Study on Hydrochemical Characteristics and Evolution Patterns of Roof Sandstone Water in the Banji Coal Mine

The Banji Mine, located in the western part of the Huainan Coalfield, is characterised by a deep burial depth and multiple aquifers. It faces significant water inflow risks from roof aquifers, especially from the sandstone aquifer above the No. 9 coal seam. To explore the hydrochemical evolution of this sandstone aquifer and address key scientific challenges in water hazard prevention, an integrated approach combining mathematical statistics, Piper trilinear diagrams, Gibbs diagrams, and principal component analysis (PCA) was employed. Results show that from 2020 to 2023, the average TDS increased from 1729.51 mg·L−1 to 2061.22 mg·L−1, and the hydrochemical types transitioned from a mix of Cl-Na (48.6% of samples) and HCO3·Cl-Na to a dominant Cl-Na type (91.1% in 2023), exhibiting high mineralisation and a distinct trend of water salinisation. The dissolution of evaporites and evaporative concentration were identified as the primary processes influencing the hydrochemical characteristics, with PCA indicating that the dominant factor (F1) explained 66.269% of the variance. Saturation index (SI) analysis revealed that calcite and dolomite were saturated to supersaturated (SI: 0.73–2.15 and 1.66–4.81, respectively), while gypsum and halite were undersaturated but showed a tendency to dissolve towards equilibrium. Over time, the cation exchange and sulfate reduction processes weakened, indicating that mining activities have disrupted the hydrochemical equilibrium of the roof sandstone aquifer, accelerating water salinisation. This study provides a theoretical foundation for identifying the causes and early warning signs of water hazards in the roof strata of the Banji Mine.