Sustainability of radiologically contaminated territories.

Decisions on appropriate countermeasures in the case of a contamination of large areas due to fallout from a weapon's detonations or a serious reactor or reprocessing plant accident depend on fallout level and the time period required for the exposure levels to fall below a given intervention limit. This is particularly relevant for countermeasures with high costs and substantial consequences to the population involved, such as relocation or soil perturbation. Natural processes which result in a decrease in activity concentrations in foodstuffs and external exposure with time, are least detrimental to the soil among all countermeasures proposed for rehabilitation of contaminated areas. The impact of these natural self-healing effects (soil sustainability) on the temporal activity reduction in foodstuffs, as well as their effect on the time period required for exposure levels to decrease to sufficiently low levels to allow unrestricted re-utilization and re-settlement of the land, are discussed. Depending on the type of foodstuff, in Central Europe this temporal decrease follows an effective half life of about 0.25-1.3 y from first to second year after fallout, followed by a decrease in milk with an effective half-life of 1-2 y in the next 7 y and about 5 y thereafter, while in cereals and vegetables in Austria a decrease by 2.3 y and in fruit by 1.3 y over a period of more than a decade is observed. In contrast, the decrease in foodstuffs derived from semi-natural environs is much slower by an effective half-life of roughly 8 y. The time-span for adequate recovery of a contaminated territory, therefore, depends on the fallout level and the radionuclides involved and the contribution of natural and semi-natural derived foodstuffs to the diet. It is shown that for 137Cs and a typical Central European diet the time span amounts to about 1 y for a deposition of 300 kBq m(-2) and to about 10 y for a deposition of 10,000 kBq m(-2). Thus it is demonstrated that natural restoration effects may contribute significantly to the environmentally safe and sustainable resettlement of an area substantially contaminated with fission products.