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Geochemical provinces and the incidence of dental diseases in Sri Lanka
The Science of the Total Environment, 13 (1979) 47--53 © Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m -- Printed in The Netherlands
47
GEOCHEMICAL P R O V I N C E S AND THE INCIDENCE OF D E N T A L DISEASES IN SRI L A N K A
C. B. D I S S A N A Y A K E
Department of Geology, University of Sri Lanka, Peradeniya (Sri Lanka) (Received N o v e m b e r 6th, 1 9 7 8 ; accepted N o v e m b e r 28th, 1978)
ABSTRACT A survey carried o u t on the i n c i d e n c e of dental diseases and the distribution of fluoride in drinking water wells in Sri L a n k a shows that 3 areas in particular had a b u n d a n t fluoride ( ~ 2ppm). Dental fluorosis w a s c o m m o n in areas with high fluoride c o n t e n t while t h o s e areas with very little or no fluoride, such as the central region of Sri Lanka, had a high incidence o f dental caries. T h e intensity of rain fall played a major role in the leaching o f fluoride ions f r o m soils a n d it is perhaps this factor which is responsible for the occurrence o f a low f l u o r i d e z o n e in the central part of Sri Lanka. The presence o f areas containing high fluoride and h e n c e dental fluorosis coincided with geochemical provinces. In these areas, mineral d e p o s i t s such as apatite and serpentine and also h o t spring regions with exhalations of fluorine a r e found.
INTRODUCTION
Among the most c o m m o n diseases prevalent in Sri Lanka are dental fluorosis and dental caries. In a developing country such as Sri Lanka, the need to study the distribution of dental diseases and the recognition of the natural causes for the o b s e r v e d distribution can hardly be overemphasized. Such a study is all the m o r e important since it has been found that a large n u m b e r of school children in Sri Lanka suffer from dental fluorosis and dental caries. This fact h a s been established by the research work carried o u t by the Faculty of Medicine, University of Sri L~nka, Peradeniya (Senewiratne et al. 1974; Senewiratne and Senewiratne, 1975). In a programme oriented towards the prevention of such dental diseases, it is of great importance to delineate t h e areas which show a high incidence of the disease and also to establish t h e geochemical causes which may be responsible for such diseases. Since t h e role of fluoride ions in the occurrence and prevention of dental fluorosis a n d dental caries has been well established, it is the aim o f this paper to correlate the distribution o f dental diseases in Sri Lanka
48 with geology and environmental geochemistry with special references to the role of fluoride. It is hoped that the delineation of geochemical provinces pertaining to fluoride and dental diseases would help in programmes o f defluoridation of water in Sri Lanka. It is shown that a study o f t h e geology needs to be carried out in addition to that for climate, soft t y p e s , vegetation and the environmental geochemistry in relation to the distribution o f diseases in an area. The island of Sri Lanka, with an area of 69450 km 2 , is primarily part o f the shield which comprises Peninsular India. Geologically and physically Sri Lanka is a southern continuation of India, only recently separated from the mainland by the shallow sea covering the Palk Strait and the G u l f o f Mannar. On the basis of height and slope characteristics, the island can be divided into 3 main morphological regions (Vitanage, 1970) (Fig. 1). Geologically the greater part (about 92%) of the country consists of Precambrian rocks, the island having remained stable over a long period of time. T h e Precambrian has been divided into a Highland Series -- consisting mainly of rocks belonging to the granulite facies such as charnockites, Vijayan Series o f granites and South-western G r o u p - - a complex of cordierite gneisses and
Ma~ra
Fig. 1. The main morphological regions and I = Lowlands; II = Uplands; III= Highlands.
the
climatic
zones
of
Sri
Lanka.
49 charnockites. For a detailed account of the Geology of Sri Lanka, the reader is referred to Cooray (19 67 ). Sri Lanka has a typical humid tropical climate, lies in the Monsoon region of South-east Asia and the island is characterized by a clearly demarcated dry and wet zone. T h e average mean temperature of the wet zone lies between 70--85 °F and in the dry zone it is approximately 90 °F.
MATERIALS AND METHODS
Collection of water In Sri Lanka pipe b o r n e water is restricted to a few large cities, the majority of the people obtain their drinking water from wells, rivers and lakes. Senewiratne and Senewiratne (1975) first carried o u t a survey on the distribution of fluoride-rich water in Sri Lanka. Subsequently, a survey on the fluoride content o f well water in the Kandy area was c o n d u c t e d by the Department of Geology, University of Sri Lanka (Dissanayake and Hapugaskumbura, 1978, in preparation.) In the first survey, water samples were collected at distances a p p r o x i m a t e l y 5 miles apart from all parts of the island. In the second s u r v e y , in the Kandy region, well water was collected at sites approximately 1A mile apart along motorable roads and f o o t paths. All water samples were collected in pre-cleaned plastic containers. Chemical analysis Fluoride was d e t e r m i n e d using an Orion ion specific electrode (Frant and Ross, 1966). The sample (5 ml) was diluted with an equal volume of Total Ionic Strength Buffer ( 5 7 ml glacial acetic acid, 58 g sodium chloride, 4 g di-sodium c y c l o h e x y l e n e dinitilotetra-acetic acid, made up to 1 litre with de-ionised water and t h e pI-I adjusted to b e t w e e n 5 and 5.5 with 5 M NaOH). The diluted sample was continuously stirred using an electromagnetic stirrer, and the electrode p o t e n t i a l was measured using an expanded scale Pye pH Meter (Model 290). Readings were taken after there had been no change in the potential for 10 m i n . This was considerably facilitated b y feeding the o u t p u t of the pH m e t e r into a Pye AR Series 2 recorder run at 60 sec per cm. Very good precision and accuracy have been obtained using this electrode (relative error o f t h e order of 0.2--0.7% and relative standard deviation 4.8--3.6%).
RESULTS AND DISCUSSION
Figure 2 shows the distribution of fluoride containing water in Sri Lanka. It can be seen that the central hill country in which Kandy is located and the south west coastal r e g i o n are relatively free of fluoride. The lowland dry zone contains a higher amount of fluoride in water with areas around Eppawala in the North Central Province, Maha Oya in the Eastern Province
50
AverogeRuoride
l
Concentrations
Mataro Fig. 2. Distribution o f fluoride-containing water in Sri Lanka (Modified f r o m Senewiratne and Senewiratne, 1975.)
and Uda Walawe in the Southern Province showing anomalous fluoride concentrations. Table I shows the average fluoride concentrations in drinking water from wells and the prevalence of dental diseases in 3 areas where a large number of wells were sampled. The inverse relationship between fluoride content and dental caries is well established (Dean, 1945; 1954). Table 1 illustrates the fact that, in areas TABLE 1 F L U O R I D E CONCENTRATIONS AND THE INCIDENCE OF D E N T A L F L U O R O S I S A N D DENTAL CARIES IN 3 AREAS O F SRI LANKA
Dental fluorosis Dental caries F l u o r i d e concentration
Anuradhapura
Polonnaruwa
Kandy
77.5% 26.2% 0.34--3.75 ppm
56.2% 26.5% 0.26--4.55 ppm
13.0% 95.9%
less than 0.2 ppm
N o t e : The maximum fluoride concentration in the Anuradhapura area was 9.0 ppm, 5.8 ppm in Maha Oya and 4.8 ppm in Uda Walawe. (see Fig. 1 for localities.)
51 where the fluoride c o n t e n t is below normal, dental caries among the inhabitants are prevalent, w h e r e a s dental fluorosis is more c o m m o n in the areas with a higher fluoride c o n t e n t . It is of interest to correlate the fluoride-rich and fluoride-poor areas delineated with natural factors such as climate and geology. Fluoride-free areas are mainly situated in the wet zone whereas the high fluoride bearing areas belong mainly to t h e dry zone (Fig. 2) It is conceivable that in the wet zone, where the average rainfall exceeds 200 inches in certain instances, the leaching of soluble salts is high. In these areas, there is a tendency for the soluble ions to be efficiently leached and carried away in solution. Fluoride is easily leached from primary and secondary minerals ( H a w k e s and Webb, 1962) and softs u n d e r the effect of high rainfall. In the dry regions, on the other hand, evaporation tends to bring the soluble ions upwards due to capillary action in soils. This, although n o t the sole explanation for the observed distribution o f the fluoride in well water in Sri Lanka, could nevertheless be an i m p o r t a n t factor. However, it is the geology o f the areas that needs special consideration. The composition of the rocks of the area, particularly the easily leached constituents coupled with the climate are the k e y factors in the geochemical distribution of elements in a tropical region. The abundance of fluoride in the rocks and the ease with which it is leached under the effect of g r o u n d water has an important bearing on the abundance of fluoride in the areas concerned and hence the prevalence of dental diseases. Among the areas containing the highest fluoride concentrations in well water, the region a r o u n d Eppawala and Anuradhapura (Fig. 2) is the largest. Senewiratne and Senewiratne (1975) reported fluoride concentrations as high as 9.0 ppm in this region. The abundance of fluoride which caused severe dental fluorosis a m o n g people of this area can be attributed to an abundance of fluorine in the rocks. It is significant that in this area there occurs an economically exploitable deposit of apatite (fluoro-hydroxy phosphate}, classified as a carbonatite, and is known to contain reserves of 23 million tons. The a p a t i t e deposit is n o w being mined and analysis shows it to contain a fluorine c o n c e n t r a t i o n of 1.5--2.4% (Jayawardena, 1976). It is a well-established fact that the fluoride ion can take the place of the h y d r o x y l ion and that a n equilibrium could be maintained. The substitution of fluoride for h y d r o x y l is to be expected from similarity of ionic radii and charges. Extensive research has been carried o u t on the fluoride--hydroxyl exchange in geological materials (Gillberg, 1964; StSrmer and Carmichael, 1971; E k s t r 6 m , 1972; Munoz and Ludington, 1974). The presence of higher concentrations of fluoride in water in this area bearing fluorine-rich rocks is t h e r e f o r e explained on the basis of F - ~ O H - interchange between minerals and water. Apart from apatite, micas which are present in abundance in this area are also known to exhibit this interchange of fluoride and h y d r o x y l (Nada and Ushio, 1964). The area in the South-east of Sri Lanka around Uda Walawe (Fig. 2), which contains high concentrations of fluoride, comprises different geological formations c o m p a r e d to the region around Anuradhapura. It has been
52 recently discovered that the area around Uda Walawe c o n s i s t s o f large deposits of serpentinites (Dissanayake and Van Riel, 1978). T h e s e o c c u r as long and narrow belts and a n u m b e r of such deposits occur to t h e n o r t h and south of Uda Walawe. Serpentine, an iron--magnesium h y d r o x y silicate also possesses the property o f exchanging the fluoride ion for the h y d r o x y l ion; it exhibits the property of taking up the fluoride ion into its s t r u c t u r e from an aqueous solution and also the release of fluoride into an a q u e o u s m e d i u m , confirmed b y the recent experimental work of Rao et al. (1975). T h i s release of exchangeable fluoride into ground water appears to be the m o s t likely explanation for the abundance of fluoride in wells of the area l y i n g in t h e d r y zone. Serpentine generally contains 1000--2000 ppm of f l u o r i d e and this is quite sufficient for an enrichment of fluoride in water in t h e vicinity, bearing in mind the large n u m b e r of deposits present. The third high fluoride zone lies around Maha Oya in the e a s t e r n part o f Sri Lanka and also lies in an area of geological and geochemical significance. A r o u n d Maha Oya a number of h o t water springs, considered t o b e derived from thermally heated circulating ground water and gaseous e m a n a t i o n s are present. Apart from the many dissolved ions, gases are also seen t o b u b b l e through the h o t water. The abundance of fluorine among these gases in such terrains is well known. Further to the north and to the south o f Maha Oya, t h e r e are other h o t springs indicating a much larger area of t h e r m a l l y h e a t e d waters. I f a more detailed survey for fluoride in water is carried o u t in t h e areas further north and south of Maha Oya, the b o u n d a r y of t h e zone m a y have to be extended. The Maha Oya topographic region, h o w e v e r , contains t h e highest number of h o t springs.
CONCLUSIONS F r o m the observations on the distribution of fluoride in d r i n k i n g wellw a t e r in Sri Lanka and the incidence of dental diseases in t h e areas concerned, it is apparent that a distinct correlation exists b e t w e e n this and t h e geology of the delineated areas. The geochemical distribution o f fluoride in t h e r o c k s and minerals along with the climatic factors are r e s p o n s i b l e for t h e prevalence of dental diseases in certain parts of Sri Lanka. T h e areas so d e m a r c a t e d are basically geochemical provinces with either an a b u n d a n c e or a l a c k of fluoride containing rocks and minerals.
ACKNOWLEDGEMENTS T h e author wishes to thank Messrs A . K . H a p u g a s k u m b u r a , T. Vinay a g a m o o r t h y , S . M . B . Amunugama and Cyril D u n u h a p p a w a for their assistance.
53 REFERENCES 1 P . G . Cooray, An I n t r o d u c t i o n to the Geology of Ceylon, National Museums o f Ceylon, Publication, 1967. 2 H . T . Dean, in W . J . Gies (Ed.), Fluorine in Dental Public Health, New York, Institute of Clinical Oral Pathology, 1945, p. 19. 3 H . T . Dean, Int. dent. J., 4 (1954) 311. 4 C.B. Dissanayake and B. J. van Riel, Geol. Mijnbouw, 57 (1978) 91. 5 T . K . EkstrSm, Contrib. Mineral. Petrol., 34 (1972) 192. 6 M. S. Frant and J. M. Ross, Jr., Science, 154 (1966) 1533. 7 M. Gillberg, Geochim. Cosmochim. Acta, 28 (1964) 495. 8 H . E . Hawbes and J. S. Webb, Geochemistry in mineral exploration, Harper and Row, New York, Evanston and L o n d o n , 1962. 9 D.E. de S. Jayawardena, Geol. Surv. Dept. Sri Lanka, Econ. Bull., 3 (1976) 1. 10 J. L. Muhoz and S. D. Ludington, Am. J. Sci., 274 (1974) 396. 11 T. Nada and T. Ushio, G e o c h e m . Int., 1 (1964) 96. 12 K . V . Rao, D. P u r u s h o t t a m and D. Vaidyanadham, Geochim. Cosmochim. Acta, 39 (1975) 1403. 13 B. Senewiratne, S. Thambipillai, J. Hettiarachchi and K. Senewiratne, Trans. Roy. Soc. Trop. Med Hyg., 68 (1974) 105. 14 B. Senewiratne and K. Senewiratne, Ind. J. Med. Res., 63 (1975) 302. 15 J . S . Stormer and I. S. E. Carmichael, Contrib. Mineral. Petrol., 34 (1972) 201. 16 P.W. Vitanage, A s t u d y of the Geomorphology and the morphotectonics of Ceylon, Proc. 2nd Seminar on Geochemical Prospecting Methods and Techniques, United Nations, New York, NY, pp. 391--405, E.72 II F.2.
47
GEOCHEMICAL P R O V I N C E S AND THE INCIDENCE OF D E N T A L DISEASES IN SRI L A N K A
C. B. D I S S A N A Y A K E
Department of Geology, University of Sri Lanka, Peradeniya (Sri Lanka) (Received N o v e m b e r 6th, 1 9 7 8 ; accepted N o v e m b e r 28th, 1978)
ABSTRACT A survey carried o u t on the i n c i d e n c e of dental diseases and the distribution of fluoride in drinking water wells in Sri L a n k a shows that 3 areas in particular had a b u n d a n t fluoride ( ~ 2ppm). Dental fluorosis w a s c o m m o n in areas with high fluoride c o n t e n t while t h o s e areas with very little or no fluoride, such as the central region of Sri Lanka, had a high incidence o f dental caries. T h e intensity of rain fall played a major role in the leaching o f fluoride ions f r o m soils a n d it is perhaps this factor which is responsible for the occurrence o f a low f l u o r i d e z o n e in the central part of Sri Lanka. The presence o f areas containing high fluoride and h e n c e dental fluorosis coincided with geochemical provinces. In these areas, mineral d e p o s i t s such as apatite and serpentine and also h o t spring regions with exhalations of fluorine a r e found.
INTRODUCTION
Among the most c o m m o n diseases prevalent in Sri Lanka are dental fluorosis and dental caries. In a developing country such as Sri Lanka, the need to study the distribution of dental diseases and the recognition of the natural causes for the o b s e r v e d distribution can hardly be overemphasized. Such a study is all the m o r e important since it has been found that a large n u m b e r of school children in Sri Lanka suffer from dental fluorosis and dental caries. This fact h a s been established by the research work carried o u t by the Faculty of Medicine, University of Sri L~nka, Peradeniya (Senewiratne et al. 1974; Senewiratne and Senewiratne, 1975). In a programme oriented towards the prevention of such dental diseases, it is of great importance to delineate t h e areas which show a high incidence of the disease and also to establish t h e geochemical causes which may be responsible for such diseases. Since t h e role of fluoride ions in the occurrence and prevention of dental fluorosis a n d dental caries has been well established, it is the aim o f this paper to correlate the distribution o f dental diseases in Sri Lanka
48 with geology and environmental geochemistry with special references to the role of fluoride. It is hoped that the delineation of geochemical provinces pertaining to fluoride and dental diseases would help in programmes o f defluoridation of water in Sri Lanka. It is shown that a study o f t h e geology needs to be carried out in addition to that for climate, soft t y p e s , vegetation and the environmental geochemistry in relation to the distribution o f diseases in an area. The island of Sri Lanka, with an area of 69450 km 2 , is primarily part o f the shield which comprises Peninsular India. Geologically and physically Sri Lanka is a southern continuation of India, only recently separated from the mainland by the shallow sea covering the Palk Strait and the G u l f o f Mannar. On the basis of height and slope characteristics, the island can be divided into 3 main morphological regions (Vitanage, 1970) (Fig. 1). Geologically the greater part (about 92%) of the country consists of Precambrian rocks, the island having remained stable over a long period of time. T h e Precambrian has been divided into a Highland Series -- consisting mainly of rocks belonging to the granulite facies such as charnockites, Vijayan Series o f granites and South-western G r o u p - - a complex of cordierite gneisses and
Ma~ra
Fig. 1. The main morphological regions and I = Lowlands; II = Uplands; III= Highlands.
the
climatic
zones
of
Sri
Lanka.
49 charnockites. For a detailed account of the Geology of Sri Lanka, the reader is referred to Cooray (19 67 ). Sri Lanka has a typical humid tropical climate, lies in the Monsoon region of South-east Asia and the island is characterized by a clearly demarcated dry and wet zone. T h e average mean temperature of the wet zone lies between 70--85 °F and in the dry zone it is approximately 90 °F.
MATERIALS AND METHODS
Collection of water In Sri Lanka pipe b o r n e water is restricted to a few large cities, the majority of the people obtain their drinking water from wells, rivers and lakes. Senewiratne and Senewiratne (1975) first carried o u t a survey on the distribution of fluoride-rich water in Sri Lanka. Subsequently, a survey on the fluoride content o f well water in the Kandy area was c o n d u c t e d by the Department of Geology, University of Sri Lanka (Dissanayake and Hapugaskumbura, 1978, in preparation.) In the first survey, water samples were collected at distances a p p r o x i m a t e l y 5 miles apart from all parts of the island. In the second s u r v e y , in the Kandy region, well water was collected at sites approximately 1A mile apart along motorable roads and f o o t paths. All water samples were collected in pre-cleaned plastic containers. Chemical analysis Fluoride was d e t e r m i n e d using an Orion ion specific electrode (Frant and Ross, 1966). The sample (5 ml) was diluted with an equal volume of Total Ionic Strength Buffer ( 5 7 ml glacial acetic acid, 58 g sodium chloride, 4 g di-sodium c y c l o h e x y l e n e dinitilotetra-acetic acid, made up to 1 litre with de-ionised water and t h e pI-I adjusted to b e t w e e n 5 and 5.5 with 5 M NaOH). The diluted sample was continuously stirred using an electromagnetic stirrer, and the electrode p o t e n t i a l was measured using an expanded scale Pye pH Meter (Model 290). Readings were taken after there had been no change in the potential for 10 m i n . This was considerably facilitated b y feeding the o u t p u t of the pH m e t e r into a Pye AR Series 2 recorder run at 60 sec per cm. Very good precision and accuracy have been obtained using this electrode (relative error o f t h e order of 0.2--0.7% and relative standard deviation 4.8--3.6%).
RESULTS AND DISCUSSION
Figure 2 shows the distribution of fluoride containing water in Sri Lanka. It can be seen that the central hill country in which Kandy is located and the south west coastal r e g i o n are relatively free of fluoride. The lowland dry zone contains a higher amount of fluoride in water with areas around Eppawala in the North Central Province, Maha Oya in the Eastern Province
50
AverogeRuoride
l
Concentrations
Mataro Fig. 2. Distribution o f fluoride-containing water in Sri Lanka (Modified f r o m Senewiratne and Senewiratne, 1975.)
and Uda Walawe in the Southern Province showing anomalous fluoride concentrations. Table I shows the average fluoride concentrations in drinking water from wells and the prevalence of dental diseases in 3 areas where a large number of wells were sampled. The inverse relationship between fluoride content and dental caries is well established (Dean, 1945; 1954). Table 1 illustrates the fact that, in areas TABLE 1 F L U O R I D E CONCENTRATIONS AND THE INCIDENCE OF D E N T A L F L U O R O S I S A N D DENTAL CARIES IN 3 AREAS O F SRI LANKA
Dental fluorosis Dental caries F l u o r i d e concentration
Anuradhapura
Polonnaruwa
Kandy
77.5% 26.2% 0.34--3.75 ppm
56.2% 26.5% 0.26--4.55 ppm
13.0% 95.9%
less than 0.2 ppm
N o t e : The maximum fluoride concentration in the Anuradhapura area was 9.0 ppm, 5.8 ppm in Maha Oya and 4.8 ppm in Uda Walawe. (see Fig. 1 for localities.)
51 where the fluoride c o n t e n t is below normal, dental caries among the inhabitants are prevalent, w h e r e a s dental fluorosis is more c o m m o n in the areas with a higher fluoride c o n t e n t . It is of interest to correlate the fluoride-rich and fluoride-poor areas delineated with natural factors such as climate and geology. Fluoride-free areas are mainly situated in the wet zone whereas the high fluoride bearing areas belong mainly to t h e dry zone (Fig. 2) It is conceivable that in the wet zone, where the average rainfall exceeds 200 inches in certain instances, the leaching of soluble salts is high. In these areas, there is a tendency for the soluble ions to be efficiently leached and carried away in solution. Fluoride is easily leached from primary and secondary minerals ( H a w k e s and Webb, 1962) and softs u n d e r the effect of high rainfall. In the dry regions, on the other hand, evaporation tends to bring the soluble ions upwards due to capillary action in soils. This, although n o t the sole explanation for the observed distribution o f the fluoride in well water in Sri Lanka, could nevertheless be an i m p o r t a n t factor. However, it is the geology o f the areas that needs special consideration. The composition of the rocks of the area, particularly the easily leached constituents coupled with the climate are the k e y factors in the geochemical distribution of elements in a tropical region. The abundance of fluoride in the rocks and the ease with which it is leached under the effect of g r o u n d water has an important bearing on the abundance of fluoride in the areas concerned and hence the prevalence of dental diseases. Among the areas containing the highest fluoride concentrations in well water, the region a r o u n d Eppawala and Anuradhapura (Fig. 2) is the largest. Senewiratne and Senewiratne (1975) reported fluoride concentrations as high as 9.0 ppm in this region. The abundance of fluoride which caused severe dental fluorosis a m o n g people of this area can be attributed to an abundance of fluorine in the rocks. It is significant that in this area there occurs an economically exploitable deposit of apatite (fluoro-hydroxy phosphate}, classified as a carbonatite, and is known to contain reserves of 23 million tons. The a p a t i t e deposit is n o w being mined and analysis shows it to contain a fluorine c o n c e n t r a t i o n of 1.5--2.4% (Jayawardena, 1976). It is a well-established fact that the fluoride ion can take the place of the h y d r o x y l ion and that a n equilibrium could be maintained. The substitution of fluoride for h y d r o x y l is to be expected from similarity of ionic radii and charges. Extensive research has been carried o u t on the fluoride--hydroxyl exchange in geological materials (Gillberg, 1964; StSrmer and Carmichael, 1971; E k s t r 6 m , 1972; Munoz and Ludington, 1974). The presence of higher concentrations of fluoride in water in this area bearing fluorine-rich rocks is t h e r e f o r e explained on the basis of F - ~ O H - interchange between minerals and water. Apart from apatite, micas which are present in abundance in this area are also known to exhibit this interchange of fluoride and h y d r o x y l (Nada and Ushio, 1964). The area in the South-east of Sri Lanka around Uda Walawe (Fig. 2), which contains high concentrations of fluoride, comprises different geological formations c o m p a r e d to the region around Anuradhapura. It has been
52 recently discovered that the area around Uda Walawe c o n s i s t s o f large deposits of serpentinites (Dissanayake and Van Riel, 1978). T h e s e o c c u r as long and narrow belts and a n u m b e r of such deposits occur to t h e n o r t h and south of Uda Walawe. Serpentine, an iron--magnesium h y d r o x y silicate also possesses the property o f exchanging the fluoride ion for the h y d r o x y l ion; it exhibits the property of taking up the fluoride ion into its s t r u c t u r e from an aqueous solution and also the release of fluoride into an a q u e o u s m e d i u m , confirmed b y the recent experimental work of Rao et al. (1975). T h i s release of exchangeable fluoride into ground water appears to be the m o s t likely explanation for the abundance of fluoride in wells of the area l y i n g in t h e d r y zone. Serpentine generally contains 1000--2000 ppm of f l u o r i d e and this is quite sufficient for an enrichment of fluoride in water in t h e vicinity, bearing in mind the large n u m b e r of deposits present. The third high fluoride zone lies around Maha Oya in the e a s t e r n part o f Sri Lanka and also lies in an area of geological and geochemical significance. A r o u n d Maha Oya a number of h o t water springs, considered t o b e derived from thermally heated circulating ground water and gaseous e m a n a t i o n s are present. Apart from the many dissolved ions, gases are also seen t o b u b b l e through the h o t water. The abundance of fluorine among these gases in such terrains is well known. Further to the north and to the south o f Maha Oya, t h e r e are other h o t springs indicating a much larger area of t h e r m a l l y h e a t e d waters. I f a more detailed survey for fluoride in water is carried o u t in t h e areas further north and south of Maha Oya, the b o u n d a r y of t h e zone m a y have to be extended. The Maha Oya topographic region, h o w e v e r , contains t h e highest number of h o t springs.
CONCLUSIONS F r o m the observations on the distribution of fluoride in d r i n k i n g wellw a t e r in Sri Lanka and the incidence of dental diseases in t h e areas concerned, it is apparent that a distinct correlation exists b e t w e e n this and t h e geology of the delineated areas. The geochemical distribution o f fluoride in t h e r o c k s and minerals along with the climatic factors are r e s p o n s i b l e for t h e prevalence of dental diseases in certain parts of Sri Lanka. T h e areas so d e m a r c a t e d are basically geochemical provinces with either an a b u n d a n c e or a l a c k of fluoride containing rocks and minerals.
ACKNOWLEDGEMENTS T h e author wishes to thank Messrs A . K . H a p u g a s k u m b u r a , T. Vinay a g a m o o r t h y , S . M . B . Amunugama and Cyril D u n u h a p p a w a for their assistance.
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