The safety of groundwater resources in coastal areas is related to the sustainable development of the national economy and society. Seawater intrusion is a serious problem threatening the groundwater environment in coastal areas. Climate change, tidal effects and groundwater exploitation may destroy the balance between salt water and fresh water in coastal aquifers, leading to seawater intrusion. The threat of seawater intrusion has attracted close attention, especially in the coastal areas of northern China, and the accuracy and efficiency of seawater intrusion monitoring need to be improved. The aim of this study was to fill the blanks in seawater intrusion research in the coastal aquifer of the Daqing River Catchment, northeastern Liaodong Bay, China, and determine the extent and evolutionary characteristics of seawater intrusion in this area. In this study, historical chloride concentration data were used to trace the evolution of the salinization, and electrical resistivity tomography (ERT) was used to supplement the data in areas with limited hydrochemical data and to detect the saltwater/freshwater interface, especially in the area near the Xihai Sluice. The results show that seawater intrusion in the Daqing River Catchment is mainly caused by overexploitation of groundwater. Since 2012, strict controls have been placed on the groundwater exploitation rate, and the chloride concentration of 250 mg/L has receded year by year, with saltwater being significantly reduced by 2018. The Daqing River plays an important role in the saltwater distribution. The Xihai Sluice, located in the lower reaches of the Daqing River, intercepts and controls the seawater intrusion in a certain range by raising the level of fresh groundwater to intercept and control saltwater intrusion within a certain range. The research results also confirmed that a combination of geophysical and geochemical methods is of great value in studying seawater intrusion, especially in areas with limited available hydrochemical data. A monitoring network with ERT instruments and wells should be established to collect regular measurements of the electrical resistivity distribution, as well as the groundwater level and chemical composition.

Groundwater over-pumping in estuary cities leads to a series of groundwater environmental problems that seriously restricts economic development. On the basis of field investigation and long-term monitoring data analysis, a three-dimensional numerical model was built in the estuary of the Daqing River in Liaodong Bay, China. The Quaternary overburden can be generalized into five layers according to particle composition and parameters in the vertical direction. There are many scattered irrigation wells pumping in the second layer, and three water source areas mainly pumping groundwater in the fourth layer. Long-term over-pumping in multi-layered aquifers causes onshore layered seawater intrusion. The laws of layered intrusion under the layered pumping were calculated and analyzed with SEAWAT-2000, and the sensitivity was analyzed with the Sobol method. Results showed that the intrusion area had an obvious layered law. Layered pumping directly affected the layered intrusion area, as different permeability, tide and barrage further affected it. The prediction study showed that the cone of depression recovered after the pumping-limit of water source areas, and the intrusion area started to retreat in the fourth layer. At that time, the pumping quantity of irrigation wells became the main reason for the increase of the intrusion area. If the water source areas are used to bear part of the irrigation demand, so as to reduce the pressure of pumping in the second layer, the overall intrusion area can be reduced by about 0.23 km2 under the same pumping quantity.

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