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.

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.

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 ²¹⁰Pb_ex 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.

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.