Dose rate estimation of freshwater wildlife inhabiting irrigation ponds in restricted areas of the Fukushima Dai-ichi Nuclear Power Plant accident

To assess risks of ionising radiation to freshwater environments in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident, absorbed dose rates to aquatic organisms possibly inhabiting irrigation ponds were estimated using the ERICA Assessment Tool from the 134Cs and 137Cs radioactivity monitoring data from 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, total dose rates to amphibian, bird and pelagic fish were two orders of magnitude higher than those to plankton. Total dose rates to insect larva, 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 % ponds, dose rates to at least one taxon of freshwater organisms, 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 suggests that radioactive contamination was not likely to damage amphibians, birds, pelagic fish, benthic fish, crustaceans, and insect larvae, except for a limited number of the most severely contaminated ponds

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 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

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 134Cs + 137Cs 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