Occurrence and environmental impact of coal mine goaf water in karst areas in China

Improper exploitation and massive coal mines closures without proper solutions bring about the extensive occurrence of goaf water in China where over 3500 coal mines have been shut down in the last 30 years. Discharge of goaf water poses severe environmental impact, especially in fragile karst areas. Based on the extensively literatures reading, field investigation combined with hydrogeochemical and isotopic (δ34S) analysis, this paper reviewed the distribution, characteristics and formation of coal mine goaf water in karst areas in China. The occurrence of goaf water is reported in over 50% of the coal fields, with more than 30 water discharges. Distinct major ion chemistry in goaf water (low pH, high total dissolved solids, SO42− and negative δ34S values) are closely related to a combination of comprehensive physical, chemical and biological interactions. Recharge water, water filling channels and storage space constitute hydrogeological conditions necessarily for the produce of goaf water. Oxidation of sulfides minerals with air and water, acidic dissolution of minerals (e.g., gypsum, calcite and dolomite) and cation exchange, under the action of bacteria, are major processes in the genesis of goaf water. A case study on the environmental impact of goaf water is also done at Jinci, northern China. Our research suggests that goaf water in Jinci (TDS: 3595 mg/L - 9841 mg/L, SO42−: 2463 mg/L - 3256 mg/L; negative δ34S values < −5‰) may pollute the surface and karst water via fractures or faults evidenced by the high SO42− and low δ34S values in these waters. Finally, a conceptual model is established to demonstrate the influences of goaf water on karst water-surface water environment in karst areas.