Distributions of 137Cs and 90Sr activity concentrations in trunk of Scots pine (Pinus sylvestris L.) in the Chernobyl zone

Oscillatory Cs-137 Distribution Pattern in Scotch Pine Bark

The vertical distribution of the anthropogenic radionuclide Cs-137 in the Scots pine (Pinus sylvestris L.) bark was studied in two model trees in the radioactive contamination zone of the Bryansk region. Each tree was divided into 10-cm bars from the trunk base to a length of 17 m, and the bark with the bast was separated from each bar to obtain a separate sample. In addition to Cs-137, the natural radionuclide K-40 was measured in the bark of model tree 2 from the trunk base to a 6.5-m length. Specific activities of Cs-137 and K-40 were measured by γ-ray spectrometry. The vertical distribution of Cs-137 in the bark was for the first time observed to have a wave-like pattern with a period of approximately 1 m. The K-40 distribution showed a similar oscillatory pattern, consistent with a similar mechanism responsible for potassium and cesium behavior in woody plants. The correlation coefficient between specific activities of Cs-137 in model trees 1 and 2 was 0.80; the correlation coefficient between Cs-137 and K-40 activities in model tree 2 was 0.45. Cs-137 was assumed to provide a radiotracer to assess the intake and distribution of chemical elements in Scotch pine tissues. The oscillatory pattern observed for the vertical distributions of cesium and potassium in the pine bark has not been described in the available literature before.

Variability of activity concentrations and radial distributions of 137Cs and 90Sr in trunk wood of Scots pine and Silver birch

This study analyzes the variability of ¹³⁷Cs and ⁹⁰Sr concentrations in wood and their radial distributions in the trunks of Scots pine and Silver birch trees in the small uniformly contaminated forest stands in the Chornobyl Exclusion Zone. Concentrations of both radionuclides follow a lognormal distribution with a large scatter of values measured in the trees within the stands (GSD ranges from 1.6 to 2.0). No correlation was found between the concentrations of the two radionuclides measured in individual trees, or between their concentrations and tree diameter. The average ¹³⁷Cs and ⁹⁰Sr T_ag were 8.4 × 10^-4 m² kg^-1 and 8.8 × 10^-3 m² kg^-1 for pine, respectively, and 9.3 × 10^-4 m² kg^-1 and 1.1 × 10^-2 m² kg^-1 for birch, indicating a much higher availability of ⁹⁰Sr for uptake by the studied species. For ¹³⁷Cs, the T_ag values are within the range recommended by the IAEA Handbook (IAEA, 2010), while the values for ⁹⁰Sr exceed the recommended range for birch and are close to its upper value for pine. The highest concentrations of ¹³⁷Cs in pine at the height of 1.3 m were measured in the youngest sapwood rings; they were lower in the rest of the sapwood and decreased further in the heartwood, but remained relatively high even in annual rings that were the heartwood at the time of deposition, suggesting sapwood-to-heartwood translocation of the radionuclide by diffusion and/or ray transport. In contrast, ⁹⁰Sr concentrations increased through the sapwood from the trunk periphery in pine trees up to 80 years old and remained stable through the sapwood in older trees (except for higher concentrations in the young annual rings), but dropped to zero in physiologically inactive heartwood tissues. In most birch trees, regardless of age, ¹³⁷Cs concentrations demonstrated an increasing trend from the trunk periphery towards the pith, while concentrations of ⁹⁰Sr were relatively stable in the whole trunk except in the oldest annual rings, where they increased sharply, likely indicating active transport of the radionuclide to senescing tissues.

Late phase radiocesium dynamics in Fukushima forests post deposition

The long term dynamics of radiocesium in typical forest ecosystems was studied in the radioactive contaminated areas in Fukushima Prefecture. Six observations sites located in Yamakiya Village (Kawamata Town; since 2014), Tsushima Village (Namie Town, since 2015), and Tomioka Town (since 2017) were monitored. The forests consisted of artificial plantations of Japanese cedar (Cryptomeria japonica) at Yamakiya Village, Tsushima Village, and Tomioka Town. Tsushima Village also had a natural mixed forest dominated by Japanese red pine (Pinus densiflora), and Tomioka Town had a young and a mature artificial plantation of Japanese cypress (Chamaecyparis obtuse). Concentrations of ¹³⁷Cs were monitored in the samples collected from the main aboveground biomass compartments, fresh litterfall, forest litter, and soil. Concentrations of exchangeable forms of ¹³⁷Cs and stable K were measured in soil samples. During the observation period, the litter radiocesium inventories at all sites decreased significantly to approximately 1% or less of the total ground deposition. Approximately 80% of the total radiocesium inventory is localized in the upper 5-cm layer of soil and there is little downward migration of radiocesium. At the sites with the longest monitoring series (Yamakiya and Tsushima), the radiocesium expectation depths and expectation mass depths were relatively constant at 2-3 cm and 5-6 kg m^-2, respectively. Aboveground biomass compartments showed similar decreasing trends in radiocesium aggregated transfer factors, T_ag, in the compartments that were exposed to atmospheric fallout in March 2011 (old foliage, small branches, and outer bark). The mean T_ag in cedar stand compartments currently are in the range of 10^-3-10^-2 m² kg^-1 dw. However, the mean T_ag and their dynamic trend significantly differed in the wood compartments of the cedar stands, which may indicate root uptake differences of orders of magnitude between observation sites. The difference in radiocesium concentration in wood between the sites becomes less pronounced when normalized by the ratio of exchangeable ¹³⁷Cs/K in the soils.

Accumulation of 90SR by Betula pendula within the East Ural Radioactive Trace zone

This study was conducted in 2010-2020 at the head of the East Ural Radioactive Trace (EURT), which was formed in 1957 as a result of the Kyshtym accident at the Mayak Production Association. The main contaminant in this zone is the long-lived radionuclide Strontium-90 (90Sr). Secondary forests dominated by silver birch (Betula pendula) occupy 45% of the EURT area. Concentration of 90Sr in birch leaves and small branches was higher than that in the trunks. The 90Sr content in birch sapwood varied slightly in the radial direction and did not depend on tree age. This was due to the dynamic equilibrium of the migration processes responsible for the accumulation and horizontal transfer of 90Sr. The 90Sr concentration increases in false heartwood, which is formed as a result of the secondary metabolism of dying parenchyma in the inner part of sapwood and is characterised by a high content of ash elements. The concentration of radionuclides in the aboveground organs of birch increased and the aggregated transfer factors (Tag) decreased with an increase in the soil contamination density, in accordance with the power function. The reasons for these patterns are also discussed.

Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020

From early April 2020, wildfires raged in the highly contaminated areas around the Chernobyl nuclear power plant (CNPP), Ukraine. For about 4 weeks, the fires spread around and into the Chernobyl exclusion zone (CEZ) and came within a few kilometers of both the CNPP and radioactive waste storage facilities. Wildfires occurred on several occasions throughout the month of April. They were extinguished, but weather conditions and the spread of fires by airborne embers and smoldering fires led to new fires starting at different locations of the CEZ. The forest fires were only completely under control at the beginning of May, thanks to the tireless and incessant work of the firefighters and a period of sustained precipitation. In total, 0.7-1.2 TBq ¹³⁷Cs were released into the atmosphere. Smoke plumes partly spread south and west and contributed to the detection of airborne ¹³⁷Cs over the Ukrainian territory and as far away as Western Europe. The increase in airborne ¹³⁷Cs ranged from several hundred μBq·m^-3 in northern Ukraine to trace levels of a few μBq·m^-3 or even within the usual background level in other European countries. Dispersion modeling determined the plume arrival time and was helpful in the assessment of the possible increase in airborne ¹³⁷Cs concentrations in Europe. Detections of airborne ⁹⁰Sr (emission estimate 345-612 GBq) and Pu (up to 75 GBq, mostly ²⁴¹Pu) were reported from the CEZ. Americium-241 represented only 1.4% of the total source term corresponding to the studied anthropogenic radionuclides but would have contributed up to 80% of the inhalation dose.

Scots pine stands biomass assessment using 3D data from unmanned aerial vehicle imagery in the Chernobyl Exclusion Zone

Thirty-five years after the accident, large forest areas in the Chernobyl Exclusion Zone still contain huge amounts of radionuclides released from the Chernobyl Nuclear Power Plant Unit 4 in April 1986. An assessment of the radiological and radioecological consequences of persistent radioactive contamination and development of remediation strategies for Chernobyl forests imply acquiring comprehensive data on their contamination levels and dynamics of biomass inventories. The most accurate forest inventory data can be obtained in ground timber cruises. However, such cruises in radioactive contaminated forest ecosystems in the Chernobyl Exclusion Zone result in radiation exposures of the personnel involved, which means the need for development of the remote sensing methods. The purpose of this study is to analyze the applicability and limitations of the photogrammetric method for the remote large-scale monitoring of aboveground biomass inventories. Based on field measurements, we estimated the biomass inventories in 31 Scots pine stands including both artificial plantations and natural populations. The stands differed significantly in age (from a few years in natural populations to 115 years in the oldest plantation), productivity (from 0.4 to 19.8 kg m^-2), mean height (from 4.1 to 36 m), and other parameters. Photogrammetric data were obtained from the same stands using unmanned aerial vehicle (UAV). These data were then processed using two approaches to derive the canopy height model (CHM) parameters which were tested for correlation with the aboveground biomass inventories. In the first approach, we found that the inventories correlated well with the mean value of CHM of the site (R² = 0.79). In the second approach, the total aboveground biomass was approximated by a function of the average height of trees detected at the site and the total crown projection area (R² = 0.78). Among other local parameters, the total crown projection area was identified as the major factor impacting the accuracy of the aboveground biomass inventory estimates from the UAV survey data in both approaches. In the dense stands with the high total crown projections areas (more than 0.90), the average relative deviations of the UAV-based aboveground biomass estimates from the results of the field measurements were close to 0, which means the adequate accuracy of the UAV surveys data for radioecological monitoring purposes. The relative deviations of the UAV-based estimates in both approaches increased in the stands consisting of separated groups of trees, which indicates potential limitation of the approaches and need for their further development.