Groundwater-Seawater Interaction Along the Victoria Island Coastline, Lagos

Submarine groundwater discharge and seawater intrusion into coastal aquifers are increasingly recognised as significant processes shaping nearshore water chemistry and nutrient budgets in densely populated coastal cities. This study investigates groundwater-seawater interaction dynamics along the Victoria Island coastline, Lagos State, South West Nigeria, where rapid high-rise development, reduced pervious surfaces, and heavy groundwater abstraction are altering natural hydrological gradients. Piezometric measurements from 15 monitoring boreholes spanning the coastal belt are collected bimonthly during 2024. Hydrochemical analysis of groundwater samples measures major ions (sodium, chloride, calcium, magnesium, bicarbonate), electrical conductivity, and stable isotopes of oxygen and hydrogen to delineate freshwater-saltwater mixing zones. Chloride to bicarbonate ratios and Piper diagram classifications are used to assess the degree of seawater intrusion. Radon-222 activity measurements in nearshore coastal waters provide independent estimates of submarine groundwater discharge fluxes. Findings are expected to reveal progressive salinisation of shallow aquifers within 200 metres of the coastline, with deeper confined aquifers showing evidence of historical marine transgressions. The study quantifies the nutrient contribution of submarine groundwater discharge to Lagos coastal waters and its potential role in stimulating phytoplankton growth. Recommendations are directed at the Lagos State Water Corporation and urban planners. Keywords: submarine groundwater discharge, seawater intrusion, Victoria Island, coastal aquifer, Lagos hydrogeology