The impact of land subsidence and mitigating measures on near-surface groundwater salinities at the south bank of the Nabaki River, Japan

Land subsidence is mainly caused by over-exploitation of groundwater, and it has been resulting in several problems along the coastal areas in Japan such as damages to buildings and facilities, changes of stream slopes, and increased risks of river flooding during high tides or storm surges. At the south bank of the Nabaki River in the Kujukuri coastal plain, Japan, some inland areas have become below the mean sea-level due to land subsidence. To prevent flooding in these areas, the local authority has constructed pumping stations and ditch networks at both sides of the tidal river since 1960s. The former can discharge the unnecessary water out to keep the land surface areas dry while the latter contributes to the agricultural productions by efficiently discharge water out. However, the pumping and discharging behaviours result in lowering groundwater levels that may cause further seawater intrusion. Here, a numerical model was developed by using the HydroGeoSphere code to investigate how the land subsidence and mitigating measures affect the quality of near-surface groundwater resources. The model solved coupled surface-subsurface flow and mass transport processes with the variable-density effect. Different scenarios were designed to compare the situations with and without land subsidence and pumping activities. The simulation results showed that, although the pumping stations and ditch systems performed effectively for preventing flooding associated with land subsidence, this system can enhance seawater intrusion to the inland aquifer from the tidal river. The results suggest that the pump stations and ditch systems built for preventing floods in subsidence areas should be carefully evaluated for their potential impacts on the groundwater flow regime and water quality.