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Sato, Yuhi*; Tani, Takashi*; Ikenoue, Tsubasa; Kawamura, Hideyuki; Omori, Yuko*
Environmental Science and Pollution Research, 8 Pages, 2026/00
Tokyo Electric Power Company Holdings began the oceanic release of treated water from the Fukushima Daiichi Nuclear Power Station (FDNPS) in August 2023, in which radioactive materials were effectively removed using the Advanced Liquid Processing System (ALPS). The environmental behavior of tritium, accounting for almost all radioactivity in ALPS-treated water, is of critical scientific and social concern. The accumulation possibility of tritium in marine organisms under the release conditions of ALPS-treated water was reviewed to ensure the safety of fishery products collected off the Fukushima coast and prevent unfounded reputation damages to the products. First, previous findings from actual measurements and numerical model estimations of the distribution of tritium derived from ALPS-treated water in seawater off the Fukushima coast are summarized to discuss the impact of oceanic release on tritium levels in seawater. As a result, the impact is suggested to be highly limited, which is indistinguishable from a natural level except for within 200 km from FDNPS. Second, the accumulation possibility of organically bound tritium (OBT) in marine organisms, such as phytoplankton, seaweed, and fish, was assessed using previous findings obtained from experimental and numerical studies, resulting in far smaller OBT accumulations in those organisms compared to the food-chain guideline proposed by FAO/WHO. Finally, the risks of internal exposure through the ingestion of fishery products collected off the Fukushima coast are discussed and quantitatively explained to be minimal especially in comparison with the food-chain guideline. However, continuous environmental monitoring of the oceanic release of ALPS-treated water is considered essential.
Ikenoue, Tsubasa; Tani, Takashi*; Kawamura, Hideyuki; Sato, Yuhi*
Environmental Science & Technology, 59(38), p.20588 - 20594, 2025/09
Times Cited Count：1 Percentile：47.55(Engineering, Environmental)Since 2023, ALPS-treated water containing tritium from the Fukushima Daiichi Nuclear Power Plant (1F) accident has been released into the ocean. Monitoring of seafood collected near the Fukushima coast has shown negligible increases in the tritium concentration. However, this monitoring has limitations, including delayed data release and a limited sample size. Therefore, a predictive estimation is necessary to assess the potential for high tritium accumulation in seafood. This study estimated tritium concentrations in Japanese flounder near the Fukushima coast using numerical simulations. The estimation combined an oceanic dispersion model for tritiated water (HTO) with a tritium transfer model for the marine food web. Tritium accumulation was evaluated as organically bound tritium (OBT), the long-retaining chemical form of tritium in organisms. First, the dispersion model's ability to reproduce HTO concentrations in seawater was validated using actual measurement data from the Fukushima coast, showing good agreement. Subsequently, the OBT concentrations in flounder were estimated under the hypothetical maximum release scenario of the treated water. The estimation suggested that even within 100 km of 1F, the maximum concentration of the OBT was comparable with natural levels of tritium in environmental waters. Additionally, the maximum concentration in the flounder remained at a negligible level for internal radiation exposure through consumption.
Sato, Yuhi*; Ishizuka, Shigehiro*; Hiradate, Shuntaro*; Atarashi-Andoh, Mariko; Nagano, Hirohiko*; Koarashi, Jun
Environmental Research, 239, Part 1, p.117224_1 - 117224_9, 2023/12
Times Cited Count：9 Percentile：78.21(Environmental Sciences)The stability of soil organic matter (SOM) is important for improving our understanding of the global carbon cycle and ongoing climate change. This study examined the applicability of loss-on-ignition of soil with a stepwise increase in temperature (SIT-LOI) to evaluate the stability of the SOM using soil samples from Japan having different organic matter (OM) and mineral contents and different mean residence times (MRTs), estimated from radiocarbon analysis, for SOM. As the result of this examination, SIT-LOI data was strongly correlated with MRTs. This clearly suggests that SIT-LOI can be an indicator evaluating the stability of SOM in actual environments.
Sato, Yuhi*; Otosaka, Shigeyoshi*; Suzuki, Takashi; Nakanishi, Takahiro
Limnology and Oceanography, 68(7), p.1580 - 1594, 2023/07
Times Cited Count：8 Percentile：64.11(Limnology)To investigate the factors regulating the concentration of particulate iodine (PI) in seawater, it was measured in two Pacific coastal areas adjacent to Japan in two seasons along with parameters such as particulate organic carbon (POC) and nitrogen (PON), dissolved iodine (DI), and phytoplankton pigments. The dataset was divided in three groups namely, lower (Group A), average (Group B), and higher (Group C) atomic ratios of PI to POC (I/C). The three groups were characterized by phytoplanktonic physiological states as highly productive, steady, and senescent states, 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.
I in the seabed sediment off FukushimaOtosaka, Shigeyoshi; Sato, Yuhi*; Suzuki, Takashi; Kuwabara, Jun; Nakanishi, Takahiro
Journal of Environmental Radioactivity, 192, p.208 - 218, 2018/12
Times Cited Count：17 Percentile：42.99(Environmental Sciences)From August 2011 to October 2013, the concentration of iodine-129 (I) in the seabed sediment collected from 26 stations located within 160 km from the Fukushima Daiichi Nuclear Power Plant was estimated. The concentrations of I in seabed sediment off Fukushima in 2011 ranged between 0.02 and 0.45 mBq/kg. Although iodine is a biophilic element, the accident-derived radioiodine negligibly affects the benthic ecosystem. Until October 2013, a slightly increased activity of I in the surface sediment in the shelf-edge region (bottom depth: 200-400 m) was observed. The increase of the I concentrations in the shelf-edge sediments was affected by the (1) transport of I-bound particles from the land through rivers and (2) re-deposition of I desorbed from the contaminated coastal sediment to the shelf-edge sediments, which were considered to be dominant processes.
Otosaka, Shigeyoshi; Nakanishi, Takahiro; Suzuki, Takashi; Sato, Yuhi; Narita, Hisashi*
Environmental Science & Technology, 48(21), p.12595 - 12602, 2014/11
Times Cited Count：28 Percentile：57.48(Engineering, Environmental)From August 2011 to July 2013, a sediment trap was deployed at 100 km east of the Fukushima Daiichi Nuclear Power Plant and sinking particles were collected. Sinking flux of 
Cs decreased over time with seasonal fluctuation. The Cs fluxes were mainly affected by two principal modes. One was a rapid sinking of radiocesium-bound particles (moderate mode). This mode was dominant especially in the early post-accident stage, and was presumed to establish the distribution of radiocesium in the offshore seabed. Another was the secondary transport of particles attributed to turbulence near the seabed and was observed in winter (turbulence mode). Although the latter process would not drastically change the distribution of sedimentary radiocesium, attention should be paid as this key process redistributing the accident-derived radiocesium may cumulatively affect the long-term distribution.
Sato, Yuhi; Otosaka, Shigeyoshi; Suzuki, Takashi
Journal of Water and Environment Technology (Internet), 12(2), p.201 - 210, 2014/04
An easy and fast method for determination of total iodine in environmental samples by cathodic stripping voltammetry combined with NaClO oxidation has been developed. Adequate conditions for NaClO oxidation of 40-50
C over 2 h were determined, using three representative environmental samples (reference soil, seabed sediment, seaweed). By analyzing a mixture of thyroxin and a reference soil material, we obtained an overall recovery of more than 97% for total iodine in the concentration range of 1-7 
mol g
. This method was compared with alkaline extraction and combustion methods for solid and aqueous environmental samples. Alkaline extraction exhibited lower recovery of iodine compared with the NaClO oxidation method, indicating insufficient extraction and/or interference on determination. Combustion method also showed lower iodine recovery for some samples, probably due to a trapping efficiency and incomplete combustion of organic matter.
Sato, Yuhi; Otosaka, Shigeyoshi; Suzuki, Takashi
no journal, ,
For elucidating a dissolution process of iodine from pelagic coastal sediments, a dissolution experiment of iodine from sediments in an system simulating a natural environment was conducted using surface sediments collented from Ibaraki and Fukushima coasts and artificial seawater. In the experiment, concentrations of each chemical species of iodine and dissolved organic matter (DOM) in seawater was monitored for 70 days. The result showed characteristic behavoirs which were high concentration of iodine until 21 days, decreasing of it through 35 days, and dominance of dissolved organic iodine (DOI) after 35 days. These were potentially regulated by (1) supplying iodide caused by diffuidon of porewater, (2) supplying labile DOI within a few days, (3) attachment of iodide to sediment, and (4) exchenging of recalcitrant DOI between sediment and seawater, judging from the composition of iodine chemical species and fluorescent DOM analysis. It is suggested that these factors regulate the dissolution of iodine from sediment in a natural environment also.
Otosaka, Shigeyoshi; Sato, Yuhi; Suzuki, Takashi; Narita, Hisashi*
no journal, ,
A sediment trap experiment was conducted between August 2011 and July 2013 and sinking particles were collected at 100 km east of the Fukushima Daiichi Nuclear Power Plant. Sinking flux of Cs was the highest in September 2011 (98 mBq/m
/day), and decreased over time with seasonal fluctuation. The sinking fluxes of particulate radiocesium were controlled by two modes. One was a rapid sinking of radiocesium-bound particles (summer mode). This mode was dominant especially in the early post-accident stage, and was presumed to establish the distribution of radiocesium in the offshore seabed. Another was the secondary transport of particles attributed to turbulence near the seabed and was observed in winter (winter mode). Although the latter process would not change the distribution of sedimentary radiocesium drastically, attention should be paid as a key process redistributing the accident-derived radiocesium over a long duration.
Sato, Yuhi; Otosaka, Shigeyoshi; Suzuki, Takashi
no journal, ,
Accumulation of iodine into phytoplanktonic organic matter is important process regulating the iodine dynamics in the ocean but it has been poorly understood due to a small number of study examples. To evaluate an accumulation efficiency of iodine by phytoplankton, incubation experiments using cultivated cultures and natural populations obtained from coastal seawater were conducted. The accumulation efficiency of iodine was various in phytoplanktonic species; especially, diatom accumulated iodine significantly per organic carbon. This result suggests that diatom is the key specie regulating the accumulation into phytoplanktonic organic matter in the environment. An iodine/carbon ratio of particulate organic iodine (POI) produced by diatom was estimated, resulting in ca. 4
10
. Multiplying this ratio with net productivity of diatom, an amount of POI produced by phytoplankton in the environment may be roughly estimated.
I/Cs ratio in sedimentOtosaka, Shigeyoshi; Sato, Yuhi; Suzuki, Takashi; Kuwabara, Jun
no journal, ,
Transport processes of radionuclides derived from the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP) near the seabed are inferred from relationship between I and Cs concentrations in seabed sediment collected from August 2011 to January 2013 at totally 24 stations. I and Cs concentrations in the sediment surface (0
1 cm layer) were 0.040.45 mBq/kg and 12230 Bq/kg, respectively. The I/Cs ratio generally increased with time and the increasing tendency was remarkable in the shelf break region (bottom depth: 200400 m). These results indicate that (1) particles with higher I and Cs ratio settled to the bottom layer after 2012, and (2) such "recently settled" particles were accumulated in the shelf break region. The I/Cs in seabed sediment would be a helpful indicator to trace the fate of particulate radionuclides from coast to the open ocean.
Ikenoue, Tsubasa; Sato, Yuhi*; Kawamura, Hideyuki
no journal, ,
Fukushima Daiichi Nuclear Power Plant (1F) accident produced the radioactive-contaminated water. This water has been decontaminated using Advanced Liquid Processing System (ALPS), resulting that most radionuclides excluding tritium have been removed. The decontaminated water is called 'treated water'. Along with the planned oceanic release of the treated water from 1F since 2023, concerns for unfavorable impacts on the marine environment have been raised. Especially, the dynamics of organically bound tritium (OBT), the long-retaining chemical form of tritium in organisms, is focused on those concerns. In oceanic water column, OBT produced by plankton transfers in food webs from lower to higher trophic levels and partially settles onto seabed sediments as mainly detritus. The present study numerically reconstructed OBT dynamics in the marine ecosystem surrounding the lower trophic levels which is the base of dynamics. The reconstruction was performed using an oceanic dispersion model coupled with a lower trophic level ecosystem model. The dispersion model calculated tritiated water (HTO) concentration in coastal seawater adjacent to Fukushima under the release condition of treated water during 2023-2024, and the ecosystem model estimated OBT concentrations in phytoplankton, zooplankton, detritus, and seabed sediment using the calculation result of the former model. The estimation indicated that the OBT concentrations in the targeted organisms increased after the release of treated water, but this increase rapidly (within two months) returned to the pre-release level. The rapid decrease of OBT was caused by the fast dispersion of HTO in seawater, short lifespans of planktons, and fast decomposition of detritus. OBT concentration in the surface (0-10 cm) seabed sediment was estimated to be temporarily increased and then returned to pre-release level over time mainly due to the fast decomposition of detritus. These results suggested that the release of treated water scarcely influences to OBT accumulation in the marine ecosystem surrounding the lower trophic levels.
Otosaka, Shigeyoshi; Sato, Yuhi; Suzuki, Takashi; Kuwabara, Jun
no journal, ,
Transport processes of biohilic radionuclides, derived from the accident of Fukushima Daiichi Nuclear Power Plant, are discussed from distribution of I in seabed sediment off Fukushima. Concentrations of I in the seabed sediment in 2011 ranged between 0.02 and 0.45 mBq/kg, and were generally higher in the coastal region. In January 2013, about two years after the accident, concentrations of I in the sediment surface slightly increased in the shelf-edge region (bottom depth: 200400 m), and such a trend was not observed for radiocesium. The preferential accumulation of I in the shelf-edge sediment was explained by lateral transport of sedimentary I from the coastal to shelf-edge regions. Considering the geochemical characteristics of iodine, it can be considered that particulate organic matter near the seabed efficiently carried I to the offshore.
Otosaka, Shigeyoshi; Suzuki, Takashi; Nakanishi, Takahiro; Kuwabara, Jun; Sato, Yuhi
no journal, ,
From August to November 2011, seabed sediments were collected from off Fukushima and Ibaraki prefectures and I concentration in the sediments was measured with accelerator mass spectrometry. I concentration in the surface sediment (0-1 cm layer) ranged between 0.045 and 0.48 mBq/kg-dry, and was several times higher than that in 2009. The activity ratio of I, considered to originates from the accident of Fukushima Daiichi NPP, over Cs in seabed sediment was higher than those in surface seawater. These results indicate that radioiodine was selectively adsorbed onto particles in the surface seawater or in the sediment-water interface, and accumulated in the seabed sediments.
I near the seafloor off FukushimaOtosaka, Shigeyoshi*; Sato, Yuhi*; Suzuki, Takashi; Kuwabara, Jun; Nakanishi, Takahiro
no journal, ,
no abstracts in English
Sato, Yuhi; Otosaka, Shigeyoshi; Suzuki, Takashi
no journal, ,
It is known that iodine is a biophilic element. However, uptake processes of iodine by phytoplankton are less well understood. The present study evaluated phytoplanktonic uptake of iodine under light or dark conditions, by using cultivated cultures and natural population of phytoplankton. Cultivated cultures were incubated with iodate for 10 hours. Diatom, haptophyceae, and cryptophyceae accumulated significant amount of iodine whereas cyanobacteria did not show accumulation. Although haptophyceae and cryptophyceae incorporated iodine only under the light condition, diatom accumulated under light and dark conditions. The natural population incubation, which was conducted under sunlight and dark conditions in natural seawater, showed a significant accumulation of iodine into phytoplankton under both light and dark conditions. These results indicate that species of phytoplankton and physiological activities can affect dynamics of iodine in a diurnal scale.
