Migration of fallout 239 + 240Pu, 241Am and 137Cs in the various horizons of a forest soil under pine

At five plots in a pine stand the vertical distributions of 239+240Pu, 241Am and 137Cs from the global fallout of weapons testing in the sixties, and of 137Cs from the Chernobyl fallout, in the soil (podzol) were determined. To obtain the migration rates of the radionuclides in each soil horizon from these depth profiles, a compartment model was used. On average, the vertical migration rates of Pu and Am from the global fallout were similar and low (in general < 1 cm year−1) in all soil horizons. The smallest mobility (0.1 cm year−1) was observed in the Ofh horizon, i.e. the horizon where the pine needles are already decomposed and humic substances have been formed. In the underlying mineral horizon the migration rates increase in general with increasing depth. Radiocaesium from the global fallout is also enriched in the Ofh horizon and exhibits rather similar migration rates to the two actinides, except in the Ofh horizon, where it is more mobile by a factor of about five. While the enrichment of the actinides in the organic soil layer seems to be the result of complex formation with humic substances, the corresponding enrichment of radiocaesium in this layer is probably due to temporary immobilization and recirculation processes induced by the soil microflora. Chernobyl-derived 137Cs is presently significantly more mobile in the mineral soil than radiocaesium from the global fallout by about a factor of two.