Migration of ¹³⁷Cs and ⁹⁰Sr from Chernobyl fallout in Ukrainian, Belarussian and Russian soils

Studies carried out inside the 30-km restriction zone of the Chernobyl Nuclear Power Plant (ChNPP) and in other contaminated regions of Ukraine, Belarus and Russia have shown that much of the ¹³⁷Cs and ⁹⁰Sr deposited by the accident in 1986 has been retained in the superficial layers of the soil and is likely to remain there for a long time. However, in wet organic soils, there has been considerable downward movement. Between 1987 and 1993, laboratory and field experiments were carried out in order to determine the vertical distribution profiles of ¹³⁷Cs and ⁹⁰Sr in undisturbed soils at various locations and in various soil types. The data from these experiments were then used to calculate the vertical migration rates of the two radionuclides by two types of model. The experimental results showed that the type of soil and its water content had a significant influence on the radionuclide distribution pattern in the soil profile. In undisturbed well-drained sandy and sandy loamy soils, the radionuclides were retained in the upper soil layers. However, in peaty boggy soils and flooded meadows, there was a greater downward migration. In tilled soils, the radionuclides were distributed move or less homogeneously within the cultivated soil layer: the depth and homogeneity of the nuclide distribution depended on the soil texture and the way that the soil had been managed. The vertical migration rates of the ⁹⁰Sr were always higher than that of ¹³⁷Cs. In a comparison of migration rates between ¹³⁷Cs and ⁹⁰Sr in different types of soils, ⁹⁰Sr appeared to migrate fastest in sandy loam and sandy soils, and ¹³⁷Cs migrated fastest in peaty, boggy soils. The limitations in usage of the models are discussed, and predictions are made for the next years.