Enrichment Mechanism and Health Risk Assessment of Fluoride in Groundwater in the Oasis Zone of the Tarim Basin in Xinjiang, China

Groundwater is crucial water supply source in the Tarim Basin in Xinjiang. Due to the existence of high-fluoride groundwater, the shortage of water resources is more prominent, which brings challenge to safe groundwater supply. A total of 1326 groundwater samples were collected in the oasis zone to investigate the sources, driving factors, and health hazards of high-fluoride groundwater. Results showed that the high-fluoride groundwater was widely distributed around the edge of the oasis belt. The exceeding rates of fluoride in single-structure phreatic water (SSPW), phreatic water in confined groundwater area (PWCGA), shallow confined groundwater (SCG), and deep confined groundwater (DCG) were 31.6%, 51.5%, 35.0%, and 14.8%, respectively. Fluoride concentration showed a gradually decreasing trend from PWCGA to DCG. High-fluoride concentrations were closely related to weakly alkaline groundwater environment, high HCO3-, high Na+, and low Ca2+. The continuous dissolution of fluoride-containing minerals such as fluorite was the main driving mechanism of high-fluoride groundwater. Competitive adsorption of OH-/HCO3- and F-, evaporation concentration, precipitation of calcium-containing minerals, and cation exchange were conducive to the enrichment of groundwater fluoride. Human activities such as irrigation of high-fluoride groundwater also had a certain degree of effect on the shallow high-fluoride groundwater formation. Drinking high-fluoride groundwater would cause non-carcinogenic risks to different populations, health risks of fluoride through drinking water were increased with decreasing age. To effectively utilize groundwater resources and provide healthy drinking water for local residents, it is recommended to adopt the measures of water supply with different quality and fluoride reduction.