Development of a new model simulating contaminations of forest resources with radiocesium in Fukushima

Ota, Masakazu  ; Koarashi, Jun   

In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs. However, Cs transfer processes determining the tree contamination (particularly for stem wood, a prominent commercial resource) remain insufficiently understood. We propose a model (SOLVEG-R) for simulating dynamic behavior of Cs in a forest tree-litter-soil system and applied it to contaminated forests of cedar plantation and natural oak stand in Fukushima to elucidate relative impact of distinct Cs transfer processes determining the tree contamination. The transfer of Cs to the trees occurred mostly (99%) through surface uptake of Cs trapped by needles and bark during the fallout. Root uptake of soil Cs was several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, internal contamination of the trees proceeded through an enduring recycling (translocation) of Cs absorbed on the tree surface. A significant surface uptake of Cs through bark was suggested, contributing to 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining uptake occurred at needles) of the total uptake by the trees. It was suggested that the activity concentration of Cs in stem wood of the trees at these sites are currently (as of 2021) decreasing by 3% per year, mainly through radioactive decay of Cs and partly through dilution effect from tree growth.