Hydrogeological Investigation of Basement Complex Aquifers in Osun State, South West Nigeria

Access to safe drinking water remains a persistent challenge across rural communities in Osun State, where the crystalline basement complex presents significant hydrogeological complexity for borehole sitting. This study investigates the hydrogeological characteristics of fractured and weathered basement aquifers in selected communities of Osun State, South West Nigeria, with the objective of delineating productive groundwater zones. Vertical electrical sounding using Schlumberger configuration was carried out at fifty stations to characterise subsurface resistivity variations associated with weathered layers, fractured zones, and fresh bedrock. Interpreted sounding curves were correlated with available borehole lithological logs to calibrate the geophysical models. Aquifer parameters including transmissivity and storativity were estimated from pumping test data at selected productive boreholes. Iso-resistivity maps and depth-to-bedrock contours were generated to guide future borehole siting decisions. Hydrochemical data obtained from sampled boreholes were evaluated against regulatory standards to assess potability. The study reveals that zones of deep weathering and intersecting fracture sets, often associated with structural lineaments identifiable on aerial photographs, correlate with higher borehole yields. These findings have direct operational value for the Osun State Rural Water Supply Agency and non-governmental organisations coordinating water infrastructure development in underserved communities. The study advances understanding of basement aquifer heterogeneity in the South West geopolitical zone. Keywords: hydrogeology, basement complex, vertical electrical sounding, aquifer, Osun State.