Seasonal characteristics of groundwater discharge controlled by precipitation and its environmental effects in a coal mining subsidence lake, eastern China

Many regions have formed subsidence lakes due to underground mining in the world. However, seasonal variations of lacustrine groundwater discharge (LGD) rate and solute fluxes in the coal mining subsidence were rarely reported. In this study, we conducted four seasonal samplings in a coal mining subsidence, during which samples for stable water (δ¹⁸O) and radioactive (²²²Rn) isotopes were collected to quantify the seasonal dynamics of LGD rates. The LGD rates estimated from the ²²²Rn mass balance model were 10.2 ± 8.7, 5.5 ± 3.2, 11.5 ± 7.8, and 7.8 ± 4.5 mm d⁻¹ in summer, autumn, winter and spring, respectively. According to the ¹⁸O mass balance model, the corresponding LGD rates were 15.1, 7.3, 15.6, and 11.3 mm d⁻¹ in summer, autumn, winter and spring, respectively. We found a significant correlation between precipitation and LGD rates, suggesting precipitation was recognized as the main control factor for seasonal variations of LGD rates. Based on this correlation, the extrapolated LGD rates over a year ranged from 3.1 to 12.7 mm d⁻¹ with an average of 8.8 mm d⁻¹. Moreover, the fluxes of dissolved silicon (DSi), iron (Fe), and manganese (Mn) from LGD in autumn were (1.6 ± 0.9) × 10⁵, (1.9 ± 1.1) × 10⁴, and (1.1 ± 0.6) × 10⁴ mol a⁻¹, respectively. Correspondingly, in winter they were (3.5 ± 2.4) × 10⁵, (4.1 ± 2.8) × 10³, and (2.8 ± 1.9) × 10³ mol a⁻¹, respectively. This study demonstrated significantly seasonal variations of LGD, with precipitation being the main control factor of LGD in the coal mining subsidence lake. The fluxes of dissolved substance (DSi, Fe, Mn) from LGD need to be emphasized because they may have important impacts on the ecological stability in coal mining subsidence lakes.