Geochemical controls on the enrichment of fluoride in the mine water of the Shendong mining area, China

Abstract

Underground coal mining produces large amounts of mine water annually in the Shendong mining area of China. Due to the severe scarcity of water resources, mine water is extensively used for productive, domestic, and ecological demands. However, mine water exhibits high fluoride levels. For water-use security, reduction of fluoride exposure and environmental protection, knowledge of sources and geochemical controls of fluoride enrichment in mine water is required. The results showed that F^- concentrations of mine waters vary from 0.05 to 11.65 mg/L, with a mean value of 1.96 mg/L, and 51% of the mine waters contain F^- concentrations exceeding the Chinese drinking water standard (1 mg/L). The overall mine water quality is influenced by cation exchange, mineral dissolution, pyrite oxidation, silicate weathering and so on. The high-fluoride mine waters are all associated with Na-type, with a remarkable cation composition feature of higher Na⁺ and lower Ca²⁺ and Mg²⁺ concentrations. Overall, the high-fluoride mine waters are well-matched with the water environment with higher pH, TDS, and EC levels. PCA reveals that the geochemical controls on the enrichment of F^- in mine waters include dissolution of fluoride-bearing minerals and F^--OH^- ion exchange; the former process is mainly caused by the decrease in Ca²⁺ concentrations resulting from Na ⁺ -Ca²⁺ cation exchange and mineral precipitation, and the latter process benefits from a highly alkaline water environment, facilitating the substitution of OH^- in the mine water for F^- within or absorbed on the minerals. Evaporation also controls F^- enrichment in local areas.

Keywords: Fluoride; Geochemical controls; Geochemistry; Mine water; Shendong mining area.