Drinking water is the most significant contributor to fluoride ingestion, among all other sources in human body. Table 2 presents the statistical analysis summary of fluoride in the groundwater of all tested regions in the Cuttack district of Odisha and the value of individual site is given in Table S2 of supplementary material. The fluoride concentration of the whole study area ranged from 0.58 to 4.95 mg/L, showing higher fluoride content than the WHO-permissible limit (1. 5 mg/L). The maximum content of fluoride was recorded at Site-13 (4.95 mg/L) followed by Site-10 (4.26 mg/L), Site-8 (3.81 mg/L), Site-6 (3.24 mg/L), Site-16 (3.02 mg/L), Site-17 (2.97 mg/L), Site-5 (2.87 mg/L), Site-11 (2.71 mg/L), Site-14 (2.38 mg/L), Site-12 (2.16 mg/L), Site-15 (1.81 mg/L), Site-2 (1.25 mg/L), Site-3 (1.12 mg/L), Site-7 (1.12 mg/L), Site-4 (1.04 mg/L), Site-1 (0.82 mg/L) and Site-9 (0.81 mg/L). Among all the 17 studied sites, the groundwater of Site-6 (3.57 mg/L) has the highest mean F− concentration followed by Site-13 (3.83 mg/L), Site-10 (2.83 mg/L), Site-8 (2.77 mg/L), Site-6 (2.50 mg/L), Site-16 (2.47 mg/L), Site-5 (2.15 mg/L), Site-17 (2.03 mg/L), Site-12 (1.92 mg/L), Site-11 (1.80 mg/L), Site-14 (1.80 mg/L), Site- 15 (1.42 mg/L), Site-2 (1.04 mg/L), Site-7 (0.90 mg/L), Site-3 (0.90 mg/L), Site-4 (0.89 mg/L), Site-9 (0.71 mg/L) and Site-1 (0.69 mg/L). The mean concentration of 12 gram panchayats of Narasinghpur block was higher than the WHO-permissible limit (Fig 1), indicating severe pollution of fluoride in the groundwater of endemic regions of CKDu in the Cuttack district. Due to the geology of the local aquifers, the groundwater of the Indian subcontinent is heavily enriched with endemic fluoride concentration (Ahada and Suthar 2019). Several sampling regions had considerably varied skewness test results, indicating that the distributions of fluoride-rich minerals in this region are diverse. In approximately 45 percent of research locations, the mean value of fluoride for the studied sites was significantly higher than the area mean (1.8 mg/L), showing variation in the fluoride loading distribution in this region (Table 2).
Local geology indicates alluvial deposits along the Mahanadi river primarily in the western and central portions of the district. The fissured formation was produced by the precambrian crystallines, which mostly consist of granitic rocks, khondalites, and charnockites and occupy the western portion of the area (CGWB 2013). The concentration of fluoride in groundwater is dependent on the concentration of fluoride-bearing minerals in rock types, their depositional, dissociative, and dissolutional activities, and the length of the chemical reaction. Considered to be the primary source of fluoride in groundwater is the weathering of granite rocks, which contain fluoride-bearing minerals. Alkaline conditions (pH between 7.6 and 8.6), high HCO3- concentration (350–450 mg/L), and moderate specific conductance (750-1,750 S/cm) promote the dissolution of fluoride from fluorite into water (Saxena and Ahmed 2001; Arveti et al. 2011). The distribution of Ca2+ and, to a lesser extent, SO42-, as well as ionic strength and the presence of complex ions, appear to influence the F- concentration in groundwater. F- was found to be negatively correlated with Ca2+ and positively correlated with Na+ (Gupta et al. 2006). Due to the alkaline pH of the water, fluorides are dissolved from granitic or fluoride-bearing rocks in aquifers. In an alkaline environment, OH- rapidly replaces the F- adsorbed on the clay surface, enriching F-. Due to the similar ionic radii of hydroxyl (OH-) and fluoride (F-) under alkaline conditions, F- can be exchanged for hydroxyl (OH-) ion and transferred from fluoride-bearing rocks and clay minerals into groundwater via ion exchange (Mukherjee and Singh 2020). A number of variables influence the distribution of fluoride in ground water, including the amount of soluble and insoluble fluorine in the source rock, the temperature, precipitation, vegetation, redox potential, pH, and ion exchange activities (Das et al. 2000). The charnockitic, granitic, hornblende, and biotitic gneisses and other fluoride-bearing minerals found in the Narasinghpur block, such as micas, pyroxene, fluorite, tourmaline, topaz, sphene, and apatite, increase fluoride levels in groundwater (CRWB 2013). Increased evaporation, induced by the prevalent warm climate, raises the concentration of soluble fluorides (Moyce et al. 2020; Chawla et al. 2014). Similar results had been observed in Bolagarh block of the Khurda district, which is underlain by porphyritic granite gneisses, had fluoride values ranging from 1.4 to 8.2 mg/L 60 metres beneath the surface (Das et al. 2000).
Non-carcinogenic Health risk assessment of flouride exposure in groundwater
Environmental fluoride concentrations can have harmful effects on human health. The main source of fluoride exposure for the general public is the fluoride content of water and food in fluoride rock regions (Ahada and Suthar 2019). Children can develop kidney damage if fluoride levels in drinking water exceed 2.0 mg/L, and the severity of the damage increases with increasing fluoride levels (Schiffl 2008; Xiong et al. 2007; Liu et al. 2005). Young children retain a significantly higher proportion (80%) of an absorbed dose of fluoride than adults (50%) do, supporting the statement that "renal fluoride excretion rate is lower in children than in adults" (Mukherjee and Singh 2020). Previous researchers have noticed that infants and children exposed to high levels of fluoride by drinking water and food are exposed to greater fluoride retention in the body, consequently weakening their kidneys and increasing their vulnerability to future kidney issues (Lantz et al. 1987; Ludlow et al. 2007; Chan et al. 2013). The patient's young age, prolonged period of excessive fluoride consumption, and the absence of other causes of renal insufficiency are suggestive of a causal relationship between fluoride poisoning and renal insufficiency. The organs most seriously impacted by high fluoride exposure are the liver and kidneys (Yang and Lian 2011). In addition, dangerously excessive levels of fluorides in the diet will make persons susceptible to CKD. Individuals who work in fields and other outdoor labor-intensive occupations for extended periods typically consume more water to hydrate themselves and are exposed to excessive fluoride levels. Consuming tea adds even more fluoride to their system, worsening the condition (Dharmaratne 2015).To assess the non-carcinogenic hazard, CDIs and HQs of groundwater fluoride for each population groups were evaluated through ingestion and dermal routes are presented in Table 3.
The CDI of F- for infants, children, men, and women via ingestion varied between 0.0483 and 0.4125 mg/kg-bodyweight/day, 0.0435 and 0.3713 mg/kg-bodyweight/day, 0.0214 and 0.1828 mg/kg-bodyweight/day, and 0.0190 and 0.1620 mg/kg-bodyweight/day, respectively. The following is the average CDI (mg/kg-bodyweight/day) for various populations: Infants (0.1517)> children (0.1365)> men (0.0672)> women (0.0596). The average CDI for the study area is greater than the recommended dose for newborns (0.14 mg/kg/day) and children (0.13 mg/kg/day). Hence, the risk of health hazard owing to a higher fluoride concentration is significantly greater for infants and children than for adults. The mean HQ values of fluoride ingested are larger than one for infants (2.5276), children (2.2744), and men (1.1199), but less than one for women (0.9926). The CDI for F- administered by dermal route ranged from 0 to 0.00017, 0 to 0.00027, 0 to 0.00029, and 0 to 0.00035 mg/kg-bodyweight/day for infants, children, men, and women, respectively. The following is the average CDI (mg/kg-bodyweight/day) for various populations: women (0.00013) > men (0.00011) > children (0.00010) > infants (0.00006) had considerably lower dermal exposure reference doses than men (5.82×10-2 mg/kg bodyweight per day). CDI values are significantly greater via ingesting than dermal absorption (four orders of magnitude for infants and three orders of magnitude for other population groups). Although none of the estimated HQ values are higher than one, these HQder values imply that adults are more susceptible to the higher concentration of fluoride in groundwater through dermal contact than children and infants. In addition, the majority of locals bath in ponds, rivers, and canals. Consequently, fluoride exposure via skin contact due to the use of groundwater can be excluded for the research area. The HQ results show that the major route of fluoride exposure for the inhabitants of the study area is ingestion, and that infants and children are more susceptible to fluoride poisoning via this route than adults. This is owing to the fact that infants and children's bodies are more susceptible to health issues because of their small body weight and reduced metabolic processes. The exposure to fluoride toxicity is strongly connected with the individual's living area and long-term exposure to high-fluoride groundwater (Adimalla and Qian 2019).
The regional distribution of Total health risk (THI) owing to groundwater fluoride exposure on different population groups is presented in Figure 2(a-d) (cross validation of the model is depicted in Table S3, S4, S5, S6 of the supplemental material), indicating the vulnerable sections of the study area. For infants, children, adult men and adult women, respectively, the THI values of fluoride varied from 0.805 to 6.878 (mean 2.528) and from 0.725 to 6.192 (mean 2.276), from 0.357 to 3.051 (mean 1.121), from 0.317 to 2.706 (mean 0.995), indicating that fluoride may have harmful health effects in the following order: infants > children > men > women. While for fluoride, THI values are over the permitted USEPA limit of 1 for infants, children, and adult men and women in 92.42 percent, 86.36 percent, 51.51 percent and 43.93 percent of the areas under study, respectively. Using the identical model, similar research were conducted in different regions of India and other parts of the world. In Nirmal district of Telangana (India), Adimalla et al. (2018) reported that susceptibility to fluoride toxicity on different demographic groups follows the sequence children > women > men, where the range of fluoride in groundwater was between 0.2 and 7.1 mg/L. Similar pattern was observed in the southern districts of Panjab (Ahada and Suthar 2019) and northwest China (Li et al. 2016). This discrepancy in the outcome is attributable to the utilisation of different reference values for the identical parameters. It is possible that high water hardness and fluoride are the environmental elements that can induce kidney injury in Sri Lanka's local groundwater in a region where CKDu is common (Yang et al. 2022).
According to the findings, infants, children, and adults in the majority of research locations could be at risk for having non-cancer health issues. Children and newborns are more susceptible to the non-cancerogenic health effects of fluoride than adults. Certain renal and other enzyme pathways are inhibited by excessive fluoride, resulting in kidney and other organ failure. Children are susceptible to as little as 2 ppm of fluoride in drinking water due to their ability to retain up to 80% of ingested fluoride in the body/kidney. As a result, if the youngster continues to drink water from the same source, it could lead to the development of a sick kidney in adulthood and increased risk of developing chronic kidney disease (CKD) (Dharmaratne 2019). In order to improve the quality of life, immediate attention must be paid to the health risk/non-carcinogenic risk of the residents of the Narasinghpur block. Thus, drinking water quality should be enhanced to prevent health hazards and ensure the population' ability to live sustainably (Li and Wu 2019).