Hydrochemical Evolution and Isotopic Identification of Groundwater Contamination in a Typical Abandoned Pyrite Mining Area: A Case Study of the Jinpen Mining Area in Yunxi County

Acid mine drainage (AMD) from abandoned pyrite mines poses a serious threat to groundwater environments. This study investigated the Jinpen abandoned pyrite mine in the South Qinling metallogenic belt using hydrochemical analysis, stable hydrogen, and oxygen isotopes (δD and δ 18 O) and self‐organizing map (SOM) techniques to identify groundwater hydrochemical characteristics, evolution mechanisms, and contamination sources. SOM clustering divided the samples into four hydrochemical groups and provided a framework for subsequent ion‐ratio and isotope interpretations. G1 represents background freshwater (TDS 98.65–323.38 mg/L; HCO 3 ‐Ca or HCO 3 ‐Ca·Mg type), whereas G2 shows elevated HCO₃ − , SO 4 2− , and Ca 2+ related to mining‐induced pyrite oxidation and carbonate dissolution. G3 and G4 are severely contaminated (TDS up to 3881.33 mg/L; SO 4 ‐Ca type), with SO 4 2− , NO 3 − , Fe, and Mn exceeding China's Class V groundwater standard, indicating combined mining and anthropogenic impacts. The δD and δ 18 O values deviate from the local meteoric water line, and d‐excess values (10.2‰–23.61‰) are higher than 10‰. These isotope results suggest evaporation, mixed recharge, and possibly contributions from deeper or older groundwater through mining‐induced fractures. Ion ratios further indicate that carbonate/silicate weathering, cation exchange, pyrite oxidation, agricultural inputs, domestic sewage, and inferred alkaline neutralization jointly control groundwater chemistry. This study provides a scientific basis for pollution control and drinking‐water safety in abandoned high‐sulfur mining areas.