Geochemical characteristics and fluoride distribution in the groundwater of the Zhangye Basin in Northwestern China

To better understand geochemical processes leading to the fluoride enrichment in the groundwater of the Zhangye basin, chemical parameters, stable isotopic compositions, tritium and radiocarbon data were obtained for groundwater samples (depth 10 m-250 m) from three hydrogeological environments: Longshou and Qilian piedmont and the central fine soil plain. Groundwater (n = 7 out of a total of 8 samples) with high fluoride concentration (> 1.0 mg/l) is found in the piedmont of the Longshou Mountains, with a maximum value of 3.06 mg/l. Fluoride concentrations in the groundwater also gradually increase from the piedmont of the Qilian Mountains from 0.15 to 0.45 mg/l (n = 7) to the fine soil plain from 0.21 to 2.78 mg/l (n = 23) located in the center of the Zhangye Basin. The high fluoride concentration in groundwater correlates with the prevalence of SO4-Cl-Na-Mg and alkaline conditions. Isotope and tritium analyses indicate that streams and rivers rapidly recharge the groundwater in the piedmont of the Qilian Mountains and the fine soil plain. In contrast, the high fluoride groundwaters in the piedmont of the Longshou Mountains display 14C ages older than 8 ka, indicating a longer water-rock interaction time. Using the ionic ratio plot and the Gibbs' diagram, evaporation and dissolution of fluorine-rich minerals are shown to be major factors resulting in the elevated fluoride concentrations. The results have implications for managing groundwater resources for drinking water supply to ensure public safety in the Zhangye Basin and other inland arid basins in northwestern China.

► High F groundwaters occurred in piedmont of the Longshou Mountains. ► High F groundwaters had a longer water-rock interaction time. ► Rich-fluorine minerals dissolution and evaporation are controlling the F− contents.