Dose rate estimation of freshwater wildlife inhabiting irrigation ponds in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident.

To assess the risks of ionising radiation to freshwater environments in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident, the absorbed dose rates to aquatic organisms possibly inhabiting the irrigation ponds were estimated using the ERICA Assessment Tool from 134Cs and 137Cs radioactivity monitoring data for the period 2013 to 2017. In each year, the total dose rates to benthic organisms were in the same or higher levels compared with those to pelagic organisms. Among pelagic organisms, the total dose rates to amphibians, birds, and pelagic fish were two orders of magnitude higher than those to plankton. The total dose rates to insect larvae, which attained a maximum of 130 µGy h-1, were higher than those to the other benthic organisms. The dose rates to benthic organisms increased from 2013 to 2015 and remained constant thereafter. In 50-93% of ponds, the dose rates to at least one taxon of freshwater organism, all of which were benthic organisms, exceeded the ERICA screening level (10 µGy h-1). Comparison of the estimated dose rates with the ICRP's derived consideration reference levels (DCRLs) suggests that radioactive contamination was not likely to damage amphibians, birds, pelagic fish, benthic fish, crustaceans, and insect larvae inhabiting most of the irrigation ponds in the exclusion zone. However, this comparison also suggests that there was some chance of deleterious effects occurring to birds and benthic fish in a limited number of the most severely contaminated irrigation ponds.

Effects of chronic γ-irradiation on growth and sexual maturation of the Tohoku hynobiid salamander, Hynobius lichenatus.

There are still considerable gaps in knowledge regarding the biological effects of chronic ionising radiation exposure in amphibians. To fill these gaps, Tohoku hynobiid salamanders, Hynobius lichenatus (Amphibia, Caudata), were chronically irradiated with 137Cs γ-rays from embryonic to adult stages over 1954 days, and the effects on their growth and sexual maturation were examined under laboratory conditions. Irradiation at a dose rate of 33 µGy h-1 had some stimulatory effects on growth (body weight increase) of H. lichenatus, while growth was temporarily or permanently suppressed at 150 or 510 µGy h-1, respectively. On day 1802, secondary sexual characteristics (a tubercle at the anterior angle of the cloacal vent for males and ovisac development for females) were observed in 91% of the salamanders irradiated at 33 µGy h-1, and in a similar percentage of non-irradiated controls. At 150 and 510 µGy h-1, secondary sexual characteristics were not observed in any individuals. These results suggest that the derived consideration reference level (DCRL) of the International Commission on Radiological Protection (ICRP) for Reference Frog, i.e. 40-400 µGy h-1, is applicable for the protection of H. lichenatus, and that growth and sexual maturation of this salamander may not have been adversely affected even in the most severely contaminated area in Fukushima, where the highest dose rate to salamanders was estimated to be 50 µGy h-1. However, observations in the contaminated area are required to confirm this conclusion, considering the possible confounding factors which may make this salamander more sensitive to radiation in the natural environment than under laboratory conditions.

Radiocaesium contamination and dose rate estimation of terrestrial and freshwater wildlife in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident.

To characterise the radioactive contamination of terrestrial and freshwater wildlife caused by the Fukushima Dai-ichi Nuclear Power Plant accident, biological samples, namely, fungi, mosses, plants, amphibians, reptiles, insects, molluscs, and earthworms, were collected mainly from the forests of the exclusion zone in the Fukushima Prefecture from 2011 to 2012. Caesium-134 and 137Cs were detected by gamma spectrometry in almost all the samples. Fungi, ferns, and mosses accumulated high amounts of radiocaesium, as they did in Chernobyl, with 134Cs + 137Cs activity concentrations of 104-106 Bq kg-1 fresh mass (FM). Earthworms, amphibians, and the soft tissue of the garden snail Acusta despecta sieboldiana, also had levels as high as 104-105 Bq kg-1 FM of 134Cs + 137Cs. Most of the estimated total (internal + external) dose rates to herbaceous plants, amphibians, insects, and earthworms were below the corresponding derived consideration reference levels (DCRLs) recommended by the ICRP. This suggests that, in most cases, there was little chance of deleterious effects of ionising radiation on these organisms in the exclusion zone for the first year after the accident, though the dose rates were underestimated mainly due to the lack of consideration of short-lived radionuclides.

Radiocaesium contamination of wild boars in Fukushima and surrounding regions after the Fukushima nuclear accident.

Analysis of radioactivity data obtained under the food monitoring campaign in Japan indicates that elevated 134Cs+137Cs activity concentrations in wild boar meat remained constant or slowly decreased in Fukushima and surrounding prefectures from 2011 to 2015. The activity concentrations in some samples are still over the regulatory limit of 100 Bq kg-1 fresh weight, even in 2015. Activity concentrations of 137Cs in muscle of wild boars we captured in 2011 were higher than those in kidney, liver, spleen, heart and lung. A food processing retention factor, Fr, was 0.5 or 0.6 for 137Cs when the wild boar meat was boiled, suggesting that a parboiling process is effective for reduction of radiocaesium intake from wild boar meat.

Dose rate estimation of the Tohoku hynobiid salamander, Hynobius lichenatus, in Fukushima.

The radiological risks to the Tohoku hynobiid salamanders (class Amphibia), Hynobius lichenatus due to the Fukushima Dai-ichi Nuclear Power Plant accident were assessed in Fukushima Prefecture, including evacuation areas. Aquatic egg clutches (n = 1 for each sampling date and site; n = 4 in total), overwintering larvae (n = 1-5 for each sampling date and site; n = 17 in total), and terrestrial juveniles or adults (n = 1 or 3 for each sampling date and site; n = 12 in total) of H. lichenatus were collected from the end of April 2011 to April 2013. Environmental media such as litter (n = 1-5 for each sampling date and site; n = 30 in total), soil (n = 1-8 for each sampling date and site; n = 31 in total), water (n = 1 for each sampling date and site; n = 17 in total), and sediment (n = 1 for each sampling date and site; n = 17 in total) were also collected. Activity concentrations of (134)Cs + (137)Cs were 1.9-2800, 0.13-320, and 0.51-220 kBq (dry kg) (-1) in the litter, soil, and sediment samples, respectively, and were 0.31-220 and <0.29-40 kBq (wet kg)(-1) in the adult and larval salamanders, respectively. External and internal absorbed dose rates to H. lichenatus were calculated from these activity concentration data, using the ERICA Assessment Tool methodology. External dose rates were also measured in situ with glass dosimeters. There was agreement within a factor of 2 between the calculated and measured external dose rates. In the most severely contaminated habitat of this salamander, a northern part of Abukuma Mountains, the highest total dose rates were estimated to be 50 and 15 µGy h(-1) for the adults and overwintering larvae, respectively. Growth and survival of H. lichenatus was not affected at a dose rate of up to 490 µGy h(-1) in the previous laboratory chronic gamma-irradiation experiment, and thus growth and survival of this salamander would not be affected, even in the most severely contaminated habitat in Fukushima Prefecture. However, further studies of the adult salamanders may be required in order to examine whether the most severe radioactive contamination has any effects on sensitive endpoints, since the estimated highest dose rate to the adults exceeded some of the guidance dose rates proposed by various organisations and programmes for the protection of amphibians, which range from 4 to 400 µGy h(-1). Conversely, at one site in Nakadori, a moderately contaminated region in Fukushima Prefecture, the dose rate to the adult salamanders in spring of 2012 was estimated to be 0.2 µGy h(-1). Estimated dose rates to the overwintering larvae in spring of 2012 were 1 and 0.2 µGy h(-1) at one site in Nakadori, and in Aizu, a less contaminated region in Fukushima Prefecture, respectively. These results suggest that there is a low risk that H. lichenatus will be affected by radioactive contamination in these districts, though further studies on dose rate estimation are required for definitive risk characterisation.

Effects of chronic γ-irradiation on growth and survival of the Tohoku hynobiid salamander, Hynobius lichenatus.

The Tohoku hynobiid salamanders, Hynobius lichenatus, were chronically irradiated with γ-rays from embryonic to juvenile stages for 450 days. At 490 µGy h(-1) or lower dose rates, growth and survival were not significantly affected by irradiation, and any morphological aberrations and histological damages were not observed. At 4600 µGy h(-1), growth was severely inhibited, and all the individuals died mostly at the juvenile stage. Chronic LD50 was 42 Gy as a total dose. In the liver, the number of hematopoietic cells was significantly reduced in the living juveniles, and these cells disappeared in the dead juveniles. In the spleen, mature lymphocytes were depleted in the living larvae, and almost all the heamtopoietic cells disappeared in the dead juveniles. These results suggest that this salamander died due to acute radiation syndrome, i.e., hematopoietic damage and subsequent sepsis caused by immune depression. The death would be also attributed to skin damage inducing infection. At 18,000 µGy h(-1), morphological aberrations and severe growth inhibition were observed. All the individuals died at the larval stage due to a multiple organ failure. Chronic LD50 was 28 Gy as a total dose. Assuming that chronic LD50 was 42 Gy at lower dose rates than 4600 µGy h(-1), a chronic median lethal dose rate could be estimated to be <340 µGy h(-1) for the whole life (>14 years). These results suggest that, among guidance dose rates, i.e., 4-400 µGy h(-1), proposed by various organisations and research programmes for protection of amphibians and taxonomic groups or ecosystems including amphibians, most of them would protect this salamander but the highest value may not on the whole life scale.