Estimation of aquifer hydraulic parameters and protective capacity in basement aquifer of south-western Nigeria using geophysical techniques

Estimation of aquifer properties allows quantitative prediction of the hydraulic response of the aquifer to recharge and discharge. However, conventional methods such as pumping tests, among other limitations, are costly and time-consuming. Thus, this study aims to estimate hydraulic properties of basement aquifers of the Idi-Ishin area of Ibadan, southwest Nigeria using an indirect electrical resistivity technique. Sixty-two (62) vertical electrical soundings (VES) were carried out along eleven (11) E-W established traverse lines with a view to intercepts the fracture systems. The acquired resistivity data were subsequently interpreted using computer software, IP12WIN, for iteration and generated layered parameters. Layer parameters were further evaluated to derive the Dar-Zarrouk parameters such as longitudinal conductance (S) and transverse resistance (R) to estimate the aquifers transmissivity (T) and hydraulic conductivities (H). Pumping test was performed, and the hydraulic conductivities and transmissivities determined were correlated with the calculated values from the electrical sounding data. The subsurface geophysical investigation revealed three to four-layer systems; topsoil, weathered-partially weathered basement, fractured basement and fresh basement. The weathered-fractured basement at varied depths serves as shallow groundwater aquifers. The computed T and H ranged from 9.69 x 10(-2) to 1.71 x 10(-1) m(2)/day and 9.13 x 10(-3) to 1.86 x 10(-2) m/day which give a good relationship with the respective in-situ values from 2.46 to 6.15 x 10(-1) m(2)/day and 1.88 x 10(-2) to 4.70 x 10(-3) m/day. S and R vary between 0.014 and 0.714 ohm(-1) and 84.48-30,112.57 ohm m(2), revealing a poor to good recharge and huge aquifer thickness. Also, longitudinal resistivity (LR) and transverse resistivity (TR) range from 12.3 to 242.7 ohm m(-1) and 20,333.1-106.5 ohm m(-1) an indicative of variation in the basement lithology as coefficient of anisotropy (lambda) vary from 2.1 to 14.9, reflecting differences in basement characteristics for direction of fluid motion and accumulation. This study supports using electrical resistivities measurement to determine aquifer hydraulic conductivities, transmissivities, and protective capacity.