ИСТИНА

Three cores of bottom sediments were collected in 2018 to the depth of about 200 cm in the deep part of Schekinskoe dam reservoir on the Upa river, Russia, severely contaminated by 137Cs after the Chernobyl accident in 1986. The Upa river basin, selected as analogue of Abukuma river (Fukushima Prefecture) in terms of size and 137Cs deposition, has an area of 9,500 km2 and the mean annual rainfall is 540 mm/year, which is about 3 times lower than in Fukushima Prefecture. The collected bottom sediment cores were sliced in 2-cm layers, dried at 105oC and passed through 2-mm sieve. They were then measured for 137Cs using γ-spectrometry. Unfortunately, systematic regular monitoring of radioactive contamination of the Upa river was not carried out. The only data available are total 137Cs activity concentrations in water in the first years after the accident (1987-1991). The obtained 137Cs vertical distribution in sediment accumulation zones in the central part of the reservoir suggests that practically no vertical mixing of sediments took place, the 137Cs peak being well-defined, rather than diffused. If sediment accumulation rates after the accident are more or less uniform, layers of bottom sediments can be attributed to certain time of sedimentation. The fact that 137Cs activity concentration in bottom sediments of a specific layer is corresponding to 137Cs concentration on suspended matter at that time allowed us to obtain the dynamics of particulate 137Cs activity concentrations in the Upa from 1986 to 2017. Over the time since the accident the 137Cs concentrations have decreased by more than an order of magnitude, with only minor changes occurring during the last 15 years. Using a typical value of distribution coefficient Kd for the rivers of Chernobyl contamination zone, dissolved 137Cs activity concentrations in the Upa river have been estimated and changes of the radionuclides over 30 years after the accident have been studied. The resulting estimates of dissolved 137Cs concentrations in the Upa river are in good agreement with measurements for 1987-1991. The proposed and tested method allows us to reconstruct the long-term dependence of radionuclide concentrations in rivers and reservoirs based on their vertical distribution in bottom sediments.