Variability of activity concentrations and radial distributions of Cs-137 and( 90)Sr 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 x 10-4 m2 kg- 1 and 8.8 x 10-3 m2 kg- 1 for pine, respectively, and 9.3 x 10-4 m2 kg- 1 and 1.1 x 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 demon-strated an increasing trend from the trunk periphery towards the pith, while concentrations of 90Sr were rela-tively 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.