Empirical Study of Groundwater Salinity Intrusion Dynamics in Coastal Aquifers of Lagos Using Geophysical Methods

Seawater intrusion into coastal freshwater aquifers is a growing threat to groundwater quality in Lagos State, driven by excessive abstraction from beach ridges and barrier island aquifer systems combined with rising sea levels. This study empirically characterizes the spatial extent and temporal dynamics of saltwater intrusion in coastal aquifers of Lagos State using time-domain electromagnetic sounding and two-dimensional electrical resistivity tomography surveys complemented by hydrochemical analysis. Geophysical surveys were conducted along twelve profiles perpendicular to the shoreline across three areas: Victoria Island, Lekki Peninsula, and Badagry. Time-domain electromagnetic soundings were acquired at 80 stations and interpreted using one-dimensional layered earth inversion. Two-dimensional electrical resistivity tomography profiles of 300 m length were acquired using a 100-electrode resistivity system. Groundwater samples from 45 boreholes along survey profiles were analyzed for major ion chemistry, chloride concentration, and total dissolved solids. The freshwater-saltwater interface was mapped by integrating resistivity contours below 10 ohm-m as a proxy for saline water at each survey location. Results indicate that the saltwater interface has migrated landward by 80 to 240 m relative to estimated pre-development positions at Lekki Peninsula, with the greatest advancement corresponding to areas of highest borehole abstraction density. The study develops a saltwater intrusion vulnerability index map for coastal Lagos aquifers and identifies priority zones for managed aquifer recharge interventions. Keywords: saltwater intrusion, coastal aquifer, Lagos, electrical resistivity, hydrogeophysics