Investigation of groundwater repositories in Modakeke-Ife, southwestern, Precambrian Basement Complex, Nigeria

Geophysical assessment involving very low-frequency electromagnetic (VLF-EM) and vertical electrical sounding (VES) methods was carried out at Modakeke-Ife, within the southwestern Precambrian Basement Complex of Nigeria, to delineate possible zones of groundwater productivity and evaluate aquifer vulnerability zones. The VLF-EM was deployed at 10-m intervals along five profiles varying from 200 to 300 m in length. Twenty-two VES surveys were deployed at the locations of the anomalous zones on the VLF-EM profiles within the study area. The high peak positive anomalies (high current density) on the VLF-EM profiles indicate conductive geological features. The result from VES surveys showed four geoelectric subsurface layers inferred as topsoil (clayey sand and sand) with resistivity ranging from 78 to 830 Om a weathered layer (clayey sand, sand, sandy laterite, and laterite) with resistivity ranging from 335 to 2321 Om a partially weathered or fractured basement (323-1869 Om); and a fresh basement. The overburden thickness varies from 3 to 25 m. There is high-to-medium groundwater productivity because of the overburden thickness and low reflection coefficient. The groundwater potential map generated for the study area is categorised into three zones: low (36%), medium (14%), and high (50%). The study area's protective capacity was classified as poor, low, moderate, or high based on longitudinal conductance (LC), groundwater reservoir type, underlying lithology, depth to the groundwater reservoir (GOD), and a geoelectric layer susceptibility index (GLSI). Assessing the vulnerability of potential groundwater resource developments to contamination would reduce the environmental hazards posed by aquiferous unit contamination.