Sources, controls, and probabilistic health risk assessment of fluoride contamination in groundwater from a semi-arid region in Gujarat, Western India: An isotope–hydrogeochemical perspective

Abstract

Fluoride contamination in groundwaters of a rural region in semi-arid Western India has been studied using combination of geochemical-and-isotopic techniques, in conjunction with Health Quotient assessment approach. The objective of this study is to determine the sources and controls on fluoride content and to evaluate probabilistic non-carcinogenic risk associated with its long-term consumption. F⁻ ranges from 0.3 to 12 mg L⁻¹, shows high spatial variability, and ~ 35% of the samples have F⁻  > 1.5 mg L⁻¹ (WHO maximum limit for drinking). Two sources are identified: high F⁻ results from water–rock interaction of F-bearing minerals in granites and gneisses, while phosphate fertilizers can contribute up to ~ 0.46 mg L⁻¹ of groundwater F⁻ that can be significant for low F⁻ samples. High F⁻ samples are characterized by high pH, Na and alkalinity, and low Ca. Calcite precipitation drives the solubility of F-bearing minerals. Kinetic fractionation of water isotopes (¹⁸O and ²H) demonstrates that evaporation plays role in enriching groundwater F⁻. Non-carcinogenic risk, estimated by Hazard Quotient ($HQ_{{{\text{oral}}}}^{{\text{F}}}$), ranges from 0.13–5.72 to 0.26–11.86 for adult and children, respectively. Conservative estimate shows that ~ 0.467 million of adults and~0.073 million of children in four sub-districts are under the risk of fluorosis—while the residents of other five sub-districts remain safe from it. Finally, we suggest stakeholders to install F⁻ treatment plants to ensure the health safety of local residents in the high-risk zones, create awareness in farmers for optimum use of fertilizers, and promote rainwater harvesting, for better management of groundwater resources and quality in the region.