Hydrochemical Characteristics and Water-Quality Evaluation of Groundwater in the Ziya River Plain, Hai River Basin

The Ziya River Plain, located in the Haihe River Basin, is both a major grain-producing region and a key economic development zone in Hebei Province. Prolonged dependence on groundwater extraction has led to some issues, including water salinization and excessive hardness. Clarifying the hydrochemical characteristics and water quality status of groundwater in this region is therefore essential for the sustainable and rational utilization of local water resources. Here, a total of 104 groundwater samples were collected from deep aquifers and 287 samples from shallow aquifers. These samples were analyzed for major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺), and major anions (Cl⁻, \mathrmHCO_3^- , \mathrmSO_4^2- , \mathrmNO_3^- ), as well as additional parameters such as total hardness, ammonium nitrogen, iron, and manganese. Statistical analysis, Piper trilinear diagrams, and GIS-based spatial interpolation were employed to investigate the fundamental hydrochemical features. Additionally, an absolute principal component score–multiple linear regression (APCS-MLR) model, incorporating a modified geochemical accumulation index, was applied to identify the major pollution sources affecting the shallow aquifers. The results reveal significant spatial variation in groundwater hydrochemical types across the study area. Shallow groundwater transitions from an HCO₃-Ca·Mg type in the piedmont areas to a SO₄·Cl-Na type near coastal regions. Deep groundwater is mainly characterized by SO₄·Cl-Na and HCO₃·Cl-Na types. The overall quality of shallow groundwater is relatively poor, with Class Ⅴ water accounting for 71.25% of the samples, primarily due to elevated total dissolved solids (TDS), total hardness, and \mathrmSO_4^2- concentrations. In contrast, the quality of deep groundwater is comparatively better, although fluoride (F⁻) and manganese (Mn) are identified as key limiting factors. The APCS-MLR model analysis indicates that the primary influencing factors include seawater intrusion, mineral dissolution, and a combination of agricultural and industrial pollution sources.