Hydrogeochemical processes and groundwater quality assessment for different aquifers in the Caojiatan coal mine of Ordos Basin, northwestern China

This study explores the hydrogeochemical processes and groundwater quality of different aquifers in the Caojiatan coal mine in the Ordos Basin, northwestern China, using classical graphical methods and multivariate analysis. The study found that surface water and groundwater in the Quaternary unconsolidated aquifer (UA) and weathered bedrock zone (WZ) have a HCO₃–Ca·Mg hydrochemical type, while the groundwater in the Jurassic Zhiluo aquifer (ZA) and Jurassic Yanan aquifer (YA) are predominantly of the HCO₃–Na type. Rock weathering, silicate dissolution, and reverse cation exchange are the dominant hydrogeochemical processes controlling the water chemical variables in the study area. In addition, variations in the hydraulic conductivities and TDS in different aquifers indicate that groundwater residence time is also the main factor controlling the chemical variables. Groundwater in the UA and WZ is suitable for drinking and irrigation, with low ion concentrations. However, the groundwater in ZA and YA must be treated before drinking, because of the high pH value. The excessive concentrations of F^-, and high sodium adsorption ratio Na%, and residual sodium carbonate values also make the water unsuitable for irrigation. Mining activities can significantly impact the circulation and occurrence of natural groundwater. Once the groundwater in WZ becomes mine water, it becomes contaminated because of the mixing process with groundwater in ZA and YA. Further, undissolved coal slime and large quantities of oil leaking from fully mechanized mining equipment increase the COD_Mn levels in the mine water, making the levels higher than in natural groundwater and beyond permissible limits for drinking.