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

Distinct food-web transfers of 137Cs to fish in river and lake ecosystems: A case study focusing on masu salmon in the Fukushima evacuation zone

This study was conducted to elucidate the spatial and size variations, and food-web transfer of 137Cs in freshwater fish in the upper reaches of the Ukedo River system, a highly contaminated river system flowing through the Fukushima evacuation zone. Fish collection and environmental surveys were conducted in the summer of 2020 at five forest rivers and at the Ogaki Dam reservoir (an artificial lake) with different air dose rates (mean 0.20-3.32 μSv/h). From the river sites, two salmonid species (masu salmon and white-spotted charr) were sampled, with masu salmon generally exhibiting higher 137Cs concentrations, ranging widely (10.6 Bq/kg-wet to 13.0 kBq/kg-wet) depending on the fish size (size effect) and site. The 137Cs concentrations in masu salmon were explained by the air dose rates, 137Cs concentrations in water, sediments (excluding the lake site), and primary producers, with site-specific variations. In the rivers, masu salmon (fluvial type with parr marks) mainly fed on terrestrial insects with higher 137Cs concentrations compared with those of aquatic insects, indicating that 137Cs was transferred mainly to fish through the allochthonous forest food-web during summer. In the lake, masu salmon (lake-run type with larger size and silvery body coloration) mainly preyed on smaller fish with lower 137Cs concentrations, demonstrating that 137Cs is transferred to fish through the autochthonous lake food-web with biomagnification. Differences in 137Cs concentrations among masu salmon (mean 441 Bq/kg-wet) and other fish species (mean 74.8 Bq/kg-wet to 2.35 kBq/kg-wet) were also found in the lake. The distinct 137Cs transfers to river and lake fish were supported by stable isotope analysis: δ15N and δ13C values enriched stepwisely through the food-webs were, respectively, higher and lower in the lake. Our results obtained using multiple approaches clearly revealed the distinct food-web transfer of 137Cs in river and lake ecosystems. These findings can contribute to prediction of radioactive contamination in freshwater fish in the Fukushima evacuation zone.

Dose rate dependent genotoxic and metabolic effects predict onset of impaired development and mortality in Atlantic salmon (S. salar) embryos exposed to chronic gamma radiation

Release of radionuclides to the environment from either nuclear weapon and fuel cycles or from naturally occurring radionuclides (NORM) may cause long term contamination of aquatic ecosystems and chronic exposure of living organisms to ionizing radiation, which in turn could lead to adverse effects compromising the sustainability of populations. To address the effects of chronic ionizing radiation on the development of fish, Atlantic salmon embryos were exposed from fertilization until hatching (88 days, 550 day-degree) to dose rates from 1 to 30 mGy·h-1 gamma radiation (60Co). The lowest adopted dose rate was similar to the highest doses measured in some water bodies right after the Chernobyl accident (1 mGy·h-1), however, well above current environmentally realistic scenarios (20 μGy·h-1), or the threshold assumed for significant effects on fish population (40 μGy·h-1). Dose dependent effects were observed on survival, hatching, morbidity, DNA damage, antioxidant defenses, and metabolic status. Histopathological analysis showed dose rate dependent impairment of eye and brain tissues development and establishment of epidermal mucus cell layers accompanied by increased DNA damage at doses ≥1.3 Gy (dose rates ≥1 mGy·h-1). At ≥32.8 Gy (dose rates ≥20 mGy·h-1) deformities and developmental growth defects resulted in respective 46 and 95 % pre-hatch mortality. The 10 mGy·h-1 exposure (≥ 12 Gy total dose) caused significantly increased DNA damage, impaired eye development, and both premature and delayed hatching, while no deformities or effect on survival were observed. We observed a dose rate dependent reduction from dose rate ≥ 20 mGy·h-1 (≥ 27 Gy total dose) on antioxidant SOD, catalase and glutathione reductase enzyme activities. The reduction of antioxidant enzyme activities was in line with observed developmental delay and disturbance to time of hatching. Metabolomic profiles showed a clear shift at dose rates ≥10 mGy·h-1 (≥ 12 Gy total dose) in pathways related to oxidative stress, detoxification, DNA damage and repair. Due to gamma radiation exposure, a switch of central metabolism from glycolysis, citric acid cycle and lactate production towards pentose phosphate pathway indicated a rewiring mechanism for increased production of reductive equivalents to maintain redox homeostasis at the expense of energy output and thus embryonic development.

Effect of potassium ferric hexacyanoferrate in feed on 137Cs uptake and excretion by silver Prussian carp

The present work documents potassium ferric hexacyanoferrate (KFCF) KFe[Fe(CN)6] containing feed to be an effective and inexpensive countermeasure to reduce the 137Cs contamination of fish. Laboratory aquarium experiments were performed to investigate the effect of feed containing potassium ferric hexacyanoferrate on 137Cs uptake and excretion by silver Prussian carp (Carassius gibelio (Bloch, 1782)). After the 120-day period of 137Cs uptake with feed, reaching equilibrium 137Cs level in fish, fish in some aquariums received feeds containing either 0.1 % or 1 % KFCF for 180 days in combination with clean feed or with feed containing 137Cs. These feeds resulted in 3.6 ± 0.7 and 4.4 ± 0.9 times, respectively, lower activity of 137Cs in fish compared to control fish fed 137Cs throughout the experiment and receiving feed without KFCF. Following the first 100 days with the KFCF containing feed, the 137Cs level in fish fed contaminated feed was even lower than in fish receiving clean feed, with a half-life of 137Cs activity in fish of only T1/2 = 23-35 days. Using clean feed containing 0 %, 0.1 % and 1 % KFCF for 180 days after the 120-day 137Cs uptake period, the excretion rates for 137Cs activity in fish kb' were (6.4 ± 0.2)⋅10-3 day-1, (1.08 ± 0.08)⋅10-2 day-1, and (1.3 ± 0.1)⋅10-2 day-1, respectively (T1/2 = 108 ± 3 days, 64 ± 5 days, and 53 ± 4 days). The decrease rates for 137Cs activity concentrations in fish kb were (8.4 ± 0.3)⋅10-3 day-1, (1.3 ± 0.1)⋅10-2 day-1, and (1.5 ± 0.1)⋅10-2 day-1, respectively (T1/2 = 83 ± 3 days, 53 ± 4 days, and 46 ± 3 days). Our results demonstrate a statistically significant effect (p < 0.01) of KFCF on the excretion of 137Cs from silver Prussian carp: T1/2 decreased from 108 days with clean feeding to 53-64 days when KFCF is added.

A novel transformer-based method for predicting air absorbed dose rates in nuclear radiation environmental monitoring

Many studies have used various methods to estimate future nuclear radiation levels to control radiation contamination, provide early warnings, and protect public health and the environment. However, due to the high uncertainty and complexity of nuclear radiation data, existing prediction methods face the challenges of low prediction accuracy and short warning time. Therefore, accurate prediction of nuclear radiation levels is essential to safeguard human health and safety. This study proposes a novel Mixformer model to predict future hourly nuclear radiation data. The seasonality and trend of nuclear radiation data are extracted by data decomposition. To address the slow speed problem common in traditional methods for long-time series prediction tasks, Mixformer simplifies the decoder with convolutional layers to speed up the convergence of the model. The experiments consider the air-absorbed dose rate of nuclear radiation data, spectral data, six climatic conditions, and two other conditions. We use MSE and MAE metrics to verify the effectiveness of Mixformer prediction. The results show that the Mixformer proposed in this paper has better prediction performance compared to the currently popular models. Therefore, the proposed model is a feasible method for industrial nuclear radiation data processing and prediction.

Dose estimation to wild medaka around Fukushima Dai-ichi nuclear power plant

More than 10 y have passed since the Fukushima Dai-ichi Nuclear Power Plant (F1-NPP) accident, but there are still many areas with high levels of radioactive contamination, especially reservoirs and lakes, where the levels decrease slowly. The exposed dose to non-human biota is of great concern. In this work, the exposure dose to wild medaka (small aquarium fish) around F1-NPP was measured/estimated by two approaches: in situ measurement and with the ERICA assessment tool. S2 is a reservoir located 7.5 km from F1-NPP, and the ambient dose rate was 25.0 μSv per h in 2012, but decreased to 4.82 μSv per h in 2021. The external exposure dose rate to wild medaka living there was 0.8-11.8 μGy per h (2017-21) by in situ measurement, and the estimated dose rate with the ERICA assessment tool was 0.6-54.5 μGy per h (2012-21). The estimated and measured doses to wild medaka are not expected to cause any significant radiation effects.

Factors affecting 137Cs radioactivity and water-to-body concentration ratios of fish in river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant

To elucidate ¹³⁷Cs contamination levels and mechanisms of fish inhabiting river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant, ¹³⁷Cs activity concentrations in fish (15 species, n = 164) and water collected from Maeda River (3.3-8.9 km from the plant) and Shimofukazawa Pond (2.9 km) in 2017 were analyzed. Also, an 8-week rearing experiment using Japanese dace Pseudaspius hakonensis fed on non-contaminated pellets and the pond water (mean ¹³⁷Cs concentration of 2.0 Bq/L) was conducted to evaluate ¹³⁷Cs accumulation from water to fish. The ¹³⁷Cs concentrations in Japanese dace, the only species collected throughout five sampling sites from estuarine to upstream areas in Maeda River, were found to be correlated with ambient air dose rates and fish size, exhibiting large variations (16.5-2.6×10³ Bq/kg-wet). By contrast, dissolved ¹³⁷Cs in river waters increased from the upper to lower course (0.025-0.28 Bq/L), which caused large variations of the water-to-body concentration ratio (CR) in Japanese dace (60.0-35700 L/kg-wet). These CRs (geometric mean of 3670 L/kg-wet) were much higher than the steady-state CR of reared fish (9.7 L/kg-wet), indicating that river fish uptake ¹³⁷Cs mainly from prey items from aquatic and riparian zones, rather than from water. Statistically significant negative correlations between K⁺ concentrations in water and river fish CRs were detected, resulting in the decreasing trend of CRs from upstream to estuarine areas. These results suggest that the large heterogeneity of air dose rates, K⁺ concentration, and estuarine processes in brackish water habitats, in association with the feeding habit and size effect in fish, can engender wide variation of ¹³⁷Cs concentrations and CRs of river fish along a river course. In contrast, ¹³⁷Cs concentrations in pond fish (4.3-14.6 kBq/kg-wet) were higher than in river fish. The CRs of pond fish were constantly high but the range was smaller (1010-3440 L/kg-wet) with larger values in fish of higher trophic levels. These findings suggest that biomagnification within a pond was inferred as the main cause of ¹³⁷Cs contamination of pond fish.

Remobilisation of radiocaesium from bottom sediments to water column in reservoirs in Fukushima, Japan

Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilise in bioavailable form from sediments to water columns, which may contribute to the long-term contamination of aquatic products. This study systematically investigated the ¹³⁷Cs activities of sediment-pore water, providing a direct evidence of the remobilisation of bioavailable ¹³⁷Cs from sediments in two highly contaminated reservoirs affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. We observed that the dissolved ¹³⁷Cs activity concentration of pore water (3.0-65.8 Bq L^-1) was one to two orders of magnitude higher than that of reservoir water. Moreover, the distribution coefficient (K_d) values for the ¹³⁷Cs of sediment-pore water (2.6-14 × 10³ L kg^-1) decreased with depth. The K_d values were significantly and negatively correlated with the concentration of the major ¹³⁷Cs competing cation NH₄⁺. Our results strongly indicate a competitive ion exchange process between ¹³⁷Cs and NH₄⁺ via a highly selective interaction with the frayed edge sites of phyllosilicate minerals, which is the major reason for the variability of K_d values of sediment-pore water, even in the Fukushima case. Additionally, the sediment accumulation rates were relatively high, and the annual depositional rate of exchangeable ¹³⁷Cs prevailed over the annual diffusive flux of ¹³⁷Cs from the sediment to the overlying water. This finding indicates that even after 10 years since the FDNPP accident, the bioavailable ¹³⁷Cs is still continuously supplied from the catchment covered by mountainous forests, and reservoir sediments are a long-term important source of bioavailable ¹³⁷Cs in the riverine system. Our findings provide important parameter values for mid- and long-term assessments of the radiation impact of radionuclide discharges to freshwater environments.

Radiocaesium accumulation and fluctuating asymmetry in the Japanese mitten crab, Eriocheir japonica, along a gradient of radionuclide contamination at Fukushima

The 2011 Tohoku earthquake-tsunami and the subsequent nuclear accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) led to large-scale radionuclide contamination of the marine and freshwater environment. Monitoring studies of marine food products in the Fukushima region have generally demonstrated a declining trend in radiocaesium concentrations. However, the accumulation and elimination of radiocaesium and potential biological effects remain poorly understood for freshwater biota inhabiting highly contaminated areas at Fukushima. Consequently, the present study aimed to assess radiocaesium accumulation and developmental effects on the commercially important catadromous Japanese mitten crab, Eriocheir japonica. E. japonica were collected from four sites along a gradient of radionuclide contamination 4-44 km in distance from the FDNPS in 2017. To determine potential developmental effects, fluctuating asymmetry (FA) was used as a measure of developmental stability. Combined ¹³⁴Cs and ¹³⁷Cs values for whole E. japonica from highly contaminated sites 4 and 16 km in distance from the FDNPS were 3040 ± 521 and 2250 ± 908 Bq kg^-1 wet weight respectively, 30 and 22 times greater than the Japanese standard limit of 100 Bq kg^-1. Estimated total dose rates based on radiocaesium concentrations in whole crabs and sediment ranged from 0.016 to 37.7 μGy h^-1. No significant relationship between radiocaesium accumulation and FA was recorded, suggesting that chronic radiation exposure at Fukushima is not inducing developmental effects in E. japonica as measured using fluctuating asymmetry. Furthermore, estimated dose rates were below proposed regulatory limits where significant deleterious effects are expected. The present study will aid in the understanding of the long-term consequences of radiation exposure for non-human biota and the management of radioactively contaminated environments.

A comparative study of biota and sediments as monitors of plutonium in the Yenisei River (Siberia, Russia)

MOX-fuel production and spent nuclear fuel reprocessing started recently at the Mining-and-Chemical Combine (MCC, Zheleznogorsk, Russia) have caused an increase in controlled releases of plutonium to the Yenisei River. In this study, we analyzed time-dependent trends of plutonium (^239,240Pu and ²³⁸Pu) in biota and bottom sediments of the Yenisei during 2008-2019, to estimate comparatively the potential of abundant representatives of biota as bio-monitors of contamination of the Yenisei by plutonium. Gamma-emitting radionuclides (⁴⁰ K; ⁶⁰Co; ¹³⁷Cs; ¹⁵²Eu; ²⁴¹Am) were measured in environmental samples of the Yenisei as well. Samples of bottom sediments, water moss (Fontinalis antipyretica), shining pondweed (Potamogeton lucens), caddisfly larvae with casings (Apatania crymophila), and amphipods (Eulimnogammarus viridis and Palaseopsis cancelloides) were collected downstream and upstream of the radioactive discharge site. Environmental samples of the Yenisei collected downstream of the radioactive discharge site differed considerably in activity concentrations of plutonium but were similar in time-dependent trends of plutonium, reflecting the trends of annual discharges of plutonium. In 2018, the year of a sharp increase in controlled discharge of plutonium, the concentration of ^239,240Pu in water moss (26 Bq kg^-1 d.w.) was higher than in sediments (14 Bq kg^-1 d.w.). In other years, the highest activity concentration of ^2391,240Pu was observed in bottom sediments. In view of the higher magnitude of increase in plutonium concentration, water moss and shining pondweed can be considered as more sensitive indicators of increased fresh releases of plutonium than bottom sediments. Taking into account the food-related mechanism of plutonium uptake by amphipods, this representative of biota can be regarded as a sensitive monitor of bioavailable plutonium in the Yenisei.

Seasonal changes in uptake and depuration of 137Cs and 90Sr in silver Prussian carp (Carassius gibelio) and common rudd (Scardinius erythrophthalmus)

Dynamic transfer of radionuclides to fish was studied in a series of experiments under field condition in two lakes within the Chernobyl exclusion zone during 2016-2020. "Clean" common rudd (Scardinius erythrophthalmus) and silver Prussian carp (Carassius gibelio) were transported to the contaminated Glubokoye Lake and kept in cages during several months of exposure, while contaminated Glubokoye fish were kept in cages in the "clean" Starukha Lake. Radiocaesium (¹³⁷Cs) and radiostrontium (⁹⁰Sr) were determined in intestine contents, muscle and bone tissues based on repeated samples during several months of exposure. During summer, the activity concentrations of ¹³⁷Cs and ⁹⁰Sr increased with time of exposure in clean fish caged in the contaminated lake. During autumn and winter, however, minor changes in fish uptake occurred during several weeks of exposure to the contaminated water. Furthermore, depuration in the contaminated fish was significant during summer, while insignificant during winter when exposed in the «clean» water. The rate constant of ¹³⁷Cs uptake in muscle was between 8.0 and 22 day^-1 during summer, while 0.2 to 1.0 day^-1 during autumn-winter. Similarly, the rate constant of ⁹⁰Sr uptake in bone was between 1.4 and 1.6 day^-1, while 0.08-0.52 day^-1 during autumn-winter. Biological half-lives of ¹³⁷Cs in fish muscle tissue in summer were 77 ± 10 days, while exceeded 230 days during seasons at low water temperature. The results demonstrated that the transfer of ¹³⁷Cs and ⁹⁰Sr to fish was highly dependent upon seasons, in particular the water temperature. The transfer data obtained during low water temperature seasons deviated significantly from transfer data in literature and handbooks. Thus, seasonal changes in radionuclide transfer to fish should be taken into account when radiological impact to fish is assessed.

Impacts of freeze-thaw processes and subsequent runoff on 137Cs washoff from bare land in Fukushima

The deposited ¹³⁷Cs is one of the long-lived radionuclides, that was released following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, has been hydrologically transported as particulates in the terrestrial environment of the Fukushima region. The impact of freeze-thaw processes and subsequent runoff affecting the ¹³⁷Cs flux and concentration in sediment discharge were revealed in bare land erosion plot following the FDNPP accident by detailed monitoring and laser scanner measurement on the soil surface. We found that surface topographic changes due to the frost-heaving during the winter-spring period, and rill formation during the summer. We also found the evident seasonal changes in ¹³⁷Cs concentration; high during the early spring and gradually decreased thereafter, then surface runoff from the plot frequently occurred during spring and autumn when rainfall was high and reached a maximum in summer. From these results, the higher ¹³⁷Cs concentration in spring was caused by a mixture of unstable surface sediment following freeze-thaw processes and then transported in the early spring, but erosion amount is not significant because of the less rainfall event. The sediment with a lower ¹³⁷Cs concentration, which was supplied from the rill erosion and its expansion, was wash-offed during the summer, contributing most of the flux from erosion in bare land in Fukushima region. In case, heavy rainfall occurs in the early spring, caution is required because high concentrations of cesium may flow down into the river.

Seasonal dynamics of the activities of dissolved 137Cs and the 137Cs of fish in a shallow, hypereutrophic lake: Links to bottom-water oxygen concentrations

Remobilization of radiocesium from anoxic sediments can be an important mechanism responsible for long-term contaminations of lakes. However, it is unclear whether such remobilization occurs in shallow lakes, where concentrations of dissolved oxygen in the hypolimnion (bottom DO) change temporally in response to meteorological conditions, and whether remobilized radiocesium influences the activity in fish. We examined the seasonal dynamics of the activities of dissolved ¹³⁷Cs and ¹³⁷Cs in fish (pond smelt and crucian carp) from Lake Kasumigaura, a shallow, hypereutrophic lake, five years after the Fukushima Daiichi Nuclear Power Plant accident. The activities of both dissolved ¹³⁷Cs and ¹³⁷Cs in fish declined during that time, but the declines showed a clear seasonal pattern that included a summer peak of ¹³⁷Cs activity. The activity of dissolved ¹³⁷Cs increased when the bottom DO concentration decreased, and a nonlinear causality test revealed significant causal forcing of dissolved ¹³⁷Cs activity by bottom DO. The fact that NH₄-N concentrations in bottom waters were higher in the summer suggested that remobilization of ¹³⁷Cs from sediments could result from highly selective ion-exchange with NH₄-N. Despite the shallow depth of Lake Kasumigaura, winds had little influence bottom DO concentrations or dissolved ¹³⁷Cs activities. The fact that seasonal means of ¹³⁷Cs activities in pond smelt and crucian carp were positively correlated with the seasonal means of dissolved ¹³⁷Cs activities suggested that remobilized ¹³⁷Cs may have influenced the seasonal dynamics of radiocesium in fish through food-chain transfer, but higher feeding rates in warm water could may have also contributed to the seasonal dynamics of ¹³⁷Cs activity in fish. Our findings suggest that in shallow lakes, intermittent but repeated hypoxic events may enhance remobilization of radiocesium from sediments, and remobilized radiocesium may contributed to long-term retention of radiocesium in aquatic organisms.

Inequality in the distribution of 137Cs contamination within freshwater fish bodies and its affecting factors

Contamination of freshwater fishes with 137Cs remains as a serious problem in Japan, nearly 10 years after the Fukushima nuclear power plant accident, but there is limited information on the distribution of 137Cs contamination in fish bodies. The 137Cs distribution can be used for the estimation of internal radiation exposure through the consumption of fish and for the dose estimation of fish themselves. In this study, the 137Cs distribution in the bodies of 8 freshwater fish species was investigated as percentages of total body burden for fish inhabiting Lake Inba. Fish samples were caught in stake nets placed close to the shore approximately once a month. After the measurement of body length and fresh weight, the radioactivities of 137Cs in muscle, internal organs, spawn, milt and bone were assayed using high-purity germanium detectors. Analysis of all fish samples showed that the 137Cs distribution was highest in muscle (54 ± 12%), followed by internal organs (7.8 ± 4.6%), spawn (7.4 ± 5.4%), milt (3.2 ± 2.1%) and bone (1.2 ± 0.58%). Among fish species, the highest proportion of 137Cs in muscle was detected in largemouth bass (71 ± 1 3%), followed by snakehead (69 ± 14%), channel catfish (63 ± 17%), common carp (62 ± 14%), barbel steed (58 ± 6.5%), silver carp (57 ± 7.7%), bluegill (53 ± 4.7%), and crucian carp (50 ± 10%). These results suggested that the 137Cs in muscle was likely to be high in piscivorous fishes compared to omnivorous fishes, especially crucian carp. The proportion of 137Cs in muscle of crucian carp was not explained either by body length or fresh weight. However, a positive correlation was found between the proportion of 137Cs in muscle and the condition factor which was an indicator of nutritional status calculated from a length-weight relationship. This correlation implied that more 137Cs accumulated in muscle tissue of a fish species with high nutritional status. This is the first study to show that condition factor is more important than body length and wet weight in explaining the high proportion of 137Cs in muscle tissues, at least for crucian carp.

Transition of Radioactive Cesium Deposition in Reproductive Organs of Free-Roaming Cats in Namie Town, Fukushima

We investigated the internal contamination by radioactive cesium associated with the FDNPP accident, in the testes or uterus and ovaries of free-roaming cats (Felis silvestris catus), which were protected by volunteers in the Namie Town, Fukushima. A total of 253 samples (145 testes and 108 uterus and ovaries) obtained from adult cats and 15 fetuses from 3 pregnant female cats were measured. Free-roaming cats in Namie Town had a higher level of radioactive contamination in comparison to the control group in Tokyo, as the ¹³⁴Cs + ¹³⁷Cs activity concentration ranged from not detectable to 37,882 Bq kg^-1 in adult cats. Furthermore, the radioactivity in the fetuses was almost comparable to those in their mother's uterus and ovaries. The radioactivity was also different between several cats protected in the same location, and there was no significant correlation with ambient dose-rates and activity concentrations in soil. Moreover, radioactive cesium levels in cats decreased with each year. Therefore, it is likely that decontamination work in Namie Town and its surroundings could affect radioactive cesium accumulation, and thus possibly reduce the internal radiation exposure of wildlife living in contaminated areas. It is hence necessary to continue radioactivity monitoring efforts for the residents living in Namie Town.

Comparison of 137Cs uptake, depuration and continuous uptake, originating from feed, in five salmonid fish species

The origin of ¹³⁷Cs contamination, contamination levels, and its effective ecological half-life can differ among fish species, but until recently it was unknown whether interspecific differences in radiocaesium metabolism existed. We compare the trophic transfer of ¹³⁷Cs in five salmonid species under controlled conditions. Fish were fed a diet containing 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a fixed period of time. While there were almost no differences in contamination levels among individual fish within a species during fixed periods, the rate of ¹³⁷Cs uptake was statistically different among the species examined, for which the effective half-life of ¹³⁷Cs ranged 49-84 days. No significant difference in rate of decline in the quantity of ¹³⁷Cs was apparent among species in two experiments after correcting for differences in growth. The effective half-life was shortest in Oncorhynchus mykiss and O. masou-two species exhibiting the best growth rate in both experiments-indicating an influence of the dilution effect associated with growth. When continuously fed 200 Bq kg^-1 wet wt concentration of ¹³⁷Cs for a period of 209-294 days, contamination levels in the five species ranged 216-240 Bq kg^-1 wet wt, and reached a steady state value. Our results indicate metabolic rate, although accompanying a change of body size, did not affect ¹³⁷Cs contamination levels in fish, which implies that contamination levels in the diet strongly determined contamination levels in fish.

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.

Assessment of chromosome aberrations in large Japanese field mice (Apodemus speciosus) in Namie Town, Fukushima

PURPOSE

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in Japan on March 11 2011, the surroundings became contaminated with radionuclides. To understand the possible biological effects after chronic low dose-rate radiation in contaminated areas of Fukushima, we assessed the effects in large Japanese field mice (Apodemus speciosus) by means of chromosome aberration analysis.

MATERIALS AND METHODS

We collected A. speciosus in five sites around Namie Town, Fukushima (contaminated areas) and in two sites in Hirosaki City, Aomori (control areas, 350 km north of FDNPP) from autumn 2011 to 2013. The number of mice captured and ambient dose-rates were as follows: high (n = 11, 10.1-30.0 µGy h^-1), moderate (n = 10, 5.7-15.6 µGy h^-1), low (n = 12, 0.23-1.14 µGy h^-1) and control (n = 20, 0.04-0.07 µGy h^-1). After spleen extraction from rodents, spleen cell culture was performed to obtain metaphase spreads. Chromosome aberrations were assessed on Giemsa-stained metaphase spreads.

RESULTS

Although the mice in the contaminated areas were chronically exposed, there was no radiation-specific chromosome aberrations observed, such as dicentric chromosomes and rings. Some structural aberrations such as gaps and breaks were observed, and these frequencies decreased annually in mice from Namie Town.

CONCLUSION

These findings suggest that chromosome aberration analysis is useful to evaluate and monitor radiation effects in wild animals.

Estimation of water seepage rate in the active crater lake system of Kusatsu-Shirane volcano, Japan, using FDNPP-derived radioactive cesium as a hydrological tracer

We describe here a first attempt to estimate the water seepage rate of an active crater lake using radioactive cesium dispersed into the environment by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 as a hydrological tracer. Kusatsu-Shirane volcano is one of the most active volcanoes in Japan and has an active crater lake named Yugama. There is no outflow such as a river from Yugama crater lake. The content level of stable cesium (¹³³Cs) in Yugama water was almost constant at 15-20 nM during the sampling period of Nov. 2011 to Nov. 2014. In contrast to ¹³³Cs, however, the radioactive cesium (¹³⁴Cs and ¹³⁷Cs) concentrations in Yugama water decreased at a more rapid rate than expected by radioactive decay. Based on the decreasing rates of activity concentrations of ¹³⁴Cs and ¹³⁷Cs in Yugama water during the three years between 2012 and 2014, it is estimated that 700-800 m³ of the Yugama water, which corresponds approximately to 0.1% of the total volume, leaks through the lakebed per day. In the estimation, balance between leakage of radioactive cesium contaminated water though lake bed and geothermal inflow of radioactive cesium-free water in the volcano was taken into account. Consequently, the water seepage rate of the Yugama crater lake was calculated to be 8.1-9.3 L s^-1.

Relationship between radiocaesium in muscle and physicochemical fractions of radiocaesium in the stomach of wild boar

After the accident at the TEPCO Fukushima Daiichi Nuclear Power Plant in 2011, it became important to study radiation dynamics, assess internal radiation exposure and specify factors affecting radionuclide variation in wildlife. Therefore, it is necessary to investigate which physicochemical fractions of radiocaesium (¹³⁷Cs) are absorbed from ingested material in species with high activity concentrations of ¹³⁷Cs, such as wild boar. This study analysed the physicochemical fractions of ¹³⁷Cs in the stomach contents of wild boar to evaluate the transfer from ingested food to muscle. The ¹³⁷Cs activity concentration in muscle showed a significantly positive relationship with the ¹³⁷Cs activity concentration in the exchangeable fraction, and the sum of the ¹³⁷Cs activity concentrations in the exchangeable and bound to organic matter fractions. Seasonal variations were also found in the ¹³⁷Cs activity concentration in the exchangeable fraction, and the sum of the ¹³⁷Cs activity concentrations in the exchangeable and bound to organic matter fractions. These findings suggest that the proportions of the physicochemical fractions of ¹³⁷Cs in the exchangeable and bound to organic matter fractions in the stomach contents are important factors affecting the increases and seasonal dynamics of the activity concentrations of ¹³⁷Cs in wild boar muscle.

Different factors determine 137Cs concentration factors of freshwater fish and aquatic organisms in lake and river ecosystems

Determination of radionuclide concentration factor (CF) allows estimating the transfer of environmental radionuclides and potential risks of consuming fish contaminated with radionuclides. Although it is known that biotic and abiotic factors affect fish CF, only a few studies have examined whether these factors differ among ecosystems. We estimated radiocesium (¹³⁷Cs) CF of 30 different fish species and other aquatic organisms by monitoring three lakes and five rivers in Fukushima, 2-4 y after the Fukushima Dai-ichi Nuclear Power Plant accident. The relative effects of biotic and abiotic factors on ¹³⁷Cs CF in freshwater organisms were compared between river and lake ecosystems using generalized linear models. Our analysis demonstrated the following. (1) The factors critically affecting fish CF differed between rivers and lakes. The negative effects of suspended solid concentration (SS), total organic carbon (TOC), and salinity were significant for rivers, but not for lakes. Biomagnification of ¹³⁷Cs in piscivore fish was significant only in the lakes. (2) Fish size significantly affected the CF in both rivers and lakes. Nevertheless, the correlation between ¹³⁷Cs concentration and piscivore fish size was stronger in lakes than in rivers. (3) The SS, TOC, and salinity simultaneously influenced the CF at every trophic level. However, feeding habit was a stronger determinant of ¹³⁷Cs bioaccumulation than water chemistry in organisms at higher trophic levels, such as aquatic insects, amphibians, and fish. Our findings indicate that ¹³⁷Cs accumulation in aquatic organisms is ecosystem-dependent due to different environmental factors and food web structure.

Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of ¹³⁷Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ₁) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of ¹³⁷Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of ¹³⁷Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported ¹³⁷Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ₂) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

Numerical study of transport pathways of 137Cs from forests to freshwater fish living in mountain streams in Fukushima, Japan

The accident at the Fukushima Dai-ichi Nuclear Power Plant in 2011 released a large quantity of radiocesium into the surrounding environment. Radiocesium concentrations in some freshwater fish caught in rivers in Fukushima Prefecture in October 2018 were still higher than the Japanese limit of 100 Bq kg^-1 for general foodstuffs. To assess the uptake of ¹³⁷Cs by freshwater fish living in mountain streams in Fukushima Prefecture, we developed a compartment model for the migration of ¹³⁷Cs on the catchment scale from forests to river water. We modelled a generic forest catchment with Fukushima-like parameters to ascertain the importance of three export pathways of ¹³⁷Cs from forests to river water for the uptake of ¹³⁷Cs by freshwater fish. The pathways were direct litter fall into rivers, lateral inflow from the forest litter layer, and lateral transfer from the underlying forest soil. Simulation cases modelling only a single export pathway did not reproduce the actual trend of ¹³⁷Cs concentrations in river water and freshwater fish in Fukushima Prefecture. Simulations allowing a combined effect of the three pathways reproduced the trends well. In the latter simulations, the decreasing trend of ¹³⁷Cs in river water and freshwater fish was due to a combination of the decreasing trend in the forest leaves/needles and litter compartments, and the increasing trend in soil. The modelled ¹³⁷Cs concentrations within the forest compartments were predicted to reach an equilibrium state at around ten years after the fallout due to the equilibration of ¹³⁷Cs cycling in forests. The model suggests that long term ¹³⁷Cs concentrations in freshwater fish in mountain streams will be controlled by the transfer of ¹³⁷Cs to river water from forest organic soils.

Strong contrast of cesium radioactivity between marine and freshwater fish in Fukushima

A proper understanding of radioactive contamination levels of food resources near the Fukushima Dai-ichi Nuclear Power Plant is necessary to estimate the potential effects of radionuclide contamination on human health. This study was conducted to present a direct comparison of radiocesium (¹³⁴Cs and ¹³⁷Cs) concentrations in marine and freshwater fish inhabiting different water bodies in Fukushima Prefecture (coastal waters, 6.3-54.5 km from the plant; forest rivers and irrigation ponds, 1.4-71.6 km), and to reveal plausible contamination mechanisms for each habitat. In contrast to marine demersal fish (7 species, n = 50), which showed lower and less variable radiocesium concentrations (0.234-3.41 Bq kg^-1-wet), freshwater fish (6 species, n = 463) showed higher and more site-specific variations for each species and habitat (4.09 Bq kg^-1-wet - 25.6 kBq kg^-1-wet) in 2015-2016. The apparent concentration ratio (aCR, L/kg) of ¹³⁷Cs in fish to water is higher for fish of freshwater habitats (mean 1240-12900 for each site) than in those of coastal waters (mean 200). Radiocesium contamination is more severe and persistent in freshwater fish, especially those distributed within the designated evacuation zone (salmon in rivers and bass in ponds). Continuous radiocesium uptake through the food web in relation to fish feeding habits and size (size effect), and biotic/abiotic characteristics in water and surrounding environments are main factors affecting site/habitat-specific bioaccumulation of radiocesium in freshwater fish. By contrast, uniformly lower radiocesium concentrations in marine demersal fish are mainly attributable to decreased radiocesium transfer intensity from the benthic food web because of lowered radiocesium contamination in sediments, and low physiological ability to retain radiocesium. Our results revealed a strong contrast of radiocesium contamination levels and mechanisms between marine and freshwater fish in natural habitats. Particularly, a close relation between ¹³⁷Cs accumulation in river salmon and contamination of prey items in forest ecosystems (mainly terrestrial and aquatic insects) is peculiar to the upstream areas affected by the Fukushima accident.

A 30-year record reveals re-equilibration rates of 137Cs in marine biota after the Fukushima Dai-ichi nuclear power plant accident: Concentration ratios in pre- and post-event conditions

Concentration ratios (CRs), expressed by dividing ¹³⁷Cs activity in seawater by that in marine biota (mainly fish), were obtained from the monitoring of ¹³⁷Cs in coastal areas around Japan between 1984 and 2016. Before the TEPCO Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident (1984-2010), mean CRs of ¹³⁷Cs, mainly from global fallout (i.e. CR_GF), were almost constant for each species throughout the monitoring period, but were different among species, while the values for several species were dependent on their length (i.e. CR_GF-SIZE). Thus, CR_GF and CR_GF-SIZE values for 29 of marketable species are given here as references for conditions where marine biota are in approximate equilibrium (or steady state) with their host water with respect to ¹³⁷Cs activities in the marine environment. After the FDNPP accident (2011-2016), the impact of the accident has been sustained in eastern Japan waters as indicated by apparent CRs (CR_as) which are being used here as indicators of disequilibrium between organisms and their host water. The recession rates of this disequilibrium (the effective CR_a half-lives) ranged from 100 to 1100 days. The identified distinct variation was due to the sample locations, even for the same species, because of the change in ¹³⁷Cs activity concentrations in their host water and diet preference differences. Variation among species, even those captured from the same area, was mainly due to diet differences as well as metabolic-physiological differences in ¹³⁷Cs retention. Thus, our results from >30 years of systematically monitoring have helped quantify the recession rates of post-FDNPP disequilibrium of ¹³⁷Cs in biota for assessment of how long term is required from contaminated condition by underlying spatial, inter- and intra-species factors.

Long-term distribution and migration of 137Cs in a small lake ecosystem with organic-rich catchment: A case study of Lake Vorsee (Southern Germany)

In the shallow eutrophic Lake Vorsee with an organic-rich catchment the ¹³⁷Cs activity concentration in water samples (n = 250) and different fish species (n = 631) was determined for a period of 30 years starting soon after the Chernobyl fallout. To understand the distribution and migration of ¹³⁷Cs in the ecosystem of Lake Vorsee, also the ¹³⁷Cs inventory in soils of the catchment area, in sediments, in suspended matter, in water plants, as well as its vertical distribution in soils and sediments were studied. The time dependency of the ¹³⁷Cs activity concentration in water and fish was modelled with a compartment model (AQUASCOPE) as well as with the sum of two exponential functions. Results for effective half-lives of ¹³⁷Cs in water are T_eff1  = 1.2 a and T_eff2  = 7.2 a; for small cyprinidae T_eff1  = 0.6 a and T_eff2  = 8.9 a; and for pike T_eff1  = 1.4 a and T_eff2  = 12.1 a. The vertical distribution of ¹³⁷Cs activity concentration in soils was described by a partial differential equation (diffusion constant D = (1.05 ± 1.16) cm²⋅a^-1 and convection speed v = (0.14 ± 0.07) cm⋅a^-1) and its distribution in sediments by a system of two coupled partial differential equations (fixation rate f = 2.0 d^-1; distribution coefficient K_d^ex = (543 ± 140) L⋅kg^-1; sedimentation rate R_S = (0.030 ± 0.010) g⋅(cm²⋅a)^-1; bioturbation constant D_bio, decreasing linearly with depth with D_bioabs = (3.3 ± 1.0) cm²⋅a^-1 at the surface). According to the extended AQUASCOPE model, the following fractions of the total ¹³⁷Cs inventory were found in the different compartments of the lake ecosystem in 2016: 9.0⋅10^-1 in soils of the catchment area, 1.0⋅10^-1 in sediments, 5.5⋅10^-5 in water, 1.5⋅10^-5 in suspended matter, 1.3⋅10^-5 in watermilfoil, and 1.6⋅10^-6 in fish.

Radiocesium accumulation in aquatic organisms: A global synthesis from an experimentalist's perspective

A better understanding of the fate of radiocesium in aquatic organisms is essential for making accurate assessments of potential impacts of radiocesium contamination on ecosystems and human health. Studies of the accumulation of ¹³⁴Cs, ¹³⁶Cs and ¹³⁷Cs in diverse biota have been the subject of many field investigations; however, it may often be difficult to understand all the mechanisms underlying the observations reported. To complement field investigations, laboratory experiments allow better understanding the observations and predicting dynamics of Cs within aquatic ecosystems by accurately assessing bioaccumulation of Cs in living organisms. The present review summarizes selected relevant laboratory studies carried out on Cs bioaccumulation in aquatic organisms over a period of more than 60 years. To date, 125 experimental studies have been carried out on 227 species of aquatic organisms since 1957. The present review provides a synthesis of the existing literature by highlighting major findings and identifying gaps of key information that need to be further addressed in future works on this topic. Thus, influences of some environmental parameters such as water chemistry both for marine and freshwater ecosystems, and biotic factors such as the life-stages and size of the organisms on radiocesium bioaccumulation should be examined and become priority topics for future research on Cs accumulation in aquatic organisms.

Monitoring 137Cs concentrations in bird species occupying different ecological niches; game birds and raptors in Fukushima Prefecture

This study was conducted to assess radiocesium accumulation in birds after the accident at Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station in 2011, with a particular focus on ¹³⁷Cs, which has a long physical half-life. Results of ¹³⁷Cs monitoring in four game bird species including two pheasant species and two duck species (copper pheasant, green pheasant, spot-billed duck and mallard) were assessed in Fukushima Prefecture. We also obtained samples from rescued raptors that died during treatment or rehabilitation at the Wildlife Symbiosis Centre in Fukushima Prefecture because of severe injury. We measured the muscle concentrations of ¹³⁷Cs in four of these raptor species (black kite, northern goshawk, peregrine falcon and ural owl). Comparison of the two pheasant species showed that the copper pheasants inhabiting forested areas had higher ¹³⁷Cs concentrations in muscle (Bq/kg, fresh mass) than the green pheasants inhabiting mountainous areas near human habitation (i.e., Satoyama). No clear tendencies were observed in ¹³⁷Cs concentration in muscle of copper pheasants over time, but a tendency to decrease was observed in green pheasants over time. The difference in tendencies between species may be attributable to differences in their food habits and its ¹³⁷Cs concentration, and also differences in the situation of ¹³⁷Cs accumulation in their habitat. No significant differences were observed in ¹³⁷Cs concentration in muscle between the resident spot-billed duck and migratory mallard because of the comparatively short biological of effective half-life of radiocesium. Analysis of ¹³⁷Cs concentration in muscle of raptor revealed that the concentration was similar to, or lower than, those of pheasants and ducks.

Interspecific comparison of radiocesium trophic transfer in two tropical fish species

The trophic transfer of radiocesium (¹³⁴Cs) was investigated in two tropical fish, the silver moony Monodactylus argenteus and the spotted scat Scatophagus argus. Juveniles of both species were exposed to dietary ¹³⁴Cs using the pulse-chase feeding methodology. The food was brine shrimp (Artemia salina) previously exposed to the dissolved radiotracer. Depuration kinetics of ¹³⁴Cs were followed for 45 d. Results showed that Cs was similarly efficiently assimilated by both species (AE > 50%). The estimated trophic transfer factors in the two species ranked from 1 to 2, suggesting that ¹³⁴Cs could be biomagnified in both omnivorous species. In complement, dissections of 7 body compartments were carried out at three different times in order to highlight ¹³⁴Cs organotropism. ¹³⁴Cs organotropism was similar in both species: more than 50% of ¹³⁴Cs was quickly distributed in the muscles and skeleton (after 3 days of depuration), which is likely related to the analogous behavior between Cs and K, an essential element for muscle contractions and bone formation.

The role of salinity in the trophic transfer of 137Cs in euryhaline fish

In order to better understand the influence of changing salinity conditions on the trophic transfer of ¹³⁷Cs in marine fish that live in dynamic coastal environments, its depuration kinetics was investigated in controlled aquaria. The juvenile turbot Scophthalmus maximus was acclimated to three distinct salinity conditions (10, 25 and 38) and then single-fed with compounded pellets that were radiolabelled with ¹³⁷Cs. At the end of a 21-d depuration period, assimilation efficiencies (i.e. AEs = proportion of ¹³⁷Cs ingested that is actually assimilated by turbots) were determined from observational data acquired over the three weeks. Our results showed that AEs of ¹³⁷Cs in the turbots acclimated to the highest salinity condition were significantly lower than for the other conditions (p < 0.05). Osmoregulation likely explains the decreasing AE observed at the highest salinity condition. Indeed, observations indicate that fish depurate ingested ¹³⁷Cs at a higher rate when they increase ion excretion, needed to counterbalance the elevated salinity. Such data confirm that ambient salinity plays an important role in trophic transfer of ¹³⁷Cs in some fish species. Implications for such findings extend to seafood safety and climate change impact studies, where the salinity of coastal waters may shift in future years in response to changing weather patterns.

Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (¹³⁴Cs and ¹³⁷Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of ¹³⁷Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T_eff) of ¹³⁷Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of ¹³⁷Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of ¹³⁷Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total ¹³⁷Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T_eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The T_eff in the slow component of the TDMs could converge towards the physical half-life of ¹³⁷Cs. Nearly five and a half years after the FDNPP accident, we concluded that ¹³⁷Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of ¹³⁷Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total ¹³⁷Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate ¹³⁷Cs contamination level in the river water was high after heavy rains. Overall, ¹³⁷Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

Fractionation of radiocesium in soil, sediments, and aquatic organisms in Lake Onuma of Mt. Akagi, Gunma Prefecture using sequential extraction

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has resulted in the contamination of the environment in Gunma Prefecture with radioisotope cesium (radio-Cs, ¹³⁴Cs and ¹³⁷Cs). Concentrations of radio-Cs >500Bqkg^-1 were found in wakasagi (Hypomesus nipponensis) in Lake Onuma at the top of Mount (Mt.) Akagi in August 2011. To explain the mechanism of this contamination, monitoring studies have been conducted around Lake Onuma by measuring radio-Cs concentrations in samples of fish, aquatic plants, plankton, lake water, lake sediments, and surrounding soil. The leachability of radio-Cs was evaluated using sequential extraction by Tessier et al. The total concentration of radio-Cs in Lake Onuma ecosystems decreased gradually with time. In the brown forest soil, radio-Cs concentrations of 2000 to 6000Bqkg^-1 were detected. The abundance ratio of the easy-elution form (exchangeable and carbonate forms) in the samples was <10%. The concentrations in phytoplankton samples were 3-6 times higher than those in wakasagi samples. The ratios of easy-elution forms increased by the rank in the food chain; 37% in phytoplankton, 78% in zooplankton, and 97% in wakasagi. It is likely that the lower ratio of the easy-elution form in phytoplankton is related to the adsorption of radio-Cs on suspended substances in the lake, as suggested by the analyses of aluminum and titanium in the phytoplankton, zooplankton, and wakasagi samples. The high concentrations of radio-Cs in wakasagi would be related also to the characteristics of closed mountain lakes.

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.