Hydrochemical characteristics and processes of groundwater in the Cenozoic pore aquifer under coal mining

Mining activities considerably affect groundwater quality and resources. It is crucial to investigate water chemistry and hydrochemical processes under coal mining settings for water environment protection and sustainable utilization of groundwater resources. In this study, 36 groundwater samples from the Cenozoic pore aquifer in the Gubei coal mine in Huainan (Anhui Province, China) were analyzed using hydrochemical, multivariate statistical methods, and inverse geochemical modeling. Results revealed a hydrochemical boundary around 100 m below the ground surface, forming two flow zones participating in different hydrological cycles. In the shallow (circulation) zone above the boundary, the main hydrochemical processes are silicate dissolution and cation exchange, followed by evaporite dissolution. In this zone, the Na⁺  + K⁺, Cl^-, and SO₄^2- concentrations are low and stable, forming low-salinity water (total dissolved solids [TDS] < 1 g/L) dominated by Na*Ca*Mg-HCO₃ and Na*Ca-HCO₃ types. At greater burial depths, the pH, TDS, Na⁺  + K⁺, Cl^-, and SO₄^2- concentrations in the groundwater gradually increase, while the HCO₃^- concentration gradually decreases, which is mainly due to the enhanced evaporite dissolution and decarbonation in the deep (circulation) zone below the hydrochemical boundary. In this zone, the Na⁺  + K⁺, Cl^-, and SO₄^2- concentrations are high and variable, forming high-salinity water (TDS > 1 g/L) dominated by Na-Cl and Na-Cl*SO₄ types. A hydrochemical evolution model of the Cenozoic aquifer was finally established, which advances our understanding of the evolutionary processes of groundwater chemistry under mining drainage.