Residence times of fallout 239+240Pu, 238Pu, 241Am and 137Cs in the upper horizons of an undisturbed grassland soil

Abstract

To obtain information on the residence times of fallout 239+240Pu, 238Pu, 241Am, and 137Cs in the various layers of the soil, the authors determined at several locations in a typical undisturbed grassland (Alfisol) the vertical distribution of these radionuclides separately according to their origin (global fallout and Chernobyl-derived). The data show that Chernobyl- derived Pu and Am contributed only negligibly to the Pu- and Am-activity present in the soil from global fallout. The residence half-times τ of these radionuclides in the various soil layers were evaluated subsequently with a compartment model. They show that all three elements from the global fallout exhibit quite similar migration behavior in this soil. Even though there is a considerable spatial variability, the migration rates, defined as (thickness of soil layer i) / τ i , increase on average continually with depth from about 0.4 cm y −1 in the 0–2 cm layer to 1 cm y −1 in the 20–30 cm layer. Below 40 cm depth, none of these radionuclides was found. The mobility of Chernobyl-derived 137Cs also increased with depth, the corresponding values, however, were higher in each soil layer by a factor of 2–4.