Variation in transfer factor of radiocaesium in bank voles (Clethrionomys glareolus) in clear cut and mature forest sites after the Chernobyl accident

Bank voles that were collected between 1986 and 2004 at sites in Chernobyl fallout areas of northern Sweden showed higher 137Cs activity concentrations at the mature forest sites compared to clear cuts. This difference was not attributed to differences in ground deposition between sites but to differences in aggregated transfer rates to voles. Differences in transfer between forest types were evident for all years 1986–2004 but the change occurred at different rates in the two habitats. The apparent transfer factor between bilberry (Vaccinium myrtillus) and voles was positively related and indicated that a biomagnification was about 1.5 from vegetation to these small mammalian herbivores. The aggregated transfer factor to bank voles measured in the forest habitat, although starting at higher levels declined faster with time than clear cut sites and the differences between the forest habitat and the clear cut areas diminished with time. After the Chernobyl accident in 1986 the mean level in bank vole was 514 Bq/kg fresh mass (SD = 505) that increased to 1485 Bq/kg (SD = 881) in 1988. The activity concentration declined thereafter. The bank voles collected in similar habitats in 2004 contained on average 1022 Bq/kg (SD = 723). Still 18 years after the radionuclide fallout over Sweden high activity concentrations in voles could be found.