Water–Rock Interaction Mechanisms and Hydrochemical Evolution in the Underground Reservoirs of Coal Mines

Coal-mine underground reservoirs play pivotal roles in water purification through water-rock interactions; however, underlying mechanisms associated with water-rock interactions remain unclear to date. To address this issue, this study considered the underground reservoirs of the Daliuta coal mine, Shendong to conduct the analysis. Through onsite sampling and laboratory simulation experiments, along with hydrochemical-diagram-based analysis, multivariate statistical analysis, and hydrogeochemical simulations, the sources and evolutions of major ions associated with the water of the underground reservoirs were elucidated. Results revealed that the suspended-solid content, turbidity, electrical conductivity, and total dissolved solid concentration in the water at the outlet of the underground reservoirs decreased significantly compared to corresponding values at the inlet. Alteration in the hydrochemical type of water was observed from the Cl·HCO₃-Na·Ca, Cl-Na·Ca·Mg, and Cl·SO₄-Na types at the inlet of the reservoirs to SO₄·Cl-Na at the outlet of the reservoirs. Further, the primary hydrogeochemical processes carried out along the water flow path in the underground reservoirs included mineral dissolution (including halite, silicate, calcite, gypsum, and anhydrite dissolution), pyrite oxidation, cation exchange, and clay-mineral adsorption. Water-rock interactions in the underground reservoirs can be categorized into three stages. The first stage was dominated by dissolution and oxidation, resulting in decreased water pH and overall increased concentrations of total dissolved solids along with major ions in the water. The second stage was dominated by clay-mineral adsorption and cation exchange, resulting in increased water pH and overall concentrations of Na⁺ and K⁺ as well as decreased concentrations of Ca²⁺, Mg²⁺, and total dissolved solids in the water body. The third stage was characterized by weak water-rock interactions, wherein the concentrations of total dissolved solids along with major ions, as well as the pH of the water stabilized. Thus, our research findings provide important theoretical references for understanding water-purification mechanisms and water-storage-duration optimization in coal-mine underground reservoirs.