The effect of mining development in karst areas on water acidification and fluorine enrichment in surface watersheds

High fluoride water is a crucial driving factor for endemic fluorosis. As an important research content of hydrogeochemistry, the enrichment of fluorine in alkaline water has received a fair amount of scholarly attention, but the understanding of the migration and enrichment of fluorine in acid mine drainage (AMD) in karst area remains very limited. An analysis of 13 consecutive periods of hydrochemical samples (312 samples in total) revealed that the weathering of carbonates and sulfide-rich coal measures induced a pH as low as 2.29 in the Chetian River in Jinsha, Southwest China. The highest content of fluorine in AMD was 23.8 mg/L, and the average content in the basin was 1.4 mg/L. In terms of the seasonal variation in the whole basin, the fluorine content were higher in the rainy season than in the dry season. The mineral saturation index shows that the dissolution of fluorapatite and fluorite is an important source of fluorine. The chloro-alkaline indices displayed a strong ion exchange process in the basin, promoting the release of fluorine in silicate minerals. In comparison, the contribution of external inputs, such as atmospheric deposition, was less. Additionally, evaporation was shown to have a limited influence on fluorine enrichment. Meanwhile, pH was an essential factor driving the dynamic transformation of the mode of occurrence of fluorine in water. In the upstream alkaline water, the main occurrence form of fluorine was free F^-, while the F/Al ratio for most of the acidic samples was ≤ 1.0, indicating the main occurrence form of fluorine was likely AlF²⁺. The conclusion of this study provides a new understanding for deepening the geochemical characteristics of fluorine in karst surface water.