137Cs deposition fluxes in bottom sediments of the Yenisei River calculated using modern methods

Relevance. The long-term operation of the reactors and radiochemical plant of the Mining-and-Chemical Combine have resulted in large-scale accumulation of radionuclides in bottom sediments of the Yenisei River. To reconstruct the chronology and the rates of deposition of radionuclides in the Yenisei bottom sediments, data on sediment formation parameters are needed. For lake systems, the mass rates of sedimentation and the flow of matter are calculated using the generally accepted methods: CRS and AMDO. Under certain assumptions, these methods can be used to study sediment formation in the Yenisei River. Aim. To estimate the vertical fluxes of 137Cs deposition in bottom sediments over an extended stretch of the Yenisei River using a set of methods for calculating sedimentation rates. Material. Bottom sediments cores collected from the Yenisei River floodplain over an extended stretch of the river downstream of Krasnoyarsk, including the area affected by the Mining-and-Chemical Combine operation. Methods. Activity concentrations of radionuclides in the bottom sediments were determined using a semiconductor gamma-spectrometer coupled to a hyper-pure germanium detector (Canberra, U.S.) Sedimentation rates were determined using the standard 210Pb dating, the CRS method, and the ratio of the radionuclides, 137Cs/60Co. The vertical fluxes were calculated using the CRS method and the AMDO. Results. The ranges of sedimentation rates in the reference areas of the Yenisei River and the Mining-and-Chemical Combine -affected near zone obtained using the AMDO and the CRS method were similar. For the reference area at the village of Shivera, the values of the 137Cs vertical fluxes in the bottom sediments determined using the CRS method and the AMDO were higher than the 137Cs fluxes in the reference areas Krasnoyarsk and Esaulovo. As the village of Shivera is located in the Mining-and-Chemical Combine 30-km zone, 137Cs could be deposited in the Yenisei floodplain in the aerosol form. For two bottom sediments collection positions close to the Mining-and-Chemical Combine, sedimentation rates were calculated using different methods, but the values of the maximal 137Cs fluxes in the bottom sediments did not differ substantially from each other (60000 and 74000 Bq/(m2·yr)). Bottom sediments layers with high activity concentrations of 137Cs were detected at some positions of the floodplain, which increased the maximal 137Cs flux to 254000–680000 Bq/(m2·yr). The data obtained on the 137Cs fluxes in the Yenisei bottom sediments can be used to estimate the radiation load on the study floodplain areas in the context of the ongoing operation of the Mining-and-Chemical Combine.