Abstract:

High F^- groundwater has been found in deep aquifers of the coastal plain region, which seriously threatens the domestic water safety. To ascertain its enrichment mechanism, sixty-nine groundwater samples on site were collected for hydrochemical and isotopic analyses of the low- and high-fluoride groundwater. The results show that high fluoride groundwater mainly exists in Na-HCO₃-type groundwater, while the chemical type of low-fluoride groundwater is mainly Ca·Na-HCO₃. Saturation index calculation shows that the groundwater samples are mostly fluorite undersaturated but calcite oversaturated. This indicates that fluorite dissolution is favored with precipitations of Ca²⁺ as calcite, thus leading to higher F^- level in the groundwater. The linear regression results show that cation exchange occurred between Ca²⁺ and Na⁺ in deep groundwater, which could decrease the groundwater Ca²⁺ content and further promote the CaF₂ dissolution. There is good positive correlations between F^- and pH and {\mathrm{HCO}}_3^{-} in deep groundwater, indicating that the slightly alkaline environment and high {\mathrm{HCO}}_3^{-} content are beneficial to F^- desorption. In addition, there is strong positive F^- vs. Li correlation in deep groundwater, which also indicates that the local mixing of geothermal water may directly affect the F^- enrichment in deep groundwater. Comparing the hydrochemical and isotopic characteristics of deep groundwater with seawater, we found that the local F^- enrichment of deep groundwater is unaffected by seawater intrusion.

Key words: coastal plain, fluoride groundwater, distribution characteristic, water-rock interaction, Hebei Province