Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum 137Cs concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem 137Cs concentrations will not be affected by root uptake if active root systems occur 10 cm below the soil.

The vertical migration of 90Sr and 117Cs produced by the explosion of the atomic bomb in 1945 was investigated in an unsaturated soil layer in the Nishiyama area of Nagasaki. The in situ migration rates of 90Sr and 137Cs were estimated to be 4.2 mm yr−1 and 1.0 mm yr−1, respectively, when the rate of movement of soil water was 2500 mm yr−1. The in situ Kd values of 90Sr and 137Cs were calculated to be 0.3 m3 kg−1 and 1.2 m3 kg−1, respectively. These are probably the only results that exist for the interaction between soil and 137Cs and 90Sr over 40 yr. These results suggest that since fallout 137Cs and 90Sr have remained in the surface soil for a long period of time, we should pay significant attention to radiological effects of nuclear accidents such as the Chernobyl disaster.

Cs-134, Sr-85, and I-131 were produced by neutron irradiation of CsCl, SrCl 2 , and K 2 TeO 3 , respectively, using the Kyoto University Reactor (KUR). These radioactive nuclides were added to river water and seawater to prepare artificially contaminated samples, and the removal of these nuclides using bentonite, zeolite, and activated carbon was then investigated. In the river water samples, Cs-134 and Sr-85 were successfully removed using bentonite and zeolite, and I-131 was removed using activated carbon. In the seawater samples, Cs-134 was removed using bentonite and zeolite, whereas Sr-85 and I-131 were hardly removed at all by these adsorbents.

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