Geophysical Characterisation of Saline Intrusion Dynamics in the Lagos Coastal Aquifer Under Climate Change Projections

The Lagos coastal aquifer is under increasing threat from saline water intrusion driven by groundwater overabstraction and sea-level rise, yet the dynamic response of the freshwater-saline water interface to projected climate change and abstraction scenarios has not been quantified using a geophysically constrained numerical model. This study characterises the current saline intrusion geometry through integrated time-domain electromagnetic surveys and develops a variable-density groundwater flow model calibrated to the geophysical data to simulate the interface response under climate change scenarios. TDEM soundings at 45 stations across the coastal zone from Lagos Island to Badagry provide depth-specific resistivity information used to map the three-dimensional position of the freshwater-saline water interface. The SEAWAT variable-density flow model was constructed using a geological framework constrained by the TDEM-derived interface geometry and available borehole lithological logs. Calibration against monitored groundwater heads and chloride concentrations at 18 wells achieved a satisfactory RMS error in hydraulic head of 1.6 metres. Climate change scenarios based on CMIP6 ensemble projections for a 1.0-metre sea-level rise by 2100 combined with a 15 percent increase in recharge deficit are simulated. Results indicate that the saline interface would advance 1.8 to 3.4 kilometres landward by 2100 under the combined climate scenario, placing three major wellfields within saline intrusion risk zones. The study recommends immediate implementation of managed aquifer recharge to counteract the projected advance. Keywords: saline intrusion, TDEM, Lagos coastal aquifer, SEAWAT, climate change