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

Abstract

Contamination of groundwater with fluoride and arsenic in deep aquifers poses new health challenges worldwide. Integrated hydrogeochemical parameters with multiple isotopes (δ18O, δ2H, δ13C, δ34S, 87Sr/86Sr and 14C) 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 µg/L) above the WHO guideline of 1.5 mg/L and 10 µg/L, respectively. Deep saline groundwater normally belongs to Na-SO4-(Cl) type and is characterized by distinctly depleted δ18O and δ2H values (as low as −10.4‰ and −78.5‰, 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 δ18O and δ2H 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 δ34S values (8.74‰-9.96‰), 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.