• The highest fluoride concentrations were observed in the eastern cold springs.
• Reaction of fluorite mineral with HCO₃was likely to cause fluoride enrichment in the eastern springs.
• In the western and thermal springs, origin of fluoride was related to weathering of F-bearing silicate minerals.
• Health risk of fluoride in drinking waters were in the following order: children > teenager > adults > infants.

Abstract

Groundwater fluoride contamination is a major issue of water pollution in the world with health hazards such as dental and skeletal fluorosis. This research focused on exposure to the high concentration of fluoride in the springs water in the Bazman volcanic area, southeast Iran. The combination of chemical/isotopic analysis, geochemical modeling, health risk assessment and multivariate statistical methods were applied to investigate the contamination and sources of fluoride in the samples. Groundwater samples were collected from cold and thermal springs. Major ions, fluoride, trace elements and stable isotopes ?¹⁸O and ?D were measured in the samples using standard methods, ICP-MS and OA-ICOS, respectively. Fluoride content in springs varied from 0.5 to 3.75 mg/L with an average value of 1.66 mg/L. The highest fluoride concentrations were observed in the eastern cold springs while thermal springs showed the minimum fluoride contents. The majority of samples showed F contents higher than the calculated optimal concentration of fluoride (0.75 mg/L). Reaction of fluorite mineral with HCO₃ and replacement of F in clay minerals and metal oxy-hydroxides with OH^? in water were likely cause fluoride enrichment in the eastern springs. Whereas, in the western springs and thermal springs, origin of fluoride was related to weathering of muscovite, cryolite, apatite and fluoroapatite minerals. The ?¹⁸O and ?²H of the water samples displayed the impact on evaporation on fluoride enrichment in all spring water samples. The average value of contamination index (C_d) in the water samples was 1.94 categorizing medium risk level while springs S7, S8, S9 and S4 were above the threshold value of C_d index. The fluoride hazard quotient (HQ) showed that 25%, 44%, 56% and 0% of springs’ water resources had high risk level for age group of adults, teenager, children and infants, respectively. Therefore, health risk of fluoride in drinking water resources were in the following order: children > teenager > adults > infants.