Abstract Seawater intrusion (SI) is an increased threat to the groundwater resource in coastal regions. The cut-off wall is widely used to arrest the advance of a seawater wedge, but it is likely to cause groundwater contamination in landward aquifers. In order to alleviate the contamination problem, we proposed a new variable-permeability cut-off wall (VPFW) with a semi-permeable section at the bottom of the physical barrier. This study is the first to investigate the spatial distributions and transient behaviors of SI and nitrate pollution at field scale under the joint effects of physical barriers and denitrification. Results show the dissolved oxygen (DO) decreases almost synchronously with the intrusion of SW near the sea boundary, and the denitrification of NO3− and retreat of the dissolved organic carbon (DOC) wedge occurred in the inland aquifer. Notably, the freshwater discharge flux and nitrate discharge flux for the developed VPFW increase by 35% and 20%, respectively, while the enrichment ratio of specific volume of nitrate contamination zone (SVNR) went down by 15% on average compared with the traditional cut-off wall. Four indices were proposed to quantify the impervious barrier: heights (H), relative hydraulic conductivities (K*), and control effectiveness of SI and nitrate pollution. Moreover, we found the control effectiveness for SI and nitrate pollution was continually enhanced with the reduction of K* and increase of H; however, excessive reduction and increase may result in the opposite impacts. The optimal K* and H for the VPFW can be determined with the required value of the reduction rate of saltwater wedge length (SWR) and SVNR; but all cut-off wall cases fail to meet the demand. The results show that the VPFW is much more effective in controlling SI and nitrate pollution in comparison with the traditional cut-off wall.

中文翻译：

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变渗透全断面墙体对增强控制非承压含水层海水入侵和硝酸盐污染的见解

摘要 海水入侵（SI）是对沿海地区地下水资源的日益威胁。隔水墙被广泛用于阻止海水楔的前进，但它很可能导致向陆含水层的地下水污染。为了缓解污染问题，我们提出了一种新的可变渗透性截止墙（VPFW），在物理屏障的底部有一个半渗透部分。本研究首次在物理屏障和反硝化作用的共同作用下，研究了现场尺度 SI 和硝酸盐污染的空间分布和瞬态行为。结果表明，溶解氧 (DO) 几乎与海边界附近 SW 的入侵同步减少，内陆含水层发生了 NO3− 的反硝化作用和溶解有机碳（DOC）楔的后退。值得注意的是，与传统切割相比，开发的 VPFW 淡水排放通量和硝酸盐排放通量分别增加了 35% 和 20%，而硝酸盐污染区比容富集率（SVNR）平均下降了 15%。 -离墙。提出了四个指标来量化不透水屏障：高度 (H)、相对水力传导率 (K*) 以及 SI 和硝酸盐污染的控制效果。此外，我们发现随着K*的降低和H的增加，对SI和硝酸盐污染的控制效果不断增强；但是，过度减少和增加可能会导致相反的影响。VPFW 的最佳 K* 和 H 可以通过所需的咸水楔长 (SWR) 和 SVNR 减少率的值来确定；但所有的隔断墙案例都不能满足需求。结果表明，与传统的防渗墙相比，VPFW 在控制 SI 和硝酸盐污染方面更有效。