Hydrogeochemistry and hydrogen and oxygen stable isotope characteristics of subsidence lakes in coal mining areas with high phreatic levels in eastern China

Abstract Large-area subsidence lakes in coal mining areas with high phreatic levels in eastern China play a very important role in regulating water resources and improving the ecological environment. These coal mining-related subsidence lakes vary in formation time and connectivity with rivers. These factors may influence the hydrogeochemistry and hydrogen and oxygen stable isotope characteristics of the lake water. This study assessed the hydrogeochemical and hydrogen and oxygen stable isotope characteristics and the influencing factors of the subsidence lake water, surface river water and shallow groundwater in the Huainan coal mine area in eastern China. The results showed that the hydrochemical types of the subsidence lake water and river water are Cl-Na and HCO3•Cl-Na and that the hydrochemical types of the shallow groundwater are mainly HCO3•Cl-Na and HCO3•Cl-Ca. There are no significant differences in the hydrochemical characteristics of subsidence lakes with different subsidence ages and types. The major ions in each water body mainly come from evaporite dissolution and silicate weathering, and ion exchange occurs. Reverse ion exchange occurs in some shallow groundwater samples. The hydrogen and oxygen isotopic compositions of the subsidence lake water, river water and shallow groundwater plot below and to the right of the LMWL and are distributed along a straight line, indicating that these water bodies have a common source, namely, precipitation. With increases in the formation time of the subsidence lakes, the heavy isotopes in the lake water gradually become depleted, and the d value gradually increases, mainly driven by precipitation dilution, weakening evaporation, river recharge and groundwater recharge. The isotopic values of different types of lakes with the same subsidence time differ little. The research results are of great significance for understanding the water cycle and water resource evaluation of coal mining subsidence lakes.