Distribution and Formation Mechanism of High Fluoride Groundwater in Langfang Plain Area

Natural high-fluoride groundwater is distributing in the Langfang Plain of China. To help ensure the water security of residents in this area, further research is needed to identify the spatial distribution characteristics and formation mechanisms of the high fluoride groundwater. In this study, 76 and 100 deep and shallow groundwater samples were collected in the Langfang Plain, respectively. The pH, fluoride ions, major anions and cations in the groundwater samples were analyzed and tested to systematically analyze the distribution of high-fluoride groundwater, the fluoride source, and its hydrogeochemistry. The results show that fluorine concentrations in both deep and shallow groundwater exceeded the standard of 1.0 mg·L^-1 at a rate of 73.7% and 40%, respectively. The groundwater fluorine concentrations increased with depth, showing higher concentrations in deep groundwater (median 1.65 mg·L^-1) than in shallow groundwater (median 1.06 mg·L^-1). Fluorite dissolution in the aquifer is the main source of fluorine in the groundwater and the saturated precipitation of calcite and dolomite promotes fluorite dissolution, thus favoring fluorine enrichment in deep groundwater. The alkaline groundwater environment favors the desorption of F^- in the aquifer and the competitive adsorption of HCO₃^- can further promote the accumulation of fluorine in groundwater. Cation exchange adsorption also promotes the accumulation of fluorine in deep groundwater. Evapotranspiration concentration has obvious effects in some shallow high-fluoride groundwater. At the same time, geological processes such as the dissolution of fluorine-containing minerals and rock salt are the main mechanisms for fluorine enrichment in deep groundwater.