Groundwater recharge sources analysis of coastal aquifer under the impact of cut-off walls: A case study of the Dagu River

Water quality degradation due to seawater intrusion is a globally significant environmental and geological issue. Cut-off walls, besides mitigating seawater intrusion, can alter groundwater flow direction and rate. Re-analysis recharge sources in coastal aquifers with cut-off walls is crucial for hydrogeological research and water resource management. Our study focused on the Lower Dagu River, where cut-off walls were installed. We employed the M3-MIX method to analyze groundwater recharge sources. The qualitative assessment revealed a substantial reduction in regional hydraulic connections across the cut-off walls, although weak seasonal connections persist due to groundwater level fluctuations and wall heights. Using M3-MIX, we identified four recharge sources in the south area-three salt groundwater sources and one saline surface water source-with mixing ratios of 1.4 %, 44.3 %, 41.8 %, and 12.5 %, respectively. In the north area, three recharge sources were identified-a river water sample, a saline surface water sample, and a groundwater sample-with mixing ratios of 14.9 %, 26.8 %, and 58.3 %, respectively. Our findings underscore the impact of cut-off walls on recharge sources and highlight the importance of re-analysis these sources for effective exploitation and protection of coastal aquifer water resources.