Pan India fluoride hazard assessment in groundwater.

Safe and clean water is necessary for human life, and around 66% of the world’s population depends on groundwater for drinking and other purposes [45]. In the Indian sub-continent, around 1 billion people rely on groundwater for drinking and irrigation purposes [3]. Therefore, comprehensive knowledge of groundwater quality encompassing hydrogeochemical characteristics is essential for a holistic understanding of water toward life sustainability. However, many countries worldwide face groundwater contamination, which is of severe concern [44], [46]. Various natural and anthropogenic causes are responsible for groundwater contamination [4], [40], [73]. Under these circumstances, research on groundwater hydrogeochemistry, contamination, and associated health hazards gained attention worldwide. One of the most alarming concerns is fluoride (F¯) contamination in India’s groundwater [5], [52]. A recent assessment revealed that India is among the 25 most fluorosis-endemic countries globally [63], [64], [74].

Worldwide F¯ contamination in groundwater through geogenic and anthropogenic processes is a serious concern [22], [27], [61], [72], [86]. F¯ constitutes around 0.06 to 0.09% of Earth’s crust and is ubiquitous, naturally occurring thirteen most abundant elements [65], [66]. Fluorapatite, fluorite, and cryolite are the most familiar F¯ bearing minerals, whereas minerals like sellaite and villiaumite hold the maximum percentage (> 55%) of F¯ [69]. Groundwater from aquifers of crystalline Archean basements, mainly alkaline granites and granite gneisses, and sedimentary rocks such as limestones and shales are sensitive towards F¯ [89]. Dissolution and leaching of F¯ bearing minerals in host aquifers release the F¯ in groundwater, and the concentration primarily depends on the water’s residence time within the host aquifer [93]. Around 90% of F¯ in the water is absorbed by the gastrointestinal tract of the human body [36]. Over one-fifth of a billion people worldwide suffer from fluorosis [8].

Depending on prolonged exposure to high F¯, ingested water may develop various adverse effects in humans, from mild dental fluorosis to skeletal fluorosis [26]. Furthermore, few research findings demonstrated that high exposure to F¯ may cause carcinogenicity, neurotoxicity, thyroid dysfunctionality, genotoxicity, and fertility-related issues [32]. Some research findings also revealed that children in excessive F¯ regions exhibited growth maturity-related issues in the pre-post-natal ontogenesis stage during the first 12 months and adolescence [12]. Nonetheless, even a low F¯ content of less than 0.5 mg/L is a potential cause of tooth decay [96]. According to the World Health Organization (WHO) and the Bureau of Indian Standards (BIS), the maximum limit of F¯ for drinking purposes should be 1.5 mg/L [11], [94]. F¯ concentration greater than 1.5 mg/L is not suitable for drinking. Moreover, prolonged exposure to F¯contaminated water with values greater than 2.0 and 3.0 mg/L may lead to dental and skeletal fluorosis and is associated with high and very high risk, respectively [3]. The toxicological mechanism of F¯ still needs a deeper understanding and a more significant magnitude of investigation.

Countries like China, India, Netherlands, Spain, Sri Lanka, Iran, and Mexico usually exhibit the F¯ value above the permissible limit [22], [5], [8]. Moreover, excessive F¯ in groundwater is unsuitable for developing countries like India, primarily due to the enormous population, lack of awareness, and spatial extension. Groundwater is an integral part of Indian livelihood, as around 90% of the rural and 30% of the urban populations depend on it for drinking and other purposes [91]. However, this essential resource in many parts of India is F¯ contaminated. Shortt et al. [82] identified endemic fluorosis in the Prakasam district (formerly Guntur district) of Andhra Pradesh (formerly Madras Presidency), India. Ugran et al. [87] reported that in some regions in Karnataka, more than half of the child population is affected by dental fluorosis. Saravanan et al. [80] and Gautam et al. [24] reported the severity of dental fluorosis in parts of Tamil Nadu and Rajasthan. [75], [71], [76] comprehensively reported the severity and wide variety of fluorosis diseases in Andhra Pradesh and Telangana. Similarly, the study by Senthilkumar et al. [81] revealed the magnitude of dental and skeletal fluorosis hazards in parts of Gujarat. Narwaria and Saksena [55], Sahu et al. [79], and Marghade et al. [48] have mentioned the impact of F¯ borne diseases in Madhya Pradesh, Chhattisgarh, and Maharashtra, respectively. Adimalla and Venkatayogi [1] showcased the overall F¯ contamination and different prevailing fluorosis conditions and possibilities. Bera et al. [10] showed Jharkhand’s different fluorosis disease conditions and their effect on human society. Therefore, an F¯ health risk assessment is one of the fundamental needs of understanding the associated health hazards for humans. Advanced geostatistical and geospatial approaches in the geochemical assessment of F¯ and associated health hazards are increasingly used significantly in various regions worldwide. Various earlier studies (Table 1, Sl 1 to Sl 8) in line with F¯ health hazard outlining the significance of groundwater quality are evaluated to understand the holistic worldwide and pan India scenario. However, this present study is planned with a novel approach to focus on the pan India perspective of F¯ contamination and associated probable health risks (Table 1, Sl 9).

India has varied hydrogeological and geo-environmental settings, where every province’s groundwater regime is unique by nature. Therefore, correctly estimating the spatio-temporal variability of F¯ in groundwater is essential to knowing the degree of contamination and associated risk factors induced by geogenic activities and geo-environmental control. This is one of a kind of challenge for groundwater researchers. The exclusive circumstances, F¯ contamination, and associated geo-environmental and geogenic control need a comprehensive assessment in a pan India region. Therefore, demonstrating a comprehensive F¯ contamination in Indian landmass by local region-specific studies is not possible. Furthermore, there is a gap in the complete holistic pan India F¯ contamination study to assess the associated health hazard, which needs to be addressed. Subsequently, it is essential to have a comprehensive scientific database of F¯ contamination in India. Thus, the current study is conducted with coupled advanced geostatistical and geospatial techniques to assess (i) the F¯ contamination in India, (ii) associated probable health hazards, and (iii) the hydrogeological and geo-environmental control by using a coupled analytical approach. This study further aims to provide a F¯ management strategy integrating SWOT analysis illustrating internal aspects of strengths and weaknesses along with external aspects of opportunities and threats [67], [78]. The strengths of this study showcase the spatio-temporal variability of pan Indian F¯ contamination, the possibilities of associated health hazards, the usage of the advanced geostatistical and geospatial approach for precise results, and the hydrogeological and geo-environmental factors controlling the contamination magnitude. Weakness notifies the probability of F¯ inter-aquifer migrations, the possibility of considering multi-temporal F¯ data, and the exclusion of industrial impacts. The opportunities that emerge from this study are essential for water safety and sustainability measures, a way forward for efficient management strategies and guidelines for other harmful contaminants study. Threats caution against the effect of unaccounted influences on F¯ contaminations, the impact of climate change, faulty management practices, and non-coordinated administrative approaches. This in-depth research will be a one-of-a-kind guideline for risk mitigation related to F¯ for human health management, scientific knowledge for F¯ occurrence, and control for future adaptability.