Hydrogen and oxygen isotope characteristics of mine water in Shendong mine area and water⁃rock reactions mechanism of the formation of high⁃fluoride mine water

The fluoride(F-) mass concentrations in many mine water samples in the Shendong mining area of Shenhua exceed the national standard(> 1 mg/L)，which brings some great challenges to the storage，reuse and discharge of the mine water. In order to investigate the potential sources of fluoride in the mine water and study the forma⁃ tion mechanism of high⁃fluoride mine water，81 water samples including mine water，phreatic water of unconsolidated formation of Salawusu Formation，and surface water were collected from 8 coal mines in the Shendong mining area. Their hydrogen and oxygen isotopes and main ion mass concentrations were determined.The test results show that the F- mass concentration of mine water in the Shendong mining area ranges from 0.05 to 11.65 mg / L，with an average of 1.96 mg/L.51% of mine water is high⁃fluoride mine water.The analysis of hydrogen and oxygen isotopes shows that most of the high⁃fluoride mine water samples have relatively lighter δD and δ18 O values，moreover，the δD and δ18O values decrease with the increase of the F- mass concentrations in mine water samples. The analyses suggest that the mine water samples with lighter δD and δ18O values are relatively old and have a long residence time in the formation.They have poor hydraulic connections with the phreatic water of unconsolidated formation and surface water and are within a relatively closed groundwater environment，leading to a stagnant runoff condition and sufficient water⁃rock reactions.Once the mineralogy and geochemical conditions of the aquifer are appropriate，the F of the fluor⁃ ide⁃bearing minerals releases， resulting in the enrichment of F- in the water. The correlation analysis between the hydrochemical types of mine water and the F- mass concentration shows that the Na - HCO3 type water has the highest F- mass concentration.The F- mass concentrations in the Na⁃type mine water are obviously higher than that in the Ca⁃type mine water.With the increase of the Na+ mass concentration and the decrease of the Ca2+ mass concen⁃ tration，the mass concentration of F- increases，reflecting that the enrichment of F- is mainly related to the water⁃rock reactions of increasing the Na+ mass concentration and consuming the Ca2+ in the mine water.The comprehensive anal⁃ ysis shows that the formation of high⁃fluoride mine water in the Shendong mining area is mainly related to the cation exchange adsorption of Ca2+ in the mine water for Na+ absorbed on the minerals.Meanwhile，under higher alkaline water environment，the OH- -F- anion exchange and the Ca(OH)2 mineral precipitation also contribute to the enrich⁃ ment of F- in the mine water.In addition，the hydrogen and oxygen isotope analysis indicates that the formation of a small part of high⁃fluoride mine water is related to evaporation.