DOI: 10.5285/60782622-7bfa-4615-a9e3-0a802a9f4674
title: Soil and vegetation radionuclide activity concentrations and calculated dose rates from the Red Forest, Chernobyl, Ukraine, 2016-2017

description: Data comprise plot details and radionuclide activity concentrations for Sr-90, Cs-137, Am-241, Pu-238, Pu-239 and Pu-240 in ‘grassy’ vegetation and soil. These radionuclide activity concentrations have been used to make estimations of total weighted absorbed doses to grassy vegetation, deciduous trees and bacteria; no dose rate estimates for grassy vegetation have been made for those sites where grassy vegetation was absent.

Radiation from the 1986 Chernobyl nuclear power plant accident killed coniferous trees in a 4-6 km2 area of forest to the west of the power plant. This area is now known as the 'Red Forest’ and it has subsequently regenerated with understorey vegetation and deciduous trees; it is the most anthropogenically contaminated radioactive ecosystem on Earth. In July 2016 a severe fire burnt (to varying degrees) c. 80 percent of the Red Forest; this presented a unique opportunity to study the impact of radiation on the recovery of forest ecosystems exposed to a secondary stressor (fire). To investigate this, in September 2017 the RED FIRE project set up sixty study plots in the Red Forest (in burnt and unburnt areas) with a further nine plots established close to Buriakivka village (approximately 8 km from the Red Forest).

Vegetation samples from each plot were harvested using shears in September 2017. Each sample was sorted into ‘grassy’ and ‘other’ vegetation; these were air-dried (20-25 degrees Celsius) and the grassy vegetation samples homogenised prior to radionuclide analyses.

Soil core samples collected in September 2017 were bulked, homogenised and sub-samples taken for determination of pH and percentage moisture determined by oven drying (approximately 60 degrees Celsius) to a constant mass. The remaining soil sample was used for the determination of radionuclide activity concentrations; prior to analyses, these samples were dried at approximately 80 degrees Celsius.

This work was funded by the NERC, Grant Ref: NE/P015212/1 (RED FIRE: Radioactive Environment Damaged by fire: a Forest In Recovery)
description: Cs-137 activity concentrations were determined in grassy vegetation and soil using a high-purity germanium γ-detector (Model GC3019, Canberra-Packard, Connecticut, USA). A standard 137Cs/152Eu source (OISN-1; Applied Ecology Laboratory of Environmental Safety Centre, Odessa, Ukraine) was used for calibration. The minimal detectable activity was approximately 0.18 Bq per sample with uncertainties of approximately 10-15 percent (p=0.95) depending on the sample type. 

Sr-90 activity concentrations were determined in soil and grassy vegetation samples using a β-spectrometer (EXPRESS-01) with a thin-filmed (0.1 mm) plastic scintillator detector. No radiochemical pre-treatment was required and samples were analysed as a solid matrix. Daily calibrations were conducted. Spectra were processed by correlating the sample spectra with those obtained from standard sources and those obtained from background measurements; uncertainties were approximately 20 percent (p=0.95).
Prior to processing to determine activity concentrations of 241Am and Pu isotopes in soil samples, 242Pu and 243Am were added as yield tracers. Samples were dissolved in HF followed by treatment with HNO3, HCl, H3BO3+HNO3 and 8M HNO3. Plutonium and Am were separated using anion exchange resin (Bio Rad AG 1 × 8, 100-200 mesh). The Pu fraction was evaporated. Americium was precipitated with calcium oxalate and then separated using TRU resin columns; lanthanides were removed using an anion exchange resin column. Thin alpha sources of each separated actinide element were prepared by micro co-precipitation with neodymium fluoride and analysed using a Canberra Alpha Analyst alpha spectrometer. Counting errors were typically <20 percent.

To estimate the activity concentrations of Am-241 and Pu-isotopes for grassy vegetation transfer parameters (CRwo-soil) for Agrostis gigantea from a site within the Red Forest were used (Beresford et al. 2020; Table 4) together with the appropriate measured soil activity concentration (DM). To estimate Sr-90 and Cs-137 activity concentrations in deciduous trees CRwo-soil values from sites within the CEZ were used (Holiaka et al. 2020) together with the measured soil activity concentrations (dry mass); for Am-241 and Pu-isotopes CRwo-soil values for Pinus sylvestris (wood) from a site within the Red Forest were used (Beresford et al. 2020; Table 4).

Weighted absorbed dose rate estimations for grassy vegetation and deciduous tree wood were made using the ERICA Tool http://www.erica-tool.com/ (version 1.3) and the revised ‘R&D128’ spreadsheet model (https://wiki.ceh.ac.uk/display/rpemain/Ar+-+Kr+-+Xe+dose+calculator) was used to estimate the weighted absorbed dose to bacteria. The ambient dose rate (µSv h-1) on the soil surface was measured at each study plot using a MKS-01R meter.