Isotopic and hydrochemical evidence for sulfate transport and transformation in a confined Permian sandstone aquifer of a coal mine: evaporite dissolution and bacterial reduction.

Sulfate is a key geochemical indicator in coal mine groundwater systems. Previous studies have demonstrated that sulfate in coal mine groundwater primarily originates from sulfide oxidation and evaporite dissolution, whereas sulfate reduction mediated by sulfate-reducing bacteria (SRB) serves as a major consumption pathway. However, the coupled effects of evaporite dissolution and SRB reduction on sulfate dynamics in well-confined aquifer systems remain poorly understood. In this study, sulfate sources and transformation mechanisms in a deep confined Permian sandstone aquifer within a coal mining area were investigated using an integrated approach of hydrochemical analysis, multivariate statistics, and multi-isotope (δ34SSO4, δ18OSO4, 87Sr/86Sr, and δ13C) systematics. The results show that the target aquifer has relatively low SO₄2- concentrations (mean: 37.48 mg/L), with evaporite (gypsum) dissolution identified as the predominant source. The sulfate isotopic compositions (δ34SSO4: 25.1‰ to 35.34‰; δ18OSO4: 9.6‰ to 10.7‰) are consistent with those of evaporite minerals, confirming the contribution of endogenous dissolution. Strontium isotope ratios (87Sr/86Sr: 0.71206 to 0.71248) further preclude the significant influence of carbonate dissolution. SRB-mediated sulfate reduction, evidenced by SO₄2- depletion coupled with the generation of HCO₃-, amplifies the observed low-sulfate anomaly. Hydrogeological confinement reinforces this endogenous sulfate evolutionary pathway by limiting external inputs and sustaining anoxic conditions. This study elucidates the multiprocess coupling mechanisms governing sulfate sources in confined aquifer systems and provides a scientific basis for understanding groundwater geochemical evolution and identification of pollution sources in coal mining areas.