Occurrence and hydrogeochemical characteristics of high-fluoride groundwater in Xiji County, southern part of Ningxia Province, China

High-F- groundwater is widely distributed in Xiji County, which endangers the safety of drinking water. In order to evaluate the key factors controlling the origin and geochemical mechanisms of F- enrichment in groundwater at Xiji County, one hundred and five groundwater samples and sixty-two sediment samples were collected. Fluoride concentration in the groundwater samples ranged from 0.2 to 3.01 mg/L (mean 1.13 mg/L), with 17 % exceeding the WHO drinking water guideline value of 1.5 mg/L and 48 % exceeding the Chinese drinking water guideline value of 1.0 mg/L. High-F- groundwaters were characterized by hydrochemical types of Na-HCO3 and Na-SO4 center dot Cl, which were found in Quaternary sediment aquifer and in Tertiary clastic aquifer, respectively. Conditions favorable for F- enrichment in groundwater included weakly alkaline pH (7.2-8.9), low concentration of Ca2+, and high concentrations of HCO3 (-) and Na+. Calcite and fluorite were the main minerals controlling F- concentration in groundwaters. The hydrolysis of F-bearing minerals in aquifer sediments was the more important process for F- release in Tertiary clastic aquifer, which was facilitated by long residence time of groundwater, in comparison with Quaternary sediment aquifer. Cation exchange would also play important roles, which removed Ca2+ and Mg2+ and led to more free mobility of F- in groundwater and permitted dissolution of fluorite, especially in Tertiary clastic aquifer. However, evapotranspiration and competing adsorption of B and HCO3 (-) were the more important processes for F- enrichment in Quaternary groundwater. Groundwater in Lower Cretaceous aquifer had relatively low F- concentration, which was considered to be the potential drinking water resource.