Seawater intrusion on the west coast of Shenzhen during 1980-2020 due to the influence of anthropogenic activities.

Anthropogenic activities are considered key factors to affect the evolution of seawater intrusion (SWI) status. Understanding the relationships between anthropogenic factors and SWI development is crucial to formulate strategies that are used to mitigate groundwater salinization in coastal areas. In this study, we analyzed changes in land use on the west coast of Shenzhen, Guangdong province, China, over the recent four decades based on remote sensing data, and evaluated the SWI degrees in three historical stages during 1980-2020 based on the hydrochemistry data. Then, combining the timelines of groundwater exploitation, land use, land reclamation, and groundwater salinization, we presented the evolution of SWI affected by anthropogenic activities on the west coast of Shenzhen. It is found that the SWI can be divided into three stages: 1988-1999, a fully developing period; 2000-2009, a partly degrading period; and 2018-2020, a fully degrading period. The interface of saline and fresh groundwater paralleling with the coastline advanced 2 km inland in 20 years and took the next 20 years to retreat about 1 km. The interface advancing and retreating correspond to the excess and the prohibition of groundwater exploitation, respectively. Meanwhile, the construction and demolishment of high-position saltwater aquaculture areas, respectively, corresponded to the increase and decrease of Cl- concentrations in these areas. Besides, the correlation between seawater mixing index (SMI) values and Na+ concentrations became much lower during the desalination of groundwater, which can be considered direct evidence for the SWI retreat.