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 iscrucial 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 ({sup}(137)Cs) activiW concentrations in milk, vegetation, andsurface waters after the Chernobyl accident. Our results show that the rate of decline in {sup}(137)Cs during the years after Chernobyl is remarkably consistent in all three ecosystem components, having a mean effective half-life, T{sup}eff≈2 years. Bycomparing changes in{sup}(137)Cs availability with rates of diffusion of {sup}（40）K (a close analogue) into the lattice of an illitic clay (1) we have, for the first time, directly linked changes in the environmental availability of {sup}（137）Cs tofixation processes at a mechanistic level. These changes are consistent with declines in the exchangeable fraction of {sup}（137）Cs in soils (2, 3).
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