Anthropogenic radionuclides and heavy metals in black poplar tree (Populus nigra l.) bark sampled in one of the residential districts of Kyiv

Tree bark is known to be a good alternative biological substrate that can be successfully used in the air pollution monitoring studies, especially in urban and industrialized areas suffering from the severe anthropogenic pressure. In Kyiv black poplar is a widespread tree species, whose bark was used as a biological indicator in our research. The bark samples were collected within one of the residential districts of Kyiv and were subject to comprehensive analysis for the content of stable elements and anthropogenic radionuclides. Thermal and epicadmium NAA in short- and long-term irradiation modes, respectively, were used for the determination of concentrations of up to 40 heavy metals, while gamma spectrometry, alpha spectrometry and radiochemical extraction-ion-exchange techniques were applied to determine137Cs, 90Sr, Pu and Am radioactive isotopes in single bark samples. The analytical data obtained were subject to correlation and factor analysis, which revealed basic air pollution sources in the investigated region. It was shown that no significant correlations exist between radionuclides and any determined stable elements in the analyzed samples. All measured radioactive isotopes turned out to fall into a separate factor, which is believed to present the direct deposition of fuel microparticles from the Chernobyl NPP's Unit 4 from the atmosphere into the substratum during radioactive fallouts in spring 1986. This conclusion was supported by the evaluated isotopic ratios 137Cs/90Sr = 1.1 ± 0.4, 137Cs/239+240Pu = 100 ± 40, 239+240Pu/238Pu = 1.0 ± 0.6, as well as by the observed significant variation of the radionuclide concentrations (e.g. 10 Bq/kg - 1540 Bq/kg for 137Cs, 0.1 Bq/kg - 21 Bq/kg for 238,240Pu), which is believed to reflect a microparticle character of the pollution. The obtained data suggest that re-suspension does not play a significant role in the formation of atmospheric air pollution by radioactive substances in the investigated region. (author)