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Human health risks associated with groundwater fluoride have been assessed using USEPA method in few parts of India, but those assessments were conducted based on a single value for each parameter, which may lead to certain inaccuracy and uncertainties in results. In this study, a higher degree of accuracy in health risk assessment was achieved through Monte Carlo simulations, sensitivity analysis and uncertainty analysis. As fluoride hazards to human health are chronic, a total of 4560 water samples (N = 4560) were collected during consecutive four seasons (2 pre-monsoon and 2 post-monsoon seasons; 1140 samples/season) from the entire Birbhum district, covering all the blocks and geological settings to obtain the spatiotemporal variation of fluoride level. The Empirical Bayesian Kriging geostatistical model was employed to determine fluoride endemic areas. Amongst all blocks, Nalhati-1 had exhibited the highest fluoride level (18.25 mg/L). The study revealed that most of the blocks are vulnerable to groundwater fluoride due to its occurrences in excess level. The average and 95th percentile values of total hazard index indicate that the infants and children populations of the district are more susceptible than the adults and teens. The sensitivity analysis revealed that water ingestion rate and fluoride concentration are the most influential parameters for higher risk of fluoride-related health hazards. Health risks were evaluated through ingestion and dermal exposure routes for infants, children, teens, and adult residents. The exposure was much higher through ingestion than dermal contacts. Ingestion of defluoridated water will reduce the health risks associated with groundwater fluoride exposure., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Fluoride contamination in the groundwater has got great attention in last few decades due to their toxicity, persistent capacity and accumulation in human bodies. There are several sources of fluoride in the environment and different pathways to enter in the drinking water resources, which is responsible for potential effect on human health. Presence of high concentration of fluoride ion in groundwater is a major issue and it makes the water unsuitable for drinking purpose. Availability of fluoride in groundwater indicates various geochemical processes and subsurface contamination of a particular area. Fluoride-bearing aquifers, geological factors, rate of weathering, ion-exchange reaction, residence time and leaching of subsurface contaminants are major responsible factors for availability of fluoride in groundwater. In India, several studies have reported that the groundwater of several states are contaminated with high fluoride. The undesirable level of fluoride in groundwater is one of the most natural groundwater quality problem, which affects large portion of arid and semiarid regions of India. Rajasthan, Andhra Pradesh, Telangana, Tamil Nadu, Gujarat, and West Bengal are the relatively high-fluoride-contaminated states in India. Chronic ingestion of high doses of fluoride-rich water leads to fluorosis on human and animal. Over 66 million Indian populations are at risk due to excess fluoride-contaminated water. Therefore, groundwater contamination subject to undesirable level of fluoride needs urgent attention to understand the role of geochemistry, hydrogeology and climatic factors along with anthropogenic inputs in fluoride pollution.

This study appraised the groundwater fluoride (F−) endemicity and the exposure levels under the Central Tendency Exposure (CTE) condition and the Reasonable Maximum Exposure (RME) condition on the residents of the semi-arid parts of the Birbhum district of Peninsular India using a Variance Decomposition (Sobol Sensitivity Indices) approach combined with Monte Carlo Simulations. The study finds the national scale drinking water standard limit for F− (1.5 mg L−1) is inappropriate for the present survey area where F− concentration in groundwater varied between 0.26 and 11.82 mg L−1 and ~54.5% of the samples (N = 400) exceeded this limit. Therefore, estimated the optimum F− concentration of 0.733 mg L−1 for the region using the method recommended by the World Health Organization (WHO) to calculate the optimum F− limit at a regional scale. The average value of F− concentrations for this region (1.71 mg L−1) is considerably higher than the estimated optimum concentration or even the maximum permissible limits recommended for the subtropical regions (0.5–0.7 mg L−1). The exposure analysis revealed the infants and children as potentially vulnerable populations compared to adolescents and adults of the study area for CTE and RME scenarios. The multi-exposure pathways indicated oral intake as the main exposure pathway whereas exposure through dermal contact was insignificant for the residents of all age groups of this region. Based on the first, second and total order Sobol Sensitivity Indices, F− concentration (C) in groundwater, the groundwater ingestion rate and their combined interaction are the greatest significant parameters for the oral exposure model whereas C and its interaction effects with the proportion of the skin surface area in contact with groundwater as the utmost sensitive variables for the dermal health risks assessment model. The present study insists the inhabitants to intake defluoridated groundwater. • F− exceeded its estimated optimum value in drinking water at ~98.8% of the study area. • Infant and child age groups are more vulnerable to the toxicity of groundwater F−. • The estimated optimum F− level in drinking water for the study area is 0.733 mg L−1. • F− exposure via direct ingestion route was much higher than dermal exposure. • C , IR and their interaction are the sensitive parameters of the oral exposure model. [ABSTRACT FROM AUTHOR]

• Assessed groundwater fluoride level and its genesis in a semi-arid region. • Source of fluoride is mainly geogenic, influenced largely by pH and HCO 3 - ions. • Health risk assessed using dose-response characterization and the USEPA method. • Health risk evaluated via multiple exposure routes on different populations groups. • Infant and child age groups have more health risk than adults. The present study investigated the source, release mechanisms, mobility and controlling factors of fluoride in groundwater. It has been conducted in the semi-arid rural tract of Birbhum district, located in the eastern part of peninsular India which comprises of the granite genesis aquifers, soft to medium hard rocks with laterite and lateritic soils and alluvial aquifers. The groundwater fluoride level of the study area varied from 0.3 to 9.36 mg/L with an average value of 1.7 mg/L. In the granite region, weathering of granitic rocks, dissolution and iron-exchange processes are responsible for fluoride occurrence in groundwater and its concentration is controlled by the concentration of carbonate and bicarbonate ions. In clay, laterite soil and sedimentary aquifer regions, pH of the circulating water and clay/soil, surface charge characteristics of clay/soils, soil permeability and residence time control the groundwater fluoride through adsorption/desorption of fluoride by the clay and laterite soil. High evaporation rate of the study area also plays an important role to increase the groundwater fluoride concentration. The study also evaluated the health risks to the residents based on the dose-response characterization and USEPA human health risk assessment model through ingestion and dermal contact exposure pathways. The population of infants and children within the study area is more susceptible to the non-carcinogenic health hazards of fluoride than adults. This takes place mainly through direct ingestion route where the dominance order of the population is infants > children > men > women. [ABSTRACT FROM AUTHOR]