Vertical distribution of 137Cs in bottom sediments as representing the time changes of water contamination: Chernobyl and Fukushima

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 K_d, 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.

This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS), Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA) (JPMJSA1603), by bilateral project No. 18-55-50002 of Russian Foundation for Basic Research (RFBR) and Japan Society for the Promotion of Science (JSPS), and JSPS Project KAKENHI (B) 18H03389.