Particulate matter from water moss of a large Siberian river: Morphometric, mineral, elemental and radionuclide composition.

Particulate matter trapped by tufts of water moss Fontinalis antipyretica inhabiting fast flowing waters of the Yenisei River (Siberia, Russia) contaminated with artificial radionuclides has been studied as a potential monitor of radioactive releases to the river. Particulate matter, which was removed from wet tufts of water moss of the Yenisei by rinsing them in water, constituted at least 38% of bulk dry weight of the moss biomass sample and was similar in the contents of chemical elements, minerals, organic matter, and artificial radionuclides to bottom sediments of the Yenisei. Considerable bulk percentages of artificial radionuclides in the sample of water moss, 77% of 137Cs, 44% of 60Co, 41% of 152Eu, 55% of 154Eu, 66% of 241Am, and 34-36% of plutonium were associated with extracellular particles. The comparative study and correlation analysis suggested that 137Cs was mainly associated with mineral particles trapped by moss and that organic matter was responsible for binding plutonium in samples of water moss. Consequently, analysis of extracellular particles of water moss can provide data on contents and speciation of radionuclides transported by water current. Although a considerably high proportion of particulate matter had been washed out from tufts of water moss, some extracellular mineral particulate material and a large number of epiphytic diatoms remained attached to leaves of water moss. Our study proves that particulate matter trapped by water moss can be used as an informative monitor to trace radioactive pollutants transported by water current in running waters deficient in bottom sediments and potential biomonitors.

New data on mobility of transuranium elements in sediments of the Yenisei River.

The inflow of transuranium elements to the Yenisei River was previously associated with the production of weapons-grade plutonium at the Mining-and-Chemical Combine (MCC, Zheleznogorsk, Russia), but the source of transuranium elements in the River today is fabrication of MOX fuel that started recently at the MCC. The current study presents results of sequential chemical extraction of radionuclides from sediment samples collected in 2014 and 2020 in two areas near the MCC discharge site and compares these results with the data obtained previously by sequential extraction of sediments collected during 1999-2007. Over the study period, the strength of binding of 137Cs and 60Co in the Yenisei River sediments was high (up to 100%) and remained so, while the percentages of 241Am and 152Eu in residual solids after sequential extraction increased considerably and the percentage of 239,240Pu in residual solids decreased in samples from all study areas. In samples collected at the position located close to the MCC discharge site, the percentages of the strongly bound 241Am and 152Eu as well as 239,240Pu were lower than in the samples from the other positions. The study demonstrated an enormous increase in 239,240Pu activity concentration in the top sediment layers collected at all positions in 2020 relative to 2014. In the same period, as literature data suggest, 239,240Pu activity concentrations also increased in aquatic organisms of the Yenisei River, which can be indicative of the growing potential bioavailability of plutonium in the aquatic ecosystem, which could be caused by the presence of the mobile form of plutonium in the routine discharges from the MCC.

Monitoring and assessment of 137Cs and 90Sr radioactive isotopes in the 'soil - rhizosphere - sedge' system of the Yenisei River floodplain (near impact zone of Krasnoyarsk MCC, Russia).

Radiocaesium and radiostrontium contamination in the 'soil - rhizosphere - plants (aerial parts)' system was monitored in the floodplain ecosystem of the Yenisei River in the near impact zone of the Krasnoyarsk Mining and Chemical Combine (MCC). The monitored system included soil, rhizosphere, and sedge vegetation on islands and the river's east bank. The 137Cs and 90Sr specific activities displayed intricate space and time patterns controlled by the river water level, including the time and duration of floods and their correlation with the sedge vegetation season. The specific activities of both radionuclides, especially 137Cs, were above the background in all years of observation, except in a few cases. The soil-to-plant transfer factor (TF) patterns showed continuous 137Cs and 90Sr influx into the system and annual variations in the shares of their bioavailable and fixed forms, especially for 90Sr. The 90Sr distribution in the 'soil - rhizosphere - plants' system observed in 2014-2016 provides evidence for possible local fallout.

Distribution of 137Cs in lichens, mosses and pine needles along the transect from the north to the south of Western Siberia.

The 137Cs content in mosses (Hylocomium splendens), lichens (Cladonia stellaris) and the needles of the Siberian pine (Pinus sibirica) and the common pine (Pinus sylvestris) along the transect from the north to the south of Western Siberia from N. 67.5° to N. 55° has been investigated. The appearance of 137Cs here is linked to the nuclear weapon tests at Novaya Zemlya. The measurements have shown that at all sampling points south of N. 59.6° there is a very sharp decrease in the 137Cs specific activity in all components of the ecosystem (in mosses and lichens by about 20 times, and in the needles of conifers by 100 times) instead of a smooth decrease. This fact can be explained by the existence of the global atmospheric circulation consisting of three circulation cells in the Northern hemisphere. It is just around N. 60° that the boundary between the Polar cell and the circulation cell of midle latitudes is drawn. At this boundary, the counter surface air flows of these cells (in our case, a contaminated flow from the north and a clean one from the south) collide and generate the upward air flows here that take 137Cs away. In addition, there is water vapor condensation in the upward flows resulting in snowfalls, rains and thunderstorms. And with these precipitation events, large amounts of 137Cs should have fallen out to the north of the Polar cell boundary during the Novaya Zemlya tests. The areas south around of N. 60° have been supposed to remain clean, which is what is being observed.

Assessment of mercury levels in modern sediments of the East Siberian Sea.

Mercury (Hg) is an important environmental indicator of anthropogenic pollution. In this study, the Hg content in the bottom sediments of the East Siberian Sea was observed to range from 13 to 92 ppb, with an average of 36 ppb. Facies dependence was also observed and expressed as an increase in the Hg concentration in fine-sized sediments on the shelf edge and continental slope, compared to that in the sandy silts and sands of the inner shelf. The Hg accumulation in bottom sediments of the eastern part has increased over the past 150 years due to an increase in global emissions of anthropogenic Hg, which is caused by the transboundary transport of Hg to the Arctic. Moreover, changes in the Hg value, which occur due to the plankton arriving at the bottom sediments because of changes in hydrology and primary production, are thought to be associated with global warming.

Accumulation of natural radionuclides (7Be, 210Pb) and micro-elements in mosses, lichens and cedar and larch needles in the Arctic Western Siberia.

This is a study of the atmospheric-origin natural radionuclides (7Be and 210Pb) and a wide range of micro- and macro-element accumulation in mosses, lichens, cedar and larch needles in Arctic western Siberia (Yamal-Nenets Autonomous District). Based on the specific activities measurements of atmospheric precipitation markers (7Be and 210Pb), this study found that the concentration of dust particles in the studied objects incrementally increases in the following order, from lowest concentration to highest: cedar needles, larch needles, lichens and mosses. Concentrations of Zr, Hf, Ti, Th, Fe, V, Li, Na, Si, Be, Y, rare earth elements (REE) and Sc in this area also increase in the same ascending sequence. Enrichment factors of these elements (EF) relative to the North American Shale Composite (NASC) are close to unity, which proves their terrigenous origin. Also, the terrigenous origin of the elements in the studied biological objects is confirmed by their high correlation coefficients with Sc. This means that their concentration in the studied biological objects is the result of a background of solid atmospheric precipitation. Enrichment factors of biogenic elements and their analogues (P, Se, Mn, Mg, Ca, K, Zn, Sr, Ba, Rb, Cs) are significantly greater than unity, and this is associated with high concentrations of these elements in the biological part of the samples. A radially symmetric distribution of Pb content in biological objects is observed over the surface of the studied area (with a center located within the city of Novy Urengoy). This leads to the conclusion that there is a point source around which anthropogenic precipitation of Pb takes place. This distribution is most clearly manifested by the example of larch and cedar needles. Anthropogenic deposition of other elements has not been detected in this study area.

Radioactive particles in the Yenisei River floodplain (Russia): Characterization, leaching and potential effects in the environment.

The operation of the Mining-and-Chemical Combine (MCC), the largest producer of weapons-grade plutonium in Russia, has resulted in radioactive contamination of the Yenisei River floodplain. Investigations carried out in Novosibirsk and Krasnoyarsk institutes have shown that the floodplain of the Yenisei downstream of the MCC is contaminated by radioactive particles (RP) of various types and activities. Analytical characterization of the RP showed that most of them were fuel particles, which were carried into the Yenisei after incidents at the MCC reactors. The plutonium and caesium isotope ratios (238Pu/239,240Pu; 137Cs/134Cs) vary substantially between the particles, indicating different source terms and time intervals when the RP were formed. In addition to fuel RP, there were particles that contained activation radionuclides. The experiment on dissolution of RP using the model solution (the simulated stomach fluid) showed different cumulative extractions of radionuclides from the particles: 60Co and 137Cs extractions were the lowest, the extracted fractions of europium and americium isotopes were the largest, and plutonium occupied an intermediate position. High concentrations of radionuclides in RP are sources of exposure of organisms in aquatic and terrestrial ecosystems to low radiation doses. The plant bioassays of the effects of γ-radiation from RP showed the effect of low doses of γ-radiation on growth parameters of aquatic plant Elodea canadensis growing in the Yenisei River floodplain. The presence of RP from different sources in the Yenisei River floodplain makes this region a unique site for studying environmental effects of the particles.

Discussions on the driving mechanism of postdepositional migration of 241Am and 137Cs in organomineral sediments (Lake Krugloe, Tomsk region, Russia).

A core of bottom sediments from Lake Krugloe located within the 30 km influence zone of the Siberian Chemical Plant (located in the city of Seversk "Tomsk-7") was studied to determine scales and rates of migration of artificial radionuclides 137Cs and 241Am in organomineral sediment. It was found that the main portion of 137Cs and 241Am was contained in the sediment interval above 10 cm. This means that the horizon of 10 cm corresponds to 1950-the time of the start of widespread tests of nuclear weapons. The 210Pbex dates also confirm that this particular horizon was formed in the 1950s. Pore waters in the core above the 10 cm horizon are in oxidizing conditions. The depth of the oxidized/reduced boundary was determined from the distribution of redox-sensitive elements Fe and U dissolved in the pore solution. The core distribution of 137Cs is a slightly sloping step, with the lower edge at the 10 cm level. The smearing of the lower boundary of this distribution showing the scale of 137Cs migration made it possible to estimate the diffusion mobility of 137Cs. Its diffusion coefficient turned out to be of the order of 10-8 cm2 s-1. As shown by measurements, the scale of migration of 241Am and the scale of migration of 137Cs have similar values. Theoretical analysis carried out in this work shows that the most probable mechanism of 137Cs and 241Am migration in the pore solution of lake sediment is the migration of colloidal particles to which these radionuclides are strongly bound. Calculation of the diameter of such particles by the Stokes-Einstein formula shows that they have submicron dimensions (d ≈ 400 nm). No evidence was found that the change in redox conditions in the sediment had an effect on migration of Am ions in pore solution.

Unique diversity of radioactive particles found in the Yenisei River floodplain.

The long-term operation of three reactors and the radiochemical plant of the Mining-and-Chemical Combine (MCC), Russia's largest producer of weapons-grade plutonium, has resulted in radioactive contamination of the Yenisei River floodplain. From 1995 to 2016, we found more than 200 radioactive particles (RP) in the Yenisei floodplain, downstream of the MCC. Analytical characterization showed that most of the RP were fuel particles, which were carried into the river after incidents at the MCC reactors. Having compared the 137Cs/134Cs ratios in the particles, we determined three time intervals when the RP were formed. The plutonium isotope ratios (238Pu/239,240Pu) vary substantially between the particles and indicate several different source terms. In addition to fuel RP, we found particles that only contained activation products (60Co or europium isotopes). SEM and γ-spectrometry showed that the cobalt particles could have originated from the corrosion of the reactor coolant system and the europium particles - from the damaged compensating rods. No europium particles have been found anywhere else in the world. The presence of RP from different sources (fuel, cobalt, and europium particles) in the Yenisei River floodplain makes this region a unique site for studying environmental effects of the particles. These RP represent point sources of radioecological significance.

The artificial and natural isotopes distribution in sedge (Carex L.) biomass from the Yenisei River flood-plain: Adaptation of the sequential elution technique.

The study of migration pathways of artificial isotopes in the flood-plain biogeocoenoses, impacted by the nuclear fuel cycle plants, requires determination of isotope speciations in the biomass of higher terrestrial plants. The optimal method for their determination is the sequential elution technique (SET). The technique was originally developed to study atmospheric pollution by metals and has been applied to lichens, terrestrial and aquatic bryophytes. Due to morphological and physiological differences, it was necessary to adapt SET for new objects: coastal macrophytes growing on the banks of the Yenisei flood-plain islands in the near impact zone of Krasnoyarsk Mining and Chemical Combine (KMCC). In the first version of SET, 20 mM Na2EDTA was used as a reagent at the first stage; in the second version of SET, it was 1 M CH3COONH4. Four fractions were extracted. Fraction I included elements from the intercellular space and those connected with the outer side of the cell wall. Fraction II contained intracellular elements; fraction III contained elements firmly bound in the cell wall and associated structures; fraction IV contained insoluble residue. Adaptation of SET has shown that the first stage should be performed immediately after sampling. Separation of fractions III and IV can be neglected, since the output of isotopes into the IV fraction is at the level of error detection. The most adequate version of SET for terrestrial vascular plants is the version using 20 mM Na2EDTA at the first stage. Isotope 90Sr is most sensitive to the technique changes. Its distribution depends strongly on both the extractant used at stage 1 and duration of the first stage. Distribution of artificial radionuclides in the biomass of terrestrial vascular plants can vary from year to year and depends significantly on the age of the plant.

Distribution of 152Eu and 154Eu in the 'alluvial soil-rhizosphere-plant roots' system.

Accumulation of (152)Eu and (15)(4)Eu isotopes in bulk soil and rhizosphere soil in the near-field zone of influence of the Krasnoyarsk Mining and Chemical Combine was studied. An uneven distribution of specific activity of Eu isotopes was observed, with the gross specific activities of the isotopes in the bulk soil exceeding those of the rhizosphere. In the most contaminated locations the fine and the coarse granulometric fractions are enriched with the isotopes. A laboratory experiment indicated potential removal of soluble Eu isotopes by river flood waters may amount to 3% of the total Eu in both bulk and rhizosphere soils. The root system of plants growing in the contaminated territory accumulates (152)Eu and (154)Eu, although the isotopes were not discovered in aboveground parts of plants. Root-hairs were found to be the most contaminated.