Toshihiro Wada

Radiological impact of the nuclear power plant accident on freshwater fish in Fukushima: An overview of monitoring results.

Radionuclide ((131)I, (134)Cs, and (137)Cs) concentrations of monitored freshwater fish species collected from different habitats (rivers, lakes, and culture ponds) in Fukushima Prefecture during March 2011-December 2014 (total 16 species, nxa0=xa02692) were analyzed to present a detailed description of radionuclide contamination after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, and to elucidate species-specific spatiotemporal declining trends of (137)Cs concentration for their respective habitats. Low concentrations of (131)I (≤24xa0Bqxa0kg(-1)-wet) were detected from only 11 samples collected during March-June 2011, demonstrating that (131)I transferred to freshwater fish were not intense. In river and lake fishes, a more gradual decrease and higher radiocesium ((134)Cs, (137)Cs) concentrations were observed than in culture pond fishes, which strongly implied that radiocesium in freshwater fish species was mainly bioaccumulated through the food web in the wild. During 2011-2014, percentages above the Japanese regulatory limit of 100xa0Bqxa0kg(-1)-wet for radiocesium in river and lake fish (14.0% and 39.6%, respectively) were higher than in monitored marine fish (9.9%), indicating longer-term contamination of freshwater fish species, especially in lakes. Higher radiocesium concentrations (maximum 18.7xa0kBqxa0kg(-1)-wet in Oncorhynchus masou) were found in the northwestern areas from the FDNPP with higher deposition. However, radiocesium contamination levels were regarded as 1-2 orders of magnitude less than those after the Chernobyl accident. Lagged increase of (137)Cs concentration and longer ecological half-lives (Teco: 1.2-2.6xa0y in the central part of Fukushima Prefecture) were observed in carnivorous salmonids (O. masou, Salvelinus leucomaenis), whereas a rapid increase and decrease of (137)Cs concentration and shorter Teco (0.99 and 0.69xa0y) were found in herbivorous and planktivorous osmerids (Plecoglossus altivelis, Hypomesus nipponensis) with younger age at maturity. Comparison of Teco among salmonids, osmerids, and cyprinids suggests that, in addition to the fish feeding habits and life-cycles, hydraulic conditions in rivers and lakes (e.g., turnover time), which are expected to affect radiocesium concentration in prey items, are an important factor affecting the (137)Cs decreasing rate of freshwater fish.

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 (nxa0=xa032,492) 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 (nxa0=xa05458) were analyzed to evaluate the decreasing trends of 137Cs inside and outside (within a 20xa0km radius) of the FDNPP port. Monitoring results by Fukushima Prefecture show that percentages of samples higher than the Japanese regulatory limit of 100xa0Bqxa0kg-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 20xa0km radius area (maximum 220xa0Bqxa0kg-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 218xa0d and 386xa0d. These results show clearly that the contamination level of marine products in Fukushima Prefecture, even within the 20xa0km radius area, has decreased drastically during the five years after the FDNPP accident, although 137Cs concentrations higher than 10xa0kBqxa0kg-1-wet were still detected in some specimens of sedentary rockfishes (S.xa0cheni, 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.

Pollution of radiocesium and radiosilver in wharf roach (Ligia sp.) by the Fukushima Dai-ichi Nuclear Power Plant accident

Radionuclide concentrations in wharf roaches inhabiting coastal areas of Honshu, Japan, were investigated in October 2011 and June 2012. Relative high concentrations of 110mAg (2.1–127xa0Bqxa0kg-wet−1), 134Cs (2.6–61xa0Bqxa0kg-wet−1), and 137Cs (3.5–92xa0Bqxa0kg-wet−1) were detected in specimens from the eastern Honshu areas. Significantly lower 137Cs concentrations (0.7–1.6xa0Bqxa0kg-wet−1) were detected in specimens from western and northern Honshu. The decay-corrected 137Cs concentration was significantly inversely correlated with the distance from the Fukushima Dai-ichi Nuclear Power Plant. Thus, wharf roach may serve as a good bioindicator for monitoring radioactive contamination of its habitats.

Radiocesium in Ponds in the Near Zone of Fukushima Dai-ichi NPP

The article presents the results of studying radiocesium concentration and distribution between dissolved and particulate forms in ponds in the near zone of Fukushima Dai-ichi NPP NPP after the 2011 accident. The total concentration of 137Cs in pond water and its variations are shown to be largely governed by the concentration of particulate matter being as high as 68 Bq/L, compared with 5 Bq/L in solution. The values of the distribution coefficient Kd in the ponds are similar to those in rivers and in large flow-through lakes in the accidentally polluted zone at Fukushima Dai-ichi NPP. The contributions of the main competing ions K+ and NH4+ to radiocesium desorption from solid particles into solution were similar for the ponds; however, their relative effect on dissolved radiocesium concentration varied over time because of variations in ammonium concentration.

Behavior of 137Cs in ponds in the vicinity of the Fukushima Dai-ichi nuclear power plant

137Cs activity concentration in the water of four ponds, Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), that are within 10xa0km of the Fukushima Dai-ichi nuclear power plant were observed from April 2015 to August 2016. 137Cs inventories in soils surrounding SU, FS, IZ, and KM were 6.4, 2.9, 2.1, and 0.9xa0MBq m-2, respectively. 137Cs inventories in the bottom sediments of SU, FS, IZ, and KM were 13, 8.9, 1.6, and 1.1xa0MBq m-2, respectively. Higher 137Cs inventories in bottom sediment than those of soil in SU and FS suggest that 137Cs was delivered to and accumulated in these ponds. Mean total 137Cs activity concentrations in SU, FS, IZ, and KM were 41, 13, 9.5, and 1.4 Bq L-1, respectively. Particulate 137Cs concentration accounted for 71-90% of total 137Cs in the water samples, on average. The mean distribution coefficient, Kd, in SU, FS, IZ, and KM was 1.3xa0×xa0105, 2.1xa0×xa0105, 1.7xa0×xa0105, and 6.2xa0×xa0105xa0Lxa0kg-1, respectively. These Kd values were higher than the Kd values observed in the Chernobyl area by 1-2 orders of magnitude. Although no significant decreasing trends were found, dissolved 137Cs activity concentration tended to be low during winter in all four ponds. Dissolved 137Cs activity concentrations were proportional to K+ and DOC concentrations in all the ponds. The results from principal component analysis performed for 137Cs activity concentration and water chemistry data sets suggested that there were different mechanisms behind variability of dissolved 137Cs activity concentrations for each pond. Continuous monitoring is required to reveal temporal trends in 137Cs activity concentrations of these waters and controlling factors of such in closed water systems in Fukushima.

Horizontal movement and emigration of juvenile spotted halibut Verasper variegatus released in a shallow brackish lagoon: Matsukawa-ura, northeastern Japan, revealed by acoustic telemetry

To elucidate the post-release movement and emigration of juvenile spotted halibut Verasper variegatus, ten hatchery-reared age-0 individuals (10xa0months old, 17.0–19.7xa0cm total length) surgically implanted with acoustic transmitters were released near the center of Matsukawa-ura, a shallow brackish lagoon, on 25 November 2009 (water temperature 13.8xa0°C). They were monitored by ten acoustic receivers for 5xa0months. Of ten individuals, eight left the release site during December–March. Three of these emigrated to the outer ocean on 17 and 29 January and 30 March 2010, respectively. Juveniles probably started wintering migration in December and January when water temperatures decreased considerably in the lagoon (mean 9.5 and 6.0xa0°C, respectively). They stopped their migration in the coldest month, February (mean 5.7xa0°C), and restarted it in March when water temperatures frequently exceeded a plausible threshold for movement (≥6xa0°C). Statistical analyses revealed that the fish started migration significantly more frequently at nighttime. The migration track of an individual recorded from 11 to 30 March showed gradual nocturnal movements and a slow migration speed (estimated maximum speed 2.2xa0m/min). Our results revealed that water temperature primarily governed the seasonal timing of nocturnal migratory movements of juvenile spotted halibut.