Distribution of radioactive Cs in forests and the prediction of radioactive Cs concentrations in trees.
Interviewee
Dr Kazuyuki Sakuma
Research purpose and contents
Past studies have shown that the annual outflow of radioactive cesium (Cs) that was deposited in forests during the TEPCO Fukushima Daiichi Nuclear Power Plant (FDNPS) accident is less than 1% of the initial deposition. In other words, Cs remains in forests for a long period of time. However, the behavior of Cs in the forest of Fukushima Prefecture has not achieved equilibrium. Therefore, understanding the overall distribution and predicting the future behavior of Cs is important. Thus, we determined the survey areas to be forests in Kawamata town and Kawauchi village located in Fukushima Prefecture, where the plume from the FDNPS accident passed through. We surveyed and are currently studying Japanese cedars and Konara oaks, which are for industrial use.
Forest observation methods
In forests where the plume has passed, Cs adheres to branches, leaves, and bark, transfers to ground and soil through rain and falling litter, and is absorbed through roots. This transfer of Cs causes changes in distribution. Because the FDNPS accident occurred in March, already bare Konara oaks (deciduous broad-leaved forests) and Japanese cedars (coniferous forests) had different initial deposition distributions. Assuming forest floors that readily adsorb Cs are the reference, plots are being prepared to conduct fixed point observations within forests. Using these methods, we survey Cs inflows from tree crowns to forest floors, Cs outflows from forest floors to outside of forests, and Cs distributions in trees and soils. At this time, we examine the concentrations and migrations of Cs in Japanese cedars and Konara oaks, and we predict temporal changes in Cs distributions.
Transfer of radioactive Cs
Diagram of observation and sampling methods
Cs inflow and outflow, Cs distribution in forests, and Cs concentration dynamics
Observation results showed that the Cs concentration in trees decreased over time, excluding the soil outflow. The Cs outflow with the forest floor as a reference point either remained almost constant or decreased independent of the type of tree. Inflow was generally due to litterfall. It was almost constant after 2016 in Konara oak forests whereas it was decreasing in Japanese cedar forests. Changes in radioactive Cs distribution in forests over time are predicted by noting tree species, tree height, and trunk diameter, felling trees to collect roots, litter (leaves and branches), and soil, and then calculating the Cs deposition above ground (trees) and underground (litter and soil). As the study progressed, it was shown that more than 90% of the Cs deposition was in litter and soil for both Japanese cedar and Konara oak forests. The Cs deposition has decreased in needles and branches in Japanese cedars and sapwoods and bark in Konara oaks. Amount of Cs removed from trees should be equal to reduction in Cs in trees; however, in the case of Japanese cedar, reduction in Cs is less than Cs removed from trees. Thus, Cs is being supplied to trees above ground from an unknown source. As the Cs is being transferred from tree roots (translocation), and the Cs being transferred from underground to above ground is removed from the tree (through litterfall), the amount of Cs in trees should continue to decrease. In the case of Konara oak, they are opposite of Japanese cedars. The amount of Cs removed is less than the Cs reduction in trees. Therefore, it is assumed that Cs is being removed from trees above ground through a process other than standard Cs removal. In other words, we could estimate the transfer of Cs from the aboveground to underground parts of trees and to roots (translocation). In the future, the appearance and transfer of dissolved Cs, which is incorporated into agricultural, forestry, and fishery products with ease, will become an important issue. Therefore, we are conducting field tests and laboratory experiments on forest conditions and processes that lead to the elution of dissolved Cs from forest litter.
Prediction of equilibrium in forest
Based on the result of the present study, predictions can be made as to when equilibrium could be achieved in the forests of Fukushima Prefecture. People from difficult-to-return areas and areas where evacuation orders have been lifted are concerned about the impact on forests that have not been decontaminated. These forestry and fisheries include mushrooms, edible wild plants, and freshwater fish. At places with high initial deposition, Cs concentrations are still high. Mechanism of how Cs is being incorporated into forestry and fishery products, especially how dissolved Cs, which is easily incorporated into such products, is generated in forests and transferred, remains unclear. Thus, these mechanisms need to be elucidated. Aiming at canceling the difficult-to-return areas and providing scientific data to people living in areas where evacuation orders have been lifted, we continue to make highly reliable observations.
Related information
| Researcher | Kazuyuki Sakuma（researchmap） |
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| References | Niizato, T., Abe, H., Mitachi, K., Sasaki, Y., Ishii, Y., Watanabe, T., 2016. Input and output budgets of radiocesium concerning the forest floor in the mountain forest of Fukushima released from the TEPCO's Fukushima Dai-ichi nuclear power plant accident. J. Environ. Radioact. 161, 11–21. http://dx.doi.org/10.1016/j.jenvrad.2016.04.017. Sakuma, K., Yoshimura, K., Nakanishi, T., 2021. Leaching characteristics of 137Cs for forest floor affected by the Fukushima nuclear accident: A litterbag experiment. Chemosphere. 264, 128480. https://doi.org/10.1016/j.chemosphere.2020.128480. |
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