Six-year trends in exchangeable radiocesium in Fukushima forest soils

Temporal variations in ambient dose rates in a restricted area designated as "difficult-to-return" for residents of Tomioka Town, Fukushima Prefecture were evaluated in a car-borne survey during 2018-2019. The median dose rates in the "Decontaminated area" in the difficult-to-return zone decreased rapidly from 1.0 μSv/h to 0.32 μSv/h; however, the median dose rates in the "Non-decontaminated area" and "Radioactive waste storage area" fluctuated between 1.1-1.4 μSv/h and 0.46-0.61 μSv/h, respectively. The detected rate of the cesium-137 (137 Cs) (137 Cs-detected points per all measuring points) in the "Decontaminated area" also decreased rapidly from 64% to 6.7%, accompany with decreasing in ambient dose rates. On the other hand, the detection of 137 Cs in the "Radioactive waste storage area" and "Non-decontaminated area" decreased from 53% to 17% and 93% to 88%, respectively. We confirmed that the dose rates in the Decontaminated area dramatically decreased due to decontamination work aiming to help residents return home. Moreover, the estimated external exposure dose of workers during the present survey was 0.66 mSv/y in the Decontaminated area and 0.55 mSv/y in the Radioactive waste storage area, respectively. This case of Tomioka Town within the "difficult-to-return zone" may be the first reconstruction model for evaluating environmental contamination and radiation exposure dose rates due to artificial radionuclides derived from the nuclear disaster. The Great East Japan Earthquake (magnitude 9.0) and subsequent tsunami on March 11, 2011 caused an accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) that resulted in various radionuclides including cesium-134 (134 Cs), cesium-137 (137 Cs) and iodine-131(131 I) being released into the atmosphere and eventually depositing on land and at sea in the surrounding areas 1. The United Nations Scientific Committee on the Effects of Atomic Radiation estimated the total release of 134 Cs (half-life: 2.1 y), 137 Cs (half-life: 30 y) and 131 I (half-life: 8 d) to be 9.0, 8.8 and 120.0 petabecquerels (PBq), respectively 1. The Japanese government, municipalities and private companies have carried out environmental and individual radiation monitoring programs to confirm the radiation levels in the affected areas 2,3. More than 8 years have passed since the accident and it has been confirmed that artificial radionuclides with a relatively long half-life such as 134 Cs and 137 Cs still exist in soil and plant samples collected around the FDNPS 1-3. Environmental monitoring in Fukushima Prefecture have been carried out by many methods (the airborne survey by monitor stations and personnel, vehicle-borne survey, aerial-vehicle survey and radionuclide analysis of the environmental samples such as soils, sediments and foodstuffs) 3-9. These surveys and the collected data are extremely important to precise evaluation of environmental remediation in the affected areas. Following the FDNPS accident, residential areas, farmlands, forests close to residential areas, and roads...

Section: Discussionmentioning

confidence: 90%

Environmental Remediation of the difficult-to-return zone in Tomioka Town, Fukushima Prefecture

Cui

Taira

Matsuo

et al. 2020

“…However, the vertical distributions of 137 Cs within the soil and spatio-temporal changes in the mobility of 137 Cs in the soil are important factors in terms of quantifying the uptake of 137 Cs by trees via roots. For the uptake process, specific soil properties (e.g., minerals 28 and potassium 29,30 ) are also influential. For instance, the different concentrations of oak between Otama and Kawauchi may be attributable to the soil properties.…”

Section: Discussionmentioning

confidence: 99%

“…The trajectories of higher concentrations were adopted for the spatial evaluation in this study, and we do not deny the possibility that the future concentration may be lower than the concentration we predicted. A possible explanation is that the possible fixation of 137 Cs by minerals may lower the 137 Cs uptake by trees (but also see Manaka et al 2019 and Koarashi et al 2019) 28,33 . The model modification performed in this study may affect the trajectory; in this study, the source of root uptake was changed from the organic compartment to the mineral compartment because the organic layer is very thin in Japan, and we believe this approach is more reasonable and provides more conservative estimates in terms of continued root uptake.…”

Section: Discussionmentioning

confidence: 99%

New predictions of 137Cs dynamics in forests after the Fukushima nuclear accident

Hirano

Imamura

Kaneko

et al. 2020

Self Cite

Most of the area contaminated by the Fukushima Daiichi Nuclear Power Plant accident is covered by forest. In this paper, we updated model predictions of temporal changes in the 137Cs dynamics using the latest observation data and newly provided maps of the predicted 137Cs activity concentration for wood, which is the most commercially important part of the tree body. Overall, the previous prediction and latest observation data were in very good agreement. However, further validation revealed that the migration from the soil surface organic layer to the mineral soil was overestimated for evergreen needleleaf forests. The new prediction of the 137Cs inventory showed that although the 137Cs distribution within forests differed among forest types in the first 5 years, the difference diminished in the later phase. Besides, the prediction of the wood 137Cs activity concentrations reproduced the different trends of the 137Cs activity concentrations for cedar, oak, and pine trees. Our simulation suggests that the changes of the wood 137Cs activity concentration over time will slow down after 5–10 years. Although the model uncertainty should be considered and monitoring and model updating must continue, the study provides helpful information on the 137Cs dynamics within forest ecosystems and the changes in wood contamination.

“…The aggregated transfer factor (T ag ) values for 137 Cs in tree leaves were also generally similar across treatments (although estimated to be higher in the decontaminated area than in the untreated area in some cases) ( Table 3). These results seem reasonable given that, prior to decontamination, a large proportion of the deposited 137 Cs was immobilized in the surface mineral soil via its interactions with soil constituents [40][41][42][43][44][45] . This behavior could reduce 137 Cs availability for root uptake by plants 15,46,47 to a similar extent in both the untreated and decontaminated areas.…”

Section: Discussionmentioning

confidence: 60%

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

Koarashi

Atarashi-Andoh

Nishimura

et al. 2020

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ( 137 Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified 137 Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated 137 Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the 137 Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of 137 Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1-2 years after the accident) for Japanese forest ecosystems.The accident at the Fukushima Daiichi nuclear power plant (NPP) in March 2011 released large quantities of radionuclides into the atmosphere 1 and consequently caused serious radioactive contamination of terrestrial ecosystems over a wide area of eastern Japan 2 . Among the radionuclides deposited onto the terrestrial ecosystems, radiocesium ( 137 Cs), with a physical half-life of 30.1 years, is the primary cause for concern because 137 Cs-contaminated ecosystems can increase the radiation exposure to the local population for many years, via both an elevated ambient dose rate in the air (external exposure) and the consumption of contaminated food products (internal exposure) 3 .Decontamination is an important post-accident countermeasure to reduce the radiological impact of a nuclear accident on humans and ecosystems 3,4 . Although since the Fukushima NPP accident, decontamination efforts have been progressing in residential and agricultural areas, forested areas have received a low priority in the decontamination decision-making process, as in the case of the Chernobyl NPP accident 3 , and has not been carried out on a large scale 5 , even though forests provide a wide range of social, cultural, and economic values. This is because the forested areas are vast, occupying approximately 70% of the land in the heavily contaminated area 6 , but are generally not where people live permanently or regularly spend much time. Another reason may be that the forested areas in Japan are concentrated in mountainous and hilly regions with steep terrain, which makes them very sensitive areas to soil erosion. Technical, radiological, and econ...