Search

Although 129I discharge from watersheds is fundamental for assessing long-term radiation effects on aquatic ecosystems, 129I originating from the Fukushima nuclear accident is yet be evaluated. This study investigated the transport behavior of 129I by riverbank surveys conducted from 2013 to 2015 in a watershed where the 129I/137Cs activity ratio is low in the mountainous area and high in the plain as of 2011. Until 2015, the 129I/137Cs activity ratio of the levee crown in the studied watershed was similar to that of the surrounding area in 2011. However, the 129I/137Cs ratios of the surface riverbank sediments were all low, indicating that radionuclides transported from the mountainous area were deposited on the riverbank in the plain. The vertical distribution of the 129I/137Cs ratio in the riverbank sediments indicated that some 129I and 137Cs deposited during the accident remained in the lower layers, but most were eroded immediately after the accident. Based on the 129I/137Cs ratios of sediments deposited on the riverbank, which remained constant until 2015 after the accident, the amount of 129I discharged to the ocean was determined from the previously evaluated 137Cs discharge. It was calculated that 1.8 × 105 Bq and 1.2 × 107 Bq of 129I were discharged with sediment from the studied watershed and the contaminated river watersheds (Abukuma River and Fukushima coastal rivers, including the study river), respectively. This amount of 129I was 0.3% of the 129I released from the Fukushima Dai-ichi Nuclear Power Plant into the ocean immediately after the accident. Furthermore, a comparison of the 129I/137Cs ratio showed that the continuous 129I and 137Cs discharge from the river contribute little to their amount in the seafloor sediments along the Fukushima coast. [Display omitted] • 129I and 137Cs in riverbank affected by the Fukushima accident were investigated. • 129I/137Cs ratio is a good indicator of the sediment source in a watershed. • Most of the sediment deposited on the riverbank was transported from upstream. • River discharge contributes little to the amount of 129I in the coastal area. [ABSTRACT FROM AUTHOR]

Abstract It is a critical to examine the migration behavior of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in river systems to predict the future contamination status and propose effective countermeasures to reduce exposure. We conducted a three-year-long observation (April 2015–March 2018) of the 137Cs concentration in two rivers which located surrounding the FDNPP. The result revealed a declining trend for the dissolved and particulate 137Cs concentration in river water from four to seven years after the FDNPP accident. The dissolved and particulate 137Cs concentrations for both rivers had similar temporal patterns and showed declining trends with time. However, the dissolved 137Cs concentration had longer half-life than the particulate 137Cs concentration and large seasonal variations related to water temperature. The environmental half-life for the dissolved 137Cs concentration was longer than previous reported values within three years after the accident, suggesting that the declining trend for the dissolved 137Cs concentration is gradually decreasing with time. The temperature dependency of the dissolved 137Cs concentration became weaker year by year. From the D 10 equation we proposed, the dissolved 137Cs concentration will likely remain at the same level for several decades. The results of the present study promote our understanding of both the medium- and long-term impacts of the FDNPP accident on river systems. Highlights • 137Cs concentration in river water was continuously monitored. • The environmental half-life was different between particulate and dissolved 137Cs. • Dissolved 137Cs concentration had seasonal variation related to water temperature. • Temperature dependency of dissolved 137Cs concentration became weaker with time. [ABSTRACT FROM AUTHOR]

The release of tritium (3H) to the ocean is planned on the coastal environment in the Fukushima coastal region from Spring or Summer of 2023. Before its release, we evaluate the effect of 3H discharges from the port of Fukushima Daiichi and rivers in the Fukushima coastal region using a three-dimensional hydrodynamic model (3D-Sea-SPEC). The simulation results showed that discharges from the port of Fukushima Daiichi dominantly affected the 3H concentrations in monitoring points within approximately 1 km. Moreover, the results indicate that the effect of riverine 3H discharge was limited around the river mouth under base flow conditions. However, its impact on the Fukushima coastal regions under storm flow conditions was found, and the 3H concentrations in seawater in the Fukushima coastal region were formed around 0.1 Bq/L (mean 3H concentrations in seawater in the Fukushima coastal region) in the near shore. [Display omitted] • Tritium behavior in the Fukushima coastal region was simulated. • Tritium discharges from the port of 1F and 14 rivers in Fukushima were considered. • The simulation reproduced the 3H conc. and dispersions reasonably. • Tritium concentrations near 1F were dominantly affected by discharges from 1F. • Riverine 3H discharges impact on 3H conc. in the near shore under storm conditions. [ABSTRACT FROM AUTHOR]

It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (137Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the 137Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%–83% of secondary 137Cs migration in nearshore areas, whereas the influence of riverine 137Cs input on the sediment was only 7% per year. [Display omitted] • Particulate 137Cs flux in the coastal area was several orders of magnitude higher than that in the offshore. • The migration mechanism of sinking particles in coastal areas was classified by physical parameters. • The 137Cs flux due to riverine influence was calculated. [ABSTRACT FROM AUTHOR]

Abstract: The large amount of 137Cs deposited on the forest floor because of the Fukushima Dai-ichi Nuclear Power Plant accident represents a major potential long-term source for mobile 137Cs. To investigate 137Cs mobility in forest soils, we investigated the vertical migration of 137Cs through seepage water, using a lysimetric method. The study was conducted in a deciduous forest soil over a period spanning 2 month to 2 y after the Fukushima nuclear accident. Our observations demonstrated that the major part of 137Cs in the litter layer moved into the mineral soil within one year after the accident. On the other hand, the topsoil prevented migration of 137Cs, and only 2% of 137Cs in the leachate from litter and humus layer penetrated below a 10 cm depth. The annual migration below a 10 cm depth accounted for 0.1% of the total 137Cs inventory. Therefore, the migration of 137Cs by seepage water comprised only a very small part of the total 137Cs inventory in the mineral soil, which was undetectable from the vertical distribution of 137Cs in the soil profile. In the present and immediate future, most of the 137Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable 137Cs exists in forest surface soil. [Copyright &y& Elsevier]

Abstract: Water-extractable organic carbon (WEOC) in soil consists of a mobile and bioavailable portion of the dissolved organic carbon (DOC) pool. WEOC plays an important role in dynamics of soil organic carbon (SOC) and transport of radionuclides in forest soils. Although considerable research has been conducted on the importance of recent litter versus older soil organic matter as WEOC sources in forest soil, a more thorough evaluation of the temporal pattern of WEOC is necessary. We investigated the seasonal variation in WEOC in a Japanese cool-temperate beech forest soil by using the carbon isotopic composition (14C and 13C) of WEOC as a tracer for the carbon sources. Our observations demonstrated that fresh leaf litter DOC significantly contributed to WEOC in May (35–52%) when the spring snowmelt occurred because of the high water flux and low temperature. In the rainy season, increases in the concentration of WEOC and the proportion of hydrophobic compounds were caused by high microbial activity under wetter conditions. From summer to autumn, the WEOC in the mineral soil horizons was also dominated by microbial release from SOC (>90%). These results indicate that the origin and dynamics of WEOC are strongly controlled by seasonal events such as the spring snowmelt and the rainy season's intense rainfall. [Copyright &y& Elsevier]

Abstract: Using a sector-field ICP-MS the vertical distributions of the 99Tc concentration and 99Tc/137Cs activity ratio were measured in the coastal waters off Aomori Prefecture, Japan, where a spent-nuclear-fuel reprocessing plant has begun test operation. The 99Tc concentrations in surface water ranged from 1.8 to 2.4 mBq/m3, no greater than the estimated background level. Relatively high 99Tc/137Cs activity ratios (10–12 × 10−4) would be caused by the inflow of the high-99Tc/137Cs water mass from the Japan Sea. There is no observable contamination from the reprocessing plant in the investigated area. The 99Tc concentration and the 99Tc/137Cs activity ratio in water column showed gradual decreases with depth. Our results implied that 99Tc behaves in a more conservative manner than 137Cs in marine environments. [Copyright &y& Elsevier]

Radiocesium that originated from the Fukushima Daiichi Nuclear Power Plant accident was deposited on the ground surface and has been transported via fluvial discharge, primarily in the form of particulates, to downstream areas and eventually to the ocean. During transportation, some of the radiocesium accumulated on the riverbed. In this study, we quantified the radiocesium deposition on the riverbed in the Odaka River estuary and investigated the radiocesium sedimentation process of the river bottom. Our results show that the radiocesium inventory in the seawater intrusion area is larger than those in the freshwater and marine parts of the estuary. Moreover, the particle-size distribution in the seawater intrusion area shows a high proportion of silt and clay particles compared with the distribution in other areas. The increased radiocesium inventory in this area is attributed to the sedimentation of fine particles caused by hydrodynamic factors (negligible velocity of the river flow) rather than flocculation factor by salinity variation. • Considerable 137Cs deposition was observed in the seawater intrusion area. • We estimated the total amount of radiocesium deposited in the river's estuary. • Fine sediments with 137Cs were concluded to be deposited due to hydrodynamic processes rather than flocculation. [ABSTRACT FROM AUTHOR]

We developed a simple model to evaluate and predict the 137Cs discharge from catchments using a tank model and the L-Q equation. Using this model, the 137Cs discharge and discharge ratio from the Abukuma River and 13 other rivers in the Fukushima coastal region were estimated from immediately after the Fukushima accident up to 2017. The 137Cs discharge (and discharge ratio to the deposition inventory in the catchment) of the Abukuma River and 13 other rivers in the Fukushima coastal region during the initial six months after the accident were estimated to be 18 TBq (3.1%) and 11 TBq (0.79%), respectively. These values of 137Cs discharge ratio were 1–2 orders of magnitude higher than those observed after June 2011 in previous studies (Ueda et al., 2013; Tsuji et al., 2016; Iwagami et al., 2017a), indicating that the initial 137Cs discharge from the catchments through the rivers was significant. The simulated initial 137Cs discharge rates for the initial six months after the Fukushima accident were about 9–30 times larger in each catchment than those after that point until 2017, though initial 137Cs concentration in river water was derived from an extrapolation of data based on a two exponentially decreasing fitting. However, it was found that the impact on the ocean from the initial 137Cs discharge through the rivers can be limited because the 137Cs discharge from the Abukuma River and the 13 other rivers in the Fukushima coastal region (29 TBq) was two orders of magnitude smaller than the direct release from Fukushima Dai-ichi Nuclear Power Plant (FDNPP) into the ocean (3.5 PBq) and from atmospheric deposition into the ocean (7.6 PBq) (Kobayashi et al., 2013). This model is expected to be useful to evaluate and predict 137Cs discharge from catchments in future water management and in the estimation of 137Cs discharge into reservoirs and the ocean. • A simple model was created to predict the radiocesium discharge from catchments. • We estimated the 137Cs discharge from catchments in the first six months after the Fukushima accident. • Abukuma River had a137Cs discharge level of 18 TBq and 13 other rivers in the coastal region were at 11 TBq. • The 137Cs discharge rates during the initial six months was about 9–30 times larger than the period after that point. • However the impact of the initial 137Cs discharge through the rivers on the initial release into the ocean can be limited. [ABSTRACT FROM AUTHOR]

The fate of radiocesium ( 137 Cs) derived from the Fukushima nuclear accident and associated radiation risks are largely dependent on its migration and retention behavior in the litter–soil system of Japanese forest ecosystems. However, this behavior has not been well quantified. We established field lysimeters in a Japanese deciduous broad-leaved forest soon after the Fukushima nuclear accident to continuously monitor the downward transfer of 137 Cs at three depths: the litter–mineral soil boundary and depths of 5 cm and 10 cm in the mineral soil. Observations were conducted at two sites within the forest from May 2011 to May 2015. Results revealed similar temporal and depth-wise variations in 137 Cs downward fluxes for both sites. The 137 Cs downward fluxes generally decreased year by year at all depths, indicating that 137 Cs was rapidly leached from the forest-floor litter layer and was then immobilized in the upper (0–5 cm) mineral soil layer through its interaction with clay minerals. The 137 Cs fluxes also showed seasonal variation, which was in accordance with variations in the throughfall and soil temperature at the sites. There was no detectable 137 Cs flux at a depth of 10 cm in the mineral soil in the third and fourth years after the accident. The decreased inventory of mobile (or bioavailable) 137 Cs observed during early stages after deposition indicates that the litter–soil system in the Japanese deciduous forest provides only a temporary source for 137 Cs recycling in plants. [ABSTRACT FROM AUTHOR]

In forest ecosystems, forest litter is considered an active medium for radiocesium (137Cs). To understand discharge mechanisms of highly bioavailable dissolved 137Cs from forests to river systems, we investigated the characteristics of 137Cs leaching from forest litter as observed from litterbag experiments. Leaching experiments with conifer needle and deciduous broadleaf litters were then conducted. After soaking conifer needles and broadleaf litters for 20 min, 140 min, and 1 day, the mean values of the 137Cs leaching ratios were 0.13–2.0% and 0.81–6.6%, respectively, indicating that 137Cs leaching ratios are different between forest litter types. To elucidate the factors affecting 137Cs leaching from forest litter, a multi-regression analysis of 137Cs leaching ratios was conducted against antecedent mean precipitation and temperature before sampling the litterbag and accumulated temperature during the litterbag experiments. The 137Cs leaching ratios showed a negative correlation to the antecedent mean precipitation for both litters and the accumulated temperature for broadleaf litters, whereas it exhibited a positive correlation with the antecedent mean temperature for both litters and the accumulated temperature for conifer needle litters. It was proposed that the fraction of 137Cs in labile sites in forest litter increased/decreased due to litter decomposition by antecedent/accumulated temperature, and that this fraction can be washed off by the antecedent precipitation. The different effects of accumulated temperature on 137Cs leaching from conifer needles and broadleaf litters could be due to their different decomposition rates. Our results contribute further the understanding of the mechanisms associated with dissolved 137Cs discharge from forested catchments. Image 1 • Dissolved 137Cs leaching from forest litters was investigated. • We estimated 137Cs leaching ratios from different types of forest litters. • 137Cs leaching ratios were different from conifer needle and broadleaf litters. • Initial 137Cs leaching rates were faster than those thereafter. • 137Cs leaching from forest litters could be rationalized by environmental factors. [ABSTRACT FROM AUTHOR]

Because of their large mobility and high bioavailability, it is necessary to elucidate the origins and dynamics of dissolved radionuclides in river and reservoir systems to assess the transfer of those radionuclides from water to crops and aquatic organisms. Elution from contaminated reservoir sediments, a potential source of dissolved radionuclides, presents a long-term concern, particularly for long-lived radionuclides. In this study, we systematically investigated caesium-137 (137Cs) concentrations using a time-series suite of input and output water samples collected from 2014 to 2019 from the Ogaki Dam Reservoir, which has a catchment with a high 137Cs inventory due to the Fukushima Dai-ichi Nuclear Power Plant accident. The results of our study showed that dissolved 137Cs concentration was significantly higher in the output water than that in the main input water, and that the effective ecological half-life of dissolved 137Cs in the output water was longer than in the main input water. We quantitatively evaluated the mass balance of dissolved 137Cs in the reservoir to elucidate how much dissolved 137Cs from the rivers and production from reservoir sediments contribute to 137Cs in the reservoir output. The annual output of dissolved 137Cs was significantly higher than the total input of dissolved 137Cs, with approximately 32%–40% of the dissolved 137Cs in the output water presumably being produced from reservoir sediments. Consequently, the estimated dissolved 137Cs fluxes from reservoir sediments to overlying water were 0.57–1.3 × 104 Bq m−2 y−1. This implies that approximately 0.04%–0.09% of 137Cs accumulated in the sediments was released through elution to the overlying water each year. Reservoir sediments containing high 137Cs levels may thus become even more important as sources of bioavailable dissolved 137Cs in the future. Unlabelled Image • We quantitatively evaluated the mass balance of dissolved 137Cs in a reservoir. • We monitored 137Cs concentrations in a reservoir from 2014 to 2019. • Dissolved 137Cs in output water declined slower than that in input water. • About 40% of dissolved 137Cs in output water was derived from sediments. [ABSTRACT FROM AUTHOR]

The deposited radiocesium in the Fukushima river basin is transported in the river systems by soil particles and redistributed in the downstream areas. Although predicting the behaviors of minerals that adsorb radiocesium and of radiocesium dissolved in river water within the river systems is essential, the dominant mineral species that adsorb radiocesium have not yet been comprehensively identified. We identify herein such mineral species by investigating the 137Cs distribution and the mineral species in each size fraction that are found in the bedload sediments from an upstream reservoir to an estuary within the Tomioka river basin located east of Fukushima Prefecture in Japan. In the fine sand sediment, which is the dominant fraction in terms of the 137Cs quantity in the river bedload, the 137Cs concentrations of the felsic and mafic minerals are comparable to that of micas. The mafic minerals contain 62% of the 137Cs in the fine sand fraction in the upstream area, while the felsic minerals contain the highest quantities of 137Cs in the downstream area. These results suggest that the quantification of the mineral species and the 137Cs concentration of each size fraction are critically important in predicting the behaviors of the minerals and radiocesium within the Fukushima river basin in the future. • The fine-sand fraction of the Tomioka river bedload predominantly contained 137Cs. • Not only micas but felsic and mafic minerals could contribute to radiocesium sorption. • The 137Cs sorption on the micas was relatively low. [ABSTRACT FROM AUTHOR]

Abstract In this study, seabed sediment was collected from 26 stations located within 160 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) during the 2 years which followed the FDNPP accident of March 2011 and the concentrations of 129I and 137Cs were measured. By comparing the distribution of these two radionuclides with respect to their different geochemical behaviors in the environment, the transport of accident-derived radionuclides near the seafloor is discussed. The concentration of 129I in seabed sediment recovered from offshore Fukushima in 2011 ranged between 0.02 and 0.45 mBq kg−1, with 129I/137Cs activity ratios of (1.9 ± 0.5) × 10−6 Bq Bq−1. The initial deposition of 129I to the seafloor in the study area was 0.36 ± 0.13 GBq, and the general distribution of sedimentary 129I was established within 6 months after the accident. Although iodine is a biophilic element, the accident-derived 129I negligibly affects the benthic ecosystem. Until October 2013, a slight increase in activity of 129I in the surface sediment along the shelf-edge region (bottom depth: 200–400 m) was observed, despite that such a trend was not observed for 137Cs. The preferential increase of the 129I concentrations in the shelf-edge sediments was presumed to be affected by the re-deposition in the shelf-edge sediments of 129I desorbed from the contaminated coastal sediment. The results obtained from this study indicate that 129I/137Cs in marine particles is a useful indicator for tracking the secondary transport of accident-derived materials, particularly biophilic radionuclides, from the coast to offshore areas. Graphical abstract Image 1 Highlights • Concentration of 129I in seabed sediment off Fukushima is reported for the first time. • Deposition of the FDNPP accident-derived 129I to the seafloor was 0.36 ± 0.13 GBq • Until October 2013, 129I activity in sediment increased in the shelf-edge region. • Remobilization of 129I near the seafloor likely affected the sequential accumulation. • The accident-derived 129I is considered to negligibly affect the benthic ecosystem. [ABSTRACT FROM AUTHOR]

The Oginosawa River catchment lies 15 km south-west of the Fukushima Dai-ichi nuclear plant and covers 7.7 km 2 . Parts of the catchment were decontaminated between fall 2012 and March 2014 in preparation for the return of the evacuated population. The General-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) code was used to study sediment and 137 Cs redistribution within the catchment, including the effect of decontamination on redistribution. Fine resolution grid cells were used to model local features of the catchment, such as paddy fields adjacent to the Oginosawa River. The simulation was verified using monitoring data for river water discharge rates ( r = 0.92), suspended sediment concentrations, and particulate 137 Cs concentrations ( r = 0.40). Cesium-137 input to watercourses came predominantly from land adjacent to river channels and forest gullies, e.g. the paddy fields in the Ogi and Kainosaka districts, as the ground in these areas saturates during heavy rain and is easily eroded. A discrepancy between the simulation and monitoring results on the sediment discharge rate following decontamination may be explained by fast erosion occurring after decontamination. Forested areas far from the channels only made a minor contribution to 137 Cs input to watercourses, total erosion of between 0.001 and 0.1 mm from May 2011 to December 2015, as ground saturation is infrequent in these areas. The 2.3–6.9% y −1 decrease in the amount of 137 Cs in forest topsoil over the study period can be explained by radioactive decay (approximately 2.3% y −1 ), along with a migration downwards into subsoil and a small amount of export. The amount of 137 Cs available for release from land adjacent to rivers is expected to be lower in future than compared to this study period, as the simulations indicate a high depletion of inventory from these areas by the end of 2015. However continued monitoring of 137 Cs concentrations in river water over future years is advised, as recultivation of paddy fields by returnees may again lead to fast erosion rates and release of the remaining inventory. [ABSTRACT FROM AUTHOR]

To investigate the main factors that control the dissolved radiocesium concentration in river water in the area affected by the Fukushima Daiichi nuclear power plant accident, the correlations between the dissolved 137Cs concentrations at 66 sites normalized to the average 137Cs inventories for the watersheds with the land use, soil components, topography, and water quality factors were assessed. We found that the topographic wetness index is significantly and positively correlated with the normalized dissolved 137Cs concentration. Similar positive correlations have been found for European rivers because wetland areas with boggy organic soils that weakly retain 137Cs are mainly found on plains. However, for small Japanese river watersheds, the building area ratio in the watershed strongly affected the dissolved 137Cs concentration. One reason for this would be because the high concentrations of solutes, such as K+ and dissolved organic carbon, discharged in urban areas would inhibit 137Cs absorption to soil particles. A multiple regression equation was constructed to predict the normalized dissolved 137Cs concentration with the topography, land use, soil component, and water quality data as explanatory variables. The best model had the building land use as the primary predictor. When comparing two multiple regression models in which the explanatory variables were limited to (1) the land use and soil composition and (2) the water quality, the water quality model underestimated the high normalized dissolve 137Cs concentration in urban areas. This poor reproducibility indicates that the dissolved 137Cs concentration value in urban areas cannot be solely explained by the solid-liquid distribution of 137Cs owing to the influence of the water quality, but some specific 137Cs sources in urban areas would control the dissolved 137Cs concentration. Unlabelled Image • Dissolved 137Cs concentration decreased by one order of magnitude between 2012 and 2017. • Normalized dissolved 137Cs concentration well correlated with the building area ratio. • Topographic wetness index generally predicted dissolved 137Cs concentration. • High dissolved 137Cs concentration in urban areas cannot be explained by water quality. [ABSTRACT FROM AUTHOR]