Geophysical Delineation of Saline Intrusion in Coastal Aquifers of Bayelsa State

Coastal aquifer systems in Bayelsa State, South South Nigeria, are increasingly threatened by saline water intrusion driven by sea level variability, tidal forcing, and excessive groundwater abstraction associated with growing coastal communities and oil industry camps. This study employs electrical resistivity tomography to delineate the extent of saline intrusion in shallow coastal aquifers across three communities in Bayelsa State. Multi-electrode resistivity surveys were conducted using the Wenner-Schlumberger array along transects perpendicular to the shoreline, and two-dimensional resistivity inversion models were produced using RES2DINV software. The freshwater-saltwater interface was identified on inverted resistivity profiles as the boundary between low-resistivity saline zones and higher-resistivity freshwater-bearing sediments. Borehole electrical conductivity logs and water samples collected from existing wells were used to calibrate the resistivity interpretation. Seasonal variations in interface position were assessed by comparing survey results from wet and dry season campaigns. The study identifies three communities where the freshwater lens has been significantly thinned, with saline water reaching depths as shallow as eight metres during the dry season. These findings are communicated to the Bayelsa State Rural Water Supply and Sanitation Agency to guide borehole siting and managed aquifer recharge planning. Keywords: saline intrusion, resistivity tomography, coastal aquifer, Bayelsa State, freshwater lens.