Background The northern Yuheng mining area exhibits the widespread occurrence of water-bearing coal seams. This unique hydrogeological phenomenon poses a serious challenge to efficient mine roadway construction and safe coal mining in the mining area. The recharge sources of the coal seam water remain poorly understood, representing a primary bottleneck in the prevention and control of coal seam water hazards.

Methods This study aims to reveal the recharge sources and evolutionary mechanisms of coal seam water in the northern Yuheng mining area. By integrating methods including hydrochemical analysis, isotope analysis, and inverse hydrogeochemical simulation, this study systematically investigated the hydrogeochemical characteristics of both the coal seam water and its overlying water bodies. Accordingly, it revealed the formation mechanisms and recharge sources of the coal seam water.

Results and Conclusions Water bodies in the northern Yuheng mining area exhibit hydrochemical characteristics with distinct vertical zoning. Specifically, shallow surface water, the Quaternary groundwater, and groundwater in the Luohe Formation represent low-salinity water with hydrochemical types of HCO₃-Ca and HCO₃-Na·Ca. Their hydrochemical compositions are primarily governed by mineral dissolution. In contrast, coal seam water and deep groundwater in the Zhiluo and Yan'an formations are all high-salinity water with a hydrochemical type of SO₄-Na. Besides conventional mineral leaching, these water bodies are significantly influenced by the combined effects of cation exchange and the oxidation of sulfur-bearing minerals. With an increase in the burial depth, water bodies in the mining area exhibit gradually decreasing hydrogen and oxygen isotope values, while their ages tend to increase. Among these, deep groundwater and coal seam water show distinct depletion of hydrogen and oxygen isotopes and share highly similar corrected ¹⁴C ages. These results directly demonstrate that both water bodies have experienced intense water-rock interactions and oxygen isotope exchange and exhibit highly consistent geological periods of recharge and occurrence. Based on these findings, combined with the hydrogeological structural characteristics of the mining area, it can be inferred that the coal seam water is recharged by groundwater from the overlying Zhiluo and Yan'an formations. Inverse hydrogeochemical simulation further reveals that after migrating into the coal seam, groundwater from the overlying Zhiluo and Yan'an formations primarily underwent reverse cation exchange, gypsum precipitation, and mineral leaching. As a result, the Na⁺, Ca²⁺, and \mathrmSO_4^2- concentrations and salinity of the coal seam water significantly reduced. These simulation results are highly consistent with the geological observations of gypsum filling in coal seam fractures, further verifying the recharge sources of coal seam water. The results of this study hold great significance for gaining a scientific understanding of the recharge sources and formation mechanisms of coal seam water and guiding the prevention and control of coal seam water hazards.