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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]

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]

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: 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]

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 < 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]