Evaluation of the vulnerability of selected aquifer systems in the Eastern Dahomey basin, South Western Nigeria
Oke, Saheed Adeyinka
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This study aimed to evaluate the vulnerability of the shallow aquifer systems of the Dahomey Basin and formulate a simple vulnerability method with which data limited areas (which include the shallow unconfined aquifers in the Dahomey Basin) can be predicted. The Dahomey Basin is a transboundary aquifer which extends from Ghana to the western parts of Nigeria. The study covered the eastern section of the basin. The methodological approach involved a source–pathway–receptor vulnerability model. The Dahomey Basin was characterised through the geophysical, hydrological, litho-geochemical and hydrogeochemical approaches. The geology of the basin includes sedimentary rock types of sandstone, shale, limestone, alluvium conglomerate and the formations which are composed of sand, silt, clay, laterite and gravel. The geophysical study, which mainly aimed to estimate the depth-to-water table, identification of strata and vadose zone thickness, revealed topsoil, sandy clay, dry porous sandstone, conglomeratic sandstone, limestone and alluvium as the major lithological units in the basin. Geo-electrical curve types revealed an overlying multilayered rock. The vadose zone characterisation, which is the pathway through which contaminants infiltrate, aimed to determine the lithological properties which dictate the travel time of water. This was achieved by determining the hydraulic conductivity of the vadose lithology in the laboratory. Other important parameters such as grain size, porosity, shapes, textural classification and clay types were examined for their attenuation capacity. The hydrogeochemical investigation involving the collection and analysis of water samples from the hand-dug wells and shallow boreholes during the rainy and dry season was aimed at monitoring the groundwater quality of the basin. Ca-Mg-Cl water types and Na-K-Cl water types were delineated. Bacteriological examination of the shallow water reveals the presence of E.Coli, Heterotrophic bacteria and Salmonella/ Shigella. Precipitation which is a component of groundwater recharge ranged between 1 200–1 800 mm from the northern end to the southern end of the basin, respectively. Groundwater level were measured, monitored and average water level were delineated for the formations of the Dahomey Basin. The proposed RTt vulnerability method was applied to evaluate the groundwater vulnerability of the Dahomey Basin. The RTt method is an intrinsic physically based vulnerability method based on the concept of groundwater recharge from rainfall and travel time within the covering lithology over the aquifer. Travel time is the infiltration derived from multiplication of the slope and thickness of the vadose zone divided by fluid velocity. The fluid velocity is derived from the division of hydraulic conductivity by porosity. RTt method application results for the Dahomey Basin were presented on the RTt vulnerability map. The RTt vulnerability map was classified from very low vulnerability (12) to very high vulnerability (100). The RTt vulnerability results for the Dahomey Basin showed 18% of the areas classified as very high vulnerability, 7% of the areas classified as high vulnerability, 64% of the areas classified as moderate vulnerability and 10% of the areas classified as low vulnerability. The compared vulnerability maps of the RTt method and those of the DRASTIC, PI and AVI methods, showed similarities between the RTt method and the AVI and DRASTIC method, respectively. Areas classified as high vulnerability by these methods showed very shallow protective covers, high precipitation and porous aquifer materials, while areas classified as low vulnerability areas include thick protective cover, reduced rainfall, higher slope and higher depth-to-water. The RTt vulnerability map was validated with the hydrochemical tracer using chloride, DO and microbial loads as vulnerability indicators. This study has formulated an RTt method that can be used to predict the vulnerability of shallow unconfined aquifer systems, a key component in groundwater management. The major advantage of the RTt method is the use of less number of parameter to assess groundwater vulnerability. The method has been applied to investigate the regional aquifer of the Dahomey Basin and can be used to predict the aquifer vulnerability of similar basins across Africa with limited data.