RESIDENCE TIMES OF FALLOUT PU-239+240, PU-238, AM-241 AND CS-137 IN THE UPPER HORIZONS OF AN UNDISTURBED GRASSLAND SOIL

To obtain information on the residence times of fallout Pu-239+240, Pu-238, Am-214, and Cs-137 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-devived). 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 tau of these radionnuclides 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)tau(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 Cs-137 also increased with depth, the corresponding values, however, were higher in each soil layer by a factor of 2-4.