Long-term assessment of airborne radiocesium after the Fukushima nuclear accident: Re-suspension from bare soil and forest ecosystems
Atmospheric Chemistry and Physics Discussions
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 the contaminated area (Namie, Fukushima), and air concentration measurements inside (Namie) and outside (Tsukuba, Ibaraki) the contaminated area. The assessment period is for the year 2013 just after the start of the field experiments, December 14, 2012. The
... 012. 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 one to two 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>/d, 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>/d) as a result of radioactive decay, land surface processes (migration in the soil and biota), and decontamination. Consequently, re-suspension contributed negligibly to reducing ground radioactivity. The dust emission model could account for the air concentration of <sup>137</sup>Cs in winter, whereas the summer air concentration was underestimated by one to two orders of magnitude. Re-suspension from forests at a constant rate of 10<sup>−7</sup>/h, multiplied by the green area fraction, quantitatively accounted for 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 0.6–0.8 in the cold season and 0.1–0.4 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, and thus the current study is the first but crude estimation of the long-term assessment of radiocesium re-suspension. Further study will be needed to understand the re-suspension mechanisms and to accurately assess the re-suspension mechanisms through field experiments and numerical simulations.