Hydrogeochemistry of high fluoride groundwater in shallow aquifers, Hangjinhouqi, Hetao Plain

Hydrochemical characteristics of groundwater, the lithological features of the aquifer sediments and fluoride in rocks were studied to determine the distribution and genesis of high fluoride groundwater (> 1.5 mg/L) in Hangjinhouqi, which is an endemic fluorosis areas. Groundwater with high fluoride (> 1.5 mg/L) is mainly of HCO3-Na or HCO3-Cl-Na type for both fresh and brackish water. Different from other halide ions of Cl−, Br− and I−, F− in groundwater decreased with the TDS. Analysis of fluorine in rocks showed that the average of total fluorine in rocks in Hangjinhouqi (1278.84 mg/kg) is much higher than the background value of Inner Mongolia (316.2 mg/kg). During long-term water-rock interaction, fluoride in these rocks from Yin Mountains will be released out and then accumulated in the groundwater of sand or clay (silty clay) aquifers in the Hangjinhouqi area. All the water samples fall below the Ksp curve of CaF2. Due to CaF2 solubility control, Na-predominant water is favorable for F enrichment, that's why groundwater with high fluoride displayed low TDS in the study area.

► High fluoride shallow groundwater in Hangjinhouqi is of HCO3-Na or HCO3-Cl-Na type. ► Unlike Cl−, Br− and I−, F− in groundwater decreases with increasing salinity. ► Elevated F in rocks from Yin Mountain is a source of F to sediment.