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

Mechanisms of 137Cs leaching based on long-term observations in forested headwater catchments in Yamakiya, Fukushima Prefecture, Japan

Dissolved radiocesium (mainly 134Cs and 137Cs) is thought to be leached mainly from suspended sediment in downstream rivers, while organic matter, such as leaf litter, contributes to catchments in forested headwater streams. It is also known that dissolved 137Cs in headwater streams exhibit seasonal variation with water temperature. Some mechanisms have been proposed as the causes: ionic competition of potassium ion (K+) and ammonium ion (NH4+), leaching associated with the decomposition of organic matter, and thermodynamic adsorption-desorption processes. We investigated the relationship between K+ and the dissolved organic carbon (DOC) concentrations and seasonal changes in dissolved 137Cs concentration using a large number of samples from a headwater's small catchments. We examined temporal trends in 137Cs concentrations in coarse organic matter, suspended sediments, and dissolved forms at four sites (one decontaminated site and three undecontaminated sites) in the Yamakiya area since 2011. The distribution coefficients (Kdcss and Kdorg) of dissolved 137Cs concentrations relative to suspended sediment and coarse organic matter 137Cs concentrations were calculated, and differences in temporal changes due to decontamination were investigated. In addition, we examined the relationship between water temperature and DOC, K+, NH4+ and 137Cs concentrations in the headwater catchments. The suspended sediment 137Cs concentrations at the decontaminated headwater site (IBO) decreased significantly after decontamination and remained low thereafter. In contrast, dissolved 137Cs concentrations decreased temporarily during the decontamination period, but returned to pre-decontamination levels. Almost no NH4+ has been detected in headwater streams in our catchments. In the SET and ISH watersheds, where the distance from the groundwater spring is short, a correlation was found between DOC concentration and dissolved 137Cs concentration. In contrast, in the IBO watershed, where the distance from the groundwater spring is long, temperature dependence and a good correlation between K+ and 137Cs were observed. Therefore, microbial decomposition of organic matter may have a significant effect on the seasonal variation of dissolved 137Cs in forested headwater streams at short distances from the spring, but the influence of competing ions is expected to increase gradually as the water flows downstream.

Contrasting radiocesium transfer in the river and lake food webs: Importance of trophic level and food source

Radiocesium (¹³⁷Cs) contamination of the freshwater ecosystems adjacent to the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Japan has persisted long after the accident that occurred at the facility in March 2011. It is necessary to elucidate the dynamics of ¹³⁷Cs in various aquatic ecosystems to predict ¹³⁷Cs concentrations in fish and manage freshwater fisheries in the vicinity of FDNPP. To these ends, we applied stable isotope analysis to evaluate changes in ¹³⁷Cs levels through trophic positions and the relative importance of the ¹³⁷Cs sources at the trophic bases of two rivers and two lakes in Fukushima. The δ¹⁵N analyses disclosed that ¹³⁷Cs decreases from primary producers to fish consumers in the river food web and ¹³⁷Cs increases among fish consumers with increasing trophic position in the lake food web. The δ¹³C analysis revealed that autochthonous ¹³⁷Cs contributed to fish contamination. The periphyton-dependent and zooplankton-dependent fish had comparatively higher ¹³⁷Cs concentrations in the rivers and lakes, respectively. Cesium-137 supply from the pelagic food web was observed to contribute to greater ¹³⁷Cs levels in the fish consumers inhabiting the lakes. The results of this study show that stable isotope analysis may help clarify ¹³⁷Cs dynamics in freshwater food webs and identify the important ¹³⁷Cs sources in the food web. Identifying important ¹³⁷Cs sources and trophic transfers depending on the ecosystem help guide regulatory and management frameworks to establish profitability of the food fish stocks there and maintain food security.

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.

Contrasting seasonality of 137Cs concentrations in two stream animals that share a trophic niche

Understanding the seasonality of ¹³⁷Cs concentrations in aquatic animals is crucial for reviving local inland fisheries. The seasonality of ¹³⁷Cs concentrations in animals is expected to vary, even if focal species consume similarly contaminated foods because the ¹³⁷Cs excretion rate is species-specific, and ¹³⁷Cs uptake by foraging autochthonous food resources also vary among seasons. Here, we conducted a seasonal monitoring survey of dissolved ¹³⁷Cs concentrations as an indicator of the contamination level of food resources and measured ¹³⁷Cs concentrations in two carnivorous aquatic animals (Palaemon paucidens and Rhinogobius sp.) that share a trophic niche in a stream connected to a dam reservoir. The dissolved ¹³⁷Cs concentration had clear seasonality-high in summer and low in winter. The ¹³⁷Cs concentrations in the animals revealed a different seasonal pattern-it peaked in October in P. paucidens and peaked in February in Rhinogobius. Overall, the ¹³⁷Cs concentration was relatively higher in P. paucidens than in Rhinogobius, suggesting that P. paucidens has a lower excretion rate than Rhinogobius. Consequently, the seasonality of the ¹³⁷Cs concentration in P. paucidens showed temporal changes similar to those of the dissolved ¹³⁷Cs concentration, which were likely affected by ¹³⁷Cs uptake through foraging, whereas that in Rhinogobius was controlled by ¹³⁷Cs excretion. This study shows that the seasonality of ¹³⁷Cs concentration can differ between sympatric animals that share a trophic niche. Accumulating knowledge and comparing the seasonality of ¹³⁷Cs concentrations in fisheries species based on the balance between uptake and excretion will be valuable to determine the appropriate seasons to obtain less-contaminated products.

Radiocesium accumulation in Lake Kasumigaura by riverine input and migration following the Fukushima Dai-ichi nuclear power plant accident

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 (¹³⁷Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the ¹³⁷Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The ¹³⁷Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating ¹³⁷Cs migration. However, the rate of decrease subsequently slowed due to the ¹³⁷Cs supply from the river. The ¹³⁷Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate ¹³⁷Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in ¹³⁷Cs concentrations will be smaller. The ¹³⁷Cs concentration in the Koise River will continue to decrease; thus, the difference in the ¹³⁷Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of ¹³⁷Cs in the entire lake were 3.72 × 10¹² Bq in December 2012 and 4.18 × 10¹² Bq in August 2016. The accumulated amount of ¹³⁷Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate ¹³⁷Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of ¹³⁷Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes.

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.

Untangling radiocesium dynamics of forest-stream ecosystems: A review of Fukushima studies in the decade after the accident

Forest-stream ecosystems are widespread and biodiverse terrestrial landscapes with physical and social connections to downstream human activities. After radiocesium is introduced into these ecosystems, various material flows cause its accumulation or dispersal. We review studies conducted in the decade after the Fukushima nuclear accident to clarify the mechanisms of radiocesium transfer within ecosystems and to downstream areas through biological, hydrological, and geomorphological processes. After its introduction, radiocesium is heavily deposited in the organic soil layer, leading to persistent circulation due to biological activities in soils. Some radiocesium in soils, litter, and organisms is transported to stream ecosystems, forming contamination spots in depositional habitats. While reservoir dams function as effective traps, radiocesium leaching from sediments is a continual phenomenon causing re-contamination downstream. Integration of data regarding radiocesium dynamics and contamination sites, as proposed here, is essential for contamination management in societies depending on nuclear power to address the climate crisis.

Long-term prediction of [Formula: see text]Cs in Lake Onuma on Mt. Akagi after the Fukushima accident using fractional diffusion model

The Fukushima Daiichi Nuclear Power Plant accident also contaminates lakes in Japan. Especially in closed lakes, there is a problem of prolonged low-level [Formula: see text]Cs contamination because the activity concentration of [Formula: see text]Cs declines sharply immediately after the accident, but then begins to decrease slowly. In this paper, we derived a long-term prediction formula based on the fractional diffusion model (FDM) for the temporal variation in [Formula: see text]Cs activity concentrations of the water in Lake Onuma on Mt. Akagi, one of the closed lakes, and of pond smelt (Hypomesus nipponensis), a typical fish species inhabiting in the lake. The formula reproduced well the measured [Formula: see text]Cs activity concentration of the lake water and pond smelt for 5.4 years after the accident. Next, we performed long-term prediction for 10,000 days using this formula and compared it with the prediction results of the two-component decay function model (TDM), which is the most common model. The results suggest that the FDM prediction will lead to a longer period of contamination with low-level [Formula: see text]Cs than the TDM prediction.