Countermeasures against saltwater intrusion (SWI) are critical to prevent coastal groundwater deterioration. Among different measures to prevent SWI, subsurface dams have shown to be an effective approach, but it is likely to produce residual saltwater behind the dam in a landward aquifer. This study investigated the influences of subsurface dam design and aquifer properties on the dynamic characteristics of residual saltwater in a field‐scale aquifer and for the first time revealed the desalinization mechanism of residual saltwater behind the dams from the point of mixing zone. It was found that the low‐concentration mixing zone (LCMZ) (for the area between 10% and 50% of seawater salinity) was a major channel for the saltwater to flow over the dam to the ocean boundary while the residual salt was continuously dispersed to the LCMZ from the high‐concentration mixing zone (HCMZ) (for the area between 50% and 90% of seawater salinity) under high‐concentration gradients. Moreover, we developed two formulas of the reduction rate of saltwater wedge length (RSWL*) and the removal rate of total residual salt mass (RTSM*) to evaluate the desalination effectiveness of high‐ and low‐concentration residual saltwater, respectively. The results showed that it took much longer time for a taller dam and a dam at a closer position to the sea boundary to desalinize the high‐concentration residual saltwater in the upstream aquifer, more than 50 years for the cases of dam height beyond 16 m. On the contrary, only a slightly shorter time was needed to remove the low‐concentration saltwater behind the dams with the decrease of the distance from the sea boundary. Aquifer properties including the hydraulic gradient, hydraulic conductivity, and dispersivity strongly altered the desalinization time of the residual saltwater. The dispersivity was found to be the most critical factor influencing the removal effectiveness of saltwater retained in the landward aquifer. Increase of dispersivity from 1 to 3 m can dramatically reduce the desalinization time from more than 30 to 4 years.

Recent studies have confirmed the instability of tide‐induced upper saline plume (USP) in tidal beach aquifers. A systematic understanding of how the unstable USP contributes to groundwater flow and salinity distribution is still lacking. Here we used laboratory experiments and numerical simulations to examine the responses of the salinity distribution and submarine groundwater discharge (SGD) to tide‐induced unstable flow. The results revealed that changes in physical forcing conditions (tides and inland freshwater input) drove the transition of the USP from stable to unstable states. Meanwhile, the phase‐averaged SGD and its components fluctuated over time, and their oscillation amplitudes depended on the physical forcing conditions. There was a correlation between USP instability and the fluctuation of water exchange across the aquifer‐sea interface. The fluxes across the aquifer‐sea interface under unstable conditions usually varied considerably over time with a lower frequency in comparison to stable conditions. The extent of seawater intrusion (SWI) measured by the total salt mass stored in the aquifer also varied over time, especially under unstable conditions. The vibration tendency of the total salt mass was opposite to that of the sum of inland freshwater discharge and tide‐induced circulation efflux. For coastal aquifers affected by tides, SWI was dominated by the sum of inland freshwater discharge and tide‐induced circulation efflux, followed by the sum of inland freshwater input and tide‐induced circulation influx. These findings lay the foundation for gaining a better understanding of tide‐induced unstable flow and its potential implications.