Since the beginning of the decade, the number of tube wells with hand pumps being built in rural community water delivery projects in Sri Lanka has grown dramatically. On the island, there are currently approximately 13,000 tube wells with hand pumps, most of which are located in the dry zone. The challenges of tube well water quality have grown in importance as the number of tube wells has increased. The high fluoride concentration in groundwater, particularly in low-lying parts of the dry zone, is one of the primary water quality issues. Fluorine is one of the most common elements in the geogenic environment, and its ionic form fluoride is widespread in natural water. Although fluoride is an important component of human health in small amounts, excessive fluoride has negative health consequences. Sri Lanka, like many other tropical nations, has high amounts of fluoride in its groundwater, which is the primary supply of drinking water. The dry zone has naturally occurring high-fluoride groundwater, notably in Sri Lanka's North Central Province (Anuradhapura and Polonnaruwa). Girandurukotte, Nikawewa, Medawachchiya, and Padaviya are the fluoride hotspots. The WHO recommends a fluoride level of 0.5–1.5 mg/L in drinking water. Sri Lanka's maximum permissible fluoride content in drinking water is 1.0 mg/L.
The effects of fluoride on human health are still being disputed, although its toxicity has raised serious concerns in many areas where fluoride is present in high amounts in the drinking water. However, the optimum range of fluoride varies in a very narrow margin causing fluoride imbalances, often in populations in tropical countries where people consume large volumes of water with higher levels of fluoride. Food is the primary source of most trace elements required by humans, although fluoride enters the body through water. Fluorosis has the greatest effect on developing teeth, and children under the age of seven are most sensitive. Dental fluorosis develops at levels exceeding 1.5 mg/L as a result of excessive fluoride exposure throughout childhood, particularly during tooth development. Enamel gets tougher and discolored as a consequence.
Dental Fluorosis
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Skeletal fluorosis is defined as the hardening of bones, joint discomfort, and limb motor dysfunction caused by long-term and continuous exposure to a high amount of fluoride (>4 mg/L). Fluorosis can be caused by long-term exposure to fluoride in the environment. Patients with skeletal fluorosis and spinal cord compression can develop after consuming high fluoride water regularly for around 20 years. Chronic exposure to high amounts of environmental fluoride may affect the functions of the liver, kidney, heart, lungs, brain, thyroid gland, chromosomes, nervous system development, and reproductive capacities, in addition to hard tissue health concerns.
Geologically, Sri Lanka is dominated by Precambrian high-grade metamorphic rocks and can be divided into three major lithotectonic units namely, the Highland Complex, the Vijayan Complex, and the Wanni Complex. Among these, the Highland Complex is the largest unit and forms the backbone of the Precambrian bedrock of Sri Lanka. The rocks comprising the Highland Complex were mostly metamorphosed under granulite facies conditions. Quartz - feldspar - garnet -sillimanite - graphite schists, quartzites, marbles, and calc-silicate gneisses are the other prominent rock types. These complexes consist of mainly biotite hornblende gneisses, scattered bands of metasediments, charnockitic gneisses, and granites. Most of the rocks in Precambrian Sri Lanka consist of fluoride-bearing minerals such as micas, hornblende, sphene, and apatite. Further, minerals such as fluorite, tourmaline, and topaz are also found in many locations and these also contribute to the general geochemical cycle of fluorine in the physical environment. Because of its strong electronegativity and solubility, fluorine is found in natural waters as fluoride. Fluoride is a prominent geochemical reactant in groundwater due to its strong reactivity. The geochemistry of the F- ion (ionic radius 1.36A) is comparable to that of the OH- ion (ionic radius 1.40 A), and exchange between them is possible. The interchange of fluoride and hydroxyl ions in groundwater occurs when it travels through aquifers, resulting in fluoride enrichment. Under tropical humid climatic conditions, rock weathering is intense and this contributes fluorides readily into the solution. The Dry Zone of Sri Lanka contains high levels of fluorides in groundwater, although it does not differ from the Wet Zone as far as the types of rocks and minerals are concerned. However, the climate and the hydrological conditions are markedly different and these appear to play a major role in the geochemical cycling of fluoride in the groundwater. The area has a tropical climate with average annual precipitation below 1000mm, and average annual evaporation of 1400mm. The average annual temperature is 33 °C and an average of 2555 h of sunshine annually. The mean annual rainfall in the wet zone is higher than the dry zone (> 2500 mm) and its average annual temperature is low (25 0C). 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 the solution remains in equilibrium with calcite and alkalinity is greater than hardness. In the dry zone, evaporation tends to carry soluble ions upwards via capillary action. Dissolution of evaporative salts deposited in the 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.
Reference:
• Chandrajith, R., Diyabalanage, S. and Dissanayake, C.B., 2020. Geogenic fluoride and arsenic in groundwater of Sri Lanka and its implications to community health. Groundwater for Sustainable Development,
• Dharmagunawardhane, H.A., and Dissanayake, C.B., 1993. Fluoride Problems in Sri Lanka. Environmental Management and Health, 4(2), pp.9–16.
• Dissanayake, C.B., 1991. The fluoride problem in the groundwater of Sri Lanka-environmental management and health. International Journal of Environmental Studies, 38(2–3), pp.137–155.
• C.B. Dissanayake, R. C., 1998. Medical geochemistry of tropical environments. Earth-Science Reviews, 47(28 April 1999), p. 219–258.
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