Combining long-term monitoring system data with on-site experiments to analyze groundwater salinization and seawater intrusion in the coastal areas of Yunlin, TAIWAN

Groundwater serves as the main freshwater resources for water supply in coastal area of Yunlin, Taiwan. However, the development of agriculture, fisheries, and industry, has experienced excessive exploitation of groundwater. This has led to an imbalance in pressure between groundwater and seawater, resulting in an escalating issue of groundwater salinization. Clearly defining the interface between groundwater and seawater and examining the mechanisms behind groundwater salinization is an urgent task for water resources management. This study aims to establish a conceptual model for hydrogeology and seawater intrusion in the coastal areas of Yunlin using various types of data and investigation methods. The methods conducted in this study includes the electrical resistivity tomography (ERT), temperature-depth (TD) profiles, and the numerical model of Quasi-2D to delineate the potential region of salinized groundwater in the coastal area of Yunlin. Based on the long-term water quality data, Yiwu is identified as the potential impact zone of seawater intrusion where the salinity of groundwater exceeds the standard of freshwater. ERT was conducted in the shallow aquifer of Yiwu area and the results indicated that the aquifer is influenced by tidal effects. The thermal contour, derived from TD profiles obtained from nearby observation wells, depict the interface of freshwater and seawater. Subsequently, a Quasi-2D numerical model using TD profiles is employed to simulate groundwater flow directions and velocities around the interface. Ultimately, the hydrogeological conceptual model for seawater intrusion is developed. This study demonstrates that utilizing the long-term measurements and aforementioned series of methods is effective in delineating the interface of freshwater and seawater and developing the conceptual model of seawater intrusion.   Keywords: seawater intrusion, electrical resistivity tomography, temperature, numerical model