| Summary: | Karst groundwater in the Niangziguan spring fields is the main source to supply domestic and industrial water demands in Yangquan City, China. However, the safety of water supply in this region has recently suffered from deteriorating quality levels. Therefore, identifying pollution sources and causes is crucial for maintaining a reliable water supply. In this study, a systematic sample collection for the karst groundwater in the Niangziguan spring fields was implemented to identify hydrochemical characteristics of the karst groundwater through comprehensive analyses of hydrochemistry (piper diagram, and ion ratios,) and stable isotopes (S and H-O). The results show that the karst groundwater in the Niangziguan spring fields was categorized as SO<sub>4</sub>·HCO<sub>3</sub>-Ca·Mg, HCO<sub>3</sub>·SO<sub>4</sub>-Ca·Mg, and SO<sub>4</sub>-Ca types. K<sub>+</sub>, Cl<sup>-</sup>, and Na<sup>+</sup> are mainly sourced from urban sewage and coal mine drainage. In addition, SO<sub>4</sub><sup>2− </sup>was mainly supplied by the dissolution of gypsum and the oxidation of FeS<sub>2</sub> in coal-bearing strata. It is noteworthy that, based on H-O and S isotopes, 75% of the karst groundwater was contaminated by acidic water in coal mines at different degrees. In the groundwater of the Niangziguan spring field, the proportions of SO<sub>4</sub><sup>2−</sup> derived from FeS<sub>2</sub> oxidation were 60.6% (N50, Chengxi spring), 30.3% (N51, Wulong spring), and 26.0% (N52, Four springs mixed with water). Acid mine drainage directly recharges and pollutes karst groundwater through faults or abandoned boreholes, or discharges to rivers, and indirectly pollutes karst groundwater through river infiltration in carbonate exposed areas. The main source of rapid increase of sulfate in karst groundwater is acid water from abandoned coal mines.
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