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

Abstract

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 (<600 μm) were mixed in ultrapure water (pH = 7) for 2000 h. A prominent variation, which ranged between 0.22 and 0.95 mgL−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.