The Occurrence of Fluoride in Natural Waters and the Environment

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Seneviratne (1974) has observed that the teeth of many school children in the north central province of the country exhibited the distinctive changes of dental fluorosis whereas those in the central province did not. A study was therefore conducted to investigate the extent and degree of dental fluorosis in schoolchildren in above mentioned areas. The results have revealed that endemic dental fluorosis is very common in the north central province where 55.2-77.0% of school children have mottled teeth. The fluoride concentration of samples of drinking water ranged from 0.2 – 9 ppm. There was no fluoride in the water in the central province. There was a striking difference in the prevalence of caries in the two areas. Although there was a good correlation between dental fluorosis and the presence of fluorine in drinking water, in some subjects the fluoride in drinking water could not explain the presence of dental fluorosis.

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Dissanayake (1991) examined the distribution of fluoride in the groundwater of Sri Lanka. The author stated that the high fluoride content in the groundwater (sometimes in excess of 5 mg/l) was related to high frequency of dental fluorosis, while very low fluoride content of water results in dental caries. The highest percentage of deep wells with excessive fluoride contents were recorded from Anuradhapura, Polonnaruwa, Ampara and Kurunegala districts, which belongs to north eastern part of the country. He further tried to correlate the fluoride-rich and fluoride-poor areas delineated, with natural factors such as climate and geology. The clear climatic zones of Sri Lanka i.e. Wet Zone and the Dry Zone, also coincide with the areas of low fluoride and high fluoride, respectively. It is also apparent that dental caries is widely prevalent in the Wet Zone while dental fluorosis is more widely observed in the Dry Zone. The levels of fluoride in the groundwater in the various districts of Sri Lanka correspond well with the incidence of dental caries and dental fluorosis reported.

Warnakulasuriya et al., (1992) examined the prevalence of dental caries and dental fluorosis 14-yr-old children living in four geographic areas of Sri Lanka with water F levels of 0.09-8.O ppm. Based on the study results, authors stated that their data are comparable with findings from other tropical countries, e.g. Kenya and Senegal, and reaffirm that WHO guidelines for the upper limit of F in drinking water may be unsuitable for developing countries with a hot, dry climate. Dharmagunawardhane and Dissanayake, (1993) compared the fluoride-rich and fluoride-poor areas with climatic, geomorphological and geological factors prevailing in the country. The low fluoride concentrations in groundwater are common in the wet zone where annual average rainfall in general exceeds 5,000 mm and high fluoride is common in the dry zone. Physiographically, high fluoride zones lie within the low plains of the island whereas the low fluoride areas are mostly confined to the central highlands. This situation, though possibly caused by many other factors, appears to be partially explained by the fact that, in the elevated wet zone areas with high rainfall, fluoride is leached from primary and secondary minerals in rocks and soils whereas, in the dry zone, evaporation tends to bring soluble ions upwards by capillary action. The slow rate of groundwater movement in the low plains also tends to increase the fluoride concentration because the contact time of groundwater with a particular geological formation is comparatively long. Geologically, high fluoride concentrations are observed in tube well water in association with rock types such as charnockites, hornblende-biotite gneiss, intrusive granites, granitic gneiss, granulites and calc gneiss, whereas low fluoride concentrations in tube wells have been observed in association with quartzite and marble.

The drinking water analysis carried out in endemic CKDue areas in Sri Lanka illustrates that most of the drinking water contains medium to high level of fluoride. The high fluoride is not only a possible risk factor but it is possible that there is some relationship with the disease or it could even increase the severity of the disease. For instance, the variability in the Na and Ca in the groundwater and the Ca saturation factor in the presence of fluoride may prove to be an important parameter in the cause of the disease.

High fluoride in drinking water particularly in the Dry Zone of Sri Lanka, affects over 8 million people and is a serious concern. During the last two decades, most of the Dry Zone communities have shifted from surface water sources to groundwater resources and it is noted that both shallow and deep wells show similar levels of natural contamination with fluoride. Even within the Dry Zone regions, some low fluoride bearing zones can be observed and this could be due to excessive mixing of groundwater with surface water, which is diverted from the Wet Zone region for irrigation purposes. The fluoride distribution map given here (Figure 1) will provide a better understanding of the problem of water quality in Sri Lanka. The output of the present work may be useful to the local health authorities as well as those responsible for the management of water supply to rural communities in the Dry Zone of Sri Lanka.Jayawardana et al (2012) Seventy four representative residual soil samples were collected from the zone of accumulation in relatively fluoride-rich groundwater sites in the areas of Madirigiriya and Talawa (depth range 0.6–3.0 m) and fluoride-poor sites at Padaviya (depth range 0.6–4.6 m), using a hand auger. Three replicate samples were collected from each site. Fluoride concentration in the groundwater, geomorphology, basement geology and anthropogenic impacts were the major factors in the selection of sampling sites. Samples were collected from immature, moderately-weathered layers which are mostly sandy clay loams.

The geochemistry of soils from the Madirigiriya, Talawa, and Padaviya districts suggests that fluoride is not readily retained, leading to high values in groundwater. The high mobility of fluoride from soil to water is facilitated by immature soils which have sandy clay loam texture containing some primaryminerals and rock fragments. The results suggest thatweathering of heavy minerals including zirconium, apatite, fluorite, monazite and garnet are the dominant mineral hosts in the soils in fluoride-rich groundwater districts. Relative to the basements rocks, Zr, Nb and Th are stable, whereas F, CaO and P2O5 are depleted. Loss of CaO provides favorable conditions for the leaching of fluorine to groundwater. Conversely, in fluoride-poor groundwater districts the soils are enriched in TiO2, Fe2O3, MnO, Cr, V and Sc, reflecting the weathering of biotite, hornblende, garnet and pyroxenes in the basement. The primary minerals present in these soils are the main cause for the enrichment of those elements. The soil geochemistry may also reflect original magmatic contrasts in the meta-igneous rocks, which then led to variations in fluoride in the dry zone of Sri Lanka. The greater influence of the primary fluoride-rich fluid that formed the metaigneous rocks in the fluoride-rich groundwater districts caused enrichment of fluoride-bearing heavy minerals.

Longer residence time in aquifers within fractured crystalline bedrocks may enhance fluoride levels in the groundwater in these areas. In addition, elevated fluoride concentrations in shallow groundwater in intensive agricultural areas appear to be related to the leaching of fluoride from soils due to successive irrigation. Many factors contribute to the high fluoride concentrations in groundwater in the study area. The major contributing factor toward high fluoride is the nature of the underlying basement rocks such as granitic gneiss, hornblende biotite gneiss, and biotite gneiss. The deep groundwater found in the fractured crystalline bedrocks has long residence time, and this also seems to enhance the fluoride concentrations. Agricultural practices may also elevate the fluoride levels in the waters. The difference of Mg, Ca, and HCO3 – ion concentrations in the Giribawa, Nochchiyagama and Dambulla, Kakirawa areas seems to be strongly controlled by calcite, dolomite, and plagioclase which are the underline geology.Case studies from worldRajasthan has an arid climate with low but highly variable annual rainfall. In Sirohi district, fluoride concentrations up to 16 mg L–1 have been found in groundwater from dug wells and boreholes at depths between 25–75 m during geochemical exploration for uranium. These dykes act as barriers, which slow down groundwater flow and permit prolonged contact times to raise concentrations of groundwater fluoride. L–1) Low-fluoride areas in the east (<1.5 mg are associated with carbonate rocks and higher calcium may inhibit an excess of fluoride. It is not clear from the study to what extent the alluvium acts as a source (or a conduit) for the fluoride-bearing water and no groundwater flow data are shown.Villagers in some parts of the Bolgatanga area suffer from dental fluorosis as a result of the high concentrations of dissolved fluoride. The problem is particularly prevalent in Bongo and Sekoti districts, although the numbers of people affected, and the regional extent is not known in detail. Highest fluoride concentrations occur in groundwater from the Bongo area in the west, with values reaching up to 3.8 mg L–1, and from Sekoti district in the east, with values reaching up to 3.2 mg L–1 (Figure 7). The higher concentrations from any given rock type are generally found in borehole waters. Water from shallow hand-dug wells typically (but not always) has very low fluoride concentrations (<0.4 mg L–1) as these have had much shorter reaction times with the host rocks.

Common situation in many aquifers is to encounter increasing fluoride concentrations in the groundwaters down the flow gradient. This situation arises initially as a result of continuous dissolution of fluoride from minerals in the carbonate aquifers, up to the limit of fluorite solubility, followed by ion-exchange reactions involving removal of calcium. The phenomenon is particularly well-illustrated in the Jurassic Lincolnshire Limestone aquifer of eastern England.

Some of the highest concentrations of fluoride ever recorded have been found in water from the East African Rift Valley. The Rift extends through Eritrea, Djibouti, Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi and Malawi and excessive fluoride concentrations have been found in groundwaters, hot springs, alkaline lakes and some river systems in all these countries (Table 3). The region has well-documented cases of severe dental and skeletal fluorosis.

In the area of Mount Meru, a Neogene volcano in northern Tanzania, Nanyaro et al. (1984) reported fluoride concentrations of 12–76 mg L–1 in rivers draining the volcano’s slopes (Table 3) and 15–63 mg L–1 in associated springs. They attributed the high concentrations to weathering of fluorine-rich alkaline igneous rocks and to contributions from fumaroles and gases as well as to the re-dissolution of fluorine-rich trona (Na2CO3.NaHCO3.2H2O), which occurs as a seasonal encrustation in low-lying river valleys and lake margins as a result of extreme evaporation. The river water chemistry varies significantly seasonally as a result of dilution following periods of heavy rainfall.

Remediation of high-fluoride groundwater

As a means of combating the fluoride problem, several methods of water treatment using various media have been tested and a number are in use in various parts of the world. Some of the common methods are listed in Table 5 and reviews have been published by Heidweiller (1990) and Mohapatra et al. (2009). Most low-technology methods rely on precipitation or flocculation, or adsorption/ion-exchange processes. Probably the best-known and most established method is the Nalgonda technique, named after the Nalgonda District in Andhra Pradesh, India where the technique has been developed and is in use today. The technique uses a combination of alum (or aluminium chloride) and lime (or sodium aluminate), together with bleaching powder, which are added to high-fluoride water, stirred and left to settle. Fluoride is subsequently removed by flocculation, sedimentation and filtration. The method can be used at a domestic scale (in buckets) or community scale (fill-and-draw type defluoridation plants). It has moderate costs and uses materials which are usually easily available. It is therefore preferred for small defluoridation units used exclusively for drinking water. Aluminium polychloride sulphate has been found to have technical advantages over alum as use of alum results in increased concentrations of SO4 and suspended particles in the treated water.

Other precipitation methods include the use of gypsum, dolomite or calcium chloride. Most methods tested are capable in principle of reducing fluoride in treated water to a concentration below the WHO guideline value. Vanderdonck and Van Kesteren (1993) reported that the calcium chloride method is capable of reducing fluoride concentrations up to 20 mg L–1 to acceptable concentrations.The most common ion-exchange removal methods tested are activated carbon, activated alumina, ion-exchange resins (e.g. Defluoron 2), plant carbon, clay minerals, clay pots, crushed bone or bone char. Activated alumina and bone materials are among the most effective appropriate-technology removal methods (with highest removal capacity, Table 5). These also have drawbacks however: activated alumina may not always be available or affordable and bone products are not readily acceptable in some cultures. Use of fired clay-pot shards has proved a promising approach in some developing countries. The use of fly ash has also been tested favourably for fluoride removal, although the concentrations of other solutes in the water after treatment with fly ash need to be ascertained. Some success has also been demonstrated experimentally using aluminium-rich volcanic soils (Ando soils) from the East African Rift of Kenya, although field testing of the technique has not been reported.Other highly efficient methods of removal include electro-dialysis, reverse osmosis and nanofiltration. These methods tend to involve higher technology and higher costs (Table 5) and are therefore less suitable for many applications in developing countries.

Most methods designed for village-scale fluoride removal have drawbacks in terms of removal efficiency, cost, local availability of materials, chemistry of resultant treated water and disposal of treatment chemicals. Local circumstances will dictate which methods, if any, are the most appropriate. In addition, many of the defluoridation methods have only been tested at pilot scale or in the laboratory. In practice, remediation techniques such as those outlined above often meet with disappointing success when put into operation. Success rates depend on efficacy, user acceptance, ease of maintenance, and degree of community participation, availability and cost of raw materials. An additional problem lies with monitoring. In most rural communities, it will be almost impossible to monitor the initial fluoride concentrations so that dosing can be accurate. It is also not practicable at village level to chemically monitor the progress of treatment schemes, such as knowing when to recycle or replace the media involved in the treatment.

As an example of problems with the water treatment approach, defluoridation schemes have been in operation in Wonji, Ethiopia since 1962. In the past, these principally involved bone char, but most recently a resin-activated-alumina adsorption method has been used. Only two of twelve plants were still in operation in 1997 and in the years of operation, only four of the plants consistently reduced fluoride below 1.5 mg L–1. The major problems encountered included efficiencies in the operation of the plant, lack of spare parts and materials and lack of community involvement. The affected areas are currently served by a piped water scheme, but in more remote areas, interest in defluoridation continues.

Given the potential drawbacks of water treatment, alternative fluoride mitigation approaches could prove more effective, such as careful borehole siting, groundwater management and use of alternative sources such as rainwater harvesting. It should be borne in mind that the main issue is for the provision of rather small amounts of water for drinking purposes so rainwater harvesting from roof collection or augmenting aquifer recharge provides a straightforward and low-cost solution.

Factors worth considering in borehole siting are local geology and variations in groundwater fluoride concentration with depth. Groundwater management includes consideration of optimum pumping rates, especially where there exists the possibility of mixing of groundwater with deep fluoride-rich groundwater (e.g. old groundwater or hydrothermal solutions), which would be increasingly drawn upwards at high pumping rates. Possibilities for enhanced recharge of low-fluoride surface water to aquifers could also give benefits to shallow groundwater quality. Examples include the charco dams of Tanzania. A fully-fledged artificial recharge scheme would offer similar benefits. Considerations with rainwater harvesting include availability of rainfall in arid and semi-arid areas that tend to be worst-affected by fluoride problems, and in maintenance of clean storage facilities for the water supply.

In view of the site-specific nature of each incidence of high fluoride and also the problems of applying remediation, especially in small communities, the supply of small quantities of imported low-fluoride water should also be considered. This could be achieved by importation of water in bottles or tankers to central distribution points and may prove cost-effective, since the high-fluoride source can still be used for sanitation and possibly also for most forms of agriculture.

In low-rainfall areas with fluoride problems, it is important to assess the hydrogeological situation very carefully. The generation of high-fluoride groundwaters usually requires considerable residence times in the aquifer, for reasons outlined in the preceding sections. Thus, it is likely that younger, shallow groundwaters, for example those recharged rapidly below wadis or stream channels, may have lower fluoride concentrations than the bulk groundwater. They may be exploitable as a resource overlying older groundwater. Exploitation may require ‘skimming’ the shallow water table rather than abstraction from deeper penetrating boreholes. The harvesting of rainwater, either directly in cisterns or by collection in small recharge dams, offers a potentially attractive alternative solution.

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