Elucidating the Pollution Sources and Groundwater Evolution in Typical Seawater Intrusion Areas Using Hydrochemical and Environmental Stable Isotope Technique: A Case Study for Shandong Province, China

Abstract

Seawater intrusion has a serious impact on industry, agriculture, and people’s daily life. Thus, the present study was designed to elucidate the pollution sources and groundwater evolution in typical intrusion areas of Shandong Province by hydrochemistry and environmental isotope techniques. The water samples were collected to analyze the groundwater evolution under different groundwater level dropping funnel caused by overexploitation was the direct causation of seawater intrusion in the three typical intruded areas. The groundwater evolution paths demonstrated that the groundwater in the south of Laizhou Bay had the fastest evolution rate and the highest degree of evolution, followed by the Dagu River Basin. The groundwater evolution extent and fitting of mixing lines indicated that the groundwater in the south of Laizhou Bay, Longkou, and Dagu River Basin was dominated by palaeosaltwater intrusion, modern seawater intrusion, and sea-saltwater mixed intrusion, respectively. Palaeosaltwater mixing produces a more severe salinization effect compared to seawater mixing. Meanwhile, the isotopes are gradually enriched with the deepening of intrusion, while the decrease of isotopes is delayed compared with the saltwater retreat. This is caused by that the stable isotopes enriched in the aquiclude due to the chemical permeation effect will be released into the aquifer after the salinity attenuates in the aquifer. The palaeosaltwater intrusion caused by anthropogenic activities has promoted serious fluorine pollution in the south of Laizhou Bay, while the groundwater nitrate pollution in Longkou was the most serious, followed by the Dagu River Basin due to high-density agricultural and domestic activity.