Salinization Characteristics and Controlling Mechanisms of Coastal Groundwater in Tianjin, China

Coastal groundwater systems are highly susceptible to seawater intrusion and salinization. However, complex depositional settings and multiple geochemical processes have made it difficult to understand the mechanism of groundwater salinization. To characterize coastal groundwater salinization and elucidate its controlling mechanisms in Tianjin, China, a total of 228 groundwater samples were collected. The degree of groundwater salinization and associated hydrochemical evolution was systematically investigated using the groundwater quality index for seawater intrusion (GQI_SWI) and some hydrochemical indicators and evolutionary diagrams. Hydrochemical analyses revealed an overall evolution from HCO₃-Ca type to Cl-Na type groundwater, with distinct salinization geochemical evolution pathways among different aquifers. The GQI_SWI assessment indicated that both shallow and deep groundwater in Tianjin are dominated by fresh and brackish waters, with only minor differences in the overall degree of salinization along the vertical profile. Salinization of shallow groundwater is mainly concentrated in coastal areas and generally decreases progressively from the coastline toward inland areas. In contrast, the spatial distribution of salinity in deep groundwater is characterized primarily by localized high-salinity anomalies, without the development of a continuous salinization zone. Groundwater salinization in Tianjin therefore cannot be attributed solely to modern seawater intrusion; rather, it is jointly controlled by modern coastal salt inputs, freshwater recharge, long-term water-rock interactions, and ion exchange. These findings provide new insights into the identification of seawater intrusion and the interpretation of salinization mechanisms in complex coastal groundwater systems and offer a scientific basis for groundwater resource protection and management.