na+ /mg/L Mg2+/ mg/L K+/ mg/L Cl- /mg/L SO4 2- /mg/L HCO3 - /mg/L NO3 - /mg/L F- /mg/L PO4 3- /mg/L Br/Cl Fsea % CBE Esakyir SW1 12.60 44.60 3.84 6.50 45.99 29.17 67.10 0.05 0.21 0.04 0.000 0 1 Ata Kwaa BH 86.40 261.50 15.36 23.50 389.97 102.17 210.45 0.05 0.35 0.03 0.137 2 3 Otabanadze BH 92.80 427.00 58.72 22.40 649.75 148.00 115.90 0.57 0.22 0.09 0.102 3 10 Gomoa Brebiano BH 118.40 173.00 13.44 101.90 435.96 98.33 45.75 0.58 0.18 0.00 0.503 2 6 Ajumako Ansa BH 103.20 299.50 24.72 18.50 417.93 119.50 158.60 0.44 0.00 0.10 0.077 2 10 Gomoa Sampa BH 18.20 78.80 8.52 9.10 79.98 23.00 76.25 0.52 0.10 0.06 0.319 0 10 Ajumako Abeadze SW1 22.40 39.20 4.84 6.70 19.99 89.83 59.78 0.72 0.49 0.14 0.144 0 -1 Ajumako Esikado SW1 20.80 36.40 4.93 6.40 17.99 87.00 61.00 0.56 0.12 0.01 2.075 0 -3 Ajumako Kyeibi BH 36.80 159.00 11.52 7.90 133.26 86.33 116.75 0.28 0.06 0.05 0.122 1 10 Ajumako Afransie BH 22.40 36.90 7.92 9.20 64.00 42.33 18.30 0.79 0.79 0.01 1.533 0 8 Dwenase BH 22.40 84.20 5.76 8.80 64.98 67.50 79.30 0.45 0.03 0.04 0.018 0 9 Enyan Abowinum BH 14.40 121.50 2.88 9.40 95.97 86.17 21.35 0.47 0.08 0.03 0.243 0 8 Gomoa Abora BH 214.40 2769.50 55.68 30.40 3599.25 298.17 268.40 0.42 0.42 0.00 0.013 19 10 Kyeibi BH 62.20 67.90 8.08 13.10 97.98 98.33 36.60 0.48 0.25 0.00 - 0 7 Baffikrom BH 11.20 38.60 4.80 8.20 19.99 62.83 48.80 0.62 0.76 0.04 - 0 2 Ekumfi Akwaakrom BH 393.00 1845.00 204.00 55.50 4798.50 158.00 231.80 0.64 0.16 0.07 - 25 -9 Ekumfi Asokwa BH 64.80 320.50 19.20 6.00 255.92 210.50 308.05 0.90 0.37 0.05 - 1 6 Ekumfi Swedru BH 116.20 623.50 86.40 46.10 949.98 97.83 280.60 0.56 0.12 0.36 - 5 10 Mankessim SW2 12.80 53.70 3.84 7.50 16.00 88.50 57.95 0.59 0.76 0.02 - - 2 Abonko HD 183.20 1429.50 41.28 7.90 2699.50 89.33 213.50 0.37 0.23 0.02 - 14 -4 Anokye HD 36.80 1000.50 168.00 468.50 2749.50 184.67 277.53 0.63 0.46 0.01 - 14 -10 Ewoyaa BH 68.00 272.50 33.60 17.60 400.00 127.00 152.50 0.61 0.39 0.07 - 2 6 University of Ghana http://ugspace.ug.edu.gh 233 APPENDIX 2 Hydrochemical parameters of the Central Region (Continued) Community Water Point Ca2+ /mg/L Na+ /mg/L Mg2+/ mg/L K+/ mg/L Cl- /mg/L SO4 2- /mg/L HCO3 - /mg/L NO3 - /mg/L F- /mg/L PO4 3- /mg/L Br/Cl Fsea % CBE Afrangua BH 52.80 72.50 14.40 3.70 56.99 111.83 97.60 0.67 0.42 0.51 - 0 8 Goekrom BH 106.80 481.50 52.80 53.70 699.75 83.67 305.00 0.44 0.08 0.03 - 4 10 Edzimber BH 94.80 295.50 30.72 13.70 549.75 63.67 149.45 0.46 0.35 0.04 - 3 3 Buranamoa BH 92.20 211.50 11.20 12.10 277.95 78.89 128.10 0.75 - - - 1 7 Nsanfo BH 115.60 254.50 29.52 62.30 447.92 84.23 219.60 0.15 - - - 2 8 Brofoyedur BH 163.20 572.50 47.52 35.00 899.75 125.32 228.75 0.46 - - - 5 9 Dwendwenbadze BH 72.40 355.50 23.04 11.30 397.94 129.00 262.50 0.65 - - - 2 8 Ekumfi Asaafa HD 62.33 165.56 30.88 105.61 266.76 116.56 115.9 0.26 - - - 1 14 Ekumfi Asaafa HD 52.64 191.18 31.4 56.7 326.5 117.04 36.6 0.59 - - - 2 10 Effutuakwa BH 80.52 127.28 51.62 12 297.59 126.16 109.8 0.26 - - - 1 5 Nkusukumopem BH 64.27 226.39 75.69 10.93 393.02 82.11 219.6 1.18 - - - 2 9 Duadze HD 9.34 143.835 43.95 12.565 315.465 20.5 12.2 0.15 - - - 2 6 Abeadze Abbankrom BH 23.06 49.24 13.38 4.685 92.845 22.61 54.9 0.61 - - - 0 6 Etsisunkwa BH 139.04 135.5 100.64 15.06 588.8 91.88 176.9 0.49 - - - 3 1 Kyekyewowere BH 59.475 46.545 19.55 5.535 73.76 47.36 140.3 0.04 - - - 0 2 Ayeldu BH 10.868 22.154 4.23 5.74 30.574 15.502 42.7 0.11 - - - - 3 Katakyiase BH 44.24 44.075 17.6 8.04 88.72 34.01 91.5 0.19 - - - 0 10 Katakyiase BH 34.09 29.25 15.105 9.55 59.845 24.835 110 0.10 - - - 0 6 Katakyiase BH 23.768 35.16 11.512 7.432 46.98 30.978 128.1 2.26 - - - 0 -3 Abeadze kwakrom BH 41.574 26.958 10.46 4.48 52.36 15.28 128.1 1.26 - - - 0 -1 Etsefawomanye BH 33.68 24.612 7.222 4.928 35.54 7.46 122 1.36 - - - 0 5 Abora Dunkwa HD 30.595 26.4 8.33 13.5 39.38 48.43 73.2 0.39 - - - 0 6 University of Ghana http://ugspace.ug.edu.gh 234 APPENDIX 2 Hydrochemical parameters of the Central Region (Continuation) Community Water Point Ca2+ /mg/L Na+ /mg/L Mg2+/ mg/L K+/ mg/L Cl- /mg/L SO4 2- /mg/L HCO3 - /mg/L NO3 - /mg/L F- /mg/L PO4 3- /mg/L Br/Cl Fsea % CBE Abora Obokor BH 345.75 626.95 291.9 43.75 2064.1 675.75 100 1.43 - - - 11 -3 Abora Kwamankese BH 58.915 65.485 27.92 7.5 159.26 30 115.9 0.41 - - - 1 2 Akwantia Kokodo BH 56.8 177.61 35.47 7.63 382.38 29.07 48.8 0.16 - - - 2 6 Abora Bando HD 48.665 78.745 22.95 4.445 125.33 35.095 115.9 0.74 - - - 1 12 Pomase BH 109.73 192.72 48.87 10.42 464.59 89.62 158.6 0.61 - - - 2 2 Obohen BH 36.31 40.105 12.325 5.915 63.92 32.855 109.8 0.03 - - - 0 5 Aborabuase BH 12.328 32.14 5.196 9.418 41.356 21.298 48.8 0.02 - - - 0 5 Empirow BH 16.795 74.525 9.185 5.91 84.905 71.505 48.8 0.23 - - - 0 3 Assin Manso BH 41.275 79.305 20.575 8.62 112.83 45.93 122 0.41 - - - 1 10 Otuam BH 38.495 105.485 26.025 10.365 179.07 78.555 115.9 0.18 - - - 1 2 Otuam BH 197.65 1362.5 329.2 5.65 2437.2 751.55 780.8 0.35 - - - 13 0 Sefara KoKodo BH 105.36 515.96 73.26 6.54 978.46 223.04 207.4 0.56 - - - 5 -2 Sefara KoKodo BH 456.7 1349.7 416.7 33.5 3605.7 544 201.3 0.13 - - - 19 0 Adansemanmu BH 117.77 219.13 59.02 86.46 445.4 203.12 408.7 0.18 - - - 2 -2 Nanaben BH 143.36 342.28 117.4 11.44 623.88 331.3 189.1 1.14 - - - 3 8 BH: Borehole, HD: Hand dug well, SW1: Surface water (Ochi Nakwa), SW2: Surface water (Ochi Amissah), Fsea: Seaw water fraction, CBE: charge balance error University of Ghana http://ugspace.ug.edu.gh 235 Appendix 3 Theory and Princip les of operation of Equip men ts Employ ed in this stud y Atomic Absorption Spectrometry The technique of atomic absorption spectroscopy (AAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 oC.During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths. The characteristic wavelengths are element specific and accurate to 0.01-0.1nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device called the photomultiplier detects the amount of reduction of the light intensity due to absorption by the analyte, and this is directly related to the amount of the element in the sample. The technique relies heavily on the Beer-Lambert‘s law. The law states that there is a logarithmic dependence between the transmissivity T, of light through a substance and the product of the absorption coefficient of the substance, α, and the distance the light travels through the material (i.e. the path length l). The absorption coefficient can, in turn, be written as a product of either a molar absorptivity of the absorber, ε, and the concentration c of absorbing species in the material, or an absorption cross section, ζ, and the (number) density N of absorbers. University of Ghana http://ugspace.ug.edu.gh 236 For liquids, these relations are usually written as: T = I Io = 10−α l = 10−εlc (1) where Io and I are the intensity or power of the incident light and that after the material respectively. The transmission (or transmissivity) is expressed in terms of an absorbance (A) which for liquid is defined as Α = − log10( Ι Ιo ) (2) This implies that the absorbance becomes linear with the concentration (or number density of absorbers) according to Α = 𝜀𝑙𝑐 = 𝛼𝑙 (3) From the measurement of the absorbances of various standards, a calibration curve showing linear relationship was drawn (Fig. A). The curve is a plot of A against C. The slope of the curve is ε. From the known value of l and ε the concentration C of the element of interest was obtained as C = A εl (4) University of Ghana http://ugspace.ug.edu.gh 237 Fig. (A) Absorbance versus concentration of (a) Mg standard and (b) Ca standard for calibration of equipment and to estimate 𝜀 the slope for determination of the concentration of Mg and Ca ions in the various water samples Flame Photometry (FP) The flame photometer operates under similar principle as the atomic absorption spectrometer except that there is no light source.It measures the wavelength and intensity of light emitted by atoms in a flame resulting from the drop from the excited state (formed due to absorption of energy from the flame) to lower states. No light source is required since the energy imparted to the atoms comes from the flame. The sample is introduced to the flame at constant rate. Filters select which colours the photometer detects and exclude the influence of other ions. The equipment was first calibrated with series of standards before determination of unknown y = 1.3292x R² = 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.5 1 A bsorbance (A ) Concen tration (C) y = 0.2521x R² = 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 2 4 A bsorbance (A ) Concen tration (C) a b University of Ghana http://ugspace.ug.edu.gh 238 concentration of elements in the sample. The concentration was determined from equation (4). The flame photometer is generally used in determining Na+, Li+, Ca2+ and K+. Ion Chromatograph Ion chromatography incorporates a mobile phase and stationary phase. The mobile phase is usually water and some buffer mixture. The stationary phase is the column which contains an active resin. The sample to be analysed is injected onto the column. The sample is pushed through the column by the force of the constant flow of the mobile phase. As the injected samples get into contact with the column, the dissolved ions in the samples develop an affinity for the column and replace less retained ions like those which make up the buffer. This exchange process occur continuously, however, the length of time various ions retain themselves on the column delays their travel through the column. Each ion has a different affinity for the column; hence some ions spend less time while others spend more time in the mobile phase. The fact that each ion has a different residence time in the mobile phase allows for its separation. Eventually, each ion come out of the column and detected by the conductivity detector. The result is a peak and the area under each peak, represents the relative amount of each ion. Computer running chromatography software connected to the equipment automatically converts each peak in a chromatogram to a sample concentration and produces a tabulated printout of the results. Liquid Water Isotope Analyser (LWIA) The instrument contains the laser analysis system and an internal computer, a CTC LC-PAL liquid autosampler, a small membrane vacuum pump, and a room air intake line that passes air University of Ghana http://ugspace.ug.edu.gh 239 through a Drierite column for moisture removal. The autosampler and theDLT-100 are connected by a ~1 m long polytetrafluoroethylene(PTFE) transfer line. A Hamilton microlitre syringe(model 7701.2N) was used to inject 0.75 litre of sample through a PTFE septum in theautosampler. The injection port of the autosampler was heated to 80°C to help vaporize thesample under vacuum immediately upon injection. The vapor then travels down the transferline into the pre-evacuated mirrored chamber for analysis.The instrument has a precision of approximately 1‰ for δ2H and 0.2‰ for δ18O. A procedure for analysis specific to the Los Gatos instrument was adopted. A 1-mL aliquot of samples and standards were pipetted into 1.5-mL autosampler glass vials and closed with PTFEseptum caps, leaving some amount of head space in the vials. The Samples and standard vials were arranged in the autosampler tray in the order shown in Fig. (B). In this procedure, a dummy sample (de-ionized water or tap water) was placed in the first position to prime the flow line. The instrument is prone to give erratic results during the start of a run and the dummy sample allows the system to stabilize before the first standards are run. The dummy vial was followed by three secondary standards (two calibration standards and a control standard), five unknowns (samples), and then another set of standards. This array of standards and unknowns was repeated up to a maximum of six times (30 unknowns) for a single run. Each standard or unknown was individually measured with six injections. Measurements and known δ values (with respect to VSMOW) of calibration standards before and after each batch of five unknowns were used for a linear regression to convert absolute isotope ratios to δ values. University of Ghana http://ugspace.ug.edu.gh 240 Fig. (B) Sample positions in the autosampler tray for a three-standard arrangement. D is the dummy sample (position 1), the yellow, purple, and green spots are for standards.DI is for post- run cleaning of the syringe. Liquid Scintillation Counter (LSC) for Trituim Determination Measurement of tritium activity in water samples consist of two major parts. These parts are: 1. pre-concentration of tritium in the analysed water sample using electrolytic enrichment and 2. detection of tritium activity in the concentrated sample using liquid scintillation counter. University of Ghana http://ugspace.ug.edu.gh 241 The first step in pre-concentrating tritium in the sampled groundwater, using electrolytic enrichment is distillation. The water samples were distilled to a conductivity less than or equal to 25µS/cm. The samples were distilled to remove impurities which may cause corrosion of the electrodes for the electrolytic process. Two grams (2g) of peroxide was added to the water samples in 500 ml standard glass flask. This was then transfered into an electrolytic cell and weighed. The mass of the empty cell was first weighed as Wce and cell plus water as Wcf. The initial mass of water was obtanied from the formular: 𝑊𝑖 =𝑊𝑐𝑓 −𝑊𝑐𝑒 (5) The weighed samples in the cells were subjected to electrolysis. The reactions taking place at the various eletrodes are: anode : 2OH- H2O + 2e - + 1/2O2 (6) cathode: 2H2O + 2e - 2OH- + H2 (7) overall reaction : 2H2O 2H2 + O2 (8) After electrolysis the cells were weighed with the mass of cell plus water represented as Wcf and final mass of water as Wf. Wf can therefore be expressed as: 𝑊𝑓 =𝑊𝑐𝑓 −𝑊𝑐𝑒 (9) The samples were then transfered from the cell into a distillation flask and neutralised with 8g of PbCl2 and subjected to distillation (final distillation). The samples were mixed with a cocktail University of Ghana http://ugspace.ug.edu.gh 242 and poured into polyethylene vials. The vials together with two standards and two dead waters were placed in the Liquid Scintililation Counter (LSC) for tritium activity measurement. The net count rate of the samples (NSA) and that of the standards (NST) were determined. After measuring the count rates the actual enrichment parameter for sample cell P, the enrichment factor for each sample Zi and the decay correction factor D were calculated. The electrolysis system was calibrated by processing three ―Spike‖ samples. The resulting enrichment parameter P was established by enriching spike water, generally in 3 cells. The enrichment parameter (P-Factor) was calculated through the following equation (10): )ln( )ln( 975.2/ )( f i fi W W E Q WW P   (10) Wi, Wf initial/ final mass of water. Q : total charge in Ah for the run.2.97545 charge to electrolyse 1g water. E : enrichment factor for tritium. Pavg : P - average of 3 spike values From P' = P*Q/2.97545 follows P'avg = Pavg*Q/2.97545 𝐸 = exp⁡( 𝑃′𝑎𝑣𝑔 𝑊𝑖−𝑊𝑓 )𝑙𝑛( 𝑊𝑖 𝑊𝑓 ) (11) The tritium activity concentration (AT) in an analysed sample wasobtained from (equation 4.18): 𝐴𝑇 = 𝑁𝑆𝐴 .𝐴𝑆𝑇 𝑁𝑆𝑇 .𝑍𝐼 𝐷 (12) University of Ghana http://ugspace.ug.edu.gh 243 NSA - net count rate of the sample (cpm) NST - net count rate of the standard (cpm) AST - activity concentration of the standard (Bq/Kg) ZI - tritium enrichment factor for the given sample D - Factor taking into account decay of tritium in the sample from the date of measurement to the date of sampling Radiocarbon (14C) Measurement The SrCO3 precipitates obtained in the field and discussed in section 4.1.2.3 were decomposed to CO2 using HCl (water solution 1:1) in a glass flaskas illustrated in equation (13). The CO2 produced was removed by nitrogen, dried and frozen. SrCO3 + 2HCl → SrCl2 + CO2 (g)↑ + H2O (13) A small portion of Cleaned CO2 was separated for 13C determination, and the rest used for benzene (C6H6)synthesis. Benzene was synthesized as follows: i) In a vacuum vessel at high temperature (750 oC), the CO2 generated was passed over Li metal resulting in the formation of Lithium Carbide (Li2C2); 2CO2 + 10Li → Li2C2 + 4Li2O (14) ii) The lithium carbide was then hydrolysedto ethyne (C2H2) and hydrogen generation; University of Ghana http://ugspace.ug.edu.gh 244 Li2C2 + 2H2O → C2H2 + 2LiOH (15) iii) From this mixture cryogeniclyC2H2 was separatedfrom H2 and dried; iv) C2H2 was trimerized to C6H6 directly in catalyst. Benzene was mixed with scintillation cocktail and measured in Liquid Scintillation Counter. The uncertainty of radiocarbon determination was 0.7 pMC (one sigma) and that of 13C determination was equal 0.08 ‰ (one sigma). University of Ghana http://ugspace.ug.edu.gh