Increase in fluoride concentration in mine water in Shendong mining area, Northwest China: Insights from isotopic and geochemical signatures

Mine water poses severe threats to the quality of the water supply and ecological environment of the Shendong mining areas owing to its excessive fluoride (F-) content. However, the geochemical behaviours and enrichment mechanisms responsible for F- enrichment during mining activities are not fully understood. In total, 18 Yanan groundwater and 45 mine water samples were collected to analyse the spatial distribution, hydrogeochemical behaviours, and formation mechanisms related to elevated F- levels by analysing the stable isotopes and water-rock interactions. In this study, F- concentrations in mine water samples varied from 0.16 to 12.75 mg/L, with a mean value of 6.10 mg/L, and 77.78% of the mine water samples had a concentration that exceeded China's national standards (1.00 mg/L) for drinking water. The F- concentration was markedly high in the mine water samples, with the mean F- concentration being 1.58 times of that in the Yanan groundwater samples. The results of stable isotopes (O-18(H2O), D, S-34(SO4), and O-18(SO4)) and water-rock interaction analyses suggested that cation exchange and competitive effects were the dominant factors responsible for elevated F- concentration in mine water during mining activities. Thus, the weathering of F-bearing minerals, agriculture, and domestic activities do not play a significant role in the secondary enrichment of F- concentration.