ИСТИНА

Lateral migration of 137Cs via temporary and constant watercourses is the main mechanism for spatial transformation of initially deposited contaminants on Chernobyl affected territories. In Central Russia, highly contaminated areas occur on densely populated territories where watercourses are exposed to major human impacts. As far as the highest concentration of 137Cs in soils and deposits can be attributed to the Chernobyl fallout, the floodplain strata turn out to a very important of landscape record. Depth increment study of floodplain deposits provides information about 137Cs concentration in suspended sediments during flood events in the post-Chernobyl period. The analysis of floodplain morphology and topography can reveal modern trends of river valley development. This information can be used for interpretation of obtained radiocesium data. Moreover, accumulation of sediments on the floodplains in contaminated areas illustrates the process of natural attenuation, which should be considered in remediation strategy development. Studies conducted in 2009-2018 in the Upa River basin (right-hand tributary of the Oka River) have made it possible to identify trends in particulate 137Cs lateral migration in different reaches of the river. The most part of sediment-associated 137Cs delivered from the cultivated slopes of the Upa River upper reaches have been trapped in Schekinskoe reservoir. As the result, incision is observed downstream and accumulation of sediment-associated 137Cs on the floodplains is decreasing dramatically. In the middle reaches, the sedimentation rates on floodplain returned to normal. The low reach of the Upa River is characterized by significant incision caused by intensive excavation of sands from the channel of the Oka River since the last decades of 20th century. However, high sedimentation rates on the Upa River floodplain is observed due to different hydrological regime of the Oka and the Upa Rivers. The results for Upa river basin are compared with data for Niida River and Abukuma River in the contaminated areas as a result of the accident on Fukushima Dai-ichi NPP. The research is funded under RFBR-JSPS Project “Assessment and prediction of sediment and radionuclide fluxes in river basin affected by severe nuclear accident”.