Multi-isotopes and hydrochemistry combined to reveal the major factors affecting Carboniferous groundwater evolution in the Huaibei coalfield, North China.

Both natural processes and anthropogenic activities have significant effects on groundwater evolution in coal mining regions. In this study, the primary controlling mechanism of the groundwater chemistry evolution for the Carboniferous groundwater in the Huaibei coalfield, North China was proposed based on the hydrogeochemical indicators combining with multiple isotope tracers. The diversity of hydrochemical types indicates the complexity of the hydrogeochemical environment in the groundwater, which is recharged by precipitation infiltration with minimal evaporation according to the distributions of δD and δO. Additionally, ion correlation analysis suggests that minerals dissolution and cation exchange between Na and Ca are the dominant processes within that groundwater. The hydrochemical and δC characteristics of the groundwater demonstrate that HCO is mainly controlled by the dissolution of carbonate minerals and soil CO, and the proportion of the latter is believed to be dominated by the hydrogeologic conditions. Similarly, the values of SO and δS indicate that a small portion of SO in the groundwater in the northern part originates from the meteoric precipitation, while it is mainly derived from the dissolution of gypsum in the southern part. Furthermore, mining activities also alter the groundwater level and flow conditions through pumping and drainage, which enhances the interaction between groundwater and aquifer lithologies, thereby affects the hydrogeochemical processes. The findings of this work are of great significance for promoting the safe exploitation of deep coal resources and the sustainable utilization of groundwater in the Huaibei coalfield, as well as the most of other coalfields in North China.