Endemic goitre, the factors controlling iodine deficiency in soils

The Science of the Total Environment, 11 (1979) 99--104 ©

Elsevier

Scientific Publishing Company, Amsterdam - -

P r i n t e d in

England

Short communication

ENDEMIC GOITRE, THE FACTORS CONTROLLING IODINE DEFICIENCY IN SOILS S. R. ASTON AND P. H. BRAZIER Department of Environmental Sciences, University of Lancaster, Lancaster, LA1 4YQ (England) (Received June 12th, 1978) ABSTRACT

Iodine concentrations in soils derived from a selection of very different parent rocks but within a small well-defined area of uniform environmental conditions have been studied. The results indicate that iodine in soils is independent of parent rock type, and that the maturing of young soils such as those found in areas of endemic goitre is the most important influence on iodine deficiency. The iodine is retained and enriched during vegetative recycling processes. INTRODUCTION The cause o f endemic goitre is n o t completely determined, but iodine deficiency is k n o w n to play an i m p o r t a n t part in the epidemiology of this disease [1--4], which has been estimated to affect up to 200 million people on a world wide basis [5]. Most areas of endemic goitre are those of relatively y o u n g glacial soils. Although statistics on the incidence of endemic goitre in this small geographical area are not available the incidence of the disease has been estimated to be 39% greater in Northwest England as a whole average for England and Wales. This estimate is based upon the consultation rate per 1000 population for the diagnosed condition [6]. The hinterland of the study area is associated with a high incidence of endemic goitre b u t on the basis of available clinical findings it is not c o m m o n within the s t u d y area. The distribution of iodine in all soils, and consequently in most foodstuffs, is related to two major sources: the parent rock from which the soils are derived by weathering, and secondly the input of atmospheric iodine mainly from marine aerosols [7]. Here, the contributions of iodine to soils in a Small well-defined area of uniform environmental conditions, but a wide variety of geological units is examined and the mode of iodine enrichment in softs is discussed. SAMPLING AND METHODS Soil profiles and parent rocks were collected in the Shap District of Northern England, where six main geological units are recognised. This

100 geographically small area is a typical upland, glaciated environment and contains source rocks ranging through igneous, sedimentary and volcanic types of ages from the Ordivician to the Pleistocene. The soils are relatively immature, having formed since the end of the last glacial phases between 10,000--14,000 years b.p. This environment is typical of those previously associated with endemic goitre in Northern England, except that it receives an input of atmospheric iodine from marine aerosols. The input of iodine by rain wash-out is ~ 0 . 3 5 ~g cm -2 yr -1 , with little variation in the average rainfall over the area of interest [8]. Iodine analyses of soil and rock samples were carried o u t by the m e t h o d of Grimaldi and Schnepfe [ 9 ] , while aluminium and titanium were determined b y atomic absorption s p e c t r o p h o t o m e t r y . Total organic carbon in the soils was determined b y the chromic acid oxidation method of E1 Wakeel and Riley [ 1 0 ] . R E S ULTS AND DISCUSSION

The average iodine concentrations in the six parent rock types and soils derived from them are shown in Table 1. The range of iodine concentrations is 0.4 to 2.5 ppm. The sedimentary rocks are generally more enriched in iodine than are the igneous (including volcanic) rocks, and all are similar to the average values reported for crustal rocks b y Mason [ 1 1 ] . The Knipe Limestone is depleted in iodine relative to other marine carbonates, b u t this m a y be explained by the fact that it is a highly crystalline variety with few marine fossils and micro-fossils which would normally provide iodine enrichment [ 1 2 , 1 3 ] . Table 1 also summarises soil iodine concentrations and enrichment factors for the t o p (0--5 cm) and b o t t o m (0--5 cm above parent rock) of soil profiles. Generally, the b o t t o m of profile soil iodine concentrations are similar to those of the parent rocks, with the exception of the soils derived from the Knipe Limestone. Iodine enrichment occurs in the surface soils relative to parent rocks and b o t t o m of profile soils for all the profiles, the degree of enrichment varying from one rock type to another. Overall, the enrichment factor for surface soil to the b o t t o m of profile soil iodine is 1.8. Two factors must be considered 'to explain this enrichment during rock weathering and soil formation. First, differential weathering and the removal of parent rock material with the net effect of increasing the iodine concentration of the residual soft material. If differential weathering is operative, the soil enrichment of iodine would be expected to vary for the extremely different rock types with their contrasting chemical and physical weathering characteristics. That is found to be the situation for the six distinct rock units of the Shap District, where weathering leads to different enrichment factors, b u t very similar iodine concentrations in surface soils. Second, the enrichment of iodine by external sources, e.g., precipitation and recycling by plant debris [14--16]. The total precipitation over the Shap District varies from 150--250 cm yr -1 , giving an iodine input of ~ 3 7 5 pg cm -2 yr -1 to ~ 6 2 4 gg cm -2 yr -I for the area.

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This is unlikely to explain the observed differences in soil iodine contents over the area, especially in view of the lack of any geographic trends in soil iodine concentrations. The extent of the vegetative recycling in the immature soils may be gauged by the total organic matter present. Figure 1 shows the relationship between total organic carbon and iodine concentrations in the soil samples of the Shap District. There is a strong correlation between iodine and organic carbon concentrations, with a linear correlation coefficient of r = 0.94, confirming earlier qualitative observations that the organic debris from !

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Fig. 2. T h e c o n c e n t r a t i o n ratios of i o d i n e / a l u m i n i u m and i o d i n e / t i t a n i u m , and conc e n t r a t i o n s o f total organic c a r b o n in a soil profile over t h e Banisdale Slate formation.

103

vegetation decay m a y be important in recycling iodine in immature soils [16--18]. Figure 2 shows the relationship of iodine to organic carbon, titanium and aluminium in a typical soil profile, and illustrates the enrichment of iodine and organic carbon relative to the resistant elements titanium and aluminium in the weathering profile. The present data illustrateseveral features of the iodine content of soilsin relation to their parent rocks and soil formation. The more important conclusions for soil iodine in relation to the epidemiology of goitre are: (I) the immature soils all show an enrichment of iodine compared to their respective parent rocks; (2) the different enrichment factors caused by the individual weathering mechanisms do not influence the eventual iodine content of surface soils; (3) the iodine and organic carbon contents of the soilsshow an excellent correlation, implying that vegetation recycling is the major influence on iodine retentional enrichment in developing soils; (4) the contribution of atmospheric iodine to the soils is unlikely to control the geographical distribution of iodine soilconcentrations in an area such as the Shap District. In summary, the iodine in soilsderived from very different parent rocks is independent of the nature of the rocks and their weathering characteristics. The maturing of young soils such as those typically found in areas of endemic goitre is probably the most important influence on iodine deficiency, the iodine being retained and enriched as vegetative decay recycles the element in surface soils.

ACKNOWLEDGEMENT We wish t o t h a n k Mrs. M. L o v e f o r h e r t e c h n i c a l assistance w i t h t h e analyses.

REFERENCES 1 P . H . J . Turton, Proc. Roy. Soc. Med., 26 (1933) 1223. 2 J.F. MeClendon, Iodine and the Incidence of Goitre, Oxford University Press, 1939, 126 pp. 3 E. M. Mason, E. M. O'Donovan and D. Kilbride, Rept. Med. Res. Council, Ireland, 1945, 7 pp. 4 A. Polanski, Wiadomasei Muz. Ziemi, 4 (1968) 33. 5 World Health Organisation Statistics, cited by Wright et al in Geochemistry, Open University Press, (1972), 106 pp. 6 Anonymous, Morbidity Statistics from General Practice 2nd National Survey H.M.S.O. London, 1974, 211 pp. 7 Y. Miyake and S. Tsunogai, J. Geophys. Res., 68 (1963) 3989. 8 D.H. Peirson, P. A. Cawse, L. Salmon, and R. S. Cambray, Nature (London), 241 (1973) 252. 9 F.S. Grimaldi and M. M. Schepfe, Anal Chim. Acta, 53 (1971) 181. 10 S.K. El Wakeel and J. P. Riley, J. du Conseil Inter. Explor. Mer., 22 (1957) 180. 11 B. Mason, Principles of Geochemistry, John Wiley, (1966), 329 pp.

104 12 J.G. Nores, Assoc. Rural Uraguay Rev. Mensual 78, (1951), 88---91. 13 A.P. Vinogradov, The Geochemistry of rare and Dispersed Chemical Elements in Soils, Consultants Bureau, N.Y., 2nd ed., 1959, pp. 51--64. 14 L. Smolik, Sbornik Ceskoslov. Akad. Zemedelske, 10 (1935) 36. 15 M. Carranza, Bol. Quim. Peruano, 1 (1945) 20. 16 A. Kappova, Sbornik Ceskoslov. Akad. Zemedelske, 21 (1949) 81. 17 G.F. Proskuryakova, N. A. Ivanov and A. Reshetnikov, Tr. Sverdlovsk Sel'Skokhov Inst., 15 (1969) 333. 18 G. I. Sinitskaya, Uch. Zap. Dal'nevost. Gos. Univ., 27 (1969) 1.