New predictions of ¹³⁷Cs 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 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.