Hydrogeochemical Characteristics of Groundwater and Influencing Factors during the Dry Season in the Xin'an River Basin, China

<title>Abstract</title> <p>Analyzing the hydrochemical characteristics and formation mechanisms of groundwater in the Xin'an River Basin is crucial for the rational development, utilization, and protection of groundwater resources, as well as for supporting the pilot objectives of the basin's ecological compensation mechanism. In November 2022, during the dry season, 165 shallow groundwater samples were collected from the Xin'an River Basin. Comprehensive analyses were conducted using mathematical statistics, graphical methods, and hydrogen-oxygen isotope techniques to examine the composition, hydrochemical types, formation origins, and recharge sources of the groundwater. The results indicate that the pH of shallow groundwater during the dry season ranges from 6.13 to 9.07, exhibiting a neutral to weakly alkaline profile; total dissolved solids (TDS) vary between 8 and 528 mg/L, confirming freshwater conditions; and total hardness (TH) ranges from 8.01 to 524 mg/L, predominantly comprising soft and hard water. A total of 15 hydrochemical types were identified in the basin's dry-season groundwater, with the HCO₃-Ca type being predominant. Gibbs plots, elementograms, and ion ratio analyses demonstrated that groundwater composition is influenced by a combination of rock weathering, cation exchange, and human activities, with rock weathering playing the dominant role. Ca²⁺ and Mg²⁺ primarily originate from the dissolution of carbonate and silicate rocks, with calcite contributing more than dolomite; Na⁺ and K⁺ mainly derive from the dissolution of evaporite salt rocks, while cation exchange is minimal. HCO₃⁻ primarily originates from carbonate rock dissolution, SO₄²⁻ from silicate rock dissolution, and Cl⁻ from evaporite salt rock dissolution. Human activities have only minor impacts on the concentrations of SO₄²⁻, Cl⁻, and NO₃⁻ in groundwater. During the dry season in the Xin'an River Basin, groundwater recharge primarily originates from atmospheric precipitation, although it is also influenced to some extent by evaporation.</p>