Hydrogeochemical vertical zonation and evolution model of the Kongjiagou coalmine in Sichuan, China

Abstract With mine drainage and activities, the hydrogeochemical environments has greatly changed in Kongjiagou coalmine, Sichuan Province, southwest China. This study aimed to summarize an ideal model of coalmine hydrogeochemical zonation and evolution. In total, 24 groundwater samples from different depth were collected to analyze their components, with the combination of methods such as ion sources, normalization index, ions ratios, cation exchange and inverse modeling. The hydrogeochemical formation main factor was water–rock reactions and more weathering by sulfuric acid. Dissolution and pyrite oxidation were the main sources of , whereas carbonate weathering was the dominant source of . The vertical extent could be distinguished to three zones by mean and standard deviation of burial depth. The normalization index, ions ratios, cation exchange and water types exhibited clear vertical zonation. In shallow groundwater, dissolution led to a Ca- type water and overall stable concentrations of ions. Redox reactions in the middle region have caused significant increases in the total dissolved solids (TDS) and and a considerable reduction in pH; hence, Ca + Mg-SO4 + HCO3 became the dominant type. Re-dissolution and cation exchange played important roles at depth and caused the pH and TDS to increase slowly. The , Ca2+, and Mg2+ concentrations increased rapidly, whereas the concentration remained basically unchanged, and the water type was either Ca + Mg-HCO3 + SO4 or Na-HCO3 + SO4. This study will be of great significance to coalmine safety production and water environments protection.