Long-lived radionuclides in the environment: On the radioecology of Iodine-129

Abstract

The environmental abundances of ¹²⁹I and ¹²⁷I in Lower Saxony, Germany, and their pathways to animals and man were investigated. The iodine isotopes are in severe disequilibrium in the different environmental compartments. Today, the environmental isotope ratios range from 10^-6 to 10^-10. The highest ratios were found in North Sea water, the lowest in deep soil samples and ground water. A differentiation by about a factor of ten between the iodine isotopes was observed for different air-borne iodine species. Time series for iodine in precipitation show a decade-long increase of ¹²⁹I fallout until the 1990ties and an ongoing constant input of ¹²⁹I with deposition densities of ~ 15 mBq m^-2 per year. In surface waters, a dilution of the fall-out iodine takes place by stable iodine which is just weakly adsorbed in the soils. The isotope ratios in soils and ground waters demonstrate a high mobility and an accumulation of ¹²⁹I in the water unsaturated soil zones and an efficient migration into water saturated soil layers and ground water. The transfer into the food chain is ruled by the complex situation in the water-soil system. Given the environmental ¹²⁹I abundances, the relatively low ¹²⁹I/¹²⁷I ratios in human thyroid glands (2 × 10^-9 - 3 × 10^-8) can only be explained by additional iodine sources with low ratios in the diet.