ИСТИНА

Three artificial reservoirs of different size and area of their catchments were investigated in the Upa River basin, which is located in Chernobyl affected area in Central Russia (Tula region, so called «Plavsk hot spot”). Study area is characterized by temperate climate, plain topography and loamy soil cover (chernozems). Arable lands occupy the main part of reservoir’s catchment areas. Soil erosion on the arable lands is the main process responsible for the 137Cs transport from catchment area to rivers and reservoirs. Evaluation of 137Cs concentration changes in bottom sediment depth profiles may provide relevant information about changes of 137Cs concentration in the different sediment sources over post-Chernobyl period. A series of sediment cores was collected in January 2018 and February 2019 using piston sampler. Cores were sliced into sections of 2-3 cm, dried, grinded and gamma-spectrometry analysis measured with spectrometer ORTEC GEM-C5060P4-B with HPGe detector (relative efficiency 20%). Peak of 137Cs concentration was identified in the all cores (see Fig.1, as an axamples). The layer with maximum 137Cs concentration can be attributed to period of the Chernobyl fallout from atmosphere. Sediments above this layer are supposed to be accumulated after 1986. 137Cs depth distribution curves have different shape but they illustrate one basic tendency. In the first years after Chernobyl fallout concentration of 137Cs in sediments drops in several times if it is compared with initial fallout and after that decrease of 137Cs concentration decline smoothly. Such interpretation is consistent with direct observations of the particulate 137Cs in the water of the Oka River and it’s tributaries (Vakulovsky et al., 1995). Nevertheless, 137Cs concentrations in sediments accumulated in reservoir drained relatively small catchment area are characterized by more high fluctuation. It is associated with more high variation in the contribution of the different sediment sources. The 137Cs depth distribution in large reservoir are more smooth due to larger catchment area. It is promoted to more uniform 137Cs concentration in river sediment due to mixing of sediments delivered from the different sediment sources. If it is assumed that the mean rate of sediment accumulation for the post-Chernobyl period was uniform, temporal changes of particulate 137Cs concentration in surface runoff in general can be described by convection-diffusion model (Crank, 1975; Prokhorov, 1981; Bulgakov et al., 2002). However it is necessary to consider hydrologic, geomorphic and landscape features of the reservoir catchment area for improving of model calculation. This work was supported by the Russian Fund for Basic Research (RFBR project No. 18-55-50002) and Russian Scientific Fund (RSF project No. 19-77-00022). Bulgakov, A.A., Konoplev A.V., Kanivets, V.V. and Voitsekhovich, O.V.: Modelling the long-term dynamics of radionuclides in rivers. Radioprotection - Colloques, 37(C1), 649-654, 2002. Crank, J. 1975 The mathematics of diffusion. London: Oxford University Press, 56-61, 1975. Prokhorov, V.M. 1981 Migration of radioactive contaminants in soils: physico-chemical mechanisms and modelling. Moscow: Energoatomizdat, 99 p., (in Russian) Vakulovsky S.M., Nikitin A.I., Chumichev V.B., Katrich I.Yu., Voitsekhovich O.V., Medinets V.I., Pisarev V.V., Bovkun L.A., Khersonsky E.S. 1994. Cesium-137 and Strontium-90 Contamination of Water Bodies in the Areas Affected by Releases from the Chernobyl Nuclear Plant Accident: An Overview. J. Environ. Radioactivity, 23, 103-122.