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The large amount of (137)Cs deposited on the forest floor because of the Fukushima Dai-ichi Nuclear Power Plant accident represents a major potential long-term source for mobile (137)Cs. To investigate (137)Cs mobility in forest soils, we investigated the vertical migration of (137)Cs 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 (137)Cs in the litter layer moved into the mineral soil within one year after the accident. On the other hand, the topsoil prevented migration of (137)Cs, and only 2% of (137)Cs 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 (137)Cs inventory. Therefore, the migration of (137)Cs by seepage water comprised only a very small part of the total (137)Cs inventory in the mineral soil, which was undetectable from the vertical distribution of (137)Cs in the soil profile. In the present and immediate future, most of the (137)Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable (137)Cs exists in forest surface soil., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The Fukushima Daiichi Nuclear Power Plant accident caused an accumulation of 137Cs in coastal sediment. The 137Cs supply from rivers to the ocean can affect the long-term fate of 137Cs in coastal sediment. Since the Fukushima coastal river basins include large decontaminated and evacuation order areas, considering the decontamination work and resumption of agriculture is important for predicting the 137Cs supply. We conducted a 30-year prediction of the 137Cs supply from the Fukushima coastal rivers to the ocean using a distributed radiocesium prediction model, considering the effects of human activities. In river basins with decontaminated and evacuation order areas, human activities reduced the total 137Cs outflow from agricultural lands, urban lands, and forest areas to the rivers and the 137Cs supply to the ocean by 5.0% and 6.0%, respectively. These results indicated that human activities slightly impacted the 137Cs outflow and supply. The 137Cs supply from rivers impacted by the accident to the coastal sediment was estimated to correspond to 11–36% of the total 137Cs in the coastal sediment in the early phase of the accident. Therefore, the 137Cs supply from rivers to the ocean is important for the long-term behavior of 137Cs in coastal sediment. [ABSTRACT FROM AUTHOR]

Cesium-rich microparticles (CsMPs) with high cesium-137 (137Cs) concentrations were released and deposited in surface soil after the Fukushima Daiichi Nuclear Power Plant accident. Radioactive materials on the soil surface layer enter rivers owing to soil erosion during rainfall. In this study, we investigated CsMPs runoff through the river via soil erosion during rainfall in the Takase River watershed in Namie Town, Fukushima Prefecture, Japan. CsMPs were rarely detected in suspended solids (SS) in water samples collected during four rainfalls between February and July 2021. Furthermore, the proportion of 137Cs concentration derived from CsMPs to 137Cs concentration in the form of SS (particulate 137Cs) in the water was ~6% on average, which suggests that 137Cs runoff in the form of CsMPs from the forest to the Takase River was not large. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

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]