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

Metal Uptake by Birches and Scots Pines Grown on a Porcelain Landfill

Potentially toxic elements (PTEs) have steadily become a serious environmental problem, especially regarding brownfields chosen for reuse, e.g., as a residential area. "Norra Hamnstaden" in Lidköping (Sweden) has a long history of industrial activity, including porcelain production with the resultant industrial waste deposited close by resulting in elevated levels of metals used for porcelain glazes, especially lead. To estimate the bioavailability of twelve PTEs (As, Ba, Pb, Cd, Co, Cu, Cr, Mn, Mo, Ni, V, Zn), their uptake by birches (Betula pendula) as well as Scots pines (Pinus sylvestris) was investigated through analyzing their leaves. Sampling was carried out on five trees once per month in the period from May to August. Different uptake patterns were observed for birches and pines, for the latter even varying with age. The birch samples showed higher contents of nickel, cobalt, molybdenum, and lead compared to the reference trees. Also, the pine needles had elevated lead levels, although by a lower factor. Birch leaves revealed surprising patterns of elevated element bioaccumulation factors, with barium reaching up to eight, offering the possibility to limit analyses to plant material for risk assessments instead of soil analysis.

90Sr and 137Cs distribution in Chornobyl forests: 30 years after the nuclear accident

The primary aim of this study was to quantify patterns in the distribution of 90Sr and 137Cs activity in pine (Pinus sylvestris L.: 18 sites) and birch (Betula pendula Roth.: 2 sites) forests within the Chornobyl exclusion zone, 30 years after the Chornobyl nuclear power plant (NPP) accident (1986). To achieve this, radionuclide activity concentrations were measured in the mineral and organic soil horizons, the green forest floor (grasses, mosses, and lichens), and trunk wood in forest plots dominated by either pine or birch trees.Our results showed that the geometric mean of the 90Sr aggregated transfer factors from mineral soil to the trunk wood of pine trees (Tag) for Scots pines was 24 x 10-3 m2 kg-1, which is an order of magnitude higher than the IAEA Handbook (2010)reference value (1.7 x 10-3 m2 kg-1), which were based on studies conducted after the Kyshtym accident (Ural region, Russia) and in the first years following the Chornobyl accident (Ukraine and Belarus). The observations suggest that the above-ground biomass (soil organic layers, green forest floor, and trees) may contribute more to the 90Sr inventory than the mineral soil at the stand level. In contrast, the 137Cs Tag values for pine and birch stands were consistent with those reported in the literature (ranging from 0.1 to 10 and 0.5-1.1 x 10-3 m2 kg-1, respectively). Both results align with the known bioavailability of radionuclides from previous studies: low for 137Cs, leading to limited soil depth migration (less than 30 cm in the mineral horizon), and higher for 90Sr, resulting in greater soil migration (up to 1 m in the mineral horizon). This study highlighted significant correlations between the radionuclides' activity concentrations in the litter layers and their content in the trunk wood of pine trees.

Effects of Large-Scale Wildfires on the Redistribution of Radionuclides in the Chornobyl River System

Wildfires in radiologically contaminated areas raise significant concerns due to potential radionuclides redistribution and increased public radiation exposure. This study examined the impact of the 2020 Chornobyl wildfire on the redistribution of radionuclides, specifically 137Cs and 90Sr, in the Chornobyl River system. We determined the quantities and speciation of 137Cs and 90Sr in charred residues and soil after wildfires and analyzed the riverine concentrations of these radionuclides based on long-term monitoring data. Our findings indicate that the inventories of 137Cs and 90Sr in the charred residues and soil decreased with increasing distance from the nuclear power plant, which is consistent with the initial deposition patterns. However, the transfer of 137Cs and 90Sr from soil to charred residues did not correspond to the distance, type of contamination source, or fire type. Speciation analysis revealed that the water-soluble fractions of 137Cs and 90Sr in the charred residues were significantly higher than those in the soil, implying increased mobility. Following the wildfires, no significant increase in 137Cs concentration was observed in a river catchment in Chornobyl. However, 90Sr concentrations showed a significant increase, exceeding the permissible levels in drinking water (2 Bq/L) in Ukraine. This increase is attributed to hydrologically driven mobilization processes: (1) during snowmelt in spring and (2) the transport of soluble 90Sr from charred residues and surface soil into the river during high suspended solid concentration events. Collectively, our findings highlight the importance of continuous monitoring of radionuclide dynamics in postwildfire environments to better assess potential radionuclide redistribution and radiation exposure risks. These results provide valuable insights into the behavior of 137Cs and 90Sr in river systems affected by wildfires, contributing to a more accurate understanding of their environmental impacts and potential countermeasures.

Long-term behaviour of Cs-137, Cs-133 and K in beech trees of French forests

In the long-term after atmospheric deposit onto a forest ecosystem, Cs-137 becomes incorporated into the biogeochemical cycle of stable elements and progressively reaches a quasi-equilibrium state. This study aimed at determining to what extent Cs-137 activity distribution in tree vegetation could be predicted from that of stable caesium (Cs-133) and potassium (K), which are known to be stable chemical analogues and competitors for Cs-137 intake in tree organs. Field campaigns that focused on beech trees (Fagus sylvatica L.) were conducted in 2021 in three French forest stands with contrasted characteristics regarding either the contribution of global vs. Chornobyl fallouts, soil or climatic conditions. Decades after Cs-137 fallouts, it was found that more than 80% of the total radioactive inventory in the system remained confined in the top 20 cm mineral layers, while organic layers and beech vegetation (including roots) contributed each to less than 1.5%. The enhanced downward migration of Cs-137 in cambisol than podzol forest sites was presumably due to migration of clay particles and bioturbation. The distribution of Cs-137 and Cs-133 inventories in beech trees was very similar among sites but differed from that of K due a higher accumulation of Cs isotopes in roots (40-50% vs. < 25% for K). The aggregated transfer factor (Tag) of Cs-137 calculated for aerial beech organs were all lower than those reported in literature more than 20 years ago, this suggesting a decrease of bioavailability in soil due to ageing processes. Regarding their variability, Tags were generally lower by a factor 5 at the cambisol site, which was fairly well explained by a much higher value of RIP (radiocesium immobilisation potential). Cs-137 concentrations in trees organs normalized by the soil exchangeable fractions were linearly correlated to those of Cs-133 and the best fit was found for the linear regression model without intercept indicating that no more contribution of the foliar uptake could be observed on long term. Provided that the vertical distribution of caesium concentrations and fine root density are properly measured or estimated, Cs-133 was shown to be a much better proxy than K to estimate the root transfer of Cs-137.

Long-term changes in 90Sr pools of Scots pine biomass in the Chornobyl Red Forest

The trenches of the waste burial site in the Chornobyl Red Forest represent a big reservoir of radionuclides for the artificial plantation of Scots pine established in that area, but the long term dynamics of tree biomass contamination, especially with 90Sr, remains unclear. The present study was conducted between 2005 and 2018 on two groups of trees of the same age. The IN group is represented by trees growing on the trench containing highly radioactive contaminated fertile soil and organic matter, while the OUT group is located outside the trench. Within a little more than one decade, the total aboveground biomass doubled in the trees of the group OUT and increased more than four times in the group IN. In the group OUT, the concentrations of 90Sr have decreased in all biomass compartments compared to 2005, while in the group IN, the concentrations demonstrated a trend to increase. Regression analysis shows that both decrease in the compartment concentrations in the group OUT (slope coefficient 0.55) and increase in the group IN (1.58) were significant. As a result of the changes in the biomass inventories and 90Sr concentrations, in absence of changes in plantation density, the contamination of total aboveground biomass by 90Sr in the group OUT would have increased slightly in 2018 (from approximately 18 GBq ha-1 to 23 GBq ha-1) compared to 2005, while in the group IN it would have increased almost 6-fold, reaching approximately 560 GBq ha-1, or about (19 ± 9) % of the total 90Sr inventory in the trench area. Trenches of the Red Forest were shown to act as long-lasting hot spots of 90Sr bioavailability for forest trees.