Aquifer characteristics and groundwater recharge pattern in a typical basement complex, Southwestern Nigeria

Abstract

A geophysical survey involving thirty four vertical electrical sounding (VES) was carried out at Federal College of Education, Osiele, Abeokuta, southwestern Nigeria using Schlumberger electrode array. The locations were selected based on the existing boreholes drilled in the past within the study area. The results revealed a maximum of five geoelectric layers, viz: topsoil, sandy clay, clayey sand, shale/clay, sandstone, fractured basement and fresh basement. Three probable aquifer units and one aquitard were delineated with clayey sand occurring in 50%, sandy clay constitutes 24%, fractured basement 24% and shale/clay 2%. VES 10, 26 and 30 with weathered layer (shale/clay) of thicknesses 14.7, 23.5 and 9.9 m respectively revealed very low yield (not productive). Borehole drilling in the study area should be executed in the peak of the dry seasons during which groundwater level is expected to be low because recharge of the existing boreholes in the study area is largely due to falling precipitation. Existing boreholes located within the study area characterized by unconfined aquifer while some are confined under pressure between relatively impermeable materials. With this, the problems of recharging and drying up of borehole can be solved.