A 3D GEOLOGICAL MODEL OF THE NASIA SUB-BASIN, NORTHERN GHANA-INTERPRETATIONS FROM THE INVERSION RESULTS OF REPROCESSED GEOTEM DATA

Inversion of regional-scale airborne electromagnetic (AEM) data was used in building a 3D geologic model of the Nasia Sub-Basin, Northern Ghana. Geological interpretation of the AEM data was guided by a priori knowledge obtained from previous research in the area, geological maps and reports. A key requirement of the modelling process was to define the subsurface stratigraphy of the area, chiefly to provide 1) a detailed stratigraphical context for hydrogeologic use, 2) a repository of comprehensive stratigraphic knowledge of the study area which will be easily accessible. The AEM measurements, consisting of GEOTEM data, was originally collected for mineral exploration purposes. The B-field data have been (re)processed to enhance the resolution and inverted by using spatial constraints to preserve the lateral and vertical coherence. During the processing and inversion phases, the regularization strategies and associated parameters have been tuned by following an iterative approach characterized by a tight collaboration between geologists and geophysicists to retrieve the geophysical model that is in the best agreement, not only with the geophysical data, but also with the geological expectations. The new (pseudo-)3D inversion showed very different features with respect to the previous Conductivity-Depth Images (CDI). The new geophysical model led to new interpretations of the geological settings and to the construction of a comprehensive 3D geomodel of the basin based on the integration of AEM and borehole information. Nevertheless, in order to have a model, suitable for hydrogeological characterization, it will be necessary to include more details regarding the upper, weathered zone as the AEM survey was optimized to have a very deep penetration and not an extremely high shallow resolution. These aspects will be remedied by the inclusion, for example, of the interpretations from Electrical Resistivity Tomography (ERT) data, which will provide the necessary resolution in the upper sections

Management of Groundwater for Productive Uses in the Afram Plains Area, Ghana

Groundwater is the main source of potable water for various uses in the Afram Plains area, Ghana. This study had the objective of assessing the quality and quantity of groundwater in the area for household and irrigation purposes. R-mode factor and hierarchical cluster analyses were combined with inverse geochemical modeling from PHREEQC and mineral stability diagrams, to determine the main contributors to the hydrochemistry of groundwater in the area. The sodium adsorption ratio, SAR, electrical conductivity, EC, sodium percent and permeability indices, PI, were then used to evaluate the suitability of groundwater in the area for irrigation purposes. A 3-Dimensional groundwater flow model was developed from MODFLOW using available hydrogeological parameters of aquifers of the southern Voltaian sedimentary rocks in the Afram Plains area. The model was simulated for both steady state and transient state conditions. An optimization model highlighting the objectives to be achieved from the exploitation of groundwater resources in the area, the various environmental and economic constraints, was linked to the transient simulation model to provide a suitable management decision support system for groundwater resources in the area. This study finds that groundwater hydrochemistry in the Afram Plains area is controlled by (i) silicate mineral weathering modulated by cation exchange processes (ii) carbonate mineral weathering and (iii) fertilizer from agricultural activities in the area. For the first time in the study area, the results of the hydrochemical modeling in this study suggests that silicate mineral weathering and cation exchange processes are the most pervasive processes in the area, and montmorillonite is the most stable mineral phase in the area. Stability in the montmorillonite field indicates that groundwater flow in the area is restricted. This finding is in consonance with the observed hydrogeology of the entire Voltaian System. In the assessment of the irrigation quality of the groundwater in the area, this study finds that more than 70% of the data used are within the range that is acceptable for irrigation of all crops in the area. This suggests that groundwater in the Afram Plains area is generally of acceptable quality will pose no or limited environmental problems when used for commercial irrigation activities in the area. This is the first comprehensive assessment of irrigation quality of groundwater in the Voltaian System of Ghana and will certainly prove handy in the management of groundwater resources in the terrain. The flow simulation model, which is the first of its kind in the area, suggests that groundwater generally flows from the central areas to the outer regions, with water levels ranging from 50 m to 200 m. When combined with the optimization model, the transient model indicates that groundwater in the area has potential to meet current household and irrigation needs and holds significant promise for future uses. Commercial extraction for irrigation activities is however, much more feasible in the outer discharge areas than the central recharge areas

Contributions to global earth sciences integration. A special issue on the 3rd Young Earth Scientists Congress

The Young Earth Scientists (YES) Network is an association of early-career geoscientists who are primarily under the age of 35 years from universities, geoscience organizations and companies from across the world (www.networkyes.org). The YES Network was formed as result of the International Year of Planet Earth in 2007 under the patronage of UNESCO. YES Congress history began in 2009, when the first event took place at the China University ofGeosciences in Beijing, China. The second YES Congress was convened in 2012 in Brisbane, Australia, in conjunction with the 34th International Geological Congress (IGC). The special issue introduced here contains some of the contributions presented at the 3rd YES Congress, held at the Mwalimu Julius Nyerere International Convention Centre in Dar es Salaam, Tanzania, from 11th to 14th August 2014. This meeting was developed in conjunction with the 25th Colloquium of African Geology (CAG25).Fil: Cónsole Gonella, Carlos Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; ArgentinaFil: Yidana, Sandow Mark. University Of Ghana; Ghan

Hydrochemical Analysis of Groundwater from the Keta Basin, Ghana

Multivariate statistical tools and mineral speciation were used to determine the main controls on the hydrochemistry of groundwater from the Keta Basin, Ghana. Groundwater was also assessed to determine its suitability for irrigation purposes. The study indicates that the hydrochemistry is controlled largely by the intrusion of seawater into the basin. Two hydrochemical facies, the SO 4 -Ca-K- Mg - NO 3 and Cl-Na- HCO 3 facies, have been defined in this study. Saturation indices calculated from mineral speciation indicate that groundwater in the area is generally saturated with respect to gypsum and anhydrite, supersaturated with respect to calcite, aragonite and dolomite, and undersaturated with respect to halite. The extremely high salinity in groundwater from some wells and boreholes in the area is attributed to the role seawater plays in groundwater hydrochemistry in the basin. Chemicals from agricultural activities play a minor role in the hydrochemistry. The sodium adsorption ratio, eltrical conductivity permability indeX, residuals sodium carbonate and sodium percent were the parameters used to assess the suitability of groundwater for irrigation. Groundwater is generally of acceptable quality for irrigation. Over 50% of the samples tested fall within the low sodicity, medium salinity range, which is an acceptable range for irrigation under well drained conditions. Moreover, more than 80% of the samples have permeability indices higher than 70%, which indicates that groundwater in the area will not adversely affect the hydraulic properties of the soils in the Keta Basin area

Hydrogeological Characterization of a Tropical Crystalline Aquifer System

This research used a numerical groundwater flow model, calibrated under steady-state conditions to develop the groundwater flow system in the West Mamprusi District of Northern Ghana. It was aimed at conceptualizing the flow system to initiate a thorough hydrogeological study of the rocks in the area. Stable isotopes were used to relate groundwater recharge in the area to recent meteoric water that had been evaporated in transit down the surficial material. On this basis, direct vertical groundwater recharge from precipitation was applied in the numerical modeling. This study suggests that the prospects of commercial development of the aquifers are high as the estimated recharge ranges between 3.3% and 29% of the annual precipitation. Estimated horizontal hydraulic conductivity ranges between 3.2 and 48 m/d in the area. The variability in the horizontal hydraulic conductivity has led to the development of four prominent groundwater flowpaths in the area. However, a prominent NE–SW flow has been observed and is in consonance with the reported structural grain of the country. A groundwater flow divide noted in the southern part of the study area has been attributed to the structural heterogeneity rather than topographical complexities as the area is largely flat

Hydrogeological and Hydrochemical Characterization of the Voltaian Basin: The Afram Plains Area, Ghana

In the Afram Plains area, groundwater is the main source of water supply for most uses. The area is underlain by aquifers of the southern Voltaian sedimentary basin, which are predominantly sandstones, mudstones, conglomerates and shale. Ordinary least squares regression analysis using 41 well-test data from aquifers in the Afram Plains portion of the Voltaian system reveals that transmissivity, T, exists in a non-linear relationship with specific capacity, Sc. The analysis reveals that T = 0.769Sc1.075 with R 2 = 0.83 for aquifers in the area. The mudstone/conglomerate aquifer in the area appears to be the most variable in terms of both specific capacity and transmissivity with transmissivity ranging from 0.18 to 197.7 m2/day and 0.5 and 148.5 m3/day/m, respectively. Horizontal fractures and joints resulting from secondary fracturing appear to be the main determinants of both transmissivity and specific capacity in the area. PHREEQC modeling and mineral stability diagrams indicate that groundwater quality in the Afram Plains area is controlled by the incongruent weathering of silicate minerals in the aquifers. These processes concentrate calcium, sodium, magnesium, potassium, bicarbonate ion and quartz, leading to calcite, dolomite and aragonite supersaturation at most locations. Hierarchical cluster analysis performed on the raw chemical data reveals two main water types or facies: the calcium-sodium-chloride-bicarbonate facies, and the magnesium-potassium-sulfate- nitrate facies for the southern and northern sections of the Afram Plains area, respectively

Groundwater Quality Evaluation for Productive Uses - the Afram Plains Area, Ghana

Groundwater is the most widely used water resource in the Afram Plains area, Ghana. The objective of this study was to determine the distribution of fluoride, sodium adsorption ratio (SAR), and salinity (EC) in groundwater from the different aquifers in the Afram Plains area. The distribution of these parameters would enable a determination of the quality of groundwater from the different aquifer units for use in households and irrigation purposes. The inverse distance weighting, interpolator with a power of 2 was applied to 143 data points of each parameter to generate prediction maps for fluoride, SAR, and EC in the area. The distribution maps from this study show that the shale aquifers that underlie the southwestern sections of the area have the highest levels of fluoride, SAR, and EC. A narrow strip of the shale aquifer in the southwestern region produces groundwater whose fluoride level (2.2 mg/L) is higher than the World Health Organization minimum allowable fluoride concentration of 1.5 mg/L in drinking water. The feldsparthic sandstone, arkose, siltstone, and mudstone aquifers in the northeastern sections of the area produce groundwater with fluoride concentration in the range of 0.0-0.3 mg/L, which fall outside the minimum range of 0.7-1.2 mg/L required in drinking water for normal bone and dental health. The highest SAR and EC are also associated with the shale aquifers to the southwestern section of the area. Using the World Food and Agriculture Organization\u27s guidelines for irrigation water quality, it was determined that groundwater from the Afram Plains area will generally have a mild to moderate effect on the hydraulic properties of soils when used as irrigation water

Evaluation of Groundwater Recharge Estimates in a Partially Metamorphosed Sedimentary Basin in a Tropical Environment: Application of Natural Tracers

This study tests the representativeness of groundwater recharge estimates through the chloride mass balance (CMB) method in a tropical environment. The representativeness of recharge estimates using this methodology is tested using evaporation estimates from isotope data, the general spatial distribution of the potential field, and the topographical variations in the area. This study suggests that annual groundwater recharge rates in the area ranges between 0.9% and 21% of annual precipitation. These estimates are consistent with evaporation rates computed from stable isotope data of groundwater and surface water in the Voltaian Basin. Moreover, estimates of groundwater recharge through numerical model calibration in other parts of the terrain appear to be consistent with the current data in this study. A spatial distribution of groundwater recharge in the area based on the estimated data takes a pattern akin to the spatial pattern of distribution of the hydraulic head, the local topography, and geology of the terrain. This suggests that the estimates at least qualitatively predicts the local recharge and discharge locations in the terrain

Hydrochemical Evaluation of the Voltaian System-the Afram Plains Area, Ghana

Inverse geochemical modeling from PHREEQC, and multivariate statistical methods were jointly used to define the genetic origin of chemical parameters of groundwater from the Voltaian aquifers in the Afram Plains area. The study finds, from hierarchical cluster analysis that there are two main hydrochemical facies namely the calcium-sodium-chloride-bicarbonate waters and the magnesium-potassium-sulfate-nitrate waters in the northern and southern sections, respectively, of the Afram Plains area. This facies differentiation is confirmed by the distribution of the SO42-/Cl- ratio, which associates groundwater from the northern and southern sections to areas influenced by contact with evaporites and seawater, respectively. Principal component analysis (PCA) with varimax rotation using the Kaiser criterion identifies four principal sources of variation in the hydrochemistry. Mineral saturation indices calculated from both major ions and trace elements, indicate saturation-supersaturation with respect to calcite, aragonite, k-mica, chlorite, rhodochrosite, kaolinite, sepiolite, and talc, and undersaturation with respect to albite, anorthite, and gypsum in the area. Inverse geochemical modeling along groundwater flowpaths indicates the dissolution of albite, anorthite and gypsum and the precipitation of kaolinite, k-mica, talc, and quartz. Both the PCA and inverse geochemical modeling identify the incongruent weathering of feldspars as the principal factors controlling the hydrochemistry in the Afram Plains area. General phase transfer equations have been developed to characterize the geochemical evolution of groundwater in the area. A very good relationship has been established between calcite and aragonite saturation indices in the Afram Plains area, with R2=1.00

Irrigation Water Resource Management for Sustainable Agriculture - the Ankobra Basin, Ghana

Two irrigation water assessment methods, the USDA classification scheme and the soil infiltration potential, were applied to water from three different locations (Ankwaso, Dominase, and Prestea) of the Ankobra River basin in Ghana, to evaluate its effectiveness as a sustainable water resource for irrigation. The study classifies water from all three locations into the low salinity, low sodicity zone with Prestea and Ankwaso having waters of the highest sodicity and salinity, respectively. A classification scheme based on effects of the water on the hydraulic properties of soils reveals that water from all locations of the basin has the potential to affect the infiltration properties of soils, especially when applied over a long period of time. Linear regression analysis indicates a strong relationship between electrical conductivity (EC) and sodium adsorption ratio (SAR) at R2 = 0.773 (n=30) for Prestea. This relationship is probably due to the fact that Na, Mg, and Ca, which are the major determinants of water SAR, are also the major contributors to the water EC at Prestea, and this is probably due to surface mining activities, which encourage the weathering of calcic and albitic feldspars. Time series analysis reveals that EC and SAR for Dominase and Prestea, respectively, have increased from 1989 to 1992. On the other hand, SAR and EC have been decreasing since 1989, for the two locations. Forecast data from the time series analyses agree well with observed data, at 0.01 level of significance. Projections were made at ten time steps ahead of 1992 using time series analysis