Spatial distribution and enrichment mechanisms of high fluoride groundwater in geotherm-affected Pliocene aquifers of the Guide basin, China

Native high fluoride (F-) groundwater is one of the most serious environmental problems facing China and even the international society, which seriously threatens human health. The confined aquifer in the Guide basin contains high concentrations of F- and is subject to multifaceted influences, stemming not only from the geothermal activity present at the basin's bedrock but also from distinctive attributes such as elevated alkalinity, significant burial depths, and limited water circulation. Consequently, the precise mechanisms governing the enrichment of F- within the groundwater in this confined aquifer remain inadequately understood. This study aims to elucidate the migration and enrichment mechanism of F- in aquifers and assess the relative contributions of different control mechanisms responsible for high F- groundwater through systematic sampling, analysis and statistical calculations. Chemical types of confined groundwater and unconfined groundwater are Na-HCO3-Cl and Ca-HCO3, respectively. High-F- groundwater mainly exists in confined aquifers, and the content of Fgradually increases along the flow path. High content of HCO3- in confined groundwater promotes the precipitation of Ca2+, then enhances the dissolution of F--containing minerals, resulting in F- being continuously released into groundwater. High pH, high Na/Ca molar ratios, geothermal and long residence time of groundwater are the additional factors to accelerate the release and migration of F- in groundwater. Results of redundancy analysis showed that competitive adsorption is the primary mechanism for F- enrichment in confined groundwater, followed by the geothermal effect, precipitation of carbonate minerals and residence time of groundwater time. The results of potential health risk assessment indicate that the potential health risks associated with confined groundwater are notably higher across all age groups compared to unconfined groundwater. The research results are not only helpful for the local government in reducing the threat of drinking-water fluorosis, but also provide a reference for other high-F- groundwater research with similar geological conditions in China and around the world.