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Actuators of a robot that physically interacts with objects or human require backdrivability to obtain force controllability, impact resistance and compactness. An electro-hydrostatic actuator (EHA) is a servo pump type hydraulic system that has a potential to satisfy backdrivability. However, the previous mechanical design of a revolute EHA did not achieve compact design compared to the other actuators. The hydraulic pump and vane motor were separately located, which made the size of EHA system larger. To solve this problem, we propose a design methodology of a revolute EHA using a frameless motor. Commercially available frameless motor is suitable for actuating the hydraulic pump. Moreover, the pump can be built in the hollow space of the frameless motor, and this design makes the EHA downsized. We present a prototype design based on the formulation of the output power. Through the experimental validations, we show that developed EHA has higher backdrivability than the comparable harmonic drive, and obtain the knowledges on design factors that are crucial to the EHA with high power-to-weight ratio. [ABSTRACT FROM AUTHOR]

To confirm factors affecting 137Cs concentration in river water after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, this study conducted monthly observations of nine rivers near the FDNPP from April 2017 to March 2018 under base-flow conditions. The annual mean dissolved and particulate 137Cs concentrations correlated well with the mean 137Cs deposition in the catchment. The normalized 137Cs concentrations in both phases by dividing by the mean 137Cs deposition in the catchment showed significant negative correlations with the 137Cs deposition ratio for forests. The inflow from the downstream plain areas increased the 137Cs concentrations near the FDNPP. [ABSTRACT FROM AUTHOR]

Nakanishi, T.; Tsuruta, T.; Misonou, T.; Shiribiki, T.; Urabe, Y., and Sanada, Y., 2023. Behavior of radiocesium (137Cs) on the coastal seafloor near the Fukushima Daiichi Nuclear Power Plant inferred from radiocesium distributions in long cores. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.-S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 161-165. Charlotte (North Carolina), ISSN 0749-0208. Continuous data of 137Cs concentrations in surface seabed sediment at several monitoring points around the Fukushima Daiichi Nuclear Power Plant (FDNPP) from 2012 to 2022 were compiled, and exponential functions fitted its temporal change. The 137Cs concentrations are gradually declining with time. However, at some monitoring points in shallow regions with a water depth of 30 m or less, long half-lives and/or large deviations of 137Cs concentrations were noticed. Long sediment cores (∼1 m) were collected at shallow regions near the FDNPP to gain insight into radiocesium dynamics in shallow seafloor, and the vertical profiles of 137Cs concentrations and particle size distribution were determined. In the sediment cores collected in the onshore zone, the 137Cs concentrations and particle size distribution were homogenized from several tens of cm to more than 1 m depth, indicating strong vertical mixing by wind waves. Therefore, onshore sediments have significant 137Cs reserves in deeper layers at present. The 137Cs supply from the deep layers and the land area might suppress this zone's surface 137Cs concentration decline. However, in sediment cores at the offshore zone, the vertical 137Cs concentration distributions were highly heterogeneous, showing the particle size dependency. Moreover, 137Cs profiles obtained at the same point for several years differed significantly, indicating local spatial distribution and temporal deposition. Therefore, it might cause a large fluctuation in 137Cs concentrations at surface sediment over time. [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]

Particulate Cs was collected from stream water for 2 years to assess the long-term trend of Cs discharge from a forest after the Fukushima Nuclear Power Plant accident. A seasonal increase in the fluvial transport load of particulate Cs in suspended solids (SS) was observed in July-October when rainfall was abundant. The Cs load was controlled by the SS load. This control was attributed to cesium affinity for phyllosilicate clay minerals as verified by the low extractability of particulate Cs. These findings indicate the fluvial particulate Cs load is significantly related to the climate and geomorphological features of Japan. [ABSTRACT FROM AUTHOR]

In order to solve difficulties in collection of river suspended solids (SS) such as frequent observations during stochastic rainfall events, a simple passive collection system of SS has been developed. It is composed of sequentially connected two large-scale filter vessels. A portion of river water flows down into the filter vessels utilizing a natural drop of streambed. The system enable us to carry out long-term, unmanned SS collection. It is also compatible with dissolved component collection. Its performance was validated in a forested catchment by applying to radiocesium and stable carbon transport. [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]

Five survey cruises were carried out from 2004 to 2007 to determine 137Cs concentrations in the water columns off Rokkasho Village, Aomori Prefecture, Japan, where the spent nuclear fuel reprocessing plant of Japan Nuclear Fuel Ltd has been undergoing test operation since March 2006. Seawater samples were collected with a large volume in situ filtration and concentration system at different depths in the water column. 137Cs in particulate form could not be detected in the survey area. Dissolved 137Cs showed temporal variation, especially in the surface water. Based on the results, it was concluded that no observable 137Cs contamination was present in the investigated area. The distribution of 137Cs concentrations was influenced by the mixing between the Tsugaru and Oyashio Currents. [ABSTRACT FROM AUTHOR]

Nakanishi, T.; Ohyama, T.; Hagiwara, H., and Sakuma, K., 2021. Impact of extreme typhoon events on the fluvial discharge of particulate radiocesium in Fukushima prefecture. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 310–314. Coconut Creek (Florida), ISSN 0749-0208. Two huge typhoons that occurred in October 2019, Hagibis and Bualoi, caused considerable flood damage in Fukushima Prefecture. There is concern about the radiocesium discharge to the coastal area related to such flood events from the river systems near the Fukushima nuclear power plant. In this study, we quantitatively evaluated the sediment and 137Cs discharges from the Ukedo River catchment based on the field observations conducted over 6 years. In 2019, approximately 90% of the annual sediment and 137Cs discharges occurred during the typhoon events (Hagibis and Bualoi). This sediment discharge was almost twice that related to the largest ever flood event since the Fukushima nuclear accident, which was caused by the typhoon Etau in September 2015. However, the 137Cs discharge related to the Hagibis and Bualoi events was two-thirds of the Etau event; the particulate 137Cs concentration in river water decreased during the observation period, showing an effective half-life of 2.2–3.9 years. Moreover, the 137Cs discharge during the two typhoon events in 2019 accounted for only 0.1% of the 137Cs deposition in the catchment, and the impact of radiocesium on the coastal area was extremely limited. [ABSTRACT FROM AUTHOR]

In this paper, we study a modified normalized least mean square (NLMS) algorithm for updating coefficients of an adaptive FIR digital filter (ADF). In the modified adaptive algorithm, filter coefficients are updated with the NLMS algorithm for each sample, but the coefficients are frozen when the input signal is smaller than a given threshold (constant). This modified NLMS has been known, but not been analyzed. In this paper, we call the modified NLMS the “conditioned NLMS” (C-NLMS) and analyze the convergence characteristics. As a result, the optimum threshold value is obtained. The simulation results and theoretical analyses show the effectiveness of the C-NLMS with the proposed threshold, and a good agreement between both results. The stability of the NLMS algorithm in the presence of a small input signal is improved. The convergence speed of the C-NLMS ADF under noisy circumstances is also faster than that of the unconditioned ordinary NLMS ADF in the small tap number case. © 2001 Scripta Technica, Electr Eng Jpn, 136(4): 47–57, 2001 [ABSTRACT FROM AUTHOR]

This paper proposes the parallel construction of the second-order infinite impulse response (IIR) adaptive digital filter (ADF) as an adaptive digital filter for eliminating multiple narrowband signals mixed in the broadband signal. Then the pole-control method for the filter is proposed. In the proposed pole-control, the pole radius and the pole argument are controlled separately by different algorithms. In this study, the pole argument is corrected by estimating the deviation of the central frequency of the signal to be eliminated from the convergent value of the multiplier coefficient that determines the zero. The pole radius is controlled by the method that determines the direction of correction, using the parallel-connected three ADFs with the same pole argument and a slightly different pole radius. It is verified by a simulation that those pole-control methods work effectively. It is demonstrated that the pole radius and the pole argument can be controlled simultaneously. [ABSTRACT FROM AUTHOR]

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]

To investigate the factors regulating the concentration of particulate iodine (PI) in seawater, it was measured in two Pacific coastal areas adjacent to Japan (offshore Aomori and offshore Fukushima) in two seasons (May and October) along with parameters such as particulate organic carbon (POC) and nitrogen (PON), dissolved iodine (DI), and phytoplankton pigments. The observed PI concentrations ranged from 0.0068 to 2.2 nmol L−1, with uncertain seasonal and regional differences. The atomic ratio of PI to POC (I/C) ranged from 4.4 × 10−7 to 1.1 × 10−4, in line with previous studies on marine phytoplankton, although the lower part of the range of the I/C in this study was found to be somewhat lower compared to the previous studies. The dataset was divided in three groups namely, lower (Group A), average (Group B), and higher (Group C) I/C because the I/C is an indicator of the accumulation efficiency of PI in particles in seawater and POC is the main carrier of PI. Based on the observed POC concentrations, the atomic ratio of POC to PON (C/N), and the ratio of carbon derived from chlorophyll a to POC, the three groups were characterized by phytoplanktonic physiological states as highly productive, steady, and senescent states for Groups A, B, and C, respectively. Based on the finding that PI production is associated with the phytoplankton physiological state, the seasonal and regional differences in PI concentration and I/C in the observation areas were consistently explained. Finally, we suggest that the phytoplanktonic physiological state is one of the vital factors regulating the PI concentration in seawater. [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]

The radiocesium (137Cs) distribution between dissolved and particulate phases was examined in river water and coastal seawater as a function of the 137Cs sorption behavior on suspended particles. Dissolved 137Cs activity concentrations in the Tomioka River (salinity <0.1), about 10 km south of Fukushima Dai-ichi Nuclear Power Plant, and in coastal seawater at Tomioka fishery port (salinity >30), Fukushima Prefecture, from June 2019 to October 2021 were 3.6–20 Bq/m3 (geometric mean 11 Bq/m3) and 2.4–86 Bq/m3 (13 Bq/m3), respectively. Although the suspended particle concentration was lower in the river than in seawater, the mean 137Cs activity on suspended particles was 11,000 Bq/kg-dry in the river versus 3200 Bq/kg-dry in seawater. Proportions of ion-exchangeable, organically bound, and refractory fractions of 137Cs on suspended particles were determined by sequential extraction. The ion-exchangeable fraction accounted for 0.3–2.0% (average: 1.2%) and 0.4–1.3% (0.8%) at the river and port sites, respectively. The organically bound fraction accounted for 0.3–4.8% (1.8%) and 0.1–5.5% (2.1%) at the river and port sites, respectively. In both areas, the refractory fraction accounted for >90% of 137Cs. Therefore, the small labile 137Cs fraction on suspended particles in coastal seawater indicates that the mobility of 137Cs to marine biota is quite low. This study is the first to examine radiocesium sorption forms on suspended particles in coastal seawater near the Fukushima Dai-ichi Nuclear Power Plant. It suggests immobility of 137Cs in suspended particles being incorporated to marine biota. [Display omitted] • Cs-137 distribution between dissolved and particulate phases in river water and seawater. • Sorption behavior of Cs in suspended particle in seawater was evaluated. • Ion-exchangeable fraction accounted for only 1%. • Refractory fraction accounted for >90% of 137Cs. • Small labile fraction implies immobility of 137Cs to incorporation by marine biota. [ABSTRACT FROM AUTHOR]

Large quantities of volatile radionuclides were released into the atmosphere and the hydrosphere following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March, 2011. Monitoring of radiocesium in sediment is important for evaluating the behavior of radiocesium in the environment and its effect on aquatic organisms. In this study, the radiocesium distribution in the surface sediment around the FDNPP was visualized as a radiocesium concentration map using periodical survey data from a towed gamma-ray detection system. The uncertainty of the radiocesium map was evaluated via comparison with a large amount of sediment core sample data. The characteristics of the radiocesium distribution were examined considering the seafloor topography and a geological map, which were obtained via acoustic wave survey. The characteristics of the formation of 137Cs anomaly at the estuaries were analyzed using a contour map of 137Cs concentration combined with water depth. Validation of the created map showed that it was comparable with actual sediment core samples. The map generated using the towed radiation survey depicted the 137Cs concentration distribution as the position resolution of a 1 km mesh. Finally, the 137Cs concentration decreased with time in consideration of such uncertainty. [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]

Radioactive cesium-rich microparticles (CsMPs) derived from the Fukushima Daiichi Nnuclear Power Plant accident were detected from soils and river water around Fukushima Prefecture, Japan. Because CsMPs are insoluble and rich in radioactive cesium (RCs), they may cause the overestimation of solid-water distribution coefficient (K d) for RCs in the water. Previous studies showed the proportion of RCs derived from CsMPs on RCs concentration in soils collected from areas with different contaminated levels. Because the proportion of RCs concentration derived CsMPs to the RCs concentration of soils in the less contaminated areas is higher than that in the highly contaminated areas, the effect of CsMPs on particulate RCs concentration in river water may be larger in the less contaminated areas. However, the difference in the effects of CsMPs on the particulate RCs concentration and K d in river water flowing through watersheds with different contaminated levels has not been clarified. In this study, we investigated the effect of CsMPs on the particulate RCs concentration and K d in two rivers, Takase River and Kami-Oguni River, flowing through the watersheds with different RCs contaminated levels in Fukushima Prefecture. CsMPs might enter rivers due to soil erosion because they were detected only in some samples collected from both rivers during flood events. CsMPs accounted for more than half of particulate RCs concentration in some water samples collected in the flood condition. In particular, the proportion of CsMPs in particulate RCs for the Kami-Oguni River was greater than that for the Takase River. However, when evaluating for the entire water sampling in the flood condition, a proportion of RCs concentration derived from CsMPs in the average RCs concentrations per unit mass of SS in both river waters collected in the flood condition was not large. CsMPs might temporarily increase the particulate RCs concentration and K d in the flood event, but CsMPs did not significantly affect them when evaluated throughout the event. • CsMPs entered the river via soil erosion. • CsMPs temporary increased the particulate RCs in the river water. • CsMPs did not affect K d in river water when evaluating throughout the flood event. [ABSTRACT FROM AUTHOR]

Misonou, T.; Tsuruta, T.; Shiribiki, T.; Nakanishi, T.; Nagao, S.; Ochiai, S., and Sanada, Y., 2021. Dynamics of radioactive cesium in coastal area linked with river discharge. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 315–319. Coconut Creek (Florida), ISSN 0749-0208. For understanding the dynamics of future radioactive cesium (Cs) and its distribution in the coastal area of Fukushima Prefecture, radioactive Cs supply from river discharge must be evaluated. Particulate radioactive Cs supplied by floods could significantly affect the coastal area. Therefore, the effect of particulate radioactive Cs supplied from the river was clarified by installing mooring systems (sediment traps and water quality meters) at four points (water depths of St.1: 10 m, St.2: 27 m, St.3: 30 m, and St.4: 60 m) in the coastal area of Fukushima Prefecture from October 22 to 28, 2019. During the observation period, a large typhoon (Bualoi) accompanied by torrential rain struck Fukushima Prefecture. The 137Cs concentrations of the sinking particles collected by the sediment trap at the observation point installed in front of the river (St.1) were collected immediately after typhoon Bualoi (October 25) and were approximately three times higher than before the typhoon. However, the 137Cs concentration of the sinking particles collected two days after the typhoon (October 27) was almost the same as before the typhoon. Local increases in 137Cs were not observed at other stations. The stable carbon isotope ratio (δ13C) of the sinking particles during the typhoon was approximately –30 to –26‰ at the measurement points installed in front of the Ukedo, Ohta, and Kuma Rivers (St.1, St.2, and St.3). It was assumed that there was an influence of inflow from the river on St.1, St.2, and St.3 after precipitation. The impact of rivers on coastal areas during torrential rain is extremely limited. [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]

To predict the behavior of particulate and dissolved radiocesium (137Cs) in river water, somesoil and land use categories in the watershed should be considered because these may affectthe dissolved 137Cs concentration. This recommendation is based on measurements madeafter the Chernobyl accident (Smith et al., 2004). However, it is difficult to predict thedissolved 137Cs concentration in Japanese rivers based only on studies after theChernobyl accident because 137Cs dynamics differ between the areas surroundingthe Chernobyl and Fukushima Daiichi Nuclear power plants because of climateand soil differences (Evrard et al., 2015; Konoplev et al., 2016). In this study, toinvestigate the factors that affect dissolved 137Cs concentrations in Japanese rivers, wemeasured dissolved 137Cs concentrations in 67 rivers under baseflow conditions 13–280km from the Fukushima Daiichi nuclear power plant in August and September2017. The watershed area of each sampling point was 3.2–3,600 km2 and the averaged 137Csinventory for each watershed in July 2011 was 4.7–1,700 kBq/m2(calculated from the 137Csinventory distribution map by Kato et al., under review). Land use in the watersheds variedand included forest watershed (up to 99% forest area) and urban watershed (up to 73%building area). To research the influence of water quality on the dissolved 137Csconcentration, we coincidentally measured the main coexisting solutes, pH value,and electric conductivity in the river water. Additionally, to research the effect ofthe 137Cs source, the land use ratio, soil components, and watershed topographicwetness index were calculated from geographic information data published on theInternet. The dissolved 137Cs radioactivity concentrations in river water were 0.10–120 Bq/m3andstrongly depended on the averaged 137Cs inventory in the watershed (r = 0.70, p &#x003C; 0.01).Therefore, the dissolved 137Cs concentration was normalized by dividing it by theaveraged 137Cs inventory (converted into the range 4.5×10−6–2.2×10−3 m−1) and thecorrelations among these values and each water quality factor and 137Cs source wereanalyzed. Significantly high positive and a negative correlation existed between the normalizeddissolved 137Cs concentration and building area ratio (r = 0.78) and forest area ratio (r =-0.60), respectively. The high concentration of coexisting ions in the water was a cause ofsuch a high correlation because it would impede 137Cs absorption in soil particles because thebuilding area ratio has a high positive correlation with electrical conductivity (r = 0.55). [ABSTRACT FROM AUTHOR]