Geochemical Processes of Groundwater Fluoride Evolution in Geothermal Areas: A New Insight into the Dynamics of Fluorine Levels in Geothermal Water

Geothermal areas often exhibit high levels of fluorine in the geothermal water, which poses a significant challenge to the utilization of geothermal water resources. However, the underlying mechanisms responsible for the enrichment of fluoride during the formation of geothermal water are not yet fully comprehended. To shed light on this matter, an analysis was conducted to examine the hydrochemical characteristics, as well as the δD and δ 18 O isotopes, of geothermal water at various depths in the geothermal areas. This investigation aimed to reveal the potential evolutionary process of fluoride in geothermal water. The results demonstrate that the concentration of F – in the geothermal water samples from the Tongjing mining area ranges from 0.17 to 3.02 mg/L, with an average concentration of 1.41 mg/L. Specifically, the F – concentration in geothermal water samples above 40 °C exceeds 1.5 mg/L. The presence of Ca 2+ (Mg 2+ )–Na + (K + ) exchange in groundwater causes a reduction in Ca 2+ (Mg 2+ ) content, which subsequently decreases the saturation level of minerals such as CaF 2 , CaSO 4 ‧2H 2 O, CaCO 3 , and CaMg(CO 3 ) 2 . Consequently, more fluorine-containing minerals dissolve, leading to an increased concentration of F – in the groundwater. During the formation of geothermal water in the geothermal areas, the mixing of hot and cold water elevates the temperature of shallow groundwater. This rise in temperature enhances the solubility of CaF 2 and significantly facilitates cation exchange, thereby promoting the development of a groundwater environment characterized by high temperature, high Na + concentration, and low Ca 2+ concentration. Such conditions favor the enrichment of fluoride in geothermal water. These findings offer novel insights into the impact of geothermal water temperature on the evolutionary mechanism of fluoride and elucidate the process by which fluoride evolves in geothermal water within geothermal areas.