Distribution Characteristics, Genesis, and Health Risk Assessment of Fluoride in Groundwater of the Taigemiao Mining...

This study investigates the distribution, origin, and health risks of fluoride in groundwater within the Taigemiao mining area using hydrochemical analysis, PHREEQC modeling, and the U.S. EPA noncarcinogenic risk assessment model. The key findings are as follows: (1) Groundwater hydrochemistry transitions from shallow HCO₃-Na·Ca freshwater (TDS 256 mg·L^-1) to deep Cl·SO₄-Na saline water (TDS 7344.5 mg·L^-1). Fluoride is significantly enriched in the Yan'an Formation aquifer at depths > 700 m, with a mean concentration of 4.303 mg·L^-1.Groundwater from the Yan'an Formation is characterized by high fluoride concentrations.(2) Spatially, fluoride exhibits stratified patterns: concentrations decrease from the center outward in the Zhidan Group; are higher in the south and lower in the north within the Anding and Zhiluo Formations; and show elevated levels in the west, decreasing eastward in the Yan'an Formation. Vertically, F^- initially decreases then increases markedly with depth, peaking in the Yan'an Formation below 700 m (mean 4.303 mg·L^-1). (3) Fluoride enrichment in deep groundwater (Zhiluo to Yan'an Formations) is primarily controlled by evaporation concentration, which raises Na⁺ and Cl^- and promotes strong cation exchange, reducing Ca²⁺ activity. This enhances fluorite dissolution, while calcite and dolomite precipitation further remove Ca²⁺. Together, these processes weaken fluorite dissolution inhibition, leading to substantial F^- accumulation. (4) The non-carcinogenic health risk assessment reveals that fluoride has unacceptable non-carcinogenic risks for children (mean HI 0.98, maximum HI 9.64), which are higher than those for adults (mean HI 0.50). This underscores children's greater vulnerability to fluoride exposure and indicates that the groundwater is unsuitable for their consumption.