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, Teff ≈ 2 years. By comparing changes in 137Cs availability with rates of diffusion of 40K (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 137Cs to fixation processes at a mechanistic level. These changes are co...