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Item Hydrogeochemical and Isotopic Studies of Groundwater in Coastal Aquifers of Ghana: Case Study in the Central Region(University of Ghana, 2015-03) Ganyaglo, S.Y.; Akiti, T.T.; Osae, S.K.; Armah, T.; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of Earth ScienceThe 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 occurred in the orderNa+> Ca2+> Mg2+> K+ and the anions in the order Cl-> HCO3 -> SO42-. 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 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.