Dynamics of 137Cs in the forests of the 30-km zone around the Chernobyl nuclear power plant

Dynamics of the 137Cs content in the components of the forests in the 30-km zone around the Chernobyl nuclear power plant (NPP) in 1986–1994 are associated mainly with such factors as the size of radioactive particles in the fallout, ecosystem humidification and soil type, tree age. The influence of particle size was especially noticable between 1986–1987 and was displayed by low biological availability of radionuclides in the near part of the zone (within the 10-km radius circle around the NPP) in comparison with more distant regions (within the 30-km radius circle). Later, the expression of this influence decreased and transfer factor (the ratio of 137Cs content in overground phytomass to the soil contamination density) became approximately the same for all plots with similar ecological and fallout characteristics. Humidity of landseape and soil type determined the velocity of radionuclide vertical migration in the soil and 137Cs biological availability. These parameters were maximum for the hydromorphic soils of wet landscapes enriched in organic substance and poor clayey minerals. Differences of 137Cs accumulation in overground phytomass of trees caused by tree age are displayed in the higher 137Cs concentration in structural parts of young trees as compared with old ones.