Aqueous geochemistry of high-fluoride groundwater in Datong Basin, Northern China

Poor water quality has become a public health issue as cases of fluorosis in Datong Basin, Northern China are increasing. This paper investigates the origin and the geochemical mechanisms of fluoride enrichment in groundwater at Datong Basin. The fluoride concentration in the groundwater samples from 70 wells selected for this study ranges from 0.1 to 8.3mg/L (mean 2.2mg/L), with 51% of the samples containing fluoride concentrations that exceed the WHO drinking water guideline value of 1.5mg/L. High fluoride groundwaters are characterized by hydrochemical types HCO3Na(Mg), HCO3·SO4Na(Mg) and SO4·ClNa(Mg), with low Ca2+ and high HCO3− and Na+, and occurred in transition areas between the alluvial–pluvial mountain front plain and the alluvial–lacustrine plain of Datong Basin. Conditions favorable for fluoride enrichment in groundwater include weakly alkaline pH condition (7.2–8.2), moderate TDS, and HCO3− and Na+ as the dominant ions. The hydrolysis of F-bearing minerals in aquifer sediments is the dominant process for F− release, and facilitated by alkaline conditions and long residence time of groundwater. Calcite and fluorite are the main solubility-control minerals controlling the aqueous geochemistry of high fluoride groundwater. Evapotranspiration also favors fluoride enrichment in groundwater. ► Geochemical mechanisms of high fluoride groundwater at Datong were investigated. ► High fluoride groundwater is of transitional hydrochemical types. ► pH alone seems not to be a dominant factor controlling F− enrichment. ► The hydrolysis of F-bearing minerals is the dominant process affecting F− levels. ► The evaporation to a moderate extent favors F− enrichment, but is limited.