Development of a Predictive Saltwater Intrusion Model for the Lagos Coastal Aquifer System Under Climate and Urbanisation Scenarios

Saltwater intrusion into the Lagos coastal aquifer system driven by sea level rise, excessive groundwater abstraction, and land reclamation represents an escalating freshwater security threat for Africa's largest metropolis, yet predictive modelling capable of simulating future intrusion dynamics under compound climate and urbanisation forcing scenarios has not been developed for this system. This study develops and applies a predictive saltwater intrusion model for the Lagos coastal aquifer system using a variable-density, variably-saturated numerical modelling approach implemented in SEAWAT. The model domain extends fifty kilometres inland from the shoreline and is discretised to represent the principal freshwater aquifer units within the Holocene alluvial and Pleistocene Coastal Plain Sands. Aquifer geometry and hydraulic parameters are constrained by a compiled database of over three hundred borehole lithological logs and pump test records. The model is calibrated to current saltwater-freshwater interface positions established by electrical resistivity tomography surveys and borehole salinities measured in a dedicated monitoring network. Validated model simulations are run for four combined scenarios spanning high and low sea level rise trajectories from the Intergovernmental Panel on Climate Change Sixth Assessment Report and high and low urbanisation-driven abstraction intensification projections. The study quantifies the probability distribution of interface penetration depth and lateral extent by 2050 and 2080 using ensemble simulation methods. An original contribution is made in the representation of reclaimed land hydraulics in coastal aquifer models. Keywords: saltwater intrusion, SEAWAT, Lagos coastal aquifer, climate scenarios, sea level rise.