Long-Term Monitoring of Radiocesium Concentration in Sediments and River Water along Five Rivers in Minami-Soma City during 2012–2016 Following the Fukushima Dai-ichi Nuclear Power Plant Accident

Impact evaluation of typhoons and remediation works on spatiotemporal evolution of air dose rate in two riverside parks in Fukushima, Japan after the Dai-ichi nuclear power plant accident

This study elucidated the impacts of typhoon events and remediation works on the spatiotemporal evolution of the air dose rate in riverside areas frequented by residents. Spatial distribution of the air dose rate and radiocesium concentration in the sediments were measured in two riverside parks located near each other in Fukushima Prefecture, Japan, for 2015-2020. The air dose rates measured by walk surveys were interpolated using ordinary kriging to generate air dose rate maps, to facilitate a comparison between the results at different points in time during the measurement campaigns. After the typhoons that occurred during 2015-2018, the air dose rate near the riverside in one park decreased, but not in the other, because the erosion and sediment deposition patterns differed between them. This could be due to the presence of a dam upstream, which serves a flood mitigation function. However, the extreme event of typhoon Hagibis in 2019 dropped the air dose rates near the riversides in both parks. In contrast to the typhoon events which affected the riverside areas, remediation works such as decontamination undertaken during 2015-2019 reduced the air dose rates around the garden and lawn areas which are frequently used as recreational sites. Modeling the temporal evolutions in the air dose rates for the entire area of the riverside parks revealed that 35% of the reduction was caused by physical decay of radiocesium on average, 14% by vertical migration of radiocesium in the soil through precipitation, and 51% by the three typhoons and remediation works during 2015-2019. The contribution of 20% from the strongest typhoon Hagibis highlights the fact that floods resulting from large typhoons are effective in causing natural attenuation of air dose rates in riverside parks.

Reconstruction of time changes in radiocesium concentrations in the river of the Fukushima Dai-ichi NPP contaminated area based on its depth distribution in dam reservoir's bottom sediments

Radionuclide depth distribution in bottom sediments in deep-water zones of dam reservoirs, where no sediment mixing occurs, can be used to reconstruct time changes in particulate activity concentrations of radionuclides strongly bound to bottom sediments. This approach was used to analyze the ¹³⁷Cs concentration profile in a bottom sediment core collected from Ogaki dam reservoir on the Ukedo River in the Fukushima Dai-ichi nuclear power plant contaminated zone in October 2019. The derived ¹³⁷Cs particulate concentrations provided a basis for estimating the dissolved concentration and its temporal trend in the Ukedo River, using the mean value of the apparent ¹³⁷Cs distribution coefficient. The reconstructed particulate and dissolved ¹³⁷Cs concentrations and their temporal trends are consistent with monitoring data. The annual mean particulate and dissolved ¹³⁷Cs wash-off ratios were also calculated for the period of eight years after the accident. Interestingly, the particulate ¹³⁷Cs wash-off ratios for the Ukedo River at Ogaki dam were found to be similar to those for the Pripyat River at Chernobyl in the same time period after the accident, while the dissolved ¹³⁷Cs wash-off ratios in the Ukedo River were an order of magnitude lower than the corresponding values in the Pripyat River. Both the particulate and dissolved ¹³⁷Cs wash-off ratios in the Ukedo River declined faster during the first eight years after the FDNPP accident than predicted by the diffusional model, most likely, due to greater natural attenuation and, to some extent, remediation measures implemented on the catchments in Fukushima.

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 (T_eco) 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 T_eco. Four ponds had a T_eco ranging from 3.0 ± 0.3 years to 11.4 ± 2.3 years. A forest-catchment pond exhibited a relatively long T_eco of 41.6 ± 55 years. Local variation in the T_eco 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.