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

The environmental abundances of 129 I and 127 I 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 129 I fallout until the 1990ties and an ongoing constant input of 129 I 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 129 I 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 129 I abundances, the relatively low 129 I/ 127 I 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.