Identification of hydrogeochemical processes and transport paths of a multi-aquifer system in closed mining regions

The water quality evolution of surface water and groundwater by mining activities and closed or abandoned mines is a grave public concern around the world. To understand the characteristics of hydrogeochemical processes controlling the water quality evolution after the closure of mines, a multi-aquifer system in a closed coalfield in Huainan eastern of China was investigated as an example. Based on the results of mineral characterizations, hydrogeochemistry, and stable isotopes, carbonates minerals, gypsum, and clay minerals as well as atmosphere gases-O2, CO2, and H2O were recognized as major reactants, and evaporation, dissolution/precipitation of minerals and cation exchange were potential reactions. Using inverse geochemical modeling, three major hydrogeochemical paths in the closed mines were identified. Path1 shows that water in the closed mines is mainly recharged by the overlying Quaternary aquifer and the underlying limestone aquifers through faults and fractures, accompanied by cation exchange and sulfate reduction, making the concentrations of Na+ + K+ and HCO3− increase and the concentration of SO42− decreases along the groundwater flow path. Path 2 shows that the karst groundwater flows from the shallow (recharge) zone to the deep (circulation) zone and then flows upwards to recharge the closed mine and the Permian aquifer through faults and fractures. Along Path 2, dolomite precipitates, ion exchange, and halite dissolution mainly take place, making the concentrations of Na+ + K+ and Cl− increase and the concentration of Ca2+ and HCO3− decrease and the water chemistry type change from Ca–HCO3 type to Na + K–Cl type. Path 3 shows that waters in the mining subsidence ponds (MSPs), Huai River and Quaternary loose aquifer (QLA) are connected. Although the major elements of water in QLA still meet the drinking water guidelines of World Health Organization (WHO) and U.S. Environmental Protection Agency (USEPA), considering the fact that water in QLA is a vital drinking water source for local communities, proactive measurements should be undertaken to lower the risk of contamination of QLA by MSPs through minimizing the hydraulic connection between them. Results of this study could benefit water quality prediction and management for these complex and vulnerable systems in the study area as well as other regions of the world.