Analysis of the Seawater Intrusion Process Based on Multiple Monitoring Methods: Study in the Southern Coastal Plain of Laizhou Bay, China

Abstract

Affected by natural factors and human activities, seawater intrusion has become a geo-environmental problem, significantly impacting human production and life. Seawater intrusion weakens coastal areas’ geo-environmental carrying capacity, limiting industry and commerce development potential. On the other hand, it may provide convenient deep seawater resources for coastal aquaculture development. Therefore, how to quantitatively analyze seawater intrusion’s process, scope, and influencing factors has become a hotspot for hydrogeological researchers. This study uses chemical sampling analysis, environmental isotope, fixed-point, and geophysical methods to monitor long-term seawater intrusion in the southern coastal plain of Laizhou Bay. According to the monitoring data, the chemical type of fresh groundwater changes from Ca·Mg-HCO3 to Na-HCO3·Cl, Na·Ca-HCO3·Cl from south to north, and the changing trend of brackish groundwater is from Mg·Na·Ca-HCO3, Mg·Ca-HCO3 to Na-Cl·HCO3, Na·Mg-Cl. Saline groundwater is mainly of the Na-Cl and Na·Mg-Cl type. Brine is of the Cl-Na type. The relationship between 18O, 2H, and Cl− shows that groundwater was affected by evaporation, dissolution, and mixing in the runoff process. The relationship between water level and TDS in monitoring wells at different locations and depths confirms the existence of seasonal variations and layered intrusion phenomena in groundwater recharge sources. From July 2018 to December 2019, the south intrusion and north retreat rates were approximately 213.3 m/a and 105.9 m/a, respectively. From August 2016 to December 2019, the seawater intrusion front on the Dawangdong profile generally retreated northward at approximately 27 m/a. The results of this study can provide a scientific basis for the utilization of groundwater in local production and life. Comparative analysis and mutual verification of multiple monitoring methods can provide basic ideas for constructing a multi-source monitoring system for seawater intrusion.