Geophysical assessment of seawater intrusion in Apapa-Ajegunle, coastal area of Lagos, Southwestern Nigeria

The paper looks at the interaction of seawater and freshwater in the aquifer beneath the Apapa-Ajegunle area, southwestern Nigeria. Seawater intrusion is fast becoming an increasing threat to the Lagos Apapa-Ajegunle coastal aquifers, which have been subjected to deteriorating conditions due to rapid urbanization, industrial waste discharge, and over-exploitation of groundwater in the last few years. This study combines 1D Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Tomography (ERT) alongside a geostatistical analysis to map the severity of saline intrusion and spatial extent within the shallow aquifer system. Twenty-six (26) VES points and fourteen (14) ERT traverses utilizing Schlumberger and Wenner arrays, respectively, alongside borehole data from 10 locations, were employed. The findings indicate 4-5 subsurface layers with resistivity values varying between 1.0 and 50,022 Ωm. The results indicate that the layer of saline water was encountered at depths of 1-40 m, characterized by a resistivity range of 1.0-500 Ωm, particularly in southern, eastern and western zones that represent influences from nearby water bodies. Using VES profiles, brackish layers were detected with resistivities ranging from 8 to 5022 Ωm, and saline layers with as low a resistivity as 1.8 Ωm. Freshwater aquifers were encountered with resistivities above 100 Ωm, but in many cases were covered by a saline or brackish layer, making the extraction intricate. This evidence points to the necessity of the groundwater management plan of an amazing kind. The sea intrusion causes the most threatening challenges to the availability of potable water, as well as the stability of ecosystems. To maintain the water reserves and to guarantee a sustainable water supply in the region, it is necessary that such measures be implemented as continuous geophysical monitoring, controlled groundwater abstraction, and pollution mitigation strategies.