CHANGES WITH TIME IN THE MIGRATION OF RADIOCESIUM IN THE SOIL, AS OBSERVED NEAR CHERNOBYL AND IN GERMANY, 1986-1994

At a near-field grassland site at the Chernobyl nuclear power plant (C hNPP) and at a grassland site in Germany the mean residence half-times of Chernobyl-derived Cs-137 were determined for various soil layers a s a function of time. For that purpose, the annually observed vertical distributions of this radionuclide in the soil were evaluated with a compartment model. The main difference between these two sites with re spect to the migration of radiocesium was apparent in the top soil lay er, where the residence half-time increased at the German site between 1987 and 1994 from about 0.7 to 2 years, while it decreased at the Ch NPP site from about 9 to 3 years. In the deeper soil layers the reside nce half-times increased at both sites with increasing time and decrea sed with depth in a rather similar way. The opposite behavior of the r esidence half-time in the top soil layer at the two sites results from the fact that Cs-137 was deposited at the ChNPP site predominantly in the form of rather insoluble fuel particles, while at the German site this radionuclide was attached in a more available form to natural ae rosols. The increase of the Cs-137 residence half-times with time at b oth sites in the deeper layers can be explained by the progressive fix ation of radiocesium by clay minerals of the soil.