Five-minute resolved spatial distribution of radiocesium in sea sediment derived from the Fukushima Dai-ichi Nuclear Power Plant

Cesium-137 and 137Cs/133Cs atom ratios in marine zooplankton off the east coast of Japan during 2012-2020 following the Fukushima Dai-ichi nuclear power plant accident

We measured the concentrations of cesium isotopes (¹³³Cs, ¹³⁴Cs, and ¹³⁷Cs) in zooplankton samples collected in waters off the east coast of Japan from May 2015 to June 2020. By combining these data with those obtained previously from May 2012 to February 2015, we evaluated the long-term impacts of the Fukushima Dai-ichi Nuclear Power Plant accident on marine zooplankton. Relatively high ¹³⁷Cs concentrations in zooplankton, exceeding 10 Bq/kg-dry weight, were sporadically observed until June 2016, regardless of year or station. After May-June 2017, ¹³⁷Cs concentrations decreased to below 1 Bq/kg-dry at most stations, and by May 2020, concentrations were below 0.5 Bq/kg-dry except those off Fukushima Prefecture. Since the accident, the ¹³⁷Cs/¹³³Cs atom ratios of zooplankton samples were higher than those of ambient seawater until 2019, but in May-June 2020 the ratios matched those of seawater except off Fukushima Prefecture. Highly radioactive particles were not detected in zooplankton samples by autoradiography using imaging plates after May-June 2017, although they were before. Therefore, the persistence of elevated ¹³⁷Cs/¹³³Cs ratios in zooplankton relative to seawater for nine years after the accident was probably due to the incorporation of highly radioactive particles (cesium-bearing particles or clay-mineral aggregates with highly adsorbed radiocesium) onto/into zooplankton for several years after the accident. However, since at least May-June 2017, these elevated ratios have likely been caused by small highly radioactive particles (or larger particles disaggregated into small pieces) entering the ocean from land via rivers or directly discharged from the Fukushima Nuclear Power Plant. Microplastics enriched with radiocesium with higher ¹³⁷Cs/¹³³Cs ratios than seawater may have also contributed ¹³⁷Cs to the zooplankton.

Contamination of sea urchin Mesocentrotus nudus by radiocesium released during the Fukushima Daiichi Nuclear Power Plant accident

Countless marine organisms were polluted with radioactive materials that were dispersed when the Fukushima Daiichi Nuclear Power Plant (FDNPP) was damaged in 2011 by the Great East Japan Earthquake. The aim of this study was to determine the degree to which marine herbivorous sea urchins, Mesocentrotus nudus, were contaminated with radiocesium because of the accident. We collected samples of sea urchins from four locations in Fukushima prefecture (at the coast and offshore from the Yotsukura and Ena stations) and investigated how the ¹³⁷Cs activity concentrations changed. The biological half-life (T_bio) of ¹³⁷Cs in the individual sea urchins was between 121 and 157 days. The ecological half-life (T_eco) of ¹³⁷Cs was 181–423 days and was high in places close to the FDNPP. The T_eco values in the sea urchins were longer than previously reported. The results infer that the food sources of the sea urchins around the Fukushima coast strongly influenced their uptake of ¹³⁷Cs.

Temporal variability of 137Cs concentrations in coastal sediments off Fukushima

Large amounts of radiocesium were released into marine environments following the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Released radiocesium influenced not only marine environment but also marine biota in Fukushima. Since marine biota as fisheries products is important for Japanese market, it is important to assess the distribution of radiocesium in coastal environment off Fukushima for safety concerns of radioactive contamination. Radiocesium concentrations in sediments are important for understanding fishing ground conditions and for proving the safety of fisheries products in Fukushima. In this study, monthly monitoring data collected from May 2011 to March 2020 were analyzed to describe the temporal variability of ¹³⁷Cs concentrations in coastal sediments off Fukushima (total of 3647 samples from eight lines at depths of 7-125 m off Fukushima, and three sites in Matsukawa-ura Lagoon). The ¹³⁷Cs concentration in sediment showed a decreasing trend, but our nonlinear model fitting suggested that this rate of decrease had slowed down. Additionally, ¹³⁷Cs concentrations were up to 4.08 times greater in shallow sampling sites (7, 10, 20 m depth) following heavy rainfall events (before five months vs. after five months), such as typhoons. These observations were consistent with increasing input from particulate ¹³⁷Cs fluxes from rivers and increasing dissolved ¹³⁷Cs concentrations in seawater. Finally, our numerical modeling suggested that riverine ¹³⁷Cs input could maintain ¹³⁷Cs concentrations in coastal sediment. These results indicate that riverine ¹³⁷Cs input following heavy rainfall events is the main factor for maintaining ¹³⁷Cs concentrations in coastal sediments near the Fukushima Daiichi Nuclear Power Plant.

Migration processes of radioactive cesium in the Fukushima nearshore area: Impacts of riverine input and resuspension

It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (¹³⁷Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the ¹³⁷Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%-83% of secondary ¹³⁷Cs migration in nearshore areas, whereas the influence of riverine ¹³⁷Cs input on the sediment was only 7% per year.

A storm-induced flood and associated nearshore dispersal of the river-derived suspended 137Cs

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area < 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment.

An experimental approach to assess the post-depositional mobility of 134Cs

The partitioning coefficient, K_d, which is defined by the reversible sorption processes between a solid and an aqueous phase at equilibrium, is one of the most important parameters to assess environmental transport and risk. In this study, a series of simple laboratory experiments were conducted to investigate sorption properties of ¹³⁴Cs on a model sediment under two treatments (shaken vs non-shaken) and with three (small: <75 μm, large: > 75 μm and bulk i.e., composite) particle size fractions. Vertical transport of ¹³⁴Cs across the water-sediment interface and into sediment was also evaluated. As expected, grain size had the strongest influence on ¹³⁴Cs K_d values, with the small particle size fraction yielding significantly higher K_d values than the large and bulk fractions. The mean K_d values obtained from the various experiments ranged from 89 ± 13-130 ± 5 L kg^-1 (small), 44 ± 10-91 ± 13 L kg^-1 (large), 73 ± 3-112 ± 11 L kg^-1 (bulk, shaken) and 73 ± 5-110 ± 4 L kg^-1 (bulk, non-shaken). Most of the ¹³⁴Cs partitioning processes occurred rapidly (<2 h) into the experiment. Physical mixing (shaken) did not appear to significantly affect the ¹³⁴Cs K_d values. In complement, a separate experiment on the vertical penetration of ¹³⁴Cs into a bulk sediment column showed that ¹³⁴Cs was able to penetrate up to 5 cm into the sediment column after 88 days (∼0.6 mm d^-1) and this flux rate is comparable to natural settings. Adsorption and contact time were found to be key for the ¹³⁴Cs penetration process. Results from these experiments add to the literature on post-event radionuclide transport studies in marine settings and provide an experimental perspective that can be built upon to complement field observations.

Visualization of radiocesium distribution in surface layer of seafloor around Fukushima Daiichi Nuclear Power Plant

Large quantities of volatile radionuclides were released into the atmosphere and the hydrosphere following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March, 2011. Monitoring of radiocesium in sediment is important for evaluating the behavior of radiocesium in the environment and its effect on aquatic organisms. In this study, the radiocesium distribution in the surface sediment around the FDNPP was visualized as a radiocesium concentration map using periodical survey data from a towed gamma-ray detection system. The uncertainty of the radiocesium map was evaluated via comparison with a large amount of sediment core sample data. The characteristics of the radiocesium distribution were examined considering the seafloor topography and a geological map, which were obtained via acoustic wave survey. The characteristics of the formation of ¹³⁷Cs anomaly at the estuaries were analyzed using a contour map of ¹³⁷Cs concentration combined with water depth. Validation of the created map showed that it was comparable with actual sediment core samples. The map generated using the towed radiation survey depicted the ¹³⁷Cs concentration distribution as the position resolution of a 1 km mesh. Finally, the ¹³⁷Cs concentration decreased with time in consideration of such uncertainty.

A modeling study on the oceanic dispersion and sedimentation of radionuclides off the coast of Fukushima

We developed a three-dimensional prognostic oceanic dispersion model that accounted for the phase transfer of radionuclides between seawater, suspended particles, and seabed sediments with multiscale grain sizes. A detailed hindcast of ¹³⁷Cs in the seabed sediment off the Fukushima coast was conducted to investigate the transfer mechanism of dissolved ¹³⁷Cs derived from the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident toward the seabed sediment. Extensive model-data comparison demonstrated that the model could satisfactorily reproduce the oceanic structure and ¹³⁷Cs concentrations in the seawater and seabed sediment. The model successfully reproduced the major features of the observed spatial variation of the ¹³⁷Cs activities in the sediment, which represented more than 90% of the sedimentary radiocesium existing in the coastal area off Fukushima several months after the accident. Shear stress associated with the resuspension of the seabed sediment was induced by waves near the shore and by current velocity offshore of the study area. The adsorption of ¹³⁷Cs on the seabed sediment differed depending on the particle size, with adsorption on clay being the most substantial. The distribution of ¹³⁷Cs in the sediment off the Fukushima coast was formed mainly owing to adsorption from the dissolved phase by June 2011, when the impact of the direct oceanic ¹³⁷Cs release from FNPP1 was remarkable. After June 2011, seabed sediment became a source of ¹³⁷Cs released to the seawater owing to resuspension with and desorption from the sediment.

Shoreline Extraction from WorldView2 Satellite Data in the Presence of Foam Pixels Using Multispectral Classification Method

Foam is often present in satellite images of coastal areas and can lead to serious errors in the detection of shorelines especially when processing high spatial resolution images (<20 m). This study focuses on shoreline extraction and shoreline evolution using high spatial resolution satellite images in the presence of foam. A multispectral supervised classification technique is selected, namely the Support Vector Machine (SVM) and applied with three classes which are land, foam and water. The merging of water and foam classes followed by a segmentation procedure enables the separation of land and ocean pixels. The performance of the method is evaluated using a validation dataset acquired on two study areas (south and north of the bay of Sendaï—Japan). On each site, WorldView-2 multispectral images (eight bands, 2 m resolution) were acquired before and after the Fukushima tsunami generated by the Tohoku earthquake in 2011. The consideration of the foam class enables the false negative error to be reduced by a factor of three. The SVM method is also compared with four other classification methods, namely Euclidian Distance, Spectral Angle Mapper, Maximum Likelihood, and Neuronal Network. The SVM method appears to be the most efficient to determine the erosion and the accretion resulting from the tsunami, which are societal issues for littoral management purposes.

Sorption of anthropogenic radionuclides onto river sediments and suspended solids: dependence on sediment composition

The purpose of the study is to explore the sorption behaviour of anthropogenic radionuclides in the Vltava River catchment in the Czech Republic, which could be influenced by an assumptive severe nuclear accident. Radionuclide sorption was described by distribution coefficients between water and sediment or suspended solids, respectively. Then, possible correlations between radionuclide sorption and sediment properties were investigated using simple and multiple regressions. The sorption of radionuclides was affected by sediment granularity and mineralogical composition, each radionuclide having its group of influencing sediment parameters.

Reconstruction of radiocesium levels in sediment off Fukushima: Simulation analysis of bioavailability using parameters derived from observed 137Cs concentrations

Radiocesium was released to the North Pacific coastal waters by the accident at the Fukushima Dai-ichi Nuclear Power Plant (1FNPP) of the Tokyo Electric Power Company (TEPCO) in March 2011. Since the radiocesium in the sediment off Fukushima was suggested as a possible source for the transfer of this radionuclide through the benthic food chain, we conducted numerical simulations of ¹³⁷Cs in sediments off the Fukushima coast by using a model which incorporates dynamic transfer processes between seawater and the labile and refractory fractions in sediment particles. This model reproduced the measured temporal changes of ¹³⁷Cs concentration in seabed surface sediment off Fukusima coasts, by normalizing the radiocsium transfer between seawater and sediment according to the particle diameter sizes. We found that the ¹³⁷Cs level in sediment decreased by desorption during the first several months after the accident, followed by a reduction in the labile fraction until the end of 2012. The apparent decrease of the total radiocesium level in surface sediment was estimated to occur at rates of approximately 0.2 y^-1 within a 20 km distance from the 1FNPP. The comparison of ¹³⁷Cs level decreases in the demersal fish and the simulated temporal labile fraction in fine sediment demonstrated that the consideration of radiocesium transfer via sediment is important for determining the ¹³⁷Cs depuration mechanism in some demersal fish.

The contribution of 137Cs export flux from the Tone River Japan to the marine environment

The contribution of ¹³⁷Cs transport to the marine environment via the Tone River, Japan was investigated. This river has the largest discharge among rivers on the North Pacific side of eastern Japan. The sampling site was located upstream near the river mouth and dissolved and particulate ¹³⁷Cs in the river water was measured during 2014-2015, three years after the Tokyo Electric Power Corporation Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Annual fluxes of total (dissolved and particulate) ¹³⁷Cs with considering desorption of ¹³⁷Cs from riverine particles by change of salinity from the Tone River were similar in both years (78-107 × 10⁹ Bq/y), indicating that about 0.03-0.06% of the estimated total amount of ¹³⁷Cs deposited in the catchment (1.9-2.8 × 10¹⁴ Bq) was transported to the marine environment each year. Although the annual flux was about one order of magnitude lower than the daily direct discharge into the ocean from the FDNPP (800 × 10⁹ Bq/y) during the corresponding period, continuous monitoring of rivers in the southern coastal area of east Japan on the North Pacific side are needed for the effect of ¹³⁷Cs release via the rivers in the Kanto area over the long-term.

A trial of in situ and static measurements of levels of radioactive cesium 137 on shallow rugged reefs lying close to the coastline of Fukushima

With the use of an in situ and static method for gamma-ray measurements, levels of radioactive cesium 137 on shallow rugged reefs which lie between 37.3° N and 37.4° N, from the coastline of Fukushima to 141.06° E, at a depth of around 10 m were surveyed for the first time from May 2016 to December 2017. To confirm the contact between the detector and a surface of rock, we used a fact that potassium containing minerals are abundant and uniformly distributed in the area, and thus the strength of the photoelectric peak of natural radioactive potassium 40 is nearly constant over the area. We have found that the levels of radioactive cesium 137 varied from point to point within a range from 1 × 10⁴ Bq/m² to 6 × 10⁴ Bq/m².

Distribution and fate of 129I in the seabed sediment off Fukushima

In this study, seabed sediment was collected from 26 stations located within 160 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) during the 2 years which followed the FDNPP accident of March 2011 and the concentrations of ¹²⁹I and ¹³⁷Cs were measured. By comparing the distribution of these two radionuclides with respect to their different geochemical behaviors in the environment, the transport of accident-derived radionuclides near the seafloor is discussed. The concentration of ¹²⁹I in seabed sediment recovered from offshore Fukushima in 2011 ranged between 0.02 and 0.45 mBq kg^-1, with ¹²⁹I/¹³⁷Cs activity ratios of (1.9 ± 0.5) × 10^-6 Bq Bq^-1. The initial deposition of ¹²⁹I to the seafloor in the study area was 0.36 ± 0.13 GBq, and the general distribution of sedimentary ¹²⁹I was established within 6 months after the accident. Although iodine is a biophilic element, the accident-derived ¹²⁹I negligibly affects the benthic ecosystem. Until October 2013, a slight increase in activity of ¹²⁹I in the surface sediment along the shelf-edge region (bottom depth: 200-400 m) was observed, despite that such a trend was not observed for ¹³⁷Cs. The preferential increase of the ¹²⁹I concentrations in the shelf-edge sediments was presumed to be affected by the re-deposition in the shelf-edge sediments of ¹²⁹I desorbed from the contaminated coastal sediment. The results obtained from this study indicate that ¹²⁹I/¹³⁷Cs in marine particles is a useful indicator for tracking the secondary transport of accident-derived materials, particularly biophilic radionuclides, from the coast to offshore areas.

Radiocesium in seawater, sediments, and marine megabenthic species in coastal waters off Fukushima in 2012-2016, after the 2011 nuclear disaster

In bottom-sediment samples collected in 2012 from a coastal strip (∼30 km × 120 km) off the Fukushima Daiichi Nuclear Power Plant (FDNPP), radiocesium activity concentrations were generally higher south of the FDNPP, with high activity concentration patches in the north. In periodic surveys conducted at nearshore sites during 2012-2016, no clear temporal trends were observed in radiocesium activity concentrations in seawater or bottom sediment, and activity concentrations were higher in fish than in invertebrates. During 2012-2014, radiocesium activity concentrations tended to decrease in fish, but during 2012-2013 in the south, some increases were observed. Radiocesium activity concentrations were significantly higher in some fish (e.g., Okamejei kenojei) directly offshore and south of the FDNPP than in the north. Activity concentrations in fish stomach contents were significantly correlated with those in muscle tissue, suggesting that the consumption of contaminated prey contributed greatly to radiocesium contamination in demersal fish.

The effects of sediment transport on temporal variation in radiocesium concentrations in very shallow water off the southern coast of Fukushima, Japan

We studied the very shallow coastal water off Iwaki City, Fukushima Prefecture, and investigated: (1) temporal variation in ¹³⁷Cs concentrations; (2) particle-size distribution of sediments; and (3) the effect on variation by waves, component-fractionated concentration of radiocesium and mineral composition at three sampling stations (Yotsukura, Ena rocky reef and Ena sandy station). There was a decline in ¹³⁷Cs concentrations in sediment samples at all sampling stations between 425 and 1173 days after the accident. All stations had fluctuations in ¹³⁷Cs concentrations between 425 and 800 days. At Ena sandy station and Ena rocky reef stations the declines in ¹³⁷Cs concentrations slowed from about 800 days after the accident. Fluctuations in particle median diameters were seen, as well as in ¹³⁷Cs concentrations. At Yotsukura, where the fluctuation in median diameter was small, a constant decrease in ¹³⁷Cs concentrations was observed. We considered that bioturbation may contribute the constant decrease. At Ena sandy station, where the fluctuation of the median diameter was large, the fluctuation in ¹³⁷Cs concentrations was also large. The movement of sediments was evaluated by the Shields parameter, and results indicated that at any station where the sediment was moved more frequently, the fluctuation in ¹³⁷Cs concentrations was also large. The highly contaminated small particles moved from our stations due to wave action between 425 and 800 days after the accident. The remaining relatively large particles might contribute to the slowing down in reduction of ¹³⁷Cs concentrations from 800 days after the accident. However, the ¹³⁷Cs concentrations in sediments in very shallow water off the southern coast of Fukushima may continue to decline over time.

Marine radioecology after the Fukushima Dai-ichi nuclear accident: Are we better positioned to understand the impact of radionuclides in marine ecosystems?

This paper focuses on how a community of researchers under the COMET (CO-ordination and iMplementation of a pan European projecT for radioecology) project has improved the capacity of marine radioecology to understand at the process level the behaviour of radionuclides in the marine environment, uptake by organisms and the resulting doses after the Fukushima Dai-ichi nuclear accident occurred in 2011. We present new radioecological understanding of the processes involved, such as the interaction of waterborne radionuclides with suspended particles and sediments or the biological uptake and turnover of radionuclides, which have been better quantified and mathematically described. We demonstrate that biokinetic models can better represent radionuclide transfer to biota in non-equilibrium situations, bringing more realism to predictions, especially when combining physical, chemical and biological interactions that occur in such an open and dynamic environment as the ocean. As a result, we are readier now than we were before the FDNPP accident in terms of having models that can be applied to dynamic situations. The paper concludes with our vision for marine radioecology as a fundamental research discipline and we present a strategy for our discipline at the European and international levels. The lessons learned are presented along with their possible applicability to assess/reduce the environmental consequences of future accidents to the marine environment and guidance for future research, as well as to assure the sustainability of marine radioecology. This guidance necessarily reflects on why and where further research funding is needed, signalling the way for future investigations.

Inversion analysis on vertical radiocesium distribution in pond sediment from γ-ray count measurement

Evaluation of vertical distribution of radiocesium in bottom sediment by measuring vertical γ-ray count profile was discussed. A stable inversion formula was derived based on the maximum entropy method. Efficiency of the formula was confirmed by using a low-cost apparatus composed of an array of PIN photodiodes and a single board computer with real-time inversion code. In-door experiment by using five model sediment disks showed good reproducibility of vertical radiocesium profile. On-site experiment was also carried out at a pond in Fukushima to confirm the efficiency. It was suggested that combination of the simple apparatus and MEM inversion formula gave reasonable estimates on vertical radiocesium distribution in bottom sediment of 1 kBq/kg-wet level within about 10 min.

Temporal variation of cesium isotope concentrations and atom ratios in zooplankton in the Pacific off the east coast of Japan

After the Fukushima Daiichi Nuclear Power Plant accident in March 2011, concentrations of cesium isotopes (¹³³Cs, ¹³⁴Cs, and ¹³⁷Cs) were measured in zooplankton collected in the Pacific off the east coast of Japan from May 2012 to February 2015. The time series of the data exhibited sporadic ¹³⁷Cs concentration peaks in zooplankton. In addition, the atom ratio of ¹³⁷Cs/¹³³Cs in zooplankton was consistently high compared to that in ambient seawater throughout the sampling period. These phenomena cannot be explained fully by the bioaccumulation of ¹³⁷Cs in zooplankton via ambient seawater intake, the inclusion of resuspended sediment in the plankton sample, or the taxonomic composition of the plankton. Autoradiography revealed highly radioactive particles within zooplankton samples, which could be the main factor underlying the sporadic appearance of high ¹³⁷Cs concentrations in zooplankton as well as the higher ratio of ¹³⁷Cs/¹³³Cs in zooplankton than in seawater.

Fukushima Daiichi-Derived Radionuclides in the Ocean: Transport, Fate, and Impacts

The events that followed the Tohoku earthquake and tsunami on March 11, 2011, included the loss of power and overheating at the Fukushima Daiichi nuclear power plants, which led to extensive releases of radioactive gases, volatiles, and liquids, particularly to the coastal ocean. The fate of these radionuclides depends in large part on their oceanic geochemistry, physical processes, and biological uptake. Whereas radioactivity on land can be resampled and its distribution mapped, releases to the marine environment are harder to characterize owing to variability in ocean currents and the general challenges of sampling at sea. Five years later, it is appropriate to review what happened in terms of the sources, transport, and fate of these radionuclides in the ocean. In addition to the oceanic behavior of these contaminants, this review considers the potential health effects and societal impacts.

Effects of the nuclear disaster on marine products in Fukushima: An update after five years

Original data (¹³⁴Cs and ¹³⁷Cs, and sampling location) of marine products in Fukushima Prefecture monitored during 2011-2015 (n = 32,492) were analyzed to present an updated detailed description of radiocesium contamination after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident and to examine taxon/habitat-specific decreasing trends in different areas. Furthermore, marine species data presented by the Tokyo Electric Power Company (TEPCO) during 2012-2015 (n = 5458) were analyzed to evaluate the decreasing trends of ¹³⁷Cs inside and outside (within a 20 km radius) of the FDNPP port. Monitoring results by Fukushima Prefecture show that percentages of samples higher than the Japanese regulatory limit of 100 Bq kg^-1-wet (>RL%) were higher, whereas those below the detection limit (<DL%) (mean 8.3 and 7.4 Bq kg^-1-wet for ¹³⁴Cs and ¹³⁷Cs, respectively) were lower in demersal fishes than in pelagic fish or other taxa. However, >RL% and <DL% of demersal fish respectively decreased dramatically and increased gradually to 0.06% and 86.3% in 2015, although slightly elevated radiocesium concentrations were still observed in shallow areas south of the FDNPP. The drastic decrease in radioactivity was supported by the spatiotemporal distribution of radiocesium concentrations in demersal fish, in which higher concentrations that were frequently observed in 2011 and 2012 were rarely detected in 2015, even within the 20 km radius area (maximum 220 Bq kg^-1-wet in Japanese rockfish Sebastes cheni). Statistical analyses of TEPCO data revealed that ¹³⁷Cs concentrations both inside and outside of the FDNPP port decreased exponentially with time: The respective geometric mean days of ecological half-lives were 218 d and 386 d. These results show clearly that the contamination level of marine products in Fukushima Prefecture, even within the 20 km radius area, has decreased drastically during the five years after the FDNPP accident, although ¹³⁷Cs concentrations higher than 10 kBq kg^-1-wet were still detected in some specimens of sedentary rockfishes (S. cheni, Sebastes oblongus, and Sebastes pachycephalus) in the FDNPP port. Fishing operations started on a trial basis in June 2012 have gradually expanded the target areas and species. Careful monitoring should be continued to accelerate the restoration of coastal fisheries in Fukushima Prefecture.

Radioactive cesium dynamics derived from hydrographic observations in the Abukuma River Estuary, Japan

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from <0.1 to 0.1-2.3, the mixing diagram showed that dissolved radioactive cesium concentrations increased, because of desorption. Desorption from suspended particles explained 36% of dissolved radioactive cesium in estuarine water. However, the dissolved and particulate radioactive cesium concentrations in the sea decreased sharply because of dilution. It is thought that more than 80% of the discharged particulate radioactive cesium was deposited off the river mouth, where the radioactive cesium concentrations in sediment were relatively high (217-2440 Bq kg(-1)). Radioactive cesium that was discharged to the sea was transported southward by currents driven by the density distribution.

Factors controlling the spatiotemporal variation of (137)Cs in seabed sediment off the Fukushima coast: implications from numerical simulations

We used numerical simulations to investigate major controls on spatiotemporal variations of (137)Cs activities in seabed sediments off the Fukushima coast during the first year after the Fukushima Daiichi Nuclear Power Plant accident. The numerical model we used includes (137)Cs transfer between bottom water and sediment by adsorption and desorption, and radioactive decay. The model successfully reproduced major features of the observed spatiotemporal variations of (137)Cs activities in sediments. The spatial pattern of (137)Cs in sediments, which mainly reflected the history of (137)Cs activities in bottom water overlying the sediments and the sediment particle size distribution, became established during the first several months after the accident. The simulated temporal persistence of the (137)Cs activity in the sediments was due to adsorption of (137)Cs onto the sediment mineral fraction having a long desorption timescale of (137)Cs. The simulated total (137)Cs inventory in sediments integrated over the offshore area, where most of the monitoring stations were located, was on the order of 10(13) Bq; this value is consistent with a previous estimate based on observed data. Taking into account (137)Cs activities in sediments in both the coastal area and in the vicinity of the power plant, the simulated total inventory of (137)Cs in sediments off the Fukushima coast increased to a value on the order of 10(14) Bq.