Long-term assessment of airborne radiocesium after the Fukushima nuclear accident: re-suspension from bare soil and forest ecosystems
The long-term effect of <sup>137</sup>Cs re-suspension from contaminated soil and forests due to the Fukushima nuclear accident has been quantitatively assessed by numerical simulation, a field experiment on dust emission flux in a contaminated area (town of Namie, Fukushima prefectur...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-10-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/16/13149/2016/acp-16-13149-2016.pdf |
Summary: | The long-term effect of <sup>137</sup>Cs re-suspension from contaminated soil and
forests due to the Fukushima nuclear accident has been quantitatively
assessed by numerical simulation, a field experiment on dust emission flux
in a contaminated area (town of Namie, Fukushima prefecture), and air
concentration measurements inside (Namie) and outside (city of Tsukuba, Ibaraki
prefecture) the contaminated area. In order to assess the long-term effect,
the full year of 2013 was selected to study just after the start of the
field experiments. The <sup>137</sup>Cs concentrations at Namie and Tsukuba were
approximately 10<sup>−1</sup>–1 and 10<sup>−2</sup>–10<sup>−1</sup> mBq m<sup>−3</sup>,
respectively. The observed monthly median concentration at Namie was 1 to
2 orders of magnitude larger than that at Tsukuba. This observed
difference between the two sites was consistent with the simulated
difference, indicating successful modeling of <sup>137</sup>Cs re-suspension and
atmospheric transport. The estimated re-suspension rate was approximately
10<sup>−6</sup> day<sup>−1</sup>, which was significantly lower than the decreasing rate of the
ambient gamma dose rate in Fukushima prefecture (10<sup>−4</sup>–10<sup>−3</sup> day<sup>−1</sup>)
as a result of radioactive decay, migration in the soil and biota, and
decontamination. Consequently, re-suspension contributed negligibly in
reducing ground radioactivity. The dust emission model could reproduce the
air concentration of <sup>137</sup>Cs in winter, whereas the summer air
concentration was underestimated by 1 to 2 orders of magnitude.
Re-suspension from forests at a constant rate of 10<sup>−7</sup> h<sup>−1</sup>, multiplied by
the green area fraction, could explain the air concentration of <sup>137</sup>Cs
at Namie and its seasonal variation. The simulated contribution of dust
re-suspension to the air concentration was 0.7–0.9 in the cold season and
0.2–0.4 in the warm season at both sites; the remainder of the
contribution was re-suspension from forest. The re-suspension mechanisms,
especially through the forest ecosystems, remain unknown. This is the first
study that provides a crude estimation of the long-term assessment of
radiocesium re-suspension. Additional research activities should investigate
the processes/mechanisms governing the re-suspension over the long term.
This could be achieved through conducting additional field experiments and
numerical simulations. |
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ISSN: | 1680-7316 1680-7324 |