Applying In-well ERT to characterize and monitor fresh-saltwater interface in coastal aquifer：A Laboratory and Numerical Study

Saltwater intrusion (SWI) and submarine groundwater discharge (SGD) are critical processes influencing coastal aquifer dynamics. A transition zone formed where fresh and saline groundwater mixed at the coastline. However, the position of the interface shifts due to cyclic tides and varying freshwater discharge rates. Understanding characteristics of the fresh-saltwater interface is crucial to analyze the development of groundwater salinity in land subsided coastal areas.

Here, we present the laboratory experiment of In-well Electrical Resistivity Tomography (ERT) under controlled saltwater intrusion tests. This approach leads to a straightforward detection of fresh-saltwater interface incorporating the temporal dynamics of real systems. Unlike conventional ERT, it offers higher resolution imaging at greater depths and captures more accurate subsurface data, particularly around the fresh-saltwater interface. To establish our research in investigating the local groundwater salinization distribution using in-well ERT, Numerical experiments were initially conducted using COMSOL to determine the minimum electrode spacing that would not significantly impact the results.     These simulations helped identify the optimal spacing required to maintain the accuracy and reliability of the measurements, given the limited space available in the laboratory. Based on the findings from these numerical experiments, a laboratory-scale setup was designed and implemented in a vertical direction within a cylinder tank.

In the present study, a clear interface was observed and measured, through density flow, mounted observation tubes, dyeing and a series of operations. Preliminary results show a strong correlation between the measured resistance values and the actual interface changes. Additionally, the numerical experiments simulated real well conditions, including the thickness of the internal reinforced mud cake, and incorporated a detailed electrode structure (ring structure). This work provides valuable insights through laboratory results coupled with model simulations, which are essential for the future real-site application at the lower reach of Nabaki River, Chiba, Japan—a typical tidal river system in a below-sea-level land subsided area.