Soil dust and bioaerosols as potential sources for resuspended 137Cs occurring near the Fukushima Dai-ichi nuclear power plant

One of the current pathways to radiation exposure, caused by the radionuclides discharged during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, is the inhalation of resuspended 137Cs present in the air. Although wind-induced soil particle resuspension is recognized as a primary resuspension mechanism, studies regarding the aftermath of the FDNPP accident have suggested that bioaerosols can also be a potential source of atmospheric 137Cs in rural areas, although the quantitative impact on the atmospheric 137Cs concentration is still largely unknown. We propose a model for simulating the 137Cs resuspension as soil particles and bioaerosols in the form of fungal spores, which are regarded as a potential candidate for the source of 137Cs-bearing bioaerosol emission into the air. We apply the model to the difficult-to-return zone (DRZ) near the FDNPP to characterize the relative importance of the two resuspension mechanisms. Our model calculations show that soil particle resus-pension is responsible for the surface-air 137Cs observed during winter-spring but could not account for the higher 137Cs concentrations observed in summer-autumn. Higher 137Cs concentrations are reproduced by the emission of 137Cs-bearing bioaerosols (fungal spores) that replenishes the low-level soil particle resuspension in summer-autumn. Our model results show that the accumulation of 137Cs in fungal spores and large emissions of spores characteristic of the rural environment are likely responsible for the presence of biogenic 137Cs in the air, although the former must be experimentally validated. These findings provide vital information for the assess-ment of the atmospheric 137Cs concentration in the DRZ, as applying the resuspension factor (m- 1) from urban areas, where soil particle resuspension would dominate, can lead to a biased estimate of the surface-air 137Cs concentration. Moreover, the influence of bioaerosol 137Cs on the atmospheric 137Cs concentration would last longer, because undecontaminated forests commonly exist within the DRZ.