Scientists trace the trajectory of radioactive cesium in the Fukushima ecosystem

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In the aftermath of the Fukushima nuclear accident, the Japanese government carried out intensive decontamination of the human-occupied parts of the affected area by removing surface layers of soil. But a major affected region is dense, uninhabited forest, where such decontamination strategies are not feasible. It is therefore crucial to find ways to avoid the spread of radioactive contaminants such as radiocaesium in areas of human activity located downstream of these contaminated forests.

The first step is to understand the dynamics of radiocaesium flux through riverine forest ecosystems. In the decade following the accident, a large body of research has been devoted to this. Scientists from the National Institute of Environmental Studies, Japan, sifted through the data and unraveled the threads of individual radiocaesium transport processes in forest ecosystems. “We have identified that radiocaesium accumulates primarily in the organic soil layer of forests and in standing water in streams, making them potent sources of contaminating organisms. Contamination management in these habitats is crucial to provide ecosystem services in forest streams”, says Dr. Masaru Sakai, who led the study. The results of this study were posted online on July 6, 2021 and published in volume 288 of the journal Environmental pollution on November 1, 2021.

The research team reviewed a wide range of scientific research on radiocaesium in forests and streams to identify regions of radiocaesium accumulation and storage. After the accident, radiocaesium was mainly deposited on the forest canopy and forest floor. This radiocaesium eventually reaches the earth – through rainfall and leaf fall – where it accumulates in the upper layers of the soil. Biological activities, such as those of detritivores (insects and fungi that live on leaf debris, etc.) ensure the circulation of radiocaesium in the upper layers of the soil and its subsequent incorporation into plants and fungi. This allows radiocaesium to enter the food web, eventually entering higher organisms. Radiocesium is chemically similar to potassium, an essential mineral in living organisms, contributing to its absorption by plants and animals. The “fertilization” of areas contaminated with an excess of potassium constitutes an effective strategy to suppress the biological absorption of radiocaesium.

Neighboring streams and water bodies receive their share of radiocaesium from runoff and dead leaves. Most of the radiocaesium in streams is likely to be taken up by clay minerals in the stream bed, but a small portion dissolves in water. Unfortunately, there is little information on the relationship between dissolved radiocaesium and aquatic organisms, such as fish, that could be important for the formulation of contamination management strategies. Radiocaesium in streams also accumulates in head valleys, basins and other areas of standing water. Constructions such as reservoir dams provide a way to effectively trap radiocaesium, but the regular leaching of sediment from the reservoir causes recontamination downstream.

This complex radiocaesium transport network is difficult to trace, making it impossible to develop a single solution to radiocaesium contamination. Dr. Sakai and his team recommend interdisciplinary studies to accelerate a full understanding of radiocaesium pathways in forest stream ecosystems so that measures can be developed to reduce future contamination. “This review can serve as a knowledge base to explore future contamination management strategies. The tangled radiocaesium pathways documented here may also imply the difficulties of creating successful radiation contamination management strategies after unintended nuclear accidents,” says Dr. Sakai.

Nuclear energy is often presented as a solution to the energy crisis, but it is important to plan response measures to unpredictable contamination events. To meet the essential need for clean energy in the face of the climate crisis, the management of contamination in nuclear-dependent societies is an integral part. A good understanding of the behavior of radiocaesium in ecosystems can not only lead to the successful management of existing contamination, but can also ensure the rapid containment of possible future accidents.

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Material provided by National Institute for Environmental Studies. Note: Content may be edited for style and length.

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