The Effect and Effectiveness of Decontaminating a Pond in a Residential Area of Fukushima.

ABSTRACT: Special reconstruction and revitalization bases were designated in Fukushima's difficult-to-return zone by the Japanese government as targets of intensive decontamination to drastically lower air dose rates and enable residents to return. A pond amid residences of one of these bases was targeted for decontamination, and this study aimed at evaluating the effect and effectiveness of the decontamination using decontamination factor, air dose rate reduction factor, and the additional annual effective dose to residents. Air dose rates were measured in situ with the second-generation Kyoto University Radiation Mapping system, while soil core samples were collected and measured for radioactivity in the laboratory by gamma spectrometry. Lower decontamination factors were observed for more deeply distributed radiocesium soil profiles, whereas areas covered with gravel demonstrated the largest reduction in air dose rates. Decontamination effectively lowered the radiocesium inventory and air dose rates by 51-93% and 37-91%, respectively. Moreover, the additional annual effective dose to the public changed from 1.7 ± 0.79 mSv to 1.2 ± 0.57 mSv because of decontamination representing a dose aversion of 29%. These findings demonstrate how decontaminating ponds in residential areas can help to further lower the external exposure.

Reservoir sediments as a long-term source of dissolved radiocaesium in water system; a mass balance case study of an artificial reservoir in Fukushima, Japan.

Because of their large mobility and high bioavailability, it is necessary to elucidate the origins and dynamics of dissolved radionuclides in river and reservoir systems to assess the transfer of those radionuclides from water to crops and aquatic organisms. Elution from contaminated reservoir sediments, a potential source of dissolved radionuclides, presents a long-term concern, particularly for long-lived radionuclides. In this study, we systematically investigated caesium-137 (137Cs) concentrations using a time-series suite of input and output water samples collected from 2014 to 2019 from the Ogaki Dam Reservoir, which has a catchment with a high 137Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved 137Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved 137Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved 137Cs in the reservoir to elucidate how much dissolved 137Cs from the rivers and production from reservoir sediments contribute to 137Cs in the reservoir output. The annual output of dissolved 137Cs was significantly higher than the total input of dissolved 137Cs, with approximately 32%-40% of the dissolved 137Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved 137Cs fluxes from reservoir sediments to overlying water were 0.57-1.3 × 104 Bq m-2 y-1. This implies that approximately 0.04%-0.09% of 137Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high 137Cs levels may thus become even more important as sources of bioavailable dissolved 137Cs in the future.

The ecological half-life of radiocesium in surficial bottom sediments of five ponds in Fukushima based on in situ measurements with plastic scintillation fibers.

Plastic scintillation fibers (PSFs) have been instrumental in in situ surface contamination surveys post the Fukushima Daichi Nuclear Power Plant accident. Their deployment to monitoring bottom sediments in aquatic environments provides the spatial extent of contamination over wide areas compared to discrete points as provided traditionally by sediment sampling. This study evaluated the wide area ecological half-life (Teco) of radiocesium concentration for surface sediments of five ponds in Fukushima using PSFs, monitoring data generated between 2013 and 2019. The least squares' regression method was employed to evaluate the Teco. Four ponds had a Teco ranging from 3.0 ± 0.3 years to 11.4 ± 2.3 years. A forest-catchment pond exhibited a relatively long Teco of 41.6 ± 55 years. Local variation in the Teco appears to be influenced by sedimentation as we demonstrated larger values for areas showing potential sedimentation in the forest catchment pond. This study demonstrates the importance of wide area in situ monitoring techniques, such as PSF, in providing an overview of the spatial-temporal trends of radiocesium in bottom sediments and confirms the importance of forests as secondary contaminant sources to their drainage.