MIGRATION OF FALLOUT PU-239-241 AND CS-137 IN THE VARIOUS HORIZONS OFA FOREST SOIL UNDER PINE(240, AM)

At five plots in a pine stand the vertical distributions of Pu-239+240 , Am-241 and Cs-137 from the global fallout of weapons testing in the sixties, and of Cs-137 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 wa s 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 al l 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 alread y decomposed and humic substances have been formed. In the underlying mineral horizon the migration rates increase in general with increasin g depth. Radiocaesium from the global fallout is also enriched in the Ofh horizon and exhibits rather similar migration rates to the two act inides, 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 s oil layer seems to be the result of complex formation with humic subst ances, the corresponding enrichment of radiocaesium in this layer is p robably due to temporary immobilization and recirculation processes in duced by the soil microflora. Chernobyl-derived Cs-137 is presently si gnificantly more mobile in the mineral soil than radiocaesium from the global fallout by about a factor of two.