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Accelerated radiocesium leaching from forest floor litter by heavy rainfall

Radioactive materials including 137Cs (cesium-137, half-life: 30.1 years) were released into the environment following the accident at Fukushima Daiichi Nuclear Power Plant. It has been about 10 years since the accident, but 137Cs remains in the environment, especially in forests. Many researchers have been studying the dynamics and transport processes of radioactive materials in the environment. It has been found that radioactive materials are carried along with the transfer of water and sediment. With the focus on the forested headwaters where radioactive materials remain in large quantities, it has been reported that the concentration of dissolved radiocesium in stream water increases during heavy rainfall.

Since rainwater does not contain radioactive cesium, the research group led by Assistant Professor Koichi Sakakibara of Shinshu University’s Faculty of Science was curious why the concentration of radioactive cesium in stream water increased during heavy rainfall without becoming diluted. The research team thought that radioactive cesium might have leached out from the forest litter and conducted leaching tests. They found that a large amount of radioactive cesium leached from such forest litter.

The next step was to ask the question, “Why does more radioactive cesium leach out of forest litter during heavy rainfall, when forest litter is still on the forest floor when it is not raining? (Background information: Most of the rainwater that falls on forests infiltrates into the subsurface area. The main reason for the increase in stream water volume during rainfall in forests is the discharge of groundwater. The groundwater contains almost no radioactive cesium.) So the research group set out to solve the mystery, “How is litter-derived radiocesium added to stream water during rainstorm?”

In contrast to the rainfall-runoff process, which is often focused only on rainfall and runoff, this study focused on the conversion process from rainfall to runoff, such as the variation of groundwater table level, the generation of saturated surface area at the bottom of the valley, and the variation of water quality and water age during rainfall. As a result, the answer to the problem to be solved in this study is that the main factor is the expansion of the contact area between water and litter due to the expansion of the saturated surface area caused by the rise of the groundwater table level in the forested headwater. Although previous research tended to focus only on the cause (rainfall) and the effect (runoff), Assistant Professor Sakakibara states, “we showed that the breakthrough to solve the unexplained reason lies in why the cause (rainfall) is converted into the effect (runoff).”

Uncertainty of results is inevitable when researching in the natural environment. How do results differ when the study is conducted at different times and places? How much error is there in the results due to the heterogeneity of the acquired samples from the environment? These are some of the questions that need to be answered. In the present study, the following questions were asked in-depth: 1) whether the same conclusions can be drawn for forests other than the target forest, 2) whether the samples collected for the study are representative of the Fukushima region, and 3) whether the results are affected by differences in the timing of litter falling from the trees and the degree of decomposition. Sakakibara says, “the most difficult part was to come up with a clear answer or idea to these uncertainties.”

Assistant Professor Sakakibara says, “the state and transport of radioactive materials in the environment are complex and need to be studied long-term. The half-life of 137Cs is 30 years. The results of this study only partly clarified this issue. Rivers that discharge from the forest area flow downstream to the ocean. We would like to clarify the whole picture of the pathway and process of radioactive materials originating from forests in the hydrological process from the headwater to the ocean. We believe that these findings are essential for creating a safe and secure environment and sustainable future and livelihood.”

The research was published in Science of The Total Environment.

Explore further

Dynamics of radiocesium in forests after the Fukushima disaster: Concerns and some hope

More information: Koichi Sakakibara et al, Radiocesium leaching from litter during rainstorms in the Fukushima broadleaf forest, Science of The Total Environment (2021). DOI: 10.1016/j.scitotenv.2021.148929

August 8, 2021 Posted by | Fukushima 2021 | , , | Leave a comment

A message from Forest Measurement Laboratory in Namegawa

March 6, 2021

A message from a representative of the Forest Measurement Laboratory, a group that measures radioactivity in Saitama Prefecture, just north of Tokyo. It was founded in the fall of 2012 mainly by mothers after the Fukushima nuclear disaster.

They thought that measurements by municipalities were not sufficient to protect their children from radiation exposure, so they started this project by themselves.

March 23, 2021 Posted by | Fukushima 2021 | , | Leave a comment

Dynamics of radiocesium in forests after the Fukushima disaster: Concerns and some hope

Dynamics of radiocesium in forests after the Fukushima disaster: Concerns and some hope

80% of the Fukushima prefecture are mountain forests.

February 3, 2021

Considering the massive threat posed by 137Cs to the health of both humans and ecosystems, it is essential to understand how it has distributed and how much of it still lingers.

w/reminder: there’s no such thing as ‘radioactive decontamination’ the correct term would be ‘trans-contamination’

Scientists compile available data and analyses on the flow of radionuclides to gain a more holistic understanding

Forestry and Forest Products Research Institute

After the Chernobyl disaster of 1986, the 2011 Fukushima Daiichi nuclear power plant (FDNPP) disaster was the second worst nuclear incident in history. Its consequences were tremendous for the Japanese people and now, almost a decade later, they can still be felt both there and in the rest of the world. One of the main consequences of the event is the release of large amounts of cesium-137 (137Cs)–a radioactive “isotope” of cesium–into the atmosphere, which spread farther away from the power plant through wind and rainfall.

Considering the massive threat posed by 137Cs to the health of both humans and ecosystems, it is essential to understand how it has distributed and how much of it still lingers. This is why the International Atomic Energy Agency (IAEA) has recently published a technical document on this specific issue. The fifth chapter of this “Technical Document (TECDOC),” titled “Forest ecosystems,” contains an extensive review and analysis of existing data on 137Cs levels in Fukushima prefecture’s forests following the FDNPP disaster.

The chapter is based on an extensive study led by Assoc. Prof. Shoji Hashimoto from the Forestry and Forestry Products Research Institute, Japan, alongside Dr. Hiroaki Kato from the University of Tsukuba, Japan, Kazuya Nishina from the National Institute of Environmental Studies, Japan, Keiko Tagami from the National Institutes for Quantum and Radiological Science and Technology, Japan, George Shaw from the University of Nottingham, UK, and Yves Thiry from the National Agency for Radioactive Waste Management (ANDRA), France, and several other experts in Japan and Europe.

The main objective of the researchers was to gain a better understanding of the dynamics of 137Cs flow in forests. The process is far from straightforward, as there are multiple elements and variables to consider. First, a portion of 137Cs-containing rainfall is intercepted by trees, some of which is absorbed, and the rest eventually washes down onto the forest floor. There, a fraction of the radiocesium absorbs into forest litter and the remainder flows into the various soil and mineral layers below. Finally, trees, other plants, and mushrooms incorporate 137Cs through their roots and mycelia, respectively, ultimately making it both into edible products harvested from Fukushima and wild animals.

Considering the complexity of 137Cs flux dynamics, a huge number of field surveys and gatherings of varied data had to be conducted, as well as subsequent theoretical and statistical analyses. Fortunately, the response from the government and academia was considerably faster and more thorough after the FDNPP disaster than in the Chernobyl disaster, as Hashimoto explains: “After the Chernobyl accidents, studies were very limited due to the scarce information provided by the Soviet Union. In contrast, the timely studies in Fukushima have allowed us to capture the early phases of 137Cs flow dynamics; this allowed us to provide the first wholistic understanding of this process in forests in Fukushima.”

Understanding how long radionuclides like 137Cs can remain in ecosystems and how far they can spread is essential to implement policies to protect people from radiation in Fukushima-sourced food and wood. In addition, the article also explores the effectiveness of using potassium-containing fertilizers to prevent the uptake of 137Cs in plants. “The compilation of data, parameters, and analyses we present in our chapter will be helpful for forest remediation both in Japan and the rest of the world,” remarks Hashimoto.

When preventive measures fail, the only remaining option is trying to fix the damage done–in the case of radiation control, this is only possible with a comprehensive understanding of the interplay of factors involved.

In this manner, this new chapter will hopefully lead to both timely research and more effective solutions should a nuclear disaster happen again.



Title: Environmental Transfer of Radionuclides in Japan following the Accident at the Fukushima Daiichi Nuclear Power Plant, Chapter 5 “Forest ecosystems”

Published in: International Atomic Energy Agency, IAEA TECDOC no. 1927

Link (open access):

About the Forestry and Forest Products Research Institute, Japan

Inaugurated as a unit for forest experiments in Tokyo in 1905, the Forestry and Forest Products Research Institute (FFPRI) was largely reorganized in 1988, when it received its current name. During its history of over 110 years, the FFPRI has been conducting interdisciplinary research on forests, forestry, the timber industry, and tree breeding with an agenda based around sustainable development goals. The FFPRI is currently looking to collaborate with more diverse stakeholders, such as international organizations, government agencies, and industry and academic leaders, to conduct much needed forest-related research and make sure we preserve these renewable resources. Website:

About Dr. Shoji Hashimoto from the Forestry and Forest Products Research Institute, Japan

Shoji Hashimoto obtained Master’s and PhD degrees from The University of Tokyo, Japan, in 2001 and 2004, respectively. In 2005, he joined the Forestry and Forest Products Research Institute, Japan, where he now works as Senior Researcher. He is also Associate Professor at The University of Tokyo. He has published over 50 papers and is a referee for over 30 scientific journals. His main research interests are soil and forest science, environmental dynamics, and climate change, among others. Hashimoto has also been an organizer for various events, including two Symposiums on Fukushima Forests and the Japan?Finland Joint Seminar, and serves as the coordinator of a radioecology unit in International Union of Forest Research Organizations (IUFRO).

February 14, 2021 Posted by | Fukushima 2021 | , , | Leave a comment

Fukushima forests contain ‘most of 2011 accident cesium’

March 12, 2019

A study has found that forests contain most of the radioactive cesium released during the 2011 Fukushima Daiichi nuclear accident.

About 70 percent of the cesium released into the environment is believed to have accumulated in forests near the plant.

There has been concern that the radioactive substance could spread to residential and farming areas, because little progress has been made in decontaminating the forests.

March 18, 2019 Posted by | fukushima 2019 | , , , | Leave a comment