Retention and horizontal redistribution of sedimentary Chernobyl 137Cs in a small Swedish forest lake

Abstract

The horizontal and vertical distribution of sedimentary 137Cs was examined in 20 cores to assess the long-term immobilization of 137Cs contamination in a small Swedish soft-water lake after the nuclear accident in Chemobyl in 1986. The sediment inventory of 137Cs was highly correlated with water depth, demonstrating a substantial net transport of sedimentary 137Cs from littoral areas towards the profundal zone of the lake basin. Flux estimates suggest that about half of the 137Cs has been removed from the shallow areas within the first six years after contamination, while the inventory in the deepest areas has increased by 50%. Gross loss rates are strongly related to the duration (annual number of days) of lake-water mixing. Wind-driven resuspension is thus suggested to be the main cause of the redistribution and focusing of sedimentary 137Cs. The importance of resuspension is confirmed by the horizontal uniformity of 137Cs concentrations in surficial sediment and by their similarity to 137Cs concentrations in settling material collected in sediment traps. Extra keywords: resuspension, focusing.