Аннотация:Bottom sediments of lakes and dam reservoirs can provide an insight into understanding the
dynamics of 137Cs strongly bound to sediment particles. On this premise, a number of cores of
bottom sediments were collected in deep parts of lakes Glubokoe, Azbuchin, and Cooling Pond in
close vicinity of the Chernobyl NPP in Ukraine, in Schekino reservoir (Upa River) in the Tula region
of Russia (2018) and in Ogaki reservoir (Ukedo River) in Fukushima contaminated area (2019). Each
layer of bottom sediments can be attributed to a certain time of suspended particles
sedimentation. With 137Cs activity concentration in a given layer of bottom sediments
corresponding to 137Cs concentration on suspended matter at that point in time, we were able to
reconstruct the post-accidental dynamics of particulate 137Cs activity concentrations. Using
experimental values of the distribution coefficient Kd, changes in the dissolved 137Cs activity
concentrations were estimated. The annual mean particulate and dissolved 137Cs wash-off
ratios were also calculated for the period after the accidents. Interestingly, the particulate 137Cs
wash-off ratios for the Ukedo River at Ogaki dam were found to be similar to those for the Pripyat
River at Chernobyl in the same time period after the accident, while the dissolved 137Cs wash-off
ratios in the Ukedo River were an order of magnitude lower than the corresponding values in the
Pripyat River. The estimates of particulate and dissolved 137Cs concentrations in Chernobyl cases
were in reasonable agreement with monitoring data and predictions using the semi-empirical
diffusional model. However, both the particulate and dissolved 137Cs activity concentrations and
wash-off ratios in the Ukedo River declined faster during the first eight years after the FDNPP
accident than predicted by the diffusional model, most likely, due to greater natural attenuation
and, to some extent, remediation measures implemented on the catchments in Fukushima.