Geochemical sources, hydrogeochemical behaviour of fluoride release and its health risk assessment in some fluorosis endemic areas of the Brahmaputra valley of Assam, India

Highlights: • High F⁻ concentration upto 9 mgL⁻¹ was detected in groundwater of the study area. • Micaceous minerals like biotite and muscovite were found to be the sources of F⁻. • Cationic (Ca²⁺/Na⁺) and anionic (F⁻/OH⁻) exchanges impacted groundwater F⁻ levels. • Calcium depletion due fluorite and calcite unsaturation mainly control F⁻ levels. • Health risk assessment showed children at high risk than male and female population. High fluoride (F⁻) concentration was detected in groundwater samples collected from three districts of the Brahmaputra valley of Assam, Nagaon, Kamrup Metropolitan (M) and West Karbi Anglong. The maximum F⁻ value of 9 mgL⁻¹ was found in southern Nagaon. About 38% of the total groundwater samples exceeded the WHO permissible limit of 1.5 mgL⁻¹. Petrographic analyses revealed that micaceous minerals such as biotite and muscovite are the dominant contributors of F⁻ in Nagaon and West Karbi Anglong; in Kamrup (M), it is biotite. The alkaline groundwater played a significant role in governing the dissolution and mobilization of F⁻ from its sources. Silicate weathering was more prominent in Nagaon and West Karbi Anglong, while carbonate weathering was dominant in samples collected from Kamrup (M). Both cationic (Ca²⁺/Na⁺) and anionic (F⁻/OH⁻) exchange processes occurred extensively across the studied hydrogeochemical regime. PHREEQC simulations revealed that fluorite and calcite undersaturation govern the F⁻ values in groundwater. This finding is well supported by the results of the dissolution experiment of rock samples, where powered samples (−1, was observed in the F⁻ concentration for Nagaon's rock sample solution. The experimental results gave a glimpse of real-time rock-water interaction scenarios in the study area, revealing Ca²⁺ removal processes to primarily govern F⁻ release into aquifers. A non-carcinogenic risk assessment study of F⁻ indicated a potential risk for the exposed population, especially, children in the study area.