Speciation of plutonium (238Pu, 239, 240Pu) and other radionuclides (40K, 60Co, 137Cs, 152Eu, 154Eu) in bottom sediments of the Yenisei River: a retrospective study

PurposeSpeciation of artificial radionuclides in bottom sediments (BS) collected in the Yenisei River (Siberia, Russia) was studied, to determine temporal trends in the potential mobility of radionuclides in BS, whose releases increased (238, 239, 240Pu), decreased (60Co, 137Cs), or ceased (152, 154Eu) over the period of this study.MethodsIntegrated samples of BS whose total thickness reached 20 cm were collected in the Yenisei River in September-October 2014 and 2018-2023 downstream of a controlled radioactive effluent site. A sequential extraction technique (modified Tessier method) was applied to study speciation of radionuclides in BS. Six fractions of radionuclides were extracted from sediment samples: exchangeable, associated with carbonates, associated with oxides and hydroxides of Fe, Mn, associated with organic matter, associated with amorphous silicates, and associated with residual solids. Concentrations of gamma-emitting radionuclides, plutonium, minerals, and organic matter were determined in bulk samples of BS and their fractions.ResultsThe largest percentages of 137Cs (83 +/- 9%) and Pu isotopes (67 +/- 17% of 238Pu, 64 +/- 18% of 239,240Pu) were irreversibly bound to residual solids of BS; percentages of 152Eu and 124Eu in this form were less than 27% and 11%, respectively. The percentage of 60Co in residual solids of BS was 59 +/- 24%, although the proportions of species of 60Co were quite variable, demonstrating no temporal trends. No consistent trends were revealed in the forms of Pu in BS after the upsurge of releases of Pu to the river. Easily mobilized forms of 152Eu (up to 20%) were registered in BS for up to six years after termination of Eu releases. A decrease in the percentage of easily mobilized 137Cs corresponded to a similar decrease in releases of 137Cs.ConclusionsThis study showed considerable differences in the forms of occurrence of the artificial radionuclides 60Co, 137Cs, 152Eu, 154Eu, and isotopes of Pu deposited in BS of the Yenisei River in the vicinity of a controlled radioactive discharge site over the period of this study. Temporal trends in the forms of 137Cs and 152Eu in BS related to changes in controlled releases of these isotopes were revealed.