Hydrochemical characteristics, driving factors and health risk of fluoride in groundwater from the northwestern Ordos Basin, China

Groundwater is essential for maintaining public health, promoting economic development, and ensuring ecosystem stability in arid and semi-arid regions. The northwestern Ordos Basin (China) primarily relies on groundwater from multilayered aquifer systems; however, our knowledge of the hydrochemical characteristics and water quality of groundwater in this region is limited. Here, we employed a newly collected dataset of 94 groundwater samples from different aquifers to constrain the source, controlling processes of fluoride in groundwater, and its potential health risk in the area. Groundwater is characterized by Na-Cl and Na-SO₄ types with a minor Na-HCO₃ type, which is primarily controlled by ion exchange, silicate weathering, and the dissolution of carbonate and evaporite minerals. Of the groundwater samples, 42% exceeded the fluoride limit of 1.5 mg/L established by the World Health Organization (WHO). This is mainly attributed to geogenic sources, including fluorine-bearing mineral dissolution, cation exchange, evaporation, and competitive adsorption. The water quality index suggests that most samples are unsuitable for drinking. Health risk assessment results based on the Monte Carlo simulation indicate that children face significantly higher non-carcinogenic health risks from fluoride exposure than adults (both males and females). These findings provide new insights into the complex hydrogeochemical evolution of fluoride in groundwater and the groundwater quality status in multi-aquifer systems, contributing to the sustainable development and management of groundwater resources in the Ordos Basin.