Seasonal fluctuations in the groundwater level accelerate the removal of residual saltwater upstream of subsurface dams

Subsurface dams have been applied as an effective countermeasure against saltwater intrusion in coastal aquifers. However, part of the saltwater wedge ("residual saltwater") may be trapped upstream of the dam and take years to decades to remove, hindering groundwater exploitation. This study examines the mechanism of residual saltwater removal under seasonal groundwater level fluctuations induced by intermittent precipitation or pumping. The residual saltwater removal timescale for constant-head (CH) and fluctuating head (FH) scenarios are quantified and compared using field-scale numerical simulations. The results show that seasonal fluctuations in the groundwater table enhanced the dilution of residual saltwater, thereby accelerating the removal of residual seawater compared to the CH scenario. However, the critical dam height (i.e., the minimum height for complete removal of residual saltwater) is higher in the FH scenario than in the CH scenario. The sensitivity analysis indicates that the fastest residual saltwater removal in the FH scenario is generally obtained by a dam height slightly higher than the critical dam height. The impact of seasonal groundwater level fluctuations on residual saltwater removal timescale is more pronounced with a higher amplitude of inland hydraulic head variations, a lower average inland hydraulic head over the seasonal cycle, a larger dispersivity, a greater aquifer hydraulic conductivity, or a smaller porosity. These results provide valuable insights into the design of engineered subsurface dams and the management of existing subsurface barriers in coastal aquifers.