Transfer of the Fukushima accident-derived radiocesium by hydrological processes in Japanese forests

In Japan's forests, field data on the distribution and migration of radiocesium deposited by the Fukushima Daiichi Nuclear Power Plant accident, from the initial dynamics to the distribution in the forest over ten years, have been vigorously collected. The results have been published in an IAEA technical report (IAEA- TECDOC-1927, 2020) as a database of migration parameters to be compiled by an international project (MODARIA II: Modelling and Data for Radiological Impact Assessments) promoted by the International Atomic Energy Agency (IAEA). Radiocesium's transfer mechanisms in the forest by hydrological processes and the runoff through the water system were summarized based on investigations in experimental forests and watersheds in Fukushima Prefecture. Besides, we outlined the missing links that need to be clarified by re-analysis of existing data and additional experiments based on previous studies on Fukushima and Chernobyl. Finally, we discussed the direction of future monitoring surveys.