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 Tag 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 Tag 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.

Optimising sampling strategies for emergency response: Vegetation sampling.

The lessons learned from the plant sampling campaigns implemented in the most Chernobyl affected countries are described. The variability of 137Cs activity concentrations in plants taken from a variety of sampling sites, as well as the uncertainties around the aggregated transfer factors (Tag) from soil to plants were estimated. The sampling sites covered both agricultural and natural lands in different landscapes: floodplain, plains, and watershed meadows. To determine parameters of the lognormal distribution of the 137Cs activity concentration in plants and the values of corresponding aggregated transfer factor (Tag) values, from 25 to 49 plant and soil samples were collected at each sampling site with the grid increment that varied from 1 to 10 m. The gradients of deposition i.e. monotonic changes (trends) of the contamination density conditioned by the global (in respect to study area) gradient of fallout were not observed in any of the study sites. Therefore, the variability of radionuclide contamination density (and activity concentrations in the soil) within the study sites were determined by only random factors such as microheterogeneity of radioactive deposition in a sampling point. The mean standard deviation of the logarithms of 137Cs activity concentrations in plants sampled in all such sites and the corresponding transfer factors were similar for all sites studied and were not dependent on the mean soil contamination density at the site, the type of radioactive fallout and the vegetation type. The values of the average standard deviation of the 137Cs activity concentration logarithms in plants and the corresponding transfer factors for the vegetation sampling area ≥1 m2 and the relative activity measurement uncertainties ≤10% were estimated as 0.4 ± 0.1 and 0.5 ± 0.1, respectively. A new simple method for optimization of the number of linked (conjugated) plant and soil samples as well as estimates of the activity concentration and transfer factor uncertainties when measuring composite samples were proposed. Based on the results of these studies, the recommendations were made to the sampling of plants for radionuclides.

Current radiological situation in areas of Ukraine contaminated by the Chornobyl accident: Part 2. Strontium-90 transfer to culinary grains and forest woods from soils of Ivankiv district.

Some of the highest 90Sr activity concentrations recorded beyond the Chornobyl Exclusion Zone occur in the Ivankiv district of Ukraine, located approximately 50 km south of the power plant, an area which nonetheless remains important for agricultural production. Although characterized by soils with low exchangeable calcium values, which can enhance the bioavailability of certain radionuclides, information on the transfer of 90Sr to food crops and trees in the region has remained limited to date. Analysis of 116 grain samples (wheat, rye, oat, barley or Triticale) collected from fields in 13 settlements in the region between 2011 and 2019 revealed 90Sr and 137Cs activity concentrations above Ukrainian limits in almost half of those samples, with annual averages exceeding this limit in four of those nine years (most recently in 2018) and with no clear evidence for a declining trend over time. Analysis of paired sandy soil samples from the same fields yielded concentration ratios for transfer of 90Sr from soils to grains that were on average 3 times higher than that specified by the IAEA. In addition, three quarters of wood samples collected from the trunks of trees (primarily pines) from 12 locations in the same district between 2015 and 2019 contained 90Sr activity concentrations in excess of the Ukrainian limits for firewood (60 Bq/kg), with levels more than four times that limit at one location and again no evidence for decline over time. A single sample of ash collected from a domestic wood-burning oven in the district contained 90Sr at a level 25 times higher than in the most contaminated wood sample collected in this study. Overall these results reveal additional facets of the ongoing legacy of Chornobyl contamination within the Ivankiv district, and the diversity of pathways by which local residents may be exposed to radionuclides. They also highlight the dangers associated with the current lack of routine and comprehensive environmental and food monitoring programs within the region, especially at a time in which the use of locally-sourced wood for biomass energy generation is set to expand markedly.

Effects of radiation on radial growth of Scots pine in areas highly affected by the Chernobyl accident.

The effects of radiation on radial growth of Scots pine (Pinus sylvestris L.) affected by the Chernobyl accident were studied at five sites with different deposition levels. The study sites were chosen along a high gradient of depositions at the distance of a few hundred meters in the forests with similar forest characteristics. Air dose rates varied at the sites from the background values up to 30 µGy h-1 as of December 1, 2016. Scots pine trees exposed to sub-lethal doses of 8.6-13.2 Gy at the "Red Forest" site did not demonstrate deviations in formation of annual rings 30 years after the Chernobyl accident compared to trees with lower doses and control trees. Variation with time in annual rings thickness did reveal that the effect of radiation in trees growing at the sites with different contamination not detectable in 1986 and 1987. Conversely, the effects were clear observed in a later period in 1989-1991, i.e. 3-5 years after the accident. Until 2000, there were statistically significant differences in the annual rings growth rates of Scots pine trees exposed to external absorbed dose rates of 4.4-6.7 mGy h-1 as estimated for June 1, 1986 (or 19.5-30.0 µGy h-1 as of December 1, 2016) compared with the trees of the other sites studied. The results comply with the conclusions from research with acute pine exposure conducted in the Kyshtym area in 1975-1984.