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.

中文翻译：

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无限制含水层中地下大坝后面的海水淡化残余海水的时间表和有效性

防止盐水入侵（SWI）的措施对于防止沿海地下水恶化至关重要。在防止SWI的不同措施中，地下大坝已被证明是一种有效的方法，但很可能在陆上含水层中的大坝后面产生残留的盐水。这项研究调查了地下大坝设计和含水层特性对田间规模含水层中残余咸水动力特性的影响，并首次从混合区的角度揭示了大坝后面残留咸水的脱盐机理。发现低浓度混合区（LCMZ）（用于海水盐度的10％和50％之间的区域）是盐水流过大坝到达海洋边界而残余盐连续分散的主要通道。在高浓度梯度下，从高浓度混合区（HCMZ）（海水盐度介于50％和90％之间）进入LCMZ。此外，我们开发了两个公式来减少盐水楔形长度（RSWL *）和总残留盐质量的去除率（RTSM *）分别评估高浓度和低浓度残留盐水的脱盐效果。结果表明，高坝和离海边界较近的坝要花费更长的时间来使上游含水层中的高浓度残留盐水脱盐，而对于坝高超过16 m的情况，则要花费超过50年的时间。 。相反，随着距海边界距离的减小，去除大坝后面的低浓度盐水仅需要一点时间。包括水力梯度，水力传导率和分散性在内的含水层特性极大地改变了残留盐水的脱盐时间。发现分散性是影响陆上含水层中保留的盐水去除效果的最关键因素。