Department of Earth Science

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    Site-Specific Soil Classification Of Seismic Site Effects For The Greater Accra Metropolitan Area Of Ghana Using Multichannel Analysis Of Surface Waves
    (University Of Ghana, 2021-12) Nortey, G.
    The study presents site classification for Greater Accra Metropolitan Area (GAMA) of Ghana. Multichannel analysis of surface waves (MASW) is used to calculate shear wave velocity to various depths within the study area. Calculated shear wave velocity up to a depth of 30 m has been used to assess seismic site effects and soil conditions for a major part of the GAMA. Seismic risk has become overwhelmingly apparent from many studies carried out over this area. The area is overpopulated, poorly planned, with the largest convergence of major and minor faults, as well as several historical and recent events. Despite the exposed seismic risk, a regional soils conditions map is yet to be completed for the GAMA. Dispersion data from 42 test sites was inverted and classified according to suggested seismic site classification scheme by National Earthquake Hazard Risk Program (NEHRP). From results of calculated shear wave velocity up to 30 m of soil, two main site classes have become apparent (D and C). Considering the geology of the underlying subsoils, two subdivisions for site class D is suggested (D1 and D2). The residual soils to the southwest and northwest of the study area are derived predominantly from granitoids, and their response is characteristically low. These values are generally within a range of 180 -360 m/s and thus fall into suggested site class D1, the most vulnerable within the test site. Similar to site class D1, D2 is also characterized by low shear wave velocity values (180 -360 m/s), however the underlying subsoils derived from phyllites within the study area. D2 occupies area to the southeast, lower and upper middle of the study area. Several vulnerabilities were identified for soils of site class D within the study area.
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    Hydrochemical And Isotopic Characterization Of Aquifers In The Lower Tano River Basin, Ghana, West Africa.
    (University Of Ghana, 2020-07) Edjah, A.
    The Lower Tano River basin is one of the basins endowed with minerals, oil, and gas resources in West –Africa. It covers 104,000 km2, and the major riparian countries are Ghana and Ivory Coast. The basin, underlain by the Birimian supergroup rocks and the rocks of the Apollonian has enough surface water resources to meet current water demands, but there are many challenges including high population growth rate (due to influx of oil and gas industrial workers), land degradation, mining activities, climate change etc. These challenges have put immense pressure on the groundwater resources in the basin. Also, the surface water is unreliable to meet the basin’s water demand for socio-economic development. This therefore makes groundwater the most cost effective and preferred means of supplying water to the dispersed population and rural settlements across the basin. One probable requirement for a sustainable and efficient management of groundwater is the characterization of aquifers in terms of its quantity, quality, and quantification of the recharge sources. Hence in this study, the aquifer hydraulic parameters were estimated, the overall quality of groundwater including the level of trace elements contamination, and its suitability for domestic and agricultural purposes was assessed, the hydrogeochemical processes that produces the chemical characteristics or compositions of the aquifer system was determined, the sources of recharge to the aquifer was investigated and the ages of groundwater was determined. The methodology used involved pumping tests carried out on 55 boreholes drilled across the basin, yearly sampling of surface water and groundwater resources, measurement of cations, anions, trace elements, stable isotopes of deuterium (δ2 H), oxygen-18 (δ18 O), tritium (3H) and measurement of silica. Pumping test together with Theis (1934) graphical method was used to estimate the aquifer hydraulic parameters (transmissivity, hydraulic conductivity, and specific capacity). From the estimation of the aquifer hydraulic parameters, it was observed that the mean transmissivity, hydraulic conductivity, and specific capacity values were 5.96 m2/day, 5.70 m/day and 18.80 L/min/m, respectively. The cations (calcium, sodium, potassium, magnesium) and anions (chloride, bicarbonate, nitrate, and sulphate) show that the aquifer of the Apollonian formation and the Birimian super group produces fresh groundwater with most being soft and few being moderately hard and hard. Na+ and SO42- were the dominant cation and anion in the surface water and ground water sampled in 2016. Likewise, Na+ and HCO3- were the dominant cation and anion in the surface water sampled in 2017, the groundwater sampled from 2013 to 2015, and the hand dug wells sampled in 2017. The boreholes sampled in 2017 had Na+ and SO42- as the dominant cation and anion, respectively. All the measured ions (bicarbonates, sulphate, chloride, nitrate, calcium, magnesium, sodium, potassium) in most of the surface water and groundwater sampled from the rocks of the Apollonian and the Birimian supergroup were within the WHO (2011) allowable limits for drinking water. The results of the measured trace elements that is Fe, Pb, Cd, Ni, As, and Al were high in most of the surface water samples. Also, the results of Cu, Cd, Zn, Cr, Fe, Ni, Mn, Co, Pb, As and Al in most groundwater especially those sampled from the aquifer of the Apollonian formation were higher than the WHO (2011) permissible limits for drinking water. The elevated concentrations of trace elements in the surface water and the groundwater were mostly related to geogenic sources. Based on water quality index (WQI) results, it was observed that majority of the groundwater sampled from the aquifer of the Apollonian formation were unsuitable for drinking whiles most groundwater samples taken from the aquifer of the Birimian supergroup were suitable for drinking. As per sodium adsorption ratio (SAR), 99% of the groundwater sampled from both aquifer of the Apollonian formation and the Birimian supergroup were suitable for irrigation. The sodium percent classification indicates that, minority of the groundwater sampled from the aquifer of the Apollonian formation and the Birimian supergroup falls under excellent category for irrigation. Wilcox diagram and magnesium hazard classification indicate that majority of the groundwater especially those sampled from the aquifer of the Apollonian formation gave out excellent to good water for irrigation. The residual sodium carbonate and Permeability index classification specify that all the sampled groundwater from the aquifer of the Apollonian formation and the Birimian supergroup were suitable for irrigation. The chlorinity index classification indicates that the aquifer of the Apollonian formation and the Birimian super group produce groundwater that were suitable for irrigation. Graphical including hydrogeochemical modelling and statistical approaches applied in the delineation of the major factors influencing the evolution of groundwater and the general hydrochemical characteristics revealed that the chemistry of groundwater in the Apollonian formation aquifer and the aquifer of the Birimian supergroup was mainly rock weathering and rainfall. In addition, ion exchange processes, rock-water interaction and incongruent dissolution were the major natural factors governing the formation of the groundwater chemistry in the Lower Tano River Basin. However, the occurrence of sulphate implies that sulphide mineral (pyrites, arsenopyrites and chalcopyrite) oxidation or mobilization and mining activities including illegal and legal small-scale mining possibly have an impact on the groundwater chemistry in the Lower Tano River Basin. The hydrochemistry of the sampled groundwater transited from Ca-HCO-3 to Na-Cl water type along a flow direction for the groundwater samples taken from 2014 to 2015 and 2017. For the groundwater sampled in 2016, the hydrochemistry evolved from Ca-HCO3- to Ca-SO42- water type and then to NaCl water type along the flow direction. The evolution was influenced by ion exchange processes and rock-water interaction. Based on mineral stability diagram, most groundwater in the aquifer of the Apollonian formation and the aquifer of the Birimian super group appears to be stable within the Kaolinite field suggesting ion exchange processes. Stable isotope composition (δ18O and δ2H) of rainwater indicates that the rainwater was not highly evaporated. Also, the slope and intercept of the rainfall regression lines obtained for this study were slightly similar to the local meteoric water line for Ghana and the global meteoric water line. Stable isotope composition (δ18O and δ2H) of surface water (rivers, streams, lagoon, and seawater) and groundwater (hand dug wells and boreholes) reveal that the mechanism of recharge to the aquifer of the Apollonian formation and the Birimian supergroup was rapid with the source being meteoric. Also, the surface water (rivers and streams) contributed to the groundwater recharge of the Lower Tano River Basin. Tritium in rainfall ranges from 1.27 to 10.11 TU. That of the rivers ranges from 2.04 to 3.08 TU and that of the groundwater ranges from 0.99 to 6.78 TU. The tritium results revealed that groundwater from the aquifer of the Apollonian formation and the Birimian supergroup were young, of modern recharge with short residence time. Also, the groundwater was recharged between 1960- 1965 and 1965 – 1970. The outcome of this study has added to the current hydrogeological knowledge about Lower Tano River Basin. It has also added to the characteristics of semi-confined and unconfined aquifers.
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    Geophysical And Hydrogeological Characterisation Of The Nasia Basin, Northern Ghana
    (University of Ghana, 2020-11) Abdul-Samed, A
    ABSTRACT The development of groundwater resources across the Voltaian Sedimentary Basin (VSB) is constrained by lack of knowledge on the location and suitability of aquifers for borehole construction. The main objective of this study is to evaluate the hydrogeological characteristics of the Nasia Basin to help in delineating suitable locations for groundwater exploration. An integrated geophysical investigation involving resistivity survey using 2D Electrical Resistivity Tomography (ERT), Electromagnetic Survey with EM34-3 and wireline logging of boreholes were employed to determine the resistivity of the different type of lithology occurring in the area and for providing information about the lateral and vertical extent of weathering and fracturing in the subsurface. A total of 58 ERT profiles were run at selected locations and at existing boreholes to obtain information about resistivity in different depth intervals. Majority of the profiles were run across the general strike of rocks of the VSB in the study area with length being either 400 m or 800 m. Five EM-34 ground conductivity profiles of 500-1000 m were conducted on each of two approximately parallel South-North traverses (EM campaign 1) to record the conductivity in the various geological formations and to assess its possible variation related to major geological structures. In addition to the two-parallel profile (EM campaign 1), 12 other EM-34 profiles each 500 m long were carried out at six selected communities in the central part of the study area (EM campaign 2) with the objective of locating areas within the weathered zone capable of storing groundwater for small scale pilot irrigation from dug-wells. The conclusion of the integrated geophysical investigation is that combining interpretations from various geophysical methods provides an improved characterisation of the hydrogeology. Secondly, hydrogeological investigations were conducted which involved analysis of pumping test results, evaluation of various methods of estimating aquifer transmissivity and assessment of the major factors controlling groundwater occurrence. Using remote sensing and geographical information system (GIS), thematic maps of slope, drainage density and lineament, geological formation were prepared. Other thematic maps such as static water level, regolith thickness, depth, recharge and transmissivity developed from kriging were incorporated into GIS. Finally, multi-criteria analysis and GIS techniques were used to integrate these thematic maps to delineate suitable zones to obtain a comprehensive groundwater potential map of the study area. The results also show that in certain locations such as portions of the Bimbilla formation, probability of obtaining aquifers is very low and therefore the target should be to locate thick regolith for groundwater storage. The results from the hydrogeological investigation indicate that cokriging gives better estimates of spatial aquifer transmissivity and therefore is a better approach considering the paucity of long duration pumping test data. Regression models and variographic analysis conducted confirmed the findings of previous researchers that groundwater within the study area is mainly structurally controlled and not by lithology. The groundwater potential map was classified into five zones that describe the potentiality of each cell in the study area for groundwater exploration. These classes are; very poor, poor, moderate, good and very good groundwater potential areas. It was found that 2% and 18% of the study area was classified as very good and good potential areas respectively. These are areas found to be concentrated in the Kodjari formation southwest of the Panabako sandstone formation of the study area. About 38% of the study area was classified as moderate potential which is sparsely distributed across the study area and 41% represent poor potential for groundwater exploration occurring mainly in the Bimbilla formation. Less than 1% of the study area was classified as very low potential areas and also concentrated in southeast of the study area. The reliability of the groundwater potential map was tested against successful and dry boreholes and the results showed that generally, the majority of high- and low-yielding boreholes fall in areas predicted by the map. Furthermore, a sensitivity analysis was performed to study the effect of each parameter on the overall groundwater map using the effective weight. It was found that the transmissivity was the most effective among the parameters that have the greatest influence on groundwater occurrence in the study area which is consistent with literature. The integrated geophysical method coupled with the comprehensive groundwater map has provided better information about subsurface geology of the Nasia Basin which is critical for understanding the lithological character in terms of hydrogeological conditions. The general concept of interpreting low resistivity zones as the presence of a sub-vertical fracture zone and therefore regarded as a favorable site has been refuted. A more scientific approach of interpreting geophysical results has been proposed for exploration hydrogeologist in the area and in similar geology. The groundwater potential map further shows that the potential for high yielding boreholes is limited to about 20% of the study area. Therefore, other strategies to augment for increasing irrigation such as exploring the weathered zone for groundwater storage should be considered.
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    Drai-Ahp Vulnerability Assessment of the Quaternary Aquifer in the Keta Strip
    (University Of Ghana, 2012-06) Kippo, Joseph V.
    The Quaternary aquifer in the Keta basin is a source of fresh water to the over 130,000 inhabitants living along the Keta Strip. Groundwater quality is deteriorating due to pollution from mainly agriculture, sewage, septic tanks and solid waste University of Ghana disposal in the area. This research was undertaken to assess the intrinsic vulnerability to pollution of the aquifer. The DRASTIC methodology was applied to delineate zones vulnerable to pollution. Data was gathered on the seven DRASTIC parameters which include static water levels, soil sampling to determine soil media and aquifer media types, hydraulic conductivity, elevations and water quality. Using GIS methods the data was prepared, processed and analysed. Interpolation techniques were used to generate raster layers of the parameters. The seven raster maps were combined in a GIS environment to produce a composite vulnerability map. The map was reclassified into five classes namely, very low, low, moderate, high and very high pollution potential. The resulting map shows the relative intrinsic vulnerability of the unconfined aquifer to pollution. The map was tested by plotting on it April, 2004 nitrate concentration data. Statistical correlation between nitrate values and vulnerability class values extracted from the nitrate points yielded a low Spearman rho correlation coefficient of 36.3%. The model was calibrated using statistical correlations between each of the seven parameters and nitrate values. Soil media, topography and hydraulic conductivity parameters were found not significantly correlated to nitrate pollution and were eliminated from the model parameters leaving depth to water (D), recharge (R), aquifer media (A) and influence of the vadose media (I) to form the acronym DRAI. The weights of the parameters were revised based on the seven correlation coefficients. The rating values were also revised based on the mean nitrate values for each rating class of the original DRASTIC. The model accuracy was tested by applying correlation to the vulnerability classes and nitrate concentration data. A correlation coefficient of 62 % was obtained. The Analytic Hierarchy Process (AHP) was used to calibrate the model by determining a different set of parameter weights. The model accuracy improved to 66% after correlation between nitrate values and intrinsic vulnerability classes. Based on the correlation coefficient the AHP model was selected for the study area. The AHP optimized intrinsic vulnerability map shows that 10.6%, 26.8%, 30.1%,22.6% and 9.9% of the study area have very low, low, moderate, high and very high vulnerability to groundwater pollution respectively. Nitrate concentration values sampled in July, 2003 was used to evaluate validation of the AHP model. The nitrate values were plotted on the AHP vulnerability map and statistical correlation tests between nitrate values and vulnerability values yielded a correlation coefficient of 75%, confirming a close agreement between model values and actual nitrate pollution on the ground. Anloga and Woe area where agriculture is predominant have the lowest intrinsic vulnerability. Dzelukope and Keta which are heavily populated residential areas have very high groundwater pollution potential. The validation evaluation shows that the research results are realistic and representative of the actual groundwater pollution on the ground. Therefore, the DRAI model is applicable in the study area.
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    The Sedimentology of Gambaga Massifs of the Northeastern Voltaian, Ghana.
    (University of Ghana, 2015-07) Mahamuda, A.; Asiedu, D.K.; Anani, C.Y.; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of Earth Science
    The sedimentary facies of the investigated area of the northeastern Voltaian, with respect to the sedimentary structures, composition as well as the depositional environments, can be classified into six (6) facies. The sediments are deposited dominantly in a shallow marine environments with records of shoreface/foreshore, Aeolian and tidal environments. The sediments were deposited in the various environments by both low and high energy media. The area shows a shallowing and thickening upward facies sequence. The paleocurrents of the sediments show a dominant southwestern source with minor contribution of sediments from the south and south east eastern parts. The sediments are composed of quartz, dominantly monocrystalline with undulatory extinction (strain) with minor polycrystalline, potassium feldspars, plagioclase feldspars, micas, sericites as the matrix, zircons and some accessory minerals from the petrographic study of the collected samples. The sediments studied are moderately – highly weathered in a relatively gentle – flat terrain. The composition of the clastic sediments shows high temperature regimes, dry/arid – moderate humid climatic conditions in a most probable poor rainfall environments. The sediments of the studied area from petrography, are the weathering products of granitic rocks, granitic gneisses and sedimentary rocks. The source area of the investigated area can be most likely traced to the adjacent Birimian Supergroup. Geochemically, the studied samples of the Gambaga – Nakpanduri area can be grouped into sublitharenite, subarkose and quartz arenites using log (Fe2O3/K2O) versus log (SiO2/Al2O3) bivariate plot. The weathering indices, thus, Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Thorium – Uranium ratio (Th/U) and Index of Compositional Variability (ICV) all supports a moderate – highly weathered and matured (ICV <1) sediments. Both the petrography and the geochemistry supports recycled sediments of the study area with evidence of quartz overgrowth and Th/Sc versus Zr/Sc plot respectively. The source area of the sediments has experienced semi-arid climatic conditions in flat – gentle topographic terrain with accompanying high temperature regimes. The sediments are largely the weathered products of felsic rocks with subordinate contribution of mafic and an older sedimentary source. The major elements as well as the trace elements supports an active continents margin setting and a passive margin tectonic setting. The studied area based on lithological features has been classified into three formations of lower, middle and upper parts which correlates with Tossiegou, Poubogou and Panabako formations of the Bombouaka Supergroup rock suite of earlier workers. The lower formation is made up of sandstones with asymmetric ripple marks. The middle formation is made up of sandy shales, siltstones with minor sandstones. The sandy shales are cross stratified with papery fissility, bifurcated ripple marks, flute casts, climbing ripple marks. The upper formation is made up of feldspar and quartz rich sandstones. They are medium to coarse grained with characteristic cross beds, penetrative burrows (Skolithos), over turned cross beds, ripple marks and wavy laminations.
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    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 Science
    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 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.
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    Distributed Numerical Modelling of Hydrological/Hydrogeological Processes in the Densu Basin
    (University of Ghana, 2010-07) Alfa, B. A.; Banoeng- Yakubo, B.; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of Earth Science
    The study was undertaken to develop a Densu basin integrated hydrological model capable of describing the major hydrological processes occurring in the land phase of the hydrological cycle and computation of water balance in the Densu river basin. Inadequate information on the hydrological processes as well as the regional water balance in the basin has adversely affected the sustainable development, assessment and efficient management of the basin’s water resources. A physically based numerical modeling system (MIKE SHE) was applied in an integrated approach. The MIKE SHE model provides a comprehensive framework for studying the interrelationship between upstream and downstream conditions and processes. The saturated and unsaturated zones flows were simulated using a fully implicit finite difference solution technique whereas the governing processes of the evapotranspiration were based on empirically derived equations. A conceptual model was developed using field data followed by model design, calibration and verification. This study finds that actual evapotranspiration in savannah land cover in the basin ranges between 0.5 mm/day and 4.0 mm/day. In the forest land cover, actual evapotranspiration ranges between 0.4 mm/day and 5 mm/day. Actual evapotranspiration approaches zero during the dry periods in the case of savannah land cover whereas in the Forest land cover the minimum actual evapotranspiration in the dry periods is 0.4 mm/day Results from the unsaturated zone study indicate that the Togo series soils provide a better medium for direct groundwater recharge with a maximum recharge rate of 0.7m/h and an average rate of about 0.3m/h. The recharge rate in the granite soils is an order of magnitude lower than that of the Togo series soils but longer recharge duration than in the Togo series soils. The Birimian soils are almost impermeable and therefore the recharge rate is as low as 0.003m/h. 90% of the rainfall in the Densu basin occurs within the duration of 2 to 4 hours. It was observed that the use of daily rainfall as inputs in hydrological models in tropical regions does not represent convective storm events with highly variable precipitation intensities. The use of hourly rainfall data is more appropriate if detailed description of hydrological processes is required. However for the purposes of annual water balance computations, daily rainfall data are equally appropriate. Simulation of the annual water balance indicate that, 72% of annual rainfall is lost through evapotranspiration, 21% is routed to the Densu river and its tributaries and 8% is groundwater recharge. 0.0009% is base flow and soil moisture deficit amounts to O.O 16%. The application of the MIKE SHE model in the description and characterization of the flow processes in the basin was successful with the only limiting factor being lack of long time series data for model verification
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    Solute Dispersion in Soil
    (University of Ghana, 1979-07) Laryea, K. B.; Elrick, D. E.; University of Ghana, College of Basic and Applied Sciences, School of Physical and Mathematical Sciences, Department of Earth Science.
    An experimental study of solute transport during one-dimensional sorption of a solution of KC1 into a uniformly packed horizontal soil column of Akuse clay, (a tropical clay loam from Ghana which is dominantly montmorillonitic) and Brookston clay of the Huron Catena (dominantly illitic) is reported in this thesis. A hydrodynamic equation based on the existing theories of irreversible thermodynamics is developed to incorporate the effect of anion exclusion in the usual hydrodynamic equation for one-dimensional flow of solute in soil. This equation is used to calculate the. longitudinal dispersion coefficient D^ in horizontal infiltration experiments where anion exclusion effects were observed. Also, the analysis of hydrodynamic dispersion during one-dimensional horizontal flow developed by Smiles eit al. (1978) is extended to include gravitational effects present during one-dimensional vertical infiltration by using the power series form of solution developed by Elrick _et a^L. (1979). Simulation methods using computer programs written in system/360 Continuous System Modeling Program (CSMP) are used to solve the equations used in this study. The longitudinal dispersion coefficient is in all the analyses presented in this study, assumed to be independent of the Darcy flux and a function of the water content only. In all experiments, both the soil solution content, the chloride concentration and the potassium concentration preserved similarity in terms of distance divided by square root of time. This confirmed the assumption that the longitudinal dispersion coefficient is effectively independent of the Darcy flux for the conditions of the experiments reported in this study. These results were found to be consistent with the studies of Saffman (1959) , Pfannkuch (1963) and Scotter and Raats (1970). The observed incomplete piston-like displacement of the antecedent water content suggests the presence of stagnant or 'immobile' water fractions in the soil columns considered. The derived longitudinal dispersion coefficient for Cl agreed fairly well with the derived longitudinal dispersion coefficient for K . This is an indication that under the experimental conditions imposed, the equilibrium adsorption isotherm used to describe the adsorption term in the hydrodynamic equation for solutes which interact with soil particles, is adequate. Simulation of water content, chloride concentration and concen- + - 1/2 tration of K as a function of X(=xt ) using derived data D(X), Dg (X) for Cl and Dg (X) for K+ showed very good agreement with experimental data. The simulated c(X) for Cl also showed excellent agreement with calculation of c(X) using a program for the analytical solution. Water content profile and chloride concentration profile obtained for vertical infiltration experiments indicated good agreement with theoretical water content and Cl concentration profile simulated with CSMP. The theoretical chloride concentration profiles showed a progressively dispersed 'front' with long infiltration time periods whereas the theoretical water content profiles for various time periods showed a sharp and abrupt wetting front
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    The Origin and Hydrochemical Evolution of Groundwater in the Lake Bosumtwi Area, Ghana
    (University of Ghana, 2014-06) Sena, Yvonne A. L.; Banoeng-Yakubo, B.; Asiedu, D.; Yidana, S. M.; Sakyi, A. P.; University of Ghana, College of Basic and Applied Sciences , School of Physical and Mathematical Sciences, Department of Earth Science
    Lake Bosumtwi is an important natural inland freshwater meteorite crater lake due to its scientific and socio-economic importance to both local and international communities. Although groundwater has been the main source of water supply for people living around the lake and visitors/tourists, very little work has been conducted with regard to the quality of the groundwater delivered by the aquifers within the lake basin. These aquifers are made up of the metasediments of the Birimian Supergroup and boreholes that are mostly fitted with pumps tap the groundwater. A combination of conventional graphical methods, multivariate statistical and mass balance models have been applied to surface water in Lake Bosumtwi and groundwater hydrochemical and stable isotope (δ2H and δ18O) data from Birimian aquifers around the lake. The objective was to contribute to and improve the understanding of the hydrochemistry of the lake water and groundwater in aquifers around the lake and also to understand the relationship between these two reservoirs. Results indicate that groundwater is of good to excellent quality for domestic use and is generally suitable for irrigation in comparison to the lake water that has high salinity and high sodicity and is, therefore, not suitable for irrigation. Hydrochemical and isotopic (i.e., δ2H and δ18O) compositions of the groundwater and lake water suggest that there is no apparent incipient hydraulic relationship, which benefits the main aquifer system in terms of recharge from the lake. However, the reverse process, whereby the lake receives contribution from the aquifers through subsurface flow cannot be discounted on the basis of the data from this research. iii Recharge of the aquifers appears to occur on hilltops where the water is characteristically acidic with low level of mineralization, suggesting short residence time. The groundwater within the basin has been recharged by recent meteoric water that has undergone evaporative enrichment prior to recharge. Evaporative loss in the range of 45% to 51% has been estimated for rainwater available for infiltration and subsequent recharge. The lake water is considerably enriched relative to the heavier isotopes of hydrogen and oxygen as a result of severe evaporation over the open lake surface. Estimated evaporative loss over the lake surface is about 82% resulting in the concentration of the univalent cations in the Lake. Q-mode hierarchical cluster analysis (HCA) of the hydrochemical data employed to examine the spatial distribution of the investigated samples resulted in four spatial clusters (C1 to C4). Conventional graphical plots of the hydrochemical compositions of these clusters combined with mass balance hydrochemical modelling suggested that the groundwater has evolved from Na-Mg rich water types located on hill tops to Ca-Mg rich hydrochemical facies at the lower reaches of the crater. The relevant reactive minerals within the aquifers and the geochemical processes that control the hydrochemical evolution of the groundwater and lake water were also determined. R-mode principal component analyses (PCA) of the hydrochemical data suggested dissolution of aluminosilicates followed by carbonate mineral weathering and finally anthropogenic activities as the principal processes that influenced the hydrochemistry. Mineral stability diagrams suggested kaolinite as the most stable clay mineral phase in the groundwater system thus the groundwater in the area is generally at the intermediate stage in the evolution processes. In addition to dissolution of silicates, the chemical composition of the lake appears to have been influenced by evaporation and consequent saturation of carbonate minerals. The research has demonstrated that an understanding of the hydrogeochemistry and the relationship between the lake and groundwater is important for environmental management.