Long-term assessment of airborne radiocesium after the Fukushima nuclear accident: re-suspension from bare soil and forest ecosystems

• Main Authors: M. Kajino, M. Ishizuka, Y. Igarashi, K. Kita, C. Yoshikawa, M. Inatsu
• Format: Article
• Language: English
• Published: Copernicus Publications 2016-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/16/13149/2016/acp-16-13149-2016.pdf

The long-term effect of ¹³⁷Cs 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 ¹³⁷Cs concentrations at Namie and Tsukuba were approximately 10⁻¹–1 and 10⁻²–10⁻¹ mBq m⁻³, 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 ¹³⁷Cs re-suspension and atmospheric transport. The estimated re-suspension rate was approximately 10⁻⁶ day⁻¹, which was significantly lower than the decreasing rate of the ambient gamma dose rate in Fukushima prefecture (10⁻⁴–10⁻³ day⁻¹) 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 ¹³⁷Cs 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 10⁻⁷ h⁻¹, multiplied by the green area fraction, could explain the air concentration of ¹³⁷Cs 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.