Hydrochemical and Isotopic Characteristics of Groundwater in a Near Surface Disposal Facility in China and Indicative Significance

Abstract

A near surface disposal facility in China is located in the arid region of northwest China. Groundwater at the site is predominantly bedrock fracture water, mainly occurring in the joint fractures of strongly to slightly weathered rock bodies. The aquifer is formed by Silurian-Ordovician strata, with lithologies primarily comprising metamorphic mudstone, metamorphic quartz sandstone, and marble. Hydrochemical analysis indicates the groundwater exhibits weakly alkaline with an average pH of 8.15. It has with an average Total Dissolved Solids (TDS) value of 750.9 mg/L, which belongs to freshwater. The hydrochemical type is dominated by SCvHCCb-Na-Ca type. Using Gibbs diagrams, ratio coefficient methods, and geochemical modeling techniques, it was revealed that the groundwater chemical composition is mainly controlled by carbonate dissolution, sulfate dissolution, and cation exchange. Hydrogen and oxygen stable isotope results indicate the groundwater originates from atmospheric precipitation. After entering the shallow unsaturated zone, the precipitation undergoes some non-equilibrium evaporation, though the evaporation process is not intense.