In both Chernobyl and Fukushima, a large proportion of the highly-contaminated territories is covered by forest. Knowledge about the evolution of the radionuclides deposited within these ecosystems following the accidents, as well as differing management practices regarding these environments between the two countries, enable valuable lessons on the post-accident management of forest environments to be learned.
The main characteristics of the forest ecosystems impacted by radioactive fallout differ between the two accidents. In particular, the appearance of the "red forest" is specific to the Chernobyl accident and has not been observed at Fukushima.One of the main characteristics of forest ecosystems is the high remanence of radioactive contamination. There is a very dynamic redistribution of the radionuclides. It is the result of processes that are part of the radionuclides' cycle, which is strongly correlated with that of organic matter, tree type, type of soil, climatic and anthropogenic factors as well as the characteristics of the radionuclides deposited.The interception of radioactive deposits by the canopy and transfers of radionuclides towards the leaf litter and soil are the most important processes in the early phase and first months following the accident. Little knowledge existed for the Chernobyl exclusion zone, but the data collected at Fukushima has enabled a better understanding of this process. After initial interception by the canopy, the transfer of contamination towards the soil is the result of two processes: leaching of the canopy and trunks by the rain, and the fall of biomass to form leaf litter.With the leaching of the canopies and fall of aerial biomass, the soil becomes the preponderant reservoir where radioactive caesiums (caesium-134 and caesium-137) can be found. Five years after the Fukushima accident, between 80% and 90% of these caesiums have been entrained into the top layers of soil or humus, i.e. rates comparable to those observed at Chernobyl.There is a high risk of fire in the Chernobyl exclusion zone where the forest, left to evolve naturally, can be exposed to periods of drought.
This risk is lower in Japan since it is limited by the short dry season in the spring. Nonetheless, such fires would constitute catastrophic events, leading to the mass dissemination of radionuclides on a local, and even regional, scale.
In Japan, the management of contaminated forest ecosystems differs from that applied to the forest ecosystem in the Chernobyl exclusion zone. Japan decided upon a strategy of extensive decontamination. Nevertheless, in the end the authorities decided that it was unrealistic to decontaminate the forests in their entirety, due to the volumes of waste generated and the dramatic ecological consequences that could result from mass actions to remove leaf litter or fell trees.
Henceforth, Japan distinguishes between three types of forest surfaces: those located around the residential areas with the removal and disposal of contaminated leaf litter and humus; those visited by workers on a daily basis where decontamination measures are in place and, finally, those of the "deep" forest where measures aim to restrict the dispersion of radionuclides by preventing soil erosion.
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