Mapping the saltwater-freshwater interface by integrating electrical resistivity tomography and 3D numerical...

Seawater intrusion (SI) is a special groundwater salt pollution problem. Integrating numerical model and electrical resistivity tomography (ERT) method can better study the salt-freshwater interface (SFI). However, few studies have combined them to investigate SI. This research, conducted in Longkou city, Shandong province, China, synergistically employed both methods to assess SI. In November of 2020, fifty groundwater samples were collected from monitoring wells to measure Cl^- concentration. Seventeen survey lines were deployed, and ERT inversion profiles were generated accordingly. A three-dimensional numerical model simulating groundwater flow and solute transport was established with three vertical layers. The SFI was identified from eight typical ERT inversion profiles, with the closest and farthest interfaces located 1.8 km and 4.2 km from the coastline. The model effectively captured the actual groundwater flow field. It also demonstrated high performance in simulating Cl^- concentrations, with high NES (0.76 and 0.65) and R² (0.84 and 0.83) values, and low relative RMSE at wells W1-2 and W2-2. The SFI derived from ERT was further verified through numerical simulations. The first model layer exhibited the most severe SI, and it was selected as the focus for detailed analysis. In the wet season of June to August 2020, the simulated SFI shifted landward, and vice versa in the dry season. This integrated methodology offers an effective qualitative tool for locating SFI in coastal zones, supporting informed groundwater management and SI mitigation.