Impact of River-groundwater Interactions on Residual Saltwater Pollution in Estuarine Groundwater Reservoirs

The hydrodynamics of estuarine groundwater reservoirs is governed by both subsurface dams and river systems. However, existing research has largely overlooked how river-groundwater interactions influence groundwater discharge and residual saltwater desalination. This study combines field data from the Dagu River Basin with numerical simulations to explore how surface water-groundwater interactions promote residual saltwater desalination behind subsurface dams in coastal aquifers. The results indicate that continuous river-supplied freshwater creates a hydraulic barrier, limiting saltwater intrusion and enhancing freshwater infiltration. Following the construction of dam, Cl flux at the ocean boundary stabilizes after approximately 20 years, reflecting a phased desalination process driven by hydraulic adjustments. Key factors such as riverbed sediment conductivity, sediment thickness, and dam height significantly enhance or diminish desalination efficiency. Higher riverbed hydraulic conductivity (e.g., 10 m/d) and thinner sediment layers (e.g., 5 m) accelerate freshwater penetration and salt discharge, achieving near-complete desalination within 20 years. Increasing dam height to 24-28 m raises the hydraulic gradient, enabling the total salt mass removal rate to reach 1 within 20-40 years, indicating complete desalination. Conversely, greater sediment thickness (e.g., 25 m) and low hydraulic conductivity (e.g., 0.5 m/d) slow the desalination process, extending the timeframe to nearly 100 years and increasing parameter sensitivity. Although dam height has a significant effect on desalination dynamics, the distance from the ocean has minimal influence once an effective hydraulic barrier is established. These findings emphasize optimizing dam height and sediment properties to improve desalination efficiency, offering guidance for groundwater management in saltwater-affected regions.