The Impacts of Evaporites on the Hydrogeochemical Processes of Brackish Groundwater in the Huangshui River Basin, Northwest China

The Huangshui River is a major tributary of the upper Yellow River. Understanding the hydrogeochemical processes involved in the formation of brackish groundwater in the Huangshui River Basin is essential for the rational development and utilisation of water resources, as well as for the protection of water resources and aquatic ecosystems. The chemical composition of brackish groundwater can reveal the hydrogeochemical processes it has undergone, while isotopic tracing can indicate the sources and processes of groundwater formation. This study employs hydrochemical and isotopic analyses to elucidate the contribution of groundwater–rock interactions to the formation of brackish groundwater in the Huangshui River Basin. The groundwater is classified as HCO₃–Ca·Mg type freshwater and SO₄·Cl–Ca·Mg type brackish water. Based on ion ratio coefficients and ⁸⁷Sr/⁸⁶Sr ratios, mineral dissolution, evaporation concentration and cation exchange are the dominant processes in the hydrochemical formation of brackish groundwater in the basin. The primary minerals undergoing dissolution in the basin include evaporites, carbonates and silicates. Discrimination using a hydrochemical component end-member mixing model identifies evaporites (73.6%) and carbonates (4.5%) as the main dissolved components in the brackish groundwater segment, while evaporites (67.5%) and carbonates (16.25%) are predominant in the partial dissolution of minerals in the freshwater segment. This suggests that the dissolution of evaporites plays a significant role in shaping the hydrogeochemistry of the Huangshui River Basin.