Numerical modeling of coupled hydrodynamic-chemical-biodegradation processes in coal mine water quality formation and evolution

The formation and evolution mechanisms of coal mine water quality are intricate, significantly influenced by multiple processes such as hydrodynamics, hydrochemistry, and biodegradation. A comprehensive investigation and elucidation of the mechanisms is theoretically crucial for the prevention and remediation of coal mine water pollution. The hydrogeological prototype of a goaf in a coal mine in the Ordos Basin is choosen, and a laboratorial physical model and a coupled hydrodynamic-chemical-biodegradation (HCB) milti-field numerical model for the goaf are established, focusing the water level rises and biogeochemistry processes. The research results demonstrate the significance of multi-field coupling effects on mine water quality. The water level filling up results shows that the matrix-fracture dual-porosity model effectively matches the water level in the goaf with a simulation error of 9.9%, which is much more accurate than the theoretical and the single-porosity model predictions. The simulation results of the hydrochemical field are relatively consistent with the experiments, with relative errors of 3.0%, 21.0%, and 6.2% for SO24− , HCO−3 , and pH, respectively. Results from different time periods indicate that water-rock reactions and microbial activities are not significant during the water storage process. After the goaf is filled up, the hydrodynamics almost stagnate, but the hydrochemical and microbial fields are relatively active. The pyrite oxidation reactions in the No.2 and No.3 coal seams increase the concentration of SO24− by about 24.6%. In the later stage, the water environment in the goaf evolves into weakly acidic and anaerobic reducing conditions, and the microbial degradation becomes prominent, reducing the SO24− concentration from its peak by 6.1%. A certain “self-purification” ability of mine water in the goaf after closure has been confirmed. By adjusting the microbial metabolic rate constant, the proportion of SO24− degradation can be induced up to 61.6%. In actual engineering scenarios, this target can be achieved through some strategies such as supplementing enough dissolved carbon nutrient substance and artificially establishing a closed anaerobic environment. This study expands the multi-field coupling laboratorial experiments and numerical modeling techniques to the formation and evolution of water quality in coal mine water in a goaf, and the constructive conclusions can provide theoretical guidance for the prevention and remediation of coal mine water pollution. © 2024 China Coal Society. All rights reserved.