Halogens are present and are volatile trace elements in most geological samples. Among them, iodine has the lowest abundance; less than 0.1 ppm in igneous rocks and less than several ppm in sedimentary rocks.
Iodine is least abundant of the halogens and is lithophile element. It is typically a dispersed element and is never concentrated enough in rocks or sediments to form independent minerals. The content of iodine is higher in air masses of marine origin than in those over the continents (Rankama and Sahama, 1950). The content of iodine seems to have some relationship to the salinity of the sea water as it is found to increase to the rise in salinity. Iodine is carried away from the atmosphere partly by rainwater and partly by direct adsorption into the soil and into plants. High solubility of iodine makes it enriched in soil and its highest concentration is noted in cultivated soil. According to most comprehensive observation by Goldschmidt(1954), the concentration of iodine in different media is as follows:
Igneous rock- 0.3 gm/tonne.
Cultivated soil- 2.0 gm/tonne.
Air- 0.0005 gm/tonne.
Rain water- 0.001-0.003 gm/tonne.
Sea water- 0.05 gm/tonne.
Konovalov (1959) found that rivers draining Tertiary marine sediment have higher iodine content than rivers draining other areas and this was considered to be due to iodine being easily leached from the marine sediments.
There is very marked increase in the iodine content of soils as compared to the rocks from which they derive. Many authors have suggested that much of the iodine in soils is derived from atmospheric sources, while another major source of soil iodine is that supplied by plant remains. Silty and clay soils appear to be enriched in iodine. It was found that clay fractions of soil fix iodide, a feature which is most marked for illite.
It has been generally accepted that the oceans are a major source of atmospheric origin; other sources are volcanic gases and rotting bio-materials. It has also been observed that some iodine in urban atmosphere may be derived from combustion of fossil fuels.
Iodine is a micro constituent in all plants and animals. Its influence on plant life is unknown and it may only be ballast element (Rankama and Sahama, 1950). Average content of iodine in marine life (from both plant and animal) is more than the fresh water and inland life (Cauer, 1938). In higher animals like mammals it plays a very important role, when it is present in the thyroid gland in the form of amino acid-thyroxin that controls the rate of metabolism.
Aside from tungsten, iodine is the heaviest element to be essential in living organisms, and iodine is the heaviest element thought to be needed by higher animals. About 19,000 tons are produced annually from natural sources.
A case study of Bihar State in India:
Iodine deficiency is an important global health problem with an estimated 200 million people affected by iodine related problems (Moynahan,1979). Indian Coalition for Control of Iodine Deficiency Disorders (ICCIDD) reveals that 79 million or 8 person out of every hundred, suffer from goiter in India (Hindustan Times, New Delhi, 25-11-2001).
The human body contains very little iodine (0.00004% or 0.4 ppm), yet it is essentially required to be maintained through food and water. Any disruption in iodine content jeopardizes the human metabolism. Thyroxin, a hormone secreted by the thyroid gland located on both side of the trachea contains about 65% iodine. In the absence of optimal quantity of iodine, the gland increases in size to compensate the deficiency of iodine and adversely affects the human metabolism. Water with iodine concentration less than 5-10 µg/l produces goiter.
The Gandak basin in Bihar is also known to be goiter prone since long. A detail research was carried out by Prof. N.C. Ghose (2003) of Department of Geology, Patna University on distribution of iodine in soil-water system in the Gandak Basin in Bihar.
According to Prof. Ghose, the vast tract in Gandak basin in north Bihar is known iodine deficient area and the population is prone to dreaded and endemic disease like goiter. Surface water of this area, iodine content ranges from 1.56 µg/l to 5.52 µg/l, while in groundwater which is the only source for drinking, it varies from 2.1 µg/l to 4.56 µg/l. In soil, the iodine content ranges between 3.65 µg/gm to 12.59 µg/gm. Season wise, there is considerable variation in iodine content both in surface and groundwater. During monsoon it reduces considerably in surface water due to dilution and in groundwater it reduces owing to heavy recharge of the aquifer system through infiltration. In soil, there is no definite pattern in seasonal variation in iodine content. In major part of the study area, the iodine content is deficient and ranges between 3 and 4 µg/l. The cause of low iodine is attributed to repeated floods and erosion of top soil which is the main source of iodine to the groundwater system.
The spatial variation of iodine in both surface and groundwater reveals a striking feature. It is observed that the abundance of iodine both in surface and groundwater decreases downstream from West Champaran to Vaishali. However, in surface water, the profile of iodine content of river Gandak increases again near Hajipur, where it rises to 5.52 µg/l. the high incidence of iodine at the confluence of Ganga and Gandak near Patna is due to mixing of the two river waters.
Low iodine content in water (1-4 µg/l) in the vast tract of land in East and West Champaran, Muzaffarpur and Vaishali districts of North Bihar plain makes the people vulnerable to goiter.
Rankama, K. and Sahama, Th. G. (1950) Geochemistry, Univ. Chicago Press, Chicago,912p.
Goldschmidt, V.M. (1954) Geochemistry, Clarendon Press, Oxford,730p.
Cauer, H. (1938) Chemisch-bioklimatogische Studien in der Bretagne. II. Mitteilung: Beeinflussung de mitteleuropaischen Jodmilieus durch die Breton ische Jodindustrie auf dem Wege der Luft. Biochen, Z. 299, p.69.
Moynahan, E.J. (1979) Trace elements in man. Phil. Trans. Roy. Soc. London, B-288, pp.65-79.
Konovalov, G.S. (1959) Removal of microelements by the main rivers of the U.S.S.R. Dokl. Acad. Sci. S.S.S.R. 129, p912.
Ghose, N.C., Das, K. and Saha, D. (2003) Distribution of Iodine in Soil-Water system in the Gandak Basin, Bihar, Journal of Geol. Soc. of India, pp 91-98.