Effects of chronic radiation exposure on small mammals in the Chernobyl exclusion zone

The Chernobyl reactor accident in April 1986 caused the release to atmosphere of some 2 x 10(6) TBq, or more, of total fission/activation products. Estimates of deposition vary, although it is probable that about half the activity was deposited within 20 km of the release point, predominantly following two plume trajectories to the north and west. This resulted in the death of pine trees over 400 ha, the abandonment of up to 150 000 ha of agricultural land and the establishment of an exclusion zone extending to 30 km from the site. High levels of radionuclide contamination continue to prevail within the exclusion zone. Nonetheless, recolonisation has been widespread. Mixed deciduous woodlands, with a high proportion of birch (Betula spp.) and willow (Salix spp.), have become established in the forest areas, while agricultural land has succeeded to tall grassland and scrub. Field sites investigated in this study, during 2001 2003, exhibited external gamma dose rates varying from 0.1 muSv h(-1) to 140 muSv h(-1). Corresponding mean concentrations of (CS)-C-137 in the top 20 cm of soil varied from about 6 x 10(2) to 3 x 10(6) Bq kg(-1) dw (dry weight). Little impact is evident on populations of small mammals in these areas, either for species diversity or overall abundance, although there is a slight (not statistically significant) trend for increasing spleen weight in the bank vole with increasing levels of contamination. Previous suggestions that populations contain a preponderance of juveniles and sub-adults at the highest contaminated sites are not supported.