Identifying Seawater Intrusion and Hydrochemical Processes in the Cangzhou Area of the North China Coastal Plain

ABSTRACT

Cangzhou City on the western coast of the Bohai Sea, a typical coastal plain, was investigated using major ion data from 123 groundwater samples (63 shallow and 60 deep). The Hydrochemical Facies Evolution Diagram (HFE‐D) and the Groundwater Quality Index for Seawater Intrusion (GQI _SWI ) were applied to assess seawater intrusion and its hydrochemical evolution. Groundwater is predominantly brackish, with salinity decreasing inland from the coast, and seawater influence is markedly stronger in the shallow aquifer than in the deep confined aquifer. Compared with the single Cl ⁻ indicator, GQI _SWI reduces the impact of local anomalies and more reliably reflects the regional distribution of intrusion intensity. Hydrochemical facies evolve from Ca‐HCO ₃ to Ca‐Cl and then to Na‐Cl, and HFE‐D indicates that salinization results from freshwater–seawater mixing combined with reverse cation exchange and mineral dissolution rather than simple mixing alone. The shallow aquifer acts as the main zone of salt migration and transformation, whereas deep groundwater chemistry is controlled primarily by long‐term water–rock interaction. The combined application of GQI _SWI and hydrochemical evolution analysis identifies seawater intrusion from both quantitative and process perspectives and provides a scientific basis for groundwater management and zoning in coastal plains.