Major geochemical controls on fluoride enrichment in groundwater: A case study at Datong Basin, northern China

Abstract

Hydrogeochemical investigations were carried out in the Datong (大同) Basin, northern China to understand the sources and mechanisms of fluoride enrichment in the groundwater. The results indicate that fluoride concentrations reached up to 7.2 mg/L, and the hydrochemical type changed from Ca-HCO3, Ca-Mg-HCO3 water in the recharge areas to Na-HCO3, Na-Mg-HCO3 in the flow-through area and ultimately to Na-Cl-HCO3, Na-SO4-Cl and Na-Cl in the discharge areas. The total fluorine contents in the soils ranged between 277.6 and 1 292.5 mg/kg, which can provide large amounts of fluorine from fluorine-bearing minerals, such as fluorite, biotite and amphibole. The concentration of water-soluble fluorine ranged from 3.9 to 49.9 mg/kg (mostly less than 1% of the total fluorine) in the soils and rocks that, may have released sufficient fluorine into the groundwater. The results of PHREEQC simulations indicate that most of the analyzed samples are under-saturated with respect to fluorite (CaF2) from the recharge area to the discharge area. The fluctuation of F concentration in the 11 months indicates that the seasonal variation and the agricultural activity also affect the fluoride content in the groundwater. Fluoride enrichment in groundwater is predominantly controlled by fluorite solubility, residence time, evapotranspiration intensity and seasonal variation in this arid and semi-arid area.