Combining hydrochemistry and environmental isotopes to study hydrogeochemical evolution of karst groundwater in the Jinci spring area, North China

To determine the impact of natural factors and human activities on the hydrogeochemical evolution of karst groundwater, hydrochemical surveys were conducted in the Jinci spring area, north China. In the present work, a comprehensive method, including hydrochemistry, isotopes, statistical analysis and hydrochemical modeling, was applied to analyze the origin of karst groundwater, hydrochemical reactions, and the causes of SO 4 2− pollution. Isotopic characteristics ( 2 H and 18 O) suggest that karst groundwater is majorly replenished by precipitation infiltration and surface water leakage from the exposed carbonate rock areas. The hydrochemical facies evolve from HCO 3 -Ca and HCO 3 -Ca·Mg in the recharge area, to HCO 3 ·SO 4 -Ca·Mg and SO 4 ·HCO 3 -Ca·Mg in the transitional area, and SO 4 -Ca and SO 4 -Ca·Mg in the discharge area. Naturally, dissolution or precipitation of calcite, dolomite, gypsum and halite, as well as cation exchange predominate the hydrochemical evolution. Due to long-term coal mining activities, acid mine drainage mixed with karst groundwater, resulting in strong SO 4 2− contamination of groundwater in the spring area. According to 34 S isotope fractionation, SO 4 2− is mainly from gypsum dissolution (64%), followed by pyrite oxidation (36%). In addition, the mixing processes, rock–water interactions, cation exchange, and dedolomitization were identified and confirmed by hydrochemical modeling with PHREEQC program. The findings of this work are meaningful for groundwater sustainable utilization and contamination management.