Migration of radiocaesium in Swedish soil profiles after the Chernobyl accident, 1987-1995

The study site comprises temporary and permanent grassland in areas in cent ral and northern Sweden which were strongly affected by the Chernobyl fallo ut in 1986. The aim of the study was to investigate the vertical migration of radiocaesium from 1987 to 1995 in undisturbed soil profiles under field conditions, as related to soil type and texture. The sampled sites differ i n soil types and textures, i.e. six cultivated or semi-natural mineral soil s (Dystrochrept, Haploboroll, Udorthents, Cryorthent and Haplocryod) and tw o cultivated organic soils (Sulfihemists). The ground deposition of Cs-137 ranged from 14 to 184 kBq m-(2) (average 82). Cs-137-activities were measur ed in cm-sliced, 25-cm-deep soil cores and in soil horizons of the entire s oil profiles down to 0.7-1.0 m depth. The soil cores were collected on two or three occasions between 1987 and 1995, and migration rates were calculat ed based on the median depths. Grass samples were taken from the same locat ions to measure the amount of Cs-137 transferred from soil to grass. After eight years, most (50-92%) of the Cs-137 fallout was still present in the upper 5 cm (median depth 2.3-5.1 cm), although considerable amounts ha d migrated to deeper soil layers. Downward migration was most pronounced in the organic soils and in the podzol where Cs-137 was found to about 50 cm depth. Migration rates were in the range of 0.5-1.0 cm year(-1) for the fir st year and thereafter 0.2-0.6 cm year(-1). The transfer of Cs-137 to grass was highest in two gravely sandy loam soils in the mountain region, interm ediate in two organic soils, and lowest in soils from the river and coastal areas. (C) 1999 Elsevier Science Ltd. All rights reserved.