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 (ChNPP) and at a grassland site in Germany the mean residence half-times of Chernobyl-derived Cs-137 were determined for various soil layers as 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 respect to the migration of radiocesium was apparent in the top soil layer, 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 ChNPP site from about 9 to 3 years. In the deeper soil layers the residence half-times increased at both sites with increasing time and decreased with depth in a rather similar way. The opposite behavior of the residence 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 aerosols. The increase of the Cs-137 residence half-times with time at both sites in the deeper layers can be explained by the progressive fixation of radiocesium by clay minerals of the soil.