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

The study site comprises temporary and permanent grassland in areas in central and northern Sweden which were strongly affected by the Chernobyl fallout 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 in soil types and textures, i.e. six cultivated or semi-natural mineral soils (Dystrochrept, Haploboroll, Udorthents, Cryorthent and Haplocryod) and two cultivated organic soils (Sulfihemists). The ground deposition of 137Cs ranged from 14 to 184 kBq m-2 (average 82). 137Cs-activities were measured in cm-sliced, 25-cm-deep soil cores and in soil horizons of the entire soil 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 calculated based on the median depths. Grass samples were taken from the same locations to measure the amount of 137Cs transferred from soil to grass. After eight years, most (50–92%) of the 137Cs fallout was still present in the upper 5 cm (median depth 2.3–5.1 cm), although considerable amounts had migrated to deeper soil layers. Downward migration was most pronounced in the organic soils and in the podzol where 137Cs was found to about 50 cm depth. Migration rates were in the range of 0.5–1.0 cm year-1 for the first year and thereafter 0.2–0.6 cm year-1. The transfer of 137Cs to grass was highest in two gravely sandy loam soils in the mountain region, intermediate in two organic soils, and lowest in soils from the river and coastal areas.