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During the years after a nuclear accident, the bioavailability and environmental mobility of radiocesium declines markedly, resulting in large changes in contamination of foodstuffs, vegetation, and surface waters. Predicting such changes is crucial to the determination of potential doses to affected populations and therefore to the implementation of radiological countermeasures. We have analyzed 77 data sets of radiocesium (137Cs) activity concentrations in milk, vegetation, and surface waters after the Chernobyl accident. Our results show that the rate of decline in 137Cs during the years after Chernobyl is remarkably consistent in all three ecosystem components, having a mean effective half-life, T eff ≈ 2 years. By comparing changes in 137Cs availability with rates of diffusion of 40K (a close analogue) into the lattice of an illitic clay () we have, for the first time, directly linked changes in the environmental availability of 137Cs to fixation processes at a mechanistic level. These changes are consistent with declines in the exchangeable fraction of 137Cs in soils (2, 3).

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The regularities of 137Cs distribution in trees (Pinus sylvestris and Betula pendula) growing in different types of forest ecosystems were investigated. High levels of heterogeneity of 137Cs activity concentrations in different parts of the trees, resulting from their varied metabolism have been shown. The data obtained demonstrate a non-uniform character of 137Cs distribution along the trunks, which can be explained by radionuclide fixation by the xylem vessel walls and by geometry changes along the tree trunk. It has been found that the radial distribution of 137Cs in the tree trunk is dependent on the availability of 137Cs in soil, which governs the transfer of this radionuclide via xylem sap and on the properties of the xylem. The accumulation of 137Cs by trees was influenced by the vertical distribution and availability of 137Cs in the soil as well as by the root biomass distribution in different soil horizons. A bioavailability factor, which takes into account the vertical distribution of radiocesium in soil, bioavailability of this radionuclide and distribution of root biomass in different soil horizons is proposed for comparative analyses of 137Cs transfer from soil to trees in different types of forest ecosystems.