Insight into the water–rock interaction process and purification mechanism of mine water in underground reservoir of Daliuta coal mine in China

The water quality of mine water is obviously improved after being stored in underground reservoir, but the process of water–rock interaction and the purification mechanism of mine water quality are not clear. In this study, the water samples and rock samples collected in the underground reservoir of Daliuta coal mine were taken as the research object. Based on the analysis of the hydrochemical characteristics of the reservoir water samples and the characterization of the rock samples, combined with PHREEQC analysis, the mechanism of water quality purification of mine water was discussed. The results showed that the rocks in the underground reservoir had layered silicate structure and flaky kaolinite structure, with some irregular edges and microcracks, and higher specific surface area and total pore volume. These characteristics made the rocks have a certain adsorption and removal capacity for heavy metal ions and other pollutants in the mine water. The water–rock interaction, such as the dissolution of albite and halite, the precipitation of gypsum and kaolinite, and the cation exchange, resulted in the increase of the concentration of Na+ and the decrease of the concentration of Ca2+, Mg2+, and TDS in the outlet water, and the hydrochemical type changed from SO42−-Cl−/Ca2+ type to SO42−-Cl−/Na+ type. Moreover, this study shows that PHREEQC analysis can be used to analyze the water–rock interaction of coal mine underground reservoir and can obtain more detailed information; therefore, it may have the potential ability to help assess the migration and transformation of pollutants during the storage process of mine water in underground reservoirs.