Hydrogeochemical and geothermal controls on the formation of high fluoride groundwater

•Natural high-F– groundwater mainly occurs in confined aquifers in the Guide basin.•Along the flow path, F– is gradually enriched in groundwater in confined aquifer.•High-F– groundwater is characterized by low Ca2+ and high Na+, pH, HCO3–•Precipitation of Ca2+ enhances dissolution of F–-bearing minerals and release of F–•Geothermal waters favor F– enrichment in confined groundwater.