Aquifer Characteristics and Protectivity Studies in parts of Northern Bida Basin North central Nigeria, using Geoelectric data

This Electrical resistivity investigation was carried out in parts of Northern Bida basin, North Central Nigeria. The focus of this research is to estimate the aquifer's geoelectrical properties and determine near-surface vulnerability using surface geophysics. Thirty-two Vertical Electrical Soundings (VES) were conducted using a Schlumberger array with a maximum half-current electrode (AB/2) of 100 m giving a total spread of 200 m. Data were interpreted using WINRESIST software. The number of modeled layers across the study area varies between 2-5, and their inferred lithology includes top clayey layer, top sandy layer, weathered basement, fractured basement, and un-weathered basement rocks. The thickness and resistivity of the protective layer vary between 0.5 and 10.46 m and 15 to 2668 Ωm respectively. The thickness and resistivity of the aquiferous layer vary between 1 and 65, and 13 to 5025 Ωm respectively. The last modeled layer across the area is assumed to be un-weathered basement rock whose resistivity varies between 9Ωm to 500000Ωm and the depth to un-weathered basement varies between 2 – 65 m . The hydraulic conductivity of the protective layer varies between 0.16 and 28.25 m/day and its transmissivity value is between 0.08 and 61.42m
²
/day. The hydraulic conductivity of the aquifer varies between 0.19 and 234.64 m/day and its transmissivity value is between 2.32 and 985.41m
²
/day, hence is rated as permeable (66.67%), and high permeable (33.33%). The proportion of the permeability rating of the protective layer is 78.79% for permeable and 21.21% for high permeable; these ratings make the underlying aquifer to become vulnerable to pollution due to the low residence time of contaminants passing through them. However, the transmissivity reveals that the groundwater potentials in the area are rated very low (12.12%), low (30.30%), moderate (36.36%), and high (21.21%). A comparison with Dar-zarrouk parameters shows Transverse resistance values of the aquifer are between 135-31798Ωm
²
which revealed that the weathered aquifer is rated poor (27.27%), moderate (21.21%) and high (51.52%). The average longitudinal conductance of the protective layer is 0.009Ω
^-1
confirming the protective layer to be poorly rated, hence vulnerable to pollution. This work has shown the effectiveness of surface geophysics in assessing an aquifer's hydraulic parameters in the absence of pumping test data and in identifying its susceptibility to surface pollutants.