Using reservoir sediment deposits to determine the longer-term fate of chernobyl-derived Cs fallout in the fluvial system.

Vast areas of Europe were contaminated by the fallout of Cs and other radionuclides, as a result of the Chernobyl accident in 1986. The post-fallout redistribution of Chernobyl-derived Cs was associated with erosion and sediment transport processes within the fluvial system. Bottom sediments from lakes and reservoirs can provide a valuable source of information regarding the post-fallout redistribution and fate of Cs released by the Chernobyl accident. A detailed investigation of sediment-associated Cs in the bottom sediments of a reservoir in a Chernobyl-affected area in Central Russia has been undertaken. A new approach, based on the vertical distribution of Cs activity concentrations in the reservoir bottom sediment makes it possible to separate the initially deposited bottom sediment, where the Cs activity reflects the direct fallout of Chernobyl-derived Cs to the reservoir surface and its subsequent incorporation into sediment deposited immediately after the accident, from the sediment mobilized from the catchment deposited subsequently. The deposits representing direct fallout from the atmosphere was termed the "Chernobyl peak". Its shape can be described by a diffusion equation and it can be distinguished from the remaining catchment-derived Cs associated with sediment accumulated with sediments during the post-Chernobyl period. The Cs depth distribution above the "Chernobyl peak" was used to provide a record of changes in the concentration of sediment-associated Cs transported from the upstream catchment during the post-Chernobyl period. It was found that the Cs activity concentration in the sediment deposited in the reservoir progressively decreased during the 30-year period after the accident due to a reduction in the contribution of sediment eroded from the arable land in the catchment. This reflects a reduction in both the area of cultivated land area and the reduced incidence of surface runoff from the slopes during spring snowmelt due to climate warming.