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Fluorine excess is an endemic global health problem, affecting notably. the active alkaline volcanic province of the East African Rift Valley and other geothermal areas, ancient crystalline basement rocks of large parts of India, Sri Lanka and Africa as well as groundwaters from some sedimentary aquifers. In these areas, concentrations can often exceed national and international standards for fluoride in drinking water. Low fluoride concentrations, prevalent especially in high rainfall areas, also pose health problems and give rise to controversy in relation to the need for fluoridation of water supplies. Thus, both deficiency and excess of fluoride in the human diet can have detrimental health effects and the optimal doses lie within a relatively narrow range. Drinking water still constitutes an important component of the dietary intake in many regions.
The behaviour and distribution of fluoride in water are controlled by a number of factors, the most important being geology, hydrology and climate. The element, fluorine, is present in a number of minerals and where these occur in abundance, high-fluorine provinces can be anticipated. The upper limits of fluoride concentration attained in water are controlled by mineral solubility, mainly of fluorite. Hence, dissolved fluoride concentrations may build up where calcium is deficient or in soft waters where Ca has been removed by ion exchange processes. In groundwater, residence time is also an important factor. All chemical reactions are time-dependent and progressive changes may occur with distance along the flow line. Evolution of groundwater has often taken place over centuries or millennia and older waters are more likely to have high fluoride concentrations resulting from water-rock interaction. Climatic controls on fluoride mobilisation have been demonstrated in many areas. In areas with similar rock types, high rainfall typically gives rise to fluoride-deficient waters through leaching losses and dilution, while in areas of low rainfall, concentrations of fluoride and other solutes may build up through intense water-rock interaction and evapotranspiration. In general, high fluoride concentrations in groundwaters are associated with conditions of neutral to alkaline pH (7?9), high alkalinity and where sodium is the dominant cation.
An understanding of the occurrence of fluoride in water and the environment is the most important first step in the management of fluoride-related health problems since geology, hydrology and climate can cause considerable variations in concentrations found in water at both regional and local scales. Awareness of likely at-risk (high-fluoride) aquifers allows more effective testing and monitoring of groundwater for fluoride and may also serve to identify water resources with acceptable fluoride concentrations that occur locally within fluoride endemic areas.
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