Groundwater fluoride and arsenic mobilization in a typical deep aquifer system within a semi-arid basin

Contamination of groundwater with fluoride and arsenic in deep aquifers poses new health challenges worldwide. Integrated hydrogeochemical parameters with multiple isotopes (delta O-18, delta H-2, delta C-13, delta S-34, Sr-87/Sr-86 and C-14) is applied to demonstrate the origins and evolutions of deep groundwater and mobilization of groundwater solutes, fluoride and arsenic at Yuncheng Basin, northern China. Over 80% of deep samples have elevated fluoride (up to 3.73 mg/L) and/or arsenic values (up to 24.3 mu g/L) above the WHO guideline of 1.5 mg/L and 10 mu g/L, respectively. Deep saline groundwater normally belongs to Na-SO4-(Cl) type and is characterized by distinctly depleted delta O-18 and delta H-2 values (as low as -10.4%o and -78.5%o, respectively) than deep fresh water and shallow groundwater, indicating palaeo-climatic effect. Results of ionic ratios and isotopes reveal that three types of solute sources are delineated for deep aquifers. Deep saline groundwater with old residence times of ~ 11000 yrs showed the most depleted delta O-18 and delta H-2 values, high B/Cl and low Br/Cl, pointing towards the palaeo-saline water for the locally distributed groundwater solutes in the Qiji and Wenxi County. Saline sample near the faults with residence time of 7403 yrs showed strong evidence of a geothermal origin. In the central basin, some deep groundwater yield modern ages (3696 yrs and 4348 yrs), higher salinities (up to 1200 mg/L) and lower delta S-34 values (8.74%o-9.96%o), indicating the vertical leakage of shallow groundwater suffering from pyrite oxidation. High F-As fresh groundwater, characterized by Na-HCO3(-SO4) type with diverse hydrogeochemical and isotopic compositions, are evolved in closed reducing environment and semi-closed facultative environment and controlled by palaeo-recharge and shallow leakage, respectively. Organic degradation mainly controls the enrichment of arsenic and fluoride in groundwater.

Automated summary

This study applies integrated hydrogeochemical parameters and multiple isotopes to investigate the sources, evolutions, and solubilization of fluoride and arsenic in deep groundwater in the Yuncheng Basin, northern China. The results show that over 80% of the deep samples have elevated levels of fluoride and/or arsenic above the WHO guideline values. Different types of deep groundwater have distinct characteristics, with deep saline water influenced by paleoclimate. The ionic ratios and isotopic compositions reveal the sources of solutes in deep aquifers and the effects of vertical leakage on the evolution of deep groundwater.