Groundwater discharge and dissolved inorganic carbon flux into an open-type coal mining subsidence lake in eastern China

Groundwater discharge and associated dissolved carbon fluxes are essential parameters in wetland ecosystems of coal mining subsidence lakes. Previous studies have shown that groundwater discharge considerably contributes to isolated coal mining subsidence lakes. However, groundwater discharge and associated dissolved inorganic carbon (DIC) flux into open-type subsidence lakes remain poorly understood. In this study, radioactive (²²²Rn) and stable water isotopes (¹⁸O) were used as tracers to establish isotope mass balance models, and to quantify groundwater discharge into an open-type coal mining subsidence lake. The results showed that the estimated lacustrine groundwater discharge (LGD) rates using the ²²²Rn and ¹⁸O mass balance models were 38.54 ± 21.05 mm/d and 27.89 ± 3.53 mm/d, respectively, which accounted for 15.56% and 12.30% of the total source of lake water. On this basis, combined with a DIC end-member mixing model and CO₂ diffusion model, DIC flux from LGD was 70.33 ± 1.41 mmol/(m²·d), accounting for 19.10% of the total lake DIC inventory and 65.60% of the total CO₂ evasion from the lake. The results suggest that CO₂ escape from freshwater lakes may be an important source of atmospheric carbon, which should be included in the carbon budget of lake budgets. The results provide a theoretical basis for accurate assessment of groundwater discharge and the carbon budget in open-type subsidence lakes. This is important for local water environment protection and the utilization of water resources.