Geophysical Surveys for Saltwater Intrusion Assessment Using Electrical Resistivity Tomography and Electromagnetic Induction Methods

Saltwater intrusion is as an environmental hazard in coastal lines if not appropriately managed. The over-exploitation, over-population and climate change have invited and pushed the saltwater landwards and polluted the freshwater aquifers. This research studies the results of the implemented project at the coast of Saint Andre' located in Koksijde, Belgium, to study this phenomenon through near-surface geophysics. Two geophysical methods, including Electrical Resistivity Tomography (ERT) and Electromagnetic Induction (EMI) were used to identify the saltwater intrusion. The present study aimed to investigate the possibility of saltwater intrusion, its extension and assess the government reclamation attempts to push back the saltwater. In the inversions, the Depth of Investigation Index (DOI) and the topography effect were evaluated. The subsurface conductivity of both methods was compared. The reliability of both methods to identify the saltwater intrusion has been established; however, the ERT survey provided a more comprehensive visualization than the EMI. The saltwater intrusion was found in the first 80 m of the coastal line with resistivity values of 2 to 5 Ohm.m; however, the infiltration of freshwater and the reclamation operation have stopped the further progress salinity into the dunes. Local possibilities of brackish water or clay lenses were identified with 7 to 25 Ohm.m resistivity values. The freshwater body was observed at distances between 120 and 220 m of the ERT line with values between 46 and 136 Ohm.m. The results were correlated with other studies, proving the reliability of the models.