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
Cs anomaly at the estuaries were analyzed using a contour map of
Cs 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
Cs concentration distribution as the position resolution of a 1 km mesh. Finally, the
Cs concentration decreased with time in consideration of such uncertainty.
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.
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•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.
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.
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•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.
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.
This paper summarizes the general situation of marine monitoring conducted after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the experience of development and operation of USVs, and ...the possibility of applying unmanned vessels as a tool for nuclear disaster prevention in the future. Even now, more than 10 years after the accident, large-scale marine monitoring is still being conducted in the vicinity of the Fukushima Daiichi Nuclear Power Plant. In addition, it is an important policy issue to develop monitoring tools in preparation for post-accident situations. Operational tests of three USVs have been continuously conducted for use in such environmental radiation monitoring. Development of these USVs is underway with a view to utilizing them for seawater sampling, direct measurement of the seafloor soil surface layer, and seafloor soil sampling, depending on their performance. It is necessary to promote the development of USVs for future nuclear power plant accidents.
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.
Tsuruta, T.; Shiribiki, T.; Misonou, T.; Nakanishi, T.; Sanada Y., and Urabe, Y., 2021. Vertical profiles of radioactive Cs distributions and temporal changes in seabed sediments near river mouth in ...coastal area of 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. 320–324. Coconut Creek (Florida), ISSN 0749-0208. Understanding the features of radioactive Cs (137Cs) in seabed sediments in coastal areas linked with rivers, which are major settlement areas for particulate 137Cs, is a key issue in evaluating the supply of 137Cs from river discharges. The vertical profile of 137Cs distribution in seabed sediments is an important clue for elucidating the settlement process of particulate 137Cs near river mouth. Therefore, we sampled seabed sediments using a long core sampler (vibrocoring; the maximum lengths of the core was approximately 100 cm) to clarify the entire vertical profile of the 137Cs distribution and changes in seabed sediments near the river mouth in the coastal area of Fukushima Prefecture. From 2014 through 2019, the 137Cs concentration along the entire vertical profile did not increase, and there was no remarkable migration of the 137Cs toward the deeper layers of the seabed sediments. On the contrary, the amount of 137Cs decreased by more than 70% throughout the entire depth, and the 137Cs distributions in the depth direction become uniform. These results suggest that the supply of 137Cs from the river discharge is not a major concern near the river mouth in the coastal area of Fukushima Prefecture. Resuspension of particle-bound 137Cs and offshore transport are the main factors that reduce the 137Cs concentration in the coastal seabed sediments.
Sequential methods to measure the concentrations of chemical elements in (1) acid soluble, (2) reducible, (3) oxidizable, (4) opal, and (5) refractory fractions of settling particles were developed. ...Concentrations of Cu and Ni of settling particles collected with HX-10 type sediment traps at 1.2 km and 4.8 km depths at 44°04'N, 155°03'E were measured by the methods. Copper was enriched in the reducible and oxidizable fractions, especially in the oxidizable fraction, both at 1.2 km and 4.8 km. The degree of vertical increase in Cu concentration of the oxide fraction is the largest among the five fractions. Nickel was most enriched in the oxide fraction with a concentration in the oxide fraction three times that of the oxidizable fraction. Nickel concentrations of the acid soluble and the reducible fractions decreased with depth. The results suggest that (1) Cu and Ni are incorporated in surface water into organic material and calcium carbonate particles, respectively, (2) organic material and calcium carbonate particles decompose in the water column, (3) Cu and Ni are regenerated from these particles, and (4) scavenging of Ni in intermediate and deep waters is not significant.
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