Hydrogeochemistry and isotope hydrology were carried out to investigate the spatial distribution of fluoride (F) and the mechanisms responsible for its enrichment in the western region of the Ordos basin, northwestern China. Sixty-two groundwater samples from the unconfined aquifer and fifty-six from confined aquifer were collected during the pre-monsoon (June 2016). Over 77% of groundwater samples from the unconfined aquifer (F concentration up to 13.30 mg/L) and approximately 66% from confined aquifer (with a maximum F concentration of 3.90 mg/L) exhibit F concentrations higher than the Chinese safe drinking limit (1.0 mg/L). High-F groundwater presents a distinctive hydrochemical characteristic: a high pH value and HCO3 concentration with Ca-poor and Na-rich. Mineral dissolution (e.g., feldspar, calcite, dolomite, fluorite), cation exchange and evaporation in the aquifers predominate the formation of groundwater chemistry, which are also important for F enrichment in groundwater. Mixing with unconfined groundwater is a significant mechanism resulting in the occurrence of high-F groundwater in confined aquifer. These findings indicate that physicochemical processes play crucial roles in driving F enrichment and that may be useful for studying F occurrence in groundwater in arid and semi-arid areas.

*Original abstract online at https://www.ncbi.nlm.nih.gov/pubmed/31252113