Empirical Analysis of Groundwater Recharge Dynamics and Aquifer Vulnerability in the Crystalline Basement Complex of Ekiti State

This study empirically analyses groundwater recharge dynamics and aquifer vulnerability in the weathered crystalline basement complex terrain of Ekiti State, South West Nigeria, filling a significant gap in local hydrogeological characterisation necessary for sustainable groundwater resource management. Ekiti State relies heavily on hand-dug wells and boreholes in the weathered basement aquifer for domestic and agricultural water supply, yet quantified understanding of recharge rates, spatial variability, and pollution vulnerability in this geological setting is limited. A field investigation programme was conducted involving installation and monitoring of fourteen piezometers across four catchments, rainfall measurement, soil moisture profiling, and saturated hydraulic conductivity testing. Water table fluctuation analysis, chloride mass balance, and baseflow separation methods were applied in combination to estimate annual groundwater recharge. Aquifer vulnerability was mapped using the GOD index, incorporating vadose zone lithology, degree of confinement, and depth to water table data. Stable isotope analysis of shallow groundwater samples provided additional recharge pathway characterisation. Results yield mean annual recharge estimates between 120 and 210 millimetres across the four catchments, representing 17 to 26 percent of mean annual rainfall. Recharge rates show strong positive correlation with land cover, with forest-covered catchments recharging significantly more than farmed catchments. GOD vulnerability mapping identifies extensive high-vulnerability zones in shallow, unconfined parts of the aquifer beneath agricultural areas with intensive fertiliser use. The study provides a recharge and vulnerability baseline for groundwater management planning in Ekiti State. Keywords: groundwater recharge, aquifer vulnerability, basement complex, hydrogeology, Ekiti State.