Objective Underground reservoirs in coal mines are capable of storing and purifying mine water. A deep understanding of the hydrochemical characteristics and formation mechanisms of water in underground reservoirs is identified as an important prerequisite for large-scale development of technologies for mine water treatment.

Methods This study investigated the underground reservoirs in the Daliuta coal mine within the Shendong area by testing and analyzing their influent and effluent water samples. Using methods such as the ion ratio method, multivariate statistical analyses (e.g., correlation analysis and principal component analysis (PCA)), and the positive matrix factorization (PMF) model, this study delved into the hydrogeochemical characteristics and component sources of water in the underground reservoirs.

Results and Conclusions  The results of this study indicate that compared to the influent water of the underground reservoirs, the effluent water exhibits significantly reduced heavy metal indicators including suspended solids (SS), electrical conductivity (EC), total dissolved solids (TDS), Fe³⁺, and Mn²⁺. Furthermore, the Na⁺ and Cl⁻ concentrations progressively increase while the Ca²⁺ and Mg²⁺ concentrations gradually decrease along the water flow direction. The hydrochemical type shifts from the Cl·SO₄−Ca type of the influent water to the Cl·SO₄−Na·Ca and Cl·SO₄−Na types of the effluent water. The rocks collapsing in the underground reservoirs consist primarily of mudstones and sandstones. Their minerals comprise quartz, orthoclase, albite, illite, kaolinite, chlorite, gypsum, and pyrite. The principal hydrogeochemical processes the rocks experience include mineral dissolution (i.e., dissolution of halite, silicate, and gypsum), cation exchange, adsorption, precipitation, and mixing. The hydrochemical composition of water in the underground reservoirs is predominantly controlled by three factors: mineral dissolution and cation exchange (F1), the adsorption and precipitation of ions (F2), and the mixing of different water sources (F3), which contribute to 57.2%, 22.0%, and 20.8%, respectively, on average to the mass concentrations of ions in water bodies in the underground reservoirs. Specifically, F1 contributes 86.9%, 78.8%, 79.2%, 79.7%, 74.0%, and 74.8%, respectively, to the mass concentrations of Na⁺, K⁺, Ca²⁺, Cl⁻, \mathrmSO_4^2- , and \mathrmHCO_3^- ; F2 contributes 83.9% and 70.3%, respectively, to the mass concentrations of Fe³⁺ and Mn²⁺, and F3 contributes 84.9% to the mass concentration of Mg²⁺. The results of this study provide theoretical support for a deep understanding of the water purification mechanisms for underground reservoirs in the coal mine. Additionally, these results offer technical guidance for optimizing the technologies for mine water treatment and achieving sustainable utilization of water resources.