Geoelectric layer susceptibility and vulnerability index mapping of Ifedore Local Government Area, Ondo State, Nigeria

Groundwater contamination remains a critical threat to water security, especially in areas where aquifers serve as the main source of potable water. In Ifedore Local Government Area, Ondo State, Nigeria, groundwater vulnerability has not been fully assessed, posing a risk to water resources. This study evaluates the region's vulnerability using two complementary models: the Groundwater Occurrence, Overlying Lithology, and Depth to Aquifer (GOD) model and the Geoelectric Layer Susceptibility Index (GLSI). The aim is to assess the protective capacity of aquifers and identify areas prone to contamination. Vertical Electrical Sounding (VES) data were utilized to derive geoelectrical parameters for the GLSI, while basic hydrogeological data supported the GOD model. Results from the GOD model indicate 60 % low, 3 % medium, and 37 % negligible vulnerability. The GLSI model, which integrates resistivity and vadose zone thickness, reveals 20 % low, 73 % medium, and 7 % high vulnerability. Both models indicate that vulnerability ranges from low to moderate, with some areas more susceptible to surface contaminants. Sensitivity analysis was conducted by introducing ±10 % variations to the parameters of both models. The findings show moderate sensitivity, with vulnerability classifications shifting in several VES locations, particularly in the GLSI model. This highlights the need for precise data acquisition—especially for parameters like resistivity and layer thickness—to ensure accurate assessments. Additionally, land use and land cover (LULC) analysis was integrated to evaluate anthropogenic influence on aquifer vulnerability. Areas with agricultural and urban land uses showed elevated risks, underlining the importance of managing land activities near aquifer recharge zones. The study concludes that combining GOD, GLSI, and LULC data enhances groundwater vulnerability assessments and supports the development of effective water protection and land-use strategies in data-scarce, crystalline terrains.