Time-Lapse Resistivity Monitoring of Saltwater Intrusion in the Lagos Coastal Aquifer

This study applies time-lapse electrical resistivity tomography to monitor the dynamics of saltwater intrusion in the coastal aquifer of Lagos State, South West Nigeria. Saltwater intrusion in the Lagos coastal aquifer is a progressive and accelerating threat to freshwater supply, driven by intensive groundwater abstraction exceeding recharge rates in combination with sea level rise effects. Conventional monitoring using water level and conductivity observations in boreholes provides limited spatial information about the intrusion front geometry and its temporal evolution. Time-lapse ERT provides a spatially distributed view of resistivity changes that can be related to salinity variations. This study establishes permanent ERT monitoring profiles at four coastal sites in Lagos Island, Apapa, and Ajah districts, with 48 electrodes at 5-metre spacing in a Wenner array configuration. Data are acquired quarterly over a two-year period to capture seasonal and longer-term intrusion dynamics. Baseline resistivity sections are subtracted from repeat measurements to produce time-difference sections highlighting changes. Correlation with quarterly water quality monitoring from adjacent boreholes validates resistivity-salinity relationships. Findings reveal systematic decreases in formation resistivity of 15 to 45 percent in the 10 to 25 metre depth interval at all four sites over the two-year monitoring period, consistent with progressive salinisation of the freshwater-saltwater transition zone. The intrusion rate estimated from resistivity change geometry averages 8 to 12 metres per year, accelerating in the dry season due to reduced freshwater head. One site shows evidence of lateral intrusion from an inland direction, suggesting canal connectivity. The study recommends time-lapse ERT as a cost-effective saltwater intrusion monitoring tool for Lagos groundwater management.

Keywords: time-lapse ERT, saltwater intrusion, Lagos coastal aquifer, resistivity monitoring, groundwater management.