Aquifers with high concentrations of fluoride in groundwater have been documented in many regions worldwide, a majority occurring in arid and semi-arid areas in developing countries. Over 220 million people are estimated to rely on groundwater with concentrations above the WHO guideline value and their equivalent national standards for their potable water supply. Parts of China, India, Iran, Pakistan, Sri Lanka, West Africa (Ghana, Ivory Coast, Senegal), North Africa (Algeria, Morocco, Libya, Sudan, Tunisia, West Sahara), South Africa, the East African Rift Valley (Eritrea, Ethiopia, Kenya, Uganda, Rwanda, Tanzania, Uganda, Zimbabwe), Yemen, Mexico, Brazil, and central Argentina are affected. In India alone, endemic fluorosis is a problem in 17 out of the country’s 28 states, and some 67 million people are estimated to be exposed to drinking water at concentrations above the WHO guideline value (Saxena and Sewak, 2015). Similarly, Chakraborti and others (2011) and Chakraborti and others (2016) have reported 201 fluoride endemic districts in India with a total population of 411 million and more than 66 million suffering from fluorosis. In China, 29 provinces or automonous regions have reported fluorosis (He et al., 2020). Li and others (2020b) have estimated 20 million patients with dental fluorosis and 10 million patients with skeletal fluorosis. Many studies of high-F groundwater provinces have been documented, and nearly 500 are compiled in . Only maximum F concentrations close to or above 1.5 mg/L were included.
The geochemical associations of high-F groundwaters outlined above, including Na-HCO3 conditions, low Ca concentrations, and alkaline pH (around 7-9) are not uncommonly found in geologic-basement aquifers, especially granitic and rhyolitic rocks with F-rich minerals; active volcanic terrains in association with F in lavas, ashes and hydrothermal fluids; and some sedimentary rocks with groundwaters affected by silicate hydrolysis, evapotranspiration and ion exchange, especially where fluorine-rich minerals are present. Fluoride can also occur in acidic waters and notably in some geothermal terrains.
In crystalline basement rocks, particularly those of granitic and rhyolitic composition, groundwater fluoride problems are associated with the relative abundance of fluorine-rich minerals such as micas, apatite/fluorapatite, amphiboles and fluorite. Basement aquifers cover a large part of peninsular India, and states most affected by groundwater fluoride problems are Rajasthan, Andhra Pradesh, Telangana, Uttar Pradesh, Tamil Nadu and Karnataka (Handa, 1975; Maithani et al., 1998; Rao, 2002; Reddy et al., 2010b; Suma Latha et al., 1999b). One of the highest concentrations ever recorded (90 mg/L) was from non-thermal groundwater in a now-closed well in Rajasthan (Choubisa, 2018a) (Box 1). Fluorosis has also been reported in Assam (Chakraborti et al., 2000; Kotoky et al., 2008). In Pakistan, high fluoride concentrations are a feature of aquifers of Sindh Province (Rafique et al., 2009). The Dry Zone of Sri Lanka has groundwater fluoride concentrations up to around 10 mg/L (Dissanayake, 1991). Several countries in Africa also have high groundwater fluoride concentrations in areas of geologic-basement rocks, including parts of Cameroon, Ghana, Ethiopia, Malawi, Senegal, Tanzania and South Africa (e.g., Fantong et al., 2010; McCaffrey, 1998; Tekle-Haimanot et al., 2006; Travi, 1993). Much of the high-F groundwater in Africa is associated with hydrothermal fluids and the East Africa Rift (EAR) Zone. In Muteh area, Isfahan, Iran, high concentrations (up to 9.2 mg/L) are associated with granitic and metamorphic rocks (Keshavarzi et al., 2010).
High F concentrations in groundwater and geothermal fluids from active volcanic terrains are well-documented in the western USA (Deng et al., 2011; Nordstrom and Jenne, 1977), Mexico (Morales-Arredondo et al., 2016), Iceland, New Zealand, Russia (Ellis and Mahon, 1977), France, Turkey, Algeria, Tunisia (Travi, 1993), Taiwan, Tibet (Guo et al., 2007a) and East Africa (Ayenew, 2008). The two branches of the East Africa Rift Valley form collectively one of the largest, fluoride-affected provinces in the world. Fluoride problems in Yemen, with concentrations in groundwater up to 35 mg/L, are associated with Cenozoic volcanic aquifers that represent a northward extension of the main rift (Al-Mikhlafi, 2010; Baker et al., 1997). Volcanic rocks of basaltic and rhyolitic composition are present in the region, but as observed in Ethiopia (Rango et al., 2009), the high F concentrations are associated with the rhyolitic rocks (Al-Mikhlafi, 2010).
Concentrations in groundwater in volcanic aquifers are typically up to around 15 mg/L but extremes in geothermal fluids can reach up to 1000 mg/L (Ellis, 1973). Fluoride-rich geothermal fluids are typically alkaline (up to pH 10) but acidic geothermal fluids (e.g., observed in Yellowstone, USA) can also have high F concentrations, stabilized as HF0, AlF2+, AlF2+, and AlF30 complexes (Deng et al., 2011). High concentrations of F have also been found in basaltic aquifers of Iran (Moghaddam and Fijani, 2009; Naderi et al., 2020).
In eastern Turkey, concentrations up to 12.5 mg/L were found in high-pH groundwater associated with the Tendurek Volcano. Skeletal fluorosis has been recorded in the area around the volcano (Oruc, 2008). Concentrations of F up to 4 mg/L have also been found in Isparta Province, south-west Anatolia (Turkey). Both dental and skeletal fluorosis have been recorded.
Sedimentary aquifers with high groundwater F concentrations include areas of North Africa (Algeria, Tunisia, Morocco, Libya, Sudan), West Africa (Senegal), China, Argentina, Mexico and the western USA. These areas are arid to semi-arid and the groundwaters overwhelmingly alkaline and Na-HCO3-rich; many have increased salinity (Guo et al., 2007c). Controlling processes for these aquifers have been variably ascribed to combinations of ion exchange, evapotranspiration, silicate mineral hydrolysis, calcite precipitation and sorption/desorption reactions. Groundwater of Na-SO4 (and Na-Cl) composition from an unconfined alluvial aquifer in arid central Iran has been associated with dissolution of evaporites, evaporation and ion exchange (Dehbandi et al., 2018).
Ion exchange has also been highlighted as an important influence on groundwater chemistry and downgradient evolution of F concentrations in several sedimentary aquifers in non-arid environments (as discussed in Section 9 of this book), for example in the United Kingdom (Edmunds and Walton, 1983) and Maryland, USA (Chapelle and Knobel, 1983).
Several sedimentary aquifers have had their high groundwater F concentrations attributed to the presence of fluorine-rich minerals. Travi (1993) reported F concentrations up to 13 mg/L in Cretaceous to Palaeocene aquifers from western Senegal and up to 2.3 mg/L in groundwater from the Cretaceous Complex Terminal aquifer of western Tunisia. In each case, the origin of the F was taken to be phosphorite deposits in the sediments. Concentrations in the range 1 to 3 mg/L were found in the Cenozoic Complex Terminal aquifer of Algeria (Kechiched et al., 2020; Nezli et al., 2009). Here, the groundwaters are of Na-HCO3 and Na-Cl compositions and elevated F concentrations believed influenced by evaporation. Concentrations in groundwater from sections of the underlying Lower Cretaceous Continental Intercalaire of Algeria and Tunisia were noted in the range 0.4 to 6.0 mg/L in waters of dominantly Na-SO4 and Na-Cl composition and with temperatures up to 72 °C (Besser et al., 2019; Edmunds et al., 2003), although concentrations of F were mostly < 1 mg/L. Travi (1993) also found mostly low F concentrations in groundwater from the Tunisian Continental Intercalaire.
Some F-affected sedimentary aquifers contain components of volcanic ash or volcanogenic sediment which provide a source of F. Occurrence of dissolved F in these groundwaters is often found in association with elevated concentrations of other anions and oxyanions (As, B, Mo, U, V) under the prevailing alkaline conditions (Ortega-Guerrero, 2009; Reyes-Gomez et al., 2015; Smedley et al., 2002; Alarcon-Herrera et al., 2013). In Argentina, alkaline Na-HCO3 groundwater from Quaternary sedimentary aquifers with intermixed rhyolitic volcanic ash have F concentrations up to 29 mg/L (Smedley et al., 2002). The F in these terrains is typically inferred to be derived dominantly from the ash deposits. In Mexico, high concentrations of F are reported in alkaline groundwater with increased salinity in some closed basins where sedimentary aquifers have volcanic components (Armienta and Segovia, 2008; Mahlknecht et al., 2008; Reyes-Gomez et al., 2015). High-F groundwaters in Quaternary sedimentary aquifers in China have similar characteristics with alkaline conditions and elevated salinity (Fuhong and Shuquin, 1988).