Coupled Geophysical-Hydrological Modelling of Saltwater-Freshwater Dynamics in the Niger Delta Coastal Aquifer System

This study develops an original coupled geophysical-hydrological modelling framework for simulating and predicting saltwater-freshwater dynamics in the Niger Delta coastal aquifer system, integrating time-lapse geophysical monitoring with numerical groundwater flow and solute transport modelling. The Niger Delta coastal aquifer system supports freshwater supply for over 25 million people in Rivers, Delta, Bayelsa, and Cross River States, but is under severe threat from saltwater intrusion driven by sea level rise, tidal inundation, and over-abstraction. Understanding the dynamic behaviour of the saltwater-freshwater interface requires a framework that can assimilate real-time geophysical monitoring data into a predictive numerical model. This study develops a Kalman filter data assimilation framework linking time-lapse electrical resistivity measurements from a network of permanent ERT monitoring profiles (12 profiles, 48 electrodes each) to a SEAWAT variable-density groundwater flow model calibrated for the Niger Delta coastal sedimentary sequence. The ERT forward model is embedded in the assimilation framework using a petrophysical transform linking resistivity to salinity via Archie's law with Niger Delta-specific parameters derived from laboratory core experiments. The coupled model is validated against water quality monitoring data from 24 observation boreholes over a two-year period. Predictive simulations assess interface dynamics under sea level rise scenarios (0.3 to 1.0 metre by 2100) and abstraction management strategies. Findings reveal that the assimilated model reproduces observed salinity distributions with mean absolute error of 0.8 grams per litre across the monitoring period, validating the geophysical-hydrological coupling. Under the 0.6 metre sea level rise scenario, 42 percent of current freshwater wells in the coastal zone are projected to become saline by 2080 without management intervention. The study contributes an original operational coupled monitoring-modelling system.

Keywords: saltwater intrusion, coupled modelling, Niger Delta, data assimilation, coastal aquifer.