Determining the initial Fukushima reactor accident-derived cesium-137 fallout in forested areas of municipalities in Fukushima Prefecture

This study determined the initial distribution of Fukushima reactor-derived radiocesium fallout in forest areas of Fukushima Prefecture based on analysis of airborne monitoring surveys by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). The results of the fifth airborne monitoring surveys were compared with the third airborne survey data in order to correct for variation of deposition density due to multiple factors such as weathering processes and measurement uncertainties between two surveys. Finally, the results of the third and the corrected fifth airborne surveys were combined to reconstruct the initial radiocesium fallout map over the whole Fukushima Prefecture area following the Fukushima reactor accident. Our calculation results showed that forest areas accumulated 72% of the total atmospheric input of Cs-137 to the land of Fukushima Prefecture. The deposition density in forest areas showed significant variability among municipalities. Nevertheless, forest areas accumulated a large percentage of atmospherically deposited radiocesium in many municipalities. Statistical analysis of the variability in Cs-137 deposition density indicated that deposition density varied significantly, even within a municipality. Furthermore, the inconsistency in the probability distributions of measured deposition density among municipalities makes it difficult to obtain a representative initial radiocesium fallout input following the accident. To reduce uncertainty in environmental parameters related to radiocesium transfer in forest environments, we strongly recommend that the deposition density of initial Cs-137 fallout at each location is determined by in-situ sampling survey and using the reconstructed Cs-137 fallout map presented in this study.