Chemical Characteristics and Suitability Assessment of Surface Water in the Area Surrounding the Nansi Lake

Surface water quality, serving as a key link between domestic water use and agricultural production, impacts both the drinking water safety of local residents and the sustainable use of irrigated soil. To better protect water resources and enhance their sustainable value, this study collected 50 water samples from the areas surrounding Nansi Lake. Using the Piper trilinear diagram, Gibbs model, and ion ratio analysis, the main hydrochemical types were identified. Based on this, the entropy-weighted water quality index (EWQI) was used to evaluate the water’s suitability for drinking, while irrigation water quality indicators were applied to assess its suitability for irrigation. The results indicate that during both dry and rainy seasons, Na+ and SO42− dominate the water, with average total dissolved solids (TDS) of 1279 mg/L and 1163 mg/L, respectively, indicating moderately elevated salinity. The ion concentrations follow the order: SO42− > HCO3− > Cl− > NO3− > F− and Na+ > Ca2+ > Mg2+ > K+. From a hydrochemical perspective, mixed-type and Cl-Na-type waters prevailed in both seasons. The chemical composition of surface water in the study area is largely governed by rock weathering, with ions primarily originating from the dissolution of silicate and evaporite minerals. Furthermore, cation exchange processes play a significant role in shaping the evolution of the water chemistry. The water quality evaluation indicates that surface water in the study area is generally Class II, representing good water quality. However, Class IV and Class V water exist in some areas, where the primary exceedance parameter is SO42−, which is a key factor influencing water quality. Irrigation suitability is generally good. Systematic investigation of surface water hydrochemistry and quality is of great practical significance for ensuring safe drinking and irrigation water and promoting sustainable socio-economic development.