Tracing groundwater discharge into a coal mining subsidence lake in eastern China: Observations from water stable (δD and δ¹⁸O) and radon (²²²Rn) isotopes

Many coal mining subsidence lakes have formed in eastern China due to underground coal mining. Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco-environment of lakes. However, LGD in coal mining area is rarely reported. In the study, we quantified groundwater discharge into a Huainan coal mining subsidence lake (China) using water stable (D and ¹⁸O) and radioactive (²²²Rn) isotopes. ²²²Rn and ¹⁸O mass balance models were used to independently estimate LGD. The lake water was sampled based on the horizontal and vertical profile sampling. The isotope depth profiles were weighted according to the lake bathymetry to obtain a representative isotope inventory by geographic information system (GIS) analysis. The LGD rates estimated from two models were comparable, and the values were 18 mm d⁻¹ for the ¹⁸O and 14 ± 8 mm d⁻¹ for the ²²²Rn. Compared with the LGD of Ammelshainer See lake (Germany), a subsidence lake formed by coal mining, the LGD in the study was larger, which may be related to the different hydrological conditions in different areas. By integrating the LGD rates from these two models, this study quantitatively analyzed the role of groundwater in maintaining the water balance in subsidence lakes, which is of great significance to water resources assessment and sustainable utilization of coal mining subsidence lakes in eastern China.