New predictions of (137)Cs dynamics in forests after the Fukushima nuclear accident

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 (137)Cs dynamics using the latest observation data and newly provided maps of the predicted (137)Cs 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 (137)Cs inventory showed that although the (137)Cs 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 (137)Cs activity concentrations reproduced the different trends of the (137)Cs activity concentrations for cedar, oak, and pine trees. Our simulation suggests that the changes of the wood (137)Cs 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 (137)Cs dynamics within forest ecosystems and the changes in wood contamination.