In this study, the 137Cs distribution in the wood of Japanese
56 H. Ogawa et al. / Journal of Environmental Radioactivity 161 (2016) 51e57
cedars caused by FDNP accident was examined. In May 2012, 10
trees in the Kawamata area were felled and the radial and vertical
137Cs distribution was measured. Distributions differed according
to the sampling height and depended particularly on the distance
from the treetop. These 137Cs distribution differed from that of the
equilibrium state in the wood at the steady phase and were thought
to be in the transitional process during the early phase.
The 137Cs distribution changes in the wood of Japanese cedars
were studied in the same forest from 2011 to 2013 in the Yamakiya
area. The 137Cs distribution profiles in the wood changed considerably
from 2011 to 2013 and approached to the equilibrium state
in 2013. These changes indicated that 137Cs transported activity
from sapwood to heartwood and it diffused not only radially but
also vertically in heartwood. The active transportation from
sapwood and the diffusion in heartwood was proceeded according
to the vertical position from 2011 to 2013.
The process of 137Cs distribution change in the wood of Japanese
cedars during the early phase was as follows:
(1) Just after the fallout, 137Cs was transported from sapwood to
heartwood, and diffused radially in heartwood. These process
occurred particularly from 2011 to 2012, but the progress
differed according to the vertical position
(2) As the 137Cs concentration in heartwood increased, the radial
transport from sapwood and radial diffusion in heartwood of
137Cs was reduced, and the vertical diffusion in heartwood
was relatively increased from 2012 to 2013, because of the
gradient of 137Cs concentration from treetop to ground.
(3) Because of the vertical 137Cs diffusion in heartwood, the
radial 137Cs distributions were nearly identical at any vertical
positions and approached the equilibrium state in 2013.
In Japanese cedar, these processes progressed within 3 years
after the FNDP accident. Progress might differ according to species,
but the 137Cs transportation process in the wood might be similar
for all species. These results may allow for the prediction of future
contamination level of trees, which is important for management
in the contaminated forest in Fukushima.