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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]

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]

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]

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]