Radiocaesium transfer and radiation exposure of frogs in Fukushima Prefecture

Evaluation of DNA damage and stress in wildlife chronically exposed to low-dose, low-dose rate radiation from the Fukushima Dai-ichi Nuclear Power Plant accident

The health effects associated with chronic low-dose, low-dose rate (LD-LDR) exposures to environmental radiation are uncertain. All dose-effect studies conducted outside controlled laboratory conditions are challenged by inherent complexities of ecological systems and difficulties quantifying dose to free-ranging organisms in natural environments. Consequently, the effects of chronic LD-LDR radiation exposures on wildlife health remain poorly understood and much debated. Here, samples from wild boar (Sus scrofa leucomystax) and rat snakes (Elaphe spp.) were collected between 2016 and 2018 across a gradient of radiation exposures in Fukushima, Japan. In vivo biomarkers of DNA damage and stress were evaluated as a function of multiple measurements of radiation dose. Specifically, we assessed frequencies of dicentric chromosomes (Telomere-Centromere Fluorescence in situ Hybridization: TC-FISH), telomere length (Telo-FISH, qPCR), and cortisol hormone levels (Enzyme Immunoassay: EIA) in wild boar, and telomere length (qPCR) in snakes. These biological parameters were then correlated to robust calculations of radiation dose rate at the time of capture and plausible upper bound lifetime dose, both of which incorporated internal and external dose. No significant relationships were observed between dicentric chromosome frequencies or telomere length and dose rate at capture or lifetime dose (p value range: 0.20-0.97). Radiation exposure significantly associated only with cortisol, where lower concentrations were associated with higher dose rates (r² = 0.58; p < 0.0001), a relationship that was likely due to other (unmeasured) factors. Our results suggest that wild boar and snakes chronically exposed to LD-LDR radiation sufficient to prohibit human occupancy were not experiencing significant adverse health effects as assessed by biomarkers of DNA damage and stress.

Assessing the impact of large-scale farmland abandonment on the habitat distributions of frog species after the Fukushima nuclear accident

Rice paddies function as wetlands; therefore, abandoned paddy fields cause a loss of habitats for aquatic species, such as amphibians. Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, paddy fields around the plant were abandoned and rapidly dried. To identify the impact of large-scale abandonment of paddy fields on the habitats of frogs, we investigated changes in the distributions of four frogs that breed in paddy fields using niche modeling and field surveys. The spatial distributions of suitable habitats before and after the accident for each frog were created using MaxEnt. In the area where rice cropping was restricted due to radioactive contamination, the areas of suitable habitats decreased for Pelophylax porosus porosus but increased or remained unchanged for other frogs after the accident. Additionally, field surveys conducted in 2014 indicated that the ratios of breeding sites in the area where rice cropping was restricted were significantly lower for P. p. porosus and Hyla japonica than outside this area. Thus, 3 years after the accident, one species strongly dependent upon paddy fields rapidly disappeared over a large area. Although amphibian populations or monitoring data were not available to examine changes directly after the accident in the study area, this research showed that the combination of niche modeling and field survey was effective for predicting species response after an accident and revealed that large-scale disasters sufficient to disrupt agricultural activity could markedly change the distribution of species reliant on habitats created by human activity.

Radiocesium concentration ratios and radiation dose to wild rodents in Fukushima Prefecture

Radiocesium was dispersed from the Fukushima Dai-ichi disaster in March 2011, causing comparatively high radioactive contamination in nearby environments. Radionuclide concentrations in wild rodents (Apodemus argenteus, and Apodemus speciosus) within these areas were monitored from 2012 to 2016. However, whole-organism to soil transfer parameters (i.e., concentration ratio, CR_wo-soil) for wild rodents at Fukushima were not determined and hence were lacking from the international transfer databases. We augmented the 2012-2016 data by collecting soil activity concentrations (Bq kg^-1, dry mass) from five rodent sampling sites in Fukushima Prefecture, and developed corresponding CR_wo-soil values for radiocesium (¹³⁴Cs and ¹³⁷Cs) based on rodent radioactivity concentrations (Bq kg^-1, fresh mass). The CR_wo-soil were added to the Wildlife Transfer Database (WTD; http://www.wildlifetransferdatabase.org/), supporting the development of the International Commission on Radiological Protection's (ICRP) environmental protection framework, and increasing the WTD from 84 to 477 entries for cesium and Muridae ('Reference Rat'). Significant variation occurred in CR_wo-soil values between study sites within Fukushima Prefecture. The geometric mean CR_wo-soil, in this paper, was higher than that reported for Muridae species for Chernobyl. Radiocaesium absorbed dose rates were also estimated for wild rodents inhabiting the five Fukushima study sites and ranged from 1.3 to 33 μGy h^-1. Absorbed dose rates decreased by a factor of two from 2012 to 2016. Dose rates in highly contaminated areas were within the ICRP derived consideration reference level for Reference Rat (0.1-1 mGy d^-1), suggesting the possible occurrence of deleterious effects and need for radiological effect studies in the Fukushima area.

Dose-dependent genomic DNA hypermethylation and mitochondrial DNA damage in Japanese tree frogs sampled in the Fukushima Daiichi area

The long-term consequences of the nuclear disaster at the Fukushima Daiichi Nuclear Power Plant (FDNPP) that occurred on March 2011, have been scarcely studied on wildlife. We sampled Japanese tree frogs (Dryophytes japonicus), in a 50 -km area around the FDNPP to test for an increase of DNA damages and variation of DNA methylation level. The ambient dose rate ranged between 0.4 and 2.8 μGy h^-1 and the total estimated dose rate absorbed by frogs ranged between 0.3 and 7.7 μGy h^-1. Frogs from contaminated sites exhibited a dose-dependent increase of global genomic DNA methylation level (5-mdC and 5-hmdC) and of mitochondrial DNA damages. Such DNA damages may indicate a genomic instability, which may induce physiological adaptations governed by DNA methylation changes. This study stresses the need for biological data combining targeted molecular methods and classic ecotoxicology, in order to better understand the impacts on wildlife of long term exposure to low ionizing radiation levels.

Radiocesium concentrations and GPS-coupled dosimetry in Fukushima snakes

One of the largest releases of radioactive contamination in history occurred at Japan's Fukushima Daiichi Nuclear Power Plant (FDNPP). Although the accident happened in 2011, questions still persist regarding its ecological impacts. For example, relatively little is known about radiocesium accumulation in snakes, despite their high trophic status, limited home range sizes, and close association with soil where many radionuclides accumulate. This study presents one of the most comprehensive radioecological studies of snakes published to date using a combination of whole-body radiocesium analyses, GPS transmitters, and optically stimulated luminescence (OSL) dosimeters. The objectives were to: 1) quantify whole-body radiocesium activity concentrations and internal dose rates among several common species of snakes within and around the Fukushima Exclusion Zone (FEZ), 2) determine effects of species, sex, and body size on radiocesium activity concentrations, 3) measure external dose rates using GPS-coupled dosimeters deployed on free-ranging snakes, 4) compare field-derived empirical dose rates to those generated by computer simulation software (i.e., the ERICA tool), and 5) determine if incorporating snake behavior into computer models improve simulated estimates of external dose. Whole-body radiocesium levels for snakes were highly variable among individuals (16 to 25,000 Bq/kg, FW), but were influenced more by levels of local contamination than species, sex, or size. Doses recorded by OSL dosimeters on snakes, as well as modeling in ERICA, suggest that individual movements and behavior have a substantial influence on dose rates to snakes. However, dose estimates produced with ERICA were comparable to dose received by tracked snakes. The average external plus internal dose rate for snakes captured in the FEZ was 3.6-3.9 μGy/h, with external dose contributing 80% to the total. Further research regarding reptile-specific benchmark dose rates would improve risk assessment for reptiles in radiologically contaminated areas.

Radiocesium (137Cs) concentrations in the two-toed amphiuma (Amphiuma means) and the lesser siren (Siren intermedia)

This study sought to determine radiocesium (¹³⁷Cs) concentrations in two species of aquatic salamanders - the two-toed amphiuma (Amphiuma means) and the lesser siren (Siren intermedia) on the Savannah River Site. Concentrations (¹³⁷Cs Bq/g, dry wt) of the two species were similar at both ¹³⁷Cs-contaminated (A. means = 0.733 ± 0.242, n = 5; S. intermedia = 0.839 ± 0.722, n = 5) and reference sites (A. means = 0.028 ± 0.020, n = 5; S. intermedia = 0.042 ± 0.027, n = 11). Salamanders captured in areas impacted by ¹³⁷Cs contamination exhibited significantly higher ¹³⁷Cs concentrations than individuals captured at reference sites (U = 146, p < 0.001). Salamander size (snout-vent length) was not correlated with whole-body ¹³⁷Cs. An analysis of our data using the ERICA tool suggests that negative impacts due to ¹³⁷Cs exposure are unlikely in these species. Overall, these data indicate that A. means and S. intermedia inhabiting contaminated sites bioaccumulate but do not biomagnify ¹³⁷Cs.

Estimating radiological exposure of wildlife in the field

The assessment of the ecological impact due to radionuclides at contaminated sites requires estimation of the exposure of wildlife, in order to correlate radiation dose with known radiological effects. The robust interpretation of field data requires consideration of possible confounding effects (e.g., from the tsunami at Fukushima) and an accurate and relevant quantification of radiation doses to biota. Generally, in field studies the exposure of fauna and flora has often been characterised as measurements of the ambient dose rate or activity concentrations in some components of the environment. The use of such data does not allow the establishment of a robust dose-effect relationship for wildlife exposed to ionising radiation in the field. Effects of exposure to radioactivity depend on the total amount of energy deposited into exposed organisms, which is estimated by adding doses (or dose rates) for all radionuclides and exposure pathways. Realistic dose estimation needs to reflect the entire story of the organisms of interest during their whole exposure period. The process of identifying and collecting all the related information should allow the "W" questions (Which organisms are exposed, Where, When and hoW) to be answered. Some parameters are well known to influence dose (rate): the organism life stage, its ecological characteristics (e.g. habitat, behaviour), the source term properties (e.g. discharging facility, nature of radiation), etc. The closer the collated data are to the ideal data set, the more accurate and realistic the dose (rate) assessment will be. This means characterising each exposure pathway (internal and external), the activity concentration in each exposure source, the time each organism spends in a given place, as well as the associated dose. In this paper the process of data collation in view of dose reconstruction is illustrated for Japanese birds exposed to radioactive deposition following the Fukushima accident. With respect to the Chernobyl Exclusion Zone we will also consider variability under field conditions, availability of relevant datasets and options for better estimating internal and external doses received by wildlife.

Field effects studies in the Chernobyl Exclusion Zone: Lessons to be learnt

In the initial aftermath of the 1986 Chernobyl accident there were detrimental effects recorded on wildlife, including, mass mortality of pine trees close to the reactor, reduced pine seed production, reductions in soil invertebrate abundance and diversity and likely death of small mammals. More than 30 years after the Chernobyl accident there is no consensus on the longer-term impact of the chronic exposure to radiation on wildlife in what is now referred to as the Chernobyl Exclusion Zone. Reconciling this lack of consensus is one of the main challenges for radioecology. With the inclusion of environmental protection in, for instance, the recommendations of the International Commission on Radiological Protection (ICRP), we need to be able to incorporate knowledge of the potential effects of radiation on wildlife within the regulatory process (e.g. as a basis on which to define benchmark dose rates). In this paper, we use examples of reported effects on different wildlife groups inhabiting the Chernobyl Exclusion Zone (CEZ) as a framework to discuss potential reasons for the lack of consensus, consider important factors influencing dose rates organisms receive and make some recommendations on good practice.

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.

Bioaccumulation of 137Cs in anuran larvae utilizing a contaminated effluent canal on the U.S. Department of Energy's Savannah River Site

As a result of activities such as nuclear weapons testing, nuclear power generation and waste disposal, and nuclear accidents, radiocesium (¹³⁷Cs) is a widely distributed radio-contaminant of concern that readily accumulates in exposed wildlife. Although bioaccumulation of ¹³⁷Cs is an important factor for understanding its fate within the environment, there are currently limited data available on bioaccumulation patterns of ¹³⁷Cs in amphibians, despite their widespread distribution and potential to transport contaminants between aquatic and terrestrial ecosystems. Therefore, the objective of this study was to determine the amount of time necessary for anuran larvae experimentally placed in a contaminated system to reach a steady-state whole-body ¹³⁷Cs concentration, and to determine the threshold at which that steady-state ¹³⁷Cs concentration occurred for tadpoles within our study system. By restricting uncontaminated bullfrog (Lithobates catesbeianus) larvae to three experimental enclosures located along a¹³⁷Cs contaminated effluent canal on the U.S. Department of Energy's Savannah River Site, we modeled ¹³⁷Cs uptake through time using the von Bertalanffy modification of the Richards Model. The results of our modified Richards Model indicate that bullfrog tadpoles achieved steady-state ¹³⁷Cs concentrations of 3.68-4.34 Bq/g¹³⁷Cs dry whole-body weight after 11.63-15.50 days of exposure among sampling sites, with an average of 3.94 Bq/g after 14.07 days exposure. Radiocesium accumulation in bullfrog tadpoles was more rapid than that reported for other biota studied from other contaminated systems, likely due to incidental ingestion of sediments and a diet consisting of periphyton and other items that accumulate high levels of ¹³⁷Cs. Given their rapid accumulation of ¹³⁷Cs and inability to leave aquatic environments prior to metamorphosis, our data suggest amphibian larvae may be useful indicators for monitoring ¹³⁷Cs distributions and bioavailability within aquatic systems.