Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident 137Cs fallout

In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs-137. However, Cs-137 transfer processes determining tree contamination (particularly for stem wood, a prominent commercial resource) remain insufficiently understood. We propose a model for simulating dynamic behavior of Cs-137 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-137 transfer processes determining the tree contamination. The transfer of Cs-137 to the trees occurred mostly (>99%) through surface uptake of Cs-137 trapped by needles and bark during the fallout. Root uptake of soil Cs-137 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-137 absorbed on the tree surface. A significant surface uptake of Cs-137 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, although that pathway still needs to be evaluated by experimental evidence. It was suggested that the activity concentration of Cs-137 in stem wood of the trees at these sites are currently (as of 2021) decreasing by similar to 3% per year, mainly through radioactive decay of Cs-137 and partly through dilution effect from tree growth. Although further refinement of the model is recommended, for example by including tree species specific Cs-137 transportation in stem, these findings provide vital information for planning of forestry reactivation in Fukushima; e.g., removal of forest floor organic layer will not reduce the tree contamination for a long term because of the Cs-137 absorption via the tree surface substantially greater than root uptake of Cs-137 deposited to the floor. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

Cs-137 contamination in forests affected by the Fukushima Daiichi Nuclear Power Plant accident is primarily determined by surface uptake through needles and bark, with root uptake playing a negligible role. The internal contamination of trees occurs through an enduring recycling of Cs-137 absorbed on the tree surface. The activity concentration of Cs-137 in stem wood of the trees is decreasing by approximately 3% per year, mainly due to radioactive decay and partly due to dilution effect from tree growth. These findings provide crucial information for forestry reactivation in Fukushima.