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

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.