Migration of radionuclides in the system 'soil-tree'

A growing tree changes the radionuclide distribution in soil under foliage. The highest 137Cs activity concentration in soil under the pine foliage is determined in the zone nearest to the trunk. Pine timber samples were prepared according to annual tree rings and activity concentrations of an artificial radioisotope 137Cs and natural radioisotopes 226Ra, 233Th and 40K were defined. It is determined that the 137Cs activity concentration in pine timber varies in the course of time. But the greatest pollution caused by this radioisotope and the highest activity concentrations are induced only after 10 years. In its penetration from soil into timber 137Cs competes with natural radionuclides, especially with 40K. But the ratio of activity concentrations of 137Cs and 40K in timber is 2.2 times larger than that in soil (except in a soil layer of 0-5 cm). The most effective penetration of 137Cs into pine timber is from a soil depth of 15-20 cm. It can be related to a significantly increased pine root mass at this soil depth. Activity concentrations of radioisotopes in pine needles and in the bark is by more than one order of magnitude higher than those in timber because they are caused not only by radioisotope transfer from soil through roots but also by pollution directly from the air. (author)