Geochemical factors controlling the occurrence of high-fluoride groundwater in the western region of the Ordos basin, northwestern China

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 r occurrence in groundwater in arid and semi-arid areas. (C) 2019 Elsevier Ltd. All rights reserved.