Abstract
Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest. In this paper, we updated model predictions of temporal changes in the 137Cs dynamics using the latest observation data and newly provided maps of the predicted 137Cs activity concentration for wood, which is the most commercially important part of the tree body. Overall, the previous prediction and latest observation data were in very good agreement. However, further validation revealed that the migration from the soil surface organic layer to the mineral soil was overestimated for evergreen needleleaf forests. The new prediction of the 137Cs inventory showed that although the 137Cs distribution within forests differed among forest types in the first 5 years, the difference diminished in the later phase. Besides, the prediction of the wood 137Cs activity concentrations reproduced the different trends of the 137Cs activity concentrations for cedar, oak, and pine trees. Our simulation suggests that the changes of the wood 137Cs activity concentration over time will slow down after 5–10 years. Although the model uncertainty should be considered and monitoring and model updating must continue, the study provides helpful information on the 137Cs dynamics within forest ecosystems and the changes in wood contamination.
Highlights
Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest
The overall 137Cs dynamics seen in the observation data indicate that the 137Cs captured in the tree canopy and soil surface organic layer migrated to the mineral soil in the first 5 years, and the mineral soil serves as the largest 137Cs reservoir
The comparison between the latest observation data and previous predictions showed that the simulation of the inventory was successful in predicting the dynamics of 137Cs in forest ecosystems, for deciduous broadleaf forests[10]
Summary
Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest. Forests represent the major land use of the area contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident[1], covering approximately 70% of the area. At the time of the fallout, needles/leaves, branches, and stems trapped 137Cs, and a part of the fallout was deposited directly on the soil surface[2,3] These distributions drastically change with time, within a couple of years[3]. The dynamics of 137Cs within forests represent information that is important because it captures the situation of the forest contamination and clarifies our understanding and evaluation of the changes in the ambient dose rate in forests This critical information can be provided to local peoples and to authorities of forest management so that they can implement effective countermeasures. Time (year) concentration in wood both currently and in the future are of great concern for people in terms of external and internal exposure risks and the regulation of wood use
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