Causes and Effects of a Water Spill from the Underground Pit of the Dashu Pyrite Mine, Southern Sichuan Basin, Southwest China

In March 2019, a well was drilled in the north of the Dashu pyrite mining area in southwest China to extract shale gas from below the pyritic layer. This well passed through the karst water at the top of the Maokou Formation, leading to groundwater flowing into the pit in the north of the underground mining area and producing a large amount of acid mine drainage (AMD) by water-rock interactions with sulfur-bearing minerals. On Jan. 4, 2020, under the dual influence of progressive overpressure generated by water storage and AMD erosion, the underground pit’s wall partially collapsed, resulting in over 280 × 103 m3 of highly polluted AMD entering the Dashu River. About 231 × 103 kg of Fe, 171 × 103 kg of Al, 12 × 103 kg of Ni, and a large amount of other metals and trace elements were discharged. The main purpose of this study was to explain the causes, consequences, and effects of this event. Water and sediment samples were collected from the mine, the river, and the estuary, and a hydrological model of the underground pit was developed. The spill seriously affected the water quality of ≈ 16 km of the Dashu River. Given the low pH (≈ 2.5), most of the metals (Fe, Al, Mn, Cu, Zn, etc.) migrating towards the Dashu estuary were dissolved, but because of dilution and neutralization, much of the dissolved metals precipitated as river sediments, due to Fe and Al precipitation, and adsorption and co-precipitation of other metals. The calculated water balance and monitoring of the underground pit water levels show that the water in the pit mainly came from karst water recharge at the top of the Maokou Formation, increasing mine drainage by at least 17.2 m3/h. Without effective action, serious environmental pollution or secondary geological disasters may occur in the future.