Sources

Fluoride is ubiquitous in the environment and is always present in plants, soils and phosphatic fertilizers. Various rock types contain fluoride at different levels: basalt, 360 µg/g; granites, 810 µg/g; limestone, 220 µg/g; sandstone and greywacke, 180 µg/g; shale, 800 µg/g; oceanic sediments, 730 µg/g; and soils, 285 µg/g. The F concentration in the upper continental crust is 611 ppm. It is an essential constituent in minerals such as fluorite, apatite, cryolite, and topaz. Whereas minerals such as biotite, muscovite and hornblende may contain large per cent of F and therefore, would seem to be the main source of F in surface waters. It appears, therefore, that the F content of surface water is largely dependent on the mineralogical composition of the inorganic fraction in surface soils and sediments. Apatite may perhaps exchange some of its hydroxyl ions for fluoride following reaction of the type:

Ca10(PO4)6(OH)2 + 2F–1 = Ca10(PO4)6F2 + 2OH–1 (1)

K = a2OH–1/a2F–1 = 106.6 (2)

i.e. the process converts the hydroxyl apatite of bones and calcium phosphate into fluorapatite, where K is equilibrium constant and a is activity. With increasing use of fertilizers containing fluoride, the fluoride content of surface water also increases. Approximately 20 to 400 g F per hectare is annually leached from soils, about the same amount that is added to the soil from the atmosphere, but fertilizing adds another 5 to 30 kg F per hectare annually. This fluoride accumulates in the soils. The main part of fluoride in rainwater may originate in sea aerosols: K2SiF6 (hieratite) and Na2SiF6 (malladrite), where tiny droplets of foam are caught up by the wind and may be carried far from the ocean to continental areas. The F content of various continental precipitations shows a range of 4–89 ppb and in the vicinity of cities and industrial areas, an average of 290 ppb can be found. The order of magnitude of the normal fluoride content in the air is < 0.01–0.4 µg/m3 and in industrial areas up to 5–111 µg/m3 from chemical plants producing HF, aluminium, super phosphate, brickwork and burning of low quality coal.

High pH and high bicarbonate concentration mainly control fluoride concentration in water and the major sources identified are anthropogenic activities and leaching of soil and fertilisers. The role of fertiliser leaching in groundwater is also evident by the fact that phosphate and potassium, which are the major constituents of fertilisers come together in the same factor. Dental fluorosis was observed in most of the villages even where fluoride concentration in drinking water was below 1.0 ppm while skeletal fluorosis was observed in few villages. Evidence of extensive prevalence of dental fluorosis in this area strengthen the need for further research to examine the potential role of food and other sources of fluoride to human beings and if necessary, attempts should be directed towards redefining the permissible levels of fluoride in drinking water.