Natalia V. Kuzmenkova

This study presents the first documented evidence of radioactive contamination in Western Abkhazia linked to the Chernobyl Nuclear Power Plant accident. The data obtained show that the level of ¹³⁷Cs radioactive contamination in the study area ranged from 50 to 160 kBq/m² in 1986. This corresponds to contemporary values of 25 to 79 kBq/m² when considering the radionuclide’s half-life. These measurements are highly consistent with data recorded in the adjacent Sochi region, where contamination levels varied between 40 and 185 kBq/m² in 1986. The local spatial variability of ¹³⁷Cs fallout was studied at four reference sites, located in different parts of the Mussera upland. All investigated sites demonstrated moderate variability, with value ranges of 17–25%. This heterogeneous distribution pattern is attributed to a combination of factors, including local topography, atmospheric deposition characteristics, and anthropogenic influence. Measurements of ambient dose equivalent rates ranged from 0.01 to 0.05 μSv/h. While no direct correlation was found between dose rates and the age or genesis of the underlying bedrock, a clear relationship was established between dose rates and terrain morphology. Elevated dose rates were consistently recorded in erosional landforms within topographically dissected areas. Analysis of peat cores from the Pitsunda Peninsula lagoon provided conclusive evidence of the Chernobyl disaster’s impact on Western Abkhazia, with a measured ¹³⁷Cs inventory of 20.7 kBq/m² (equivalent to 49.5 kBq/m² when corrected to 1986 values). Application of the non-equilibrium ²¹⁰Pb dating method yielded a peat accumulation rate of 0.1 cm/year. The Chernobyl accident resulted in a significant release of ¹³⁷ Cs, leading to widespread radioactive fallout. This document assesses the ¹³⁷ Cs inventory and its impact on ambient dose rates in the affected regions.

In the present study, sedimentation conditions and rates in Lake Pleshcheyevo (Yaroslavl region, Central part of the East European Plain) over the past 3600 years were reconstructed based on lithological, geochronological, isotopic, and paleobiological studies of bottom sediments from the deep part of the lake. C/N ratio analysis was used to identify the sources of organic carbon in the lake bottom sediments. Studies of anthropogenic indicators in pollen spectra provided a qualitative comparison of the two age–depth models. Deepwater mesotrophic to eutrophic sedimentation conditions were established during the epoch of cattle farming, 3600–2700 cal yr BP (1600–750 BCE), with relatively high (up to 0.04–0.05 g cm⁻² year⁻¹) mass accumulation rates (MAR) corresponding to period of high fluvial activity on the East European Plain. About 1900 cal yr BP (since the beginning of the 1st century CE), with the spread of slash-and-burn agriculture, lake eutrophication occurred, with an increase in sedimentation rates associated with deforestation and ploughing of territories. From 1300 to 200 cal yr BP (7th to the 19th century CE), sedimentation rates decreased. A sharp (up to 0.07 g cm⁻² year⁻¹) increase in MAR from the end of the 19th century to the middle of the 20th century was established. Similar trends are typical of lakes within the developed areas and are associated with expansion of cultivated agricultural land.

Accumulation of radionuclides in crop products can pose public health risks. Parameters of ¹³⁷Cs root uptake by maize were investigated at four growth stages, from leaf development to ripening (June–September), in the post-Chernobyl area of the Plavsk radioactive hotspot (the Tula region of Russia). It was established that the chernozem of the agrosystem still contains about 180 kBq ¹³⁷Cs m⁻². Seasonal trends of ¹³⁷Cs activity concentrations in above- and belowground parts of maize differ appreciably. In aerial parts, the ¹³⁷Cs accumulation rate increases as vegetative organs develop from June to July, then drops when generative organs appear in August. In the belowground biomass, the ¹³⁷Cs content increases with the growth of fine roots, which serve as relative concentrators of the radionuclide. However, at all stages of growth, radionuclide transfer to maize occurs with low intensity. It does not directly depend on changes in biomass, dry matter content, or ⁴⁰K content, and only slightly relates to the ash content. The transfer factor is estimated to be 6.2 × 10⁻³ for cob kernels (grain) and 3.8 × 10⁻² for stems and leaves, which corresponds to the IAEA-recommended values and ensures acceptable levels of ¹³⁷Cs accumulation in crop products produced in the territory.

Accumulation of radionuclides in crops products can pose public health risks. Parameters of 137Cs root uptake by maize were investigated at 4 growth stages from leaf development to ripening (June-September) in the post-Chernobyl area of the Plavsk radioactive hotspot (Tula region of Russia). It has been established that chernozem of the agrosystem still contains about 180 kBq 137Cs m-2. Seasonal trends of 137Cs activity concentrations in above- and belowground parts of maize differ appreciably. In aerial parts, 137Cs accumulation rate increases as vegetative organs develop from June to July, then they drop when generative organs appear in August. In the belowground biomass, the 137Cs content increases with the growth of fine roots, which, due to their rhizofiltration capacity, have maximal radionuclide concentration. However, at all stages of growth radionuclide transfer to maize occurs with low intensity and does not directly depend on changes in biomass, dry matter content, 40K content; and only slightly relates to the ash content. The transfer factor is estimated to be 6.2 x 10-3 for cob kernels (grain) and 3.8 x 10-2 for stems and leaves, which corresponds to the IAEA recommended values and ensures acceptable levels of 137Cs accumulation in crop products produced on the territory.

Water from four thermal springs in the Western Transcaucasia territory was studied. The main chemical parameters of the water, its isotopic, and gas composition were determined. It has been demonstrated that groundwater in mountainous regions, located between 1300 and 1750 m above sea level, exhibits characteristics of a classical nitrogen-methane therm. These characteristics are primarily attributed to the infiltration of atmospheric precipitation. The results of the geomorphological mapping demonstrated not only the radiation and chemical impact of thermal sources on the environment but also changes in the terrain and the chemogenic morpholithogenesis. The distribution of radionuclides ^226,228Ra (and their decay products) and ¹³⁷Cs entering the river with thermal water was investigated in bottom sediments from a borehole located in close proximity to Kyndyg village and downstream of the Dabakuara River. It was determined that equilibrium between ²²⁶Ra and its daughter decay products is restored at a distance of approximately 300 m from the sources. Additionally, an elevated concentration of radon decay products was observed at a distance of approximately 100 m from the thermal source. The specific activity of radium and its decay products in bottom sediments exceed the intervention levels (the level of radiation factor above which certain protective measures should be taken) by a factor of 3.5 at the direct release site. However, radioactivity decreases very quickly due to sorption of nuclides on bottom sediment particles and the decay of radon progeny. The text continues with recommendations regarding the thermal waters safe use in a recreational context.

The features of vertical distribution of ¹³⁷Cs in the profile of clayeyilluvial agrochernozem, as well as biogenic migration of the radionuclide in the system “soil–plant” of potato agrocenosis have been studied. The research was conducted on the field, occupied by potato variety “Gala”, located in the central part of Plavsky radioactive hotspot of Tula region. It is shown that 35 years after the Chernobyl accident the density of surface radioactive contamination of arable soils in the region exceeds the permissible level by ≈5 times. The depth of ¹³⁷Cs penetration down the soil profile is mainly determined by methods of agrotechnical soil treatment, and the main part of its inventories is concentrated in the upper 30-cm arable layer. This results in the direct contact of the radionuclide with underground organs of potato (roots, stolons and tubers). However, no more than 0.02% of the total ¹³⁷Cs inventories contained in the system “agrochernozem–potato” is transferred to potato plants. It was also established that potato plants are characterized by contrasting distribution of ¹³⁷Cs specific activity values wich varies with organs and tissues. Stolons and thin roots accumulate the radionuclide to the greatest extent per biomass, while tubers, on the contrary, – to the least extent. In general, the levels of ¹³⁷Cs accumulation in the raw and dry weight of potato tubers, as well as in the products that have undergone culinary processing (boiling the unpeeled and peeled potatoes), correspond to sanitaryhygienic norms, which substantiates the low ecological risks of including potatoes in crop rotations on agricultural land in the area of the Plavsky radioactive hotspot of the Tula region.

The content of237Np,239,240Pu, and241Am in seawater, suspended matter, and bottom sediment cores of the Ussuri and Amur Bays, sampled in July 2021 and August 2022, was analyzed. The activity concentrations of237Np,239,240Pu, and241Am in the water of Peter the Great Bay were determined for the first time and were found to be equal to (19–105) × 10–3, 2.0–5.3, and 24.1–33.5 mBq/m3, respectively. The activity concentrations as well as the239Pu/240Pu isotope ratio (~0.18) in the bottom sediments definitely indicate that global fallout is the main source of plutonium in the investigated territory. The sedimentation rates were determined for the Amur Bay (in the Razdolnaya River estuary), 0.9 mm/year, and for the Ussuri Bay, 4.1 mm/year. Actinide transfer coefficients were determined for Peter the Great Bay in the system dissolved forms, including colloids–suspended matter–bottom sediment.

The content of ²³⁷Np, ^239,240Pu, and ²⁴¹Am in seawater, suspended matter, and bottom sediment cores of the Ussuri and Amur Bays, sampled in July 2021 and August 2022, was analyzed. The activity concentrations of ²³⁷Np, ^239,240Pu, and ²⁴¹Am in the water of Peter the Great Bay were determined for the first time and were found to be equal to (19–105) × 10^–3, 2.0–5.3, and 24.1–33.5 mBq/m³, respectively. The activity concentrations as well as the ²³⁹Pu/²⁴⁰Pu isotope ratio (⁓0.18) in the bottom sediments definitely indicate that global fallout is the main source of plutonium in the investigated territory. The sedimentation rates were determined for the Amur Bay (in the Razdolnaya River estuary), 0.9 mm/year, and for the Ussuri Bay, 4.1 mm/year. Actinide transfer coefficients were determined for Peter the Great Bay in the system dissolved forms (including colloids)–suspended matter–bottom sediment.

The study is focused on the reconstruction of long-term dynamics of 137Cs concentration in water bodies contaminated as a result of Chernobyl and “Fukushima-1” NPP accidents. It was found that the vertical distribution of 137Cs in bottom sediments of deep-water sites could be a basis for reconstruction of the dynamics of radionuclide concentration in suspension and in solution. Columns of bottom sediments of reservoirs and experimental values of distribution coefficient Kd are used for reconstruction. The method is applicable in the absence of mixing of the deposited suspension with the underlying layers and at Kd values significantly higher than 104 l/kg. Studies in the Chernobyl and “Fukushima-1” NPP contamination zones have shown that it is possible to reconstruct the long-term changes of 137Cs concentration for rivers, lakes and ponds. The obtained data are the first assessment of the time course of 137Cs specific activity on suspended sediment in water bodies of the Chernobyl NPP zone, which is important for understanding its behavior in the soil-water system and its migration through food chains. The time dependences of 137Cs concentrations are described by the empirical two-exponential and/or semi-empirical diffusion model, applicable for predictive assessments of the long-term dynamics of radioactive contamination in water bodies after nuclear accidents. The results of the study are confirmed by monitoring data for the test water bodies, showing satisfactory agreement with the reconstructed values of 137Cs concentrations. The information obtained is important for assessing the environmental consequences of nuclear accidents and developing strategies for the management of contaminated areas. The methodology presented in the study could be adapted to study the behavior of other radionuclides in aquatic ecosystems, contributing to better prediction and management of radioecological risks over a long term horizon.