Occurrence and hydrogeochemistry of fluoride in alluvial aquifer of Weihe River, China

Fluoride (F-) has significant impacts on human health. High fluoride groundwater (up to 1.90 mg/L) has been found in upper confined aquifer underlying the first terrace of Weihe River during a hydrogeological investigation for water supply in 2005. To reveal the occurrence and hydrogeochemistry of high F- groundwater, hydrogeochemical tools such as saturation index, ionic ratios and correlation analysis were used in this study. The study shows that the concentrations of most physiochemical parameters from phreatic water, influenced by intensive evaporation and anthropogenic activities such as unregulated sewage and excreta disposal and agricultural practices in the area, are higher than those of confined water. The F- concentration in phreatic water is within the acceptable limits set by China and the World Health Organization (WHO), while that of upper confined water shows a decreasing trend northwestward as the Weihe River approaches, with F- concentration in the first terrace beyond the national and the WHO standards. High F- groundwater is observed in alkaline environment associated with high Na+, pH, HCO3 (-) and low Ca2+ and Mg2+. The enrichment of F- is controlled by geologic and hydrogeological conditions, fluorine-bearing minerals presented in alluvial formations and their dissolution/precipitation under the alkaline environment along groundwater flow. Ion exchange, human activities and the mixing of different recharge waters may influence the enrichment of F- as well.