Sulphate contamination in an abandoned coal mine in light of mining effects

Abandoned high-sulfur coal mines will become deeply buried acid-producing contamination sites, posing hazards to adjacent aquifers and surrounding environments. Furthermore, mining effects and the complex spatial distribution of abandoned mines complicate the assessment of the contamination processes and degrees in adjacent aquifers. Focusing on a certain abandoned coal mine, this study examined the migration characteristics of SO₄ ²⁻ pollution in acid mine drainage using numerical simulations by taking sulfate (SO₄ ²⁻) as the characteristic contaminant and considering the effects of the fractures formed by the mining of multiple coal seams on the aquifer structure. Furthermore, it analyzed the influence of the fluid diffusion coefficient on the transport of contaminants in the aquifers. The results show that the area covered by the SO₄ ²⁻ horizontal migration in the sandstone aquifers of the Tongziyan Formation increased with time, with increasing rate gradually decreasing. Concurrently, the vertical permeability coefficient of the sandstone aquifers increased by 10 times, suggesting elevated spatial variability. The simulation results show that 5, 10, and 15 years after the coal mine was abandoned, the maximum horizontal migration distances were 215 m, 414 m, and 612 m, and the maximum vertical migration distances were 50 m, 65 m, and 70 m, respectively. The SO₄ ²⁻ migration distance and contamination range increased with the diffusion coefficient and the acid-producing time. Compared to the horizontal direction, the variations in the SO₄ ²⁻ concentration were more sensitive to the diffusion coefficient in the vertical direction. Based on the simulation results, as well as the actual conditions of the abandoned coal mine, this study proposed the comprehensive prevention, control, and management strategy of “reducing recharge to mines, blocking contamination process, and treating contamination in the end”. The results of this study will provide a theoretical basis for the treatment, prevention, and control of contamination caused by acid mine drainage in the abandoned coal mine and offer a scientific reference for other similar mines.