Fluoride in Groundwater Worldwide

During weathering and circulation of water in rocks and soils, fluorine can be leached out and dissolved in groundwater and thermal gases. The fluorine content of groundwater varies greatly depending on the geological settings and type of rocks. The most common fluorine-bearing minerals are fluorite, apatite and micas. Therefore fluorine problems tend to occur in places where these minerals are most abundant in the host rocks.

Igneous and volcanic rocks have a fluorine concentration from 100 ppm (ultramafic) up to >1000 ppm (alkalic) ( Frencken, 1992). In general fluorine accumulates during magmatic crystallization and differentiation processes of the magma. Consequently, the residual magma is often enriched in fluorine. Groundwaters from crystalline rocks, especially (alkaline) granites (deficient in calcium) are particularly sensitive to relative high fluorine concentrations.

The fluorine, which cannot be incorporated in crystalline phase during crystallization and differentiation of magmas, will be accumulated in hydrothermal solutions. These fluids may form hydrothermal fluorite deposits and veins. Fluorine transport in these aqueous solutions is controlled mainly by the solubility of CaF₂ ( Allmann et al, 1974).

Further, of the volcanic series, the (calc-) alkaline volcanoes, typical of a continental rift (East Africa), hot spot, continental margin (Andes) or island arc (Japan), produce relative fluorine rich lava (Rosi et al, 2003).

Sedimentary rocks have a fluorine concentration from 200 ppm (limestone) up to 1000 ppm (shales) (Frencken, 1992). In carbonate sedimentary rocks the fluorine is present as fluorite. Clastic sediments have higher fluorine concentrations as the fluorine is concentrated in micas and illites in the clay fractions. High concentrations may also be found in sedimentary phosphate beds (shark teeth) or volcanic ash layers (Frencken, 1992).

Metamorphic rocks have a fluorine concentration from 100 ppm (regional metamorphism) up to > 5000 ppm (contact metamorphism). In these rocks the original minerals are enriched with fluorine by metasomatic processes (Frencken, 1992).

The ultimate concentration of fluoride in groundwater largely depends on reaction times with aquifer minerals. High fluoride concentrations can be built up in groundwaters which have long residence times in the aquifers. Such groundwaters are usually associated with deep aquifer systems and a slow groundwater movement.

Shallow aquifers which contain recently infiltrated rainwater usually have low fluoride. Exceptions can occur in shallow aquifers situated in active volcanic areas affected by hydrothermal alteration. Under such conditions, the solubility of fluorite increases with increasing temperature and fluoride may be added by dissoluition of HF gas (Frencken et al, 1992 and www.wateraid.org.uk)

Arid regions are prone to high fluoride concentrations. Here, groundwater flow is slow and the reaction times with rocks are therefore long. The fluoride contents of water may increase during evaporation if solution remains in equilibrium with calcite and alkalinity is greater than hardness. Dissolution of evaporative salts deposited in arid zone may be an important source of fluoride.

Fluoride increase is less pronounced in humid tropics because of high rainfall inputs and their diluting effect on the groundwater chemical composition (Frencken et al, 1992 and www.wateraid.org.uk)

High-fluoride groundwaters are mainly associated with a sodium-bicarbonate water type and relatively low calcium and magnesium concentrations. Such water types usually have high pH values (above 7).

Information on chemical composition of groundwater can be used as an (proxy) indicator of potential fluoride problems. The formation of fluoride rich waters is described by Frencken et al., (1992) and on .www.wateraid.org.uk

The WHO guideline value for fluoride in drinking water is 1.5 mg/l. Above 1.5 mg/l mottling of teeth may occur to an objectionable degree. Concentrations between 3 and 6 mg/l may cause skeletal fluorosis. Continued consumption of water with fluoride levels in excess of 10 mg/l can result in crippling fluorosis.

In many arid regions, drinking water is such a scarce commodity that governments have been forced to set the standard at higher levels, in order to have any drinking water at all.