Hydrogeochemical characteristics and evolution mechanism of karst groundwater in the catchment area of the Sangu Spring

The karst groundwater in the catchment area of the Sangu Spring plays an important supporting role in the residents' living and coal base construction in southeastern Shanxi Province.With the large-scale coal mining and the intensification of human activities, the hydrochemistry of the regional karst groundwater has changed drastically. In this study, 125 karst groundwater, 14 surface water and 14 rain water samples were collected and tested. The descriptive statistical analysis, coefficient of variation analysis, hydrogen and oxygen stable isotopes, Gibbs model, ion correlation, mineral saturation index and factor analysis are comprehensively used to determine the replenishment sources of the karst groundwater, the differences of hydrochemical characteristics and evolution mechanism among the subsystems. The results show that the δD and δ¹⁸O values of the karst groundwater range from −77‰ to −42‰ and from −10.6‰ to −4.5‰, respectively, and the data points fall near the local meteoric water line (LMWL), indicating that the karst groundwater are mainly recharged by infiltration from precipitation. The hydrochemical characteristics and hydrogeochemical evolution processes between subsystems Ⅰ and Ⅱ-Ⅲwere obviously different. The karst groundwater of subsystem Ⅰhas low salinity and is soft water, and 73% of the groundwater is of HCO₃—Ca(Mg) type. However, the karst groundwater in subsystems Ⅱ and Ⅲ changes from low salinity and soft water to high salinity and hard water, and 36% to 40% of the water is of HCO₃·SO₄—Ca·Mg type and 24% to 45%, of SO₄·HCO₃—Ca type. Factor analyses show that the hydrochemical evolution processes of the regional karst groundwater are mainly controlled by water-rock interactions, human activities, leakage of surface water and local fissure water. Hydrochemical compositions of \rmHCO_3^- , \rmSO_4^2- , Ca²⁺ and Mg²⁺are mainly controlled by water-rock interactions. A higher concentration of \rmSO_4^2- is controlled by mining activities, \rmNO_3^- is controlled by agriculture activities and Cl⁻and Na⁺are controlled by sewage discharge and leakage of fissure water. The results will be instructive for improving the management and utilization of groundwater resources for the local government.