RESIDENCE TIMES OF FALLOUT PU-239-238, AM-241 AND CS-137 IN THE UPPERHORIZONS OF AN UNDISTURBED GRASSLAND SOIL(240, PU)

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 author s determined at several locations in a typical undisturbed grassland ( Alfisol) the vertical distribution of these radionuclides separately a ccording to their origin (global fallout and Chernobyl-devived). The d ata show that Chernobyl-derived Pu and Am contributed only negligibly to the Pu- and Am-activity present in the soil from global fallout. Th e residence half-times tau of these radionnuclides in the various soil layers were evaluated subsequently with a compartment model. They sho w that all three elements from the global fallout exhibit quite simila r migration behavior in this soil. Even though there is a considerable spatial variability, the migration rates, defined as (thickness of so il layer i)tau(i), increase on average continually with depth from abo ut 0.4 cm y(-1) in the 0-2 cm layer to 1 cm y(-1) in the 20-30 cm laye r. Below 40 cm depth, none of these radionuclides was found. The mobil ity of Chernobyl-derived Cs-137 also increased with depth, the corresp onding values, however, were higher in each soil layer by a factor of 2-4.