Rain-induced bioecological resuspension of radiocaesium in a polluted forest in Japan

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 resuspension 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 assessment 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.

A summary of environmental radioactivity research studies by members of the Japan Society of Nuclear and Radiochemical Sciences

Many scientists who are members of the Japan Society of Nuclear and Radiochemical Sciences have been involved in academic activities in response to the Fukushima Daiichi Nuclear Power Plant accident. Projects had been implemented that include determining radionuclides in environmental samples, identifying the distribution of radionuclides by large-scale soil monitoring, tracing radionuclide discharge time series, clarifying environmental dynamics of radionuclides, etc. For the last 10 years, these results have been shared and discussed in annual workshops partly sponsored by the society. This review summarizes the studies yielding these results, and they include reconstruction of the 131I distribution on soil by long-lived 129I analysis, reconstruction of the radioactive plume transport, identification of biological resuspension sources, discovery and characterization of cesium particles, and parameterization of the environmental behavior of radiocesium for dose assessment.

Vertical distribution and transport of radiocesium via branchflow and stemflow through the canopy of cedar and oak stands in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident

Aiming to fill a need for data regarding radiocesium transport via both branchflow and stemflow through forests impacted by radioactive fallout, this study examined the vertical variation of radiocesium flux from branchflow and stemflow through the canopies of young Japanese cedar (Cryptomeria japonica (L. f.) D. Don) and Japanese oak (Quercus serrata Murray) trees in the aftermath of the Fukushima Dai-ichi Nuclear Power Plant accident. In forested areas approximately 40 km northwest of the location of the Fukushima Dai-ichi accident, the ¹³⁷Cs concentration varied significantly among sampling periods and between the two forests, with the oak stand exhibiting higher ¹³⁷Cs concentrations and depositional fluxes than the cedar stand. Expressed per unit trunk basal area, the depositional flux of ¹³⁷Cs generated from the cedar and oak stands was 375 and 2810 Bq m^-2 year^-1, respectively. Of this total, 71% and 48% originated from the cedar and oak canopy, respectively, while the remainder originated from the trunk. Accordingly, the origin of radiocesium was more balanced for the oak stand with almost half of the flux coming from the canopy (48%) and the other half from the trunk (52%). Only about a quarter (29%) of the radiocesium flux originated from the trunk in Japanese cedar. Results from this work provide needed data that can enable a more thorough conceptualization of radiocesium cycling in forests. Coupling these empirical results with a physically-based model would likely lead to better forest management and proactive strategies for rehabilitating radioactively-contaminated forests and reducing the exposure risk of radiation dose rate for those that utilize forest products.

Relationship of 137Cs with Fungal Spore Tracers in the Ambient Aerosols from Fukushima after the 2011 Nuclear Accident, East Japan

Even after 7 years of the nuclear accident that occurred in 2011 at the Fukushima Dai-ichi nuclear power plant (F1NPP), high levels of 137Cs have been detected in ambient aerosols from some polluted areas of Fukushima. Higher levels of radionuclides were often observed in the atmosphere during and after rain events. We presume that biological processes such as fungal activity associated with higher relative humidity may be involved with a possible emission of radioactivity to the atmosphere, which was originally emitted from the F1NPP accident and was deposited over the ground, forest, soil, etc. Here, we report, for the first time, relationships of 137Cs and organic tracers of fungal spores (i.e., arabitol, mannitol and trehalose) in the aerosol samples collected from Fukushima, Japan. Although we found twice-higher concentrations of 137Cs at nighttime than at daytime, fungal spore tracers did not show a consistent trend to 137Cs, that is, organic tracers at nighttime were similar with those at daytime or were even higher in daytime. This study has not clearly demonstrated that fungal spores are the important source of high levels of 137Cs at nighttime. The current unclear relationship is probably associated with the sampling strategy (four consecutive days with a sampling on/off program for day/nighttime samples) taken in this campaign, which may have caused a complicated meteorological situation.

Radiocaesium Contamination of Mushrooms at High- and Low-Level Chernobyl Exposure Sites and Its Consequences for Public Health

We compare the specific activities of 137Cs and 40K in stipes and caps of three different common mushroom species (Xerocomus badius, Russula ochroleuca and Armillariella mellea) measured at the Czech Chernobyl hot spot in the Opava area (Silesia) and at a low-exposed site at the Beskydy mountains in 2011. The highest values of 137Cs were found in caps of Xerocomus badius and Russula ochroleuca in the Opava area (11.8 and 8.77 kBq/kg, respectively). The source of 137Cs was verified by the measurement of the 134Cs/137Cs ratio. Based on our results, we estimate an effective dose per year due to radiocaesium intake in the two investigated areas for Xerocomus badius, one of the most popular edible mushrooms in the Czech Republic. In 2011, the effective dose reached the maximum value of 0.102 mSv in the Opava area and 0.004 mSv at the low-exposed site at the Beskydy mountains. Therefore, it does not represent a significant risk for public health.