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Journal Articles

Distribution and fate of $$^{129}$$I in the seabed sediment off Fukushima

Otosaka, Shigeyoshi; Sato, Yuhi*; Suzuki, Takashi; Kuwabara, Jun; Nakanishi, Takahiro

Journal of Environmental Radioactivity, 192, p.208 - 218, 2018/12

AA2017-0744.pdf:1.17MB

 Times Cited Count:9 Percentile:51.75(Environmental Sciences)

From August 2011 to October 2013, the concentration of iodine-129 ($$^{129}$$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 $$^{129}$$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 $$^{129}$$I in the surface sediment in the shelf-edge region (bottom depth: 200-400 m) was observed. The increase of the $$^{129}$$I concentrations in the shelf-edge sediments was affected by the (1) transport of $$^{129}$$I-bound particles from the land through rivers and (2) re-deposition of $$^{129}$$I desorbed from the contaminated coastal sediment to the shelf-edge sediments, which were considered to be dominant processes.

Journal Articles

Year-round variations in the fluvial transport load of particulate $$^{137}$$Cs in a forested catchment affected by the Fukushima Daiichi Nuclear Power Plant accident

Matsunaga, Takeshi; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Takeuchi, Erina; Muto, Kotomi; Tsuzuki, Katsunori; Nishimura, Shusaku; Koarashi, Jun; Otosaka, Shigeyoshi; Sato, Tsutomu*; et al.

Journal of Radioanalytical and Nuclear Chemistry, 310(2), p.679 - 693, 2016/11

AA2015-0821.pdf:3.78MB

 Times Cited Count:5 Percentile:54.39(Chemistry, Analytical)

Particulate $$^{137}$$Cs in stream water was collected continuously for two years in order to assess the long-term trend of the $$^{137}$$Cs discharge from the forest environment. Sampling was conducted from December 2011 to December 2013 in a mountainous stream, which received the $$^{137}$$Cs from the Fukushima Daiichi Nuclear Power Plant accident. A seasonal increase in fluvial transport load of particulate $$^{137}$$Cs associated with suspended solids (SS) was observed in August and September when rainfall was abundant. The particulate $$^{137}$$Cs concentration decreased at a faster rate than the rate due to radioactive decay. This decrease might be resulted from redistribution of the easily eroded and polluted soil surface due to heavy rain events such as typhoons. These findings indicate that the particulate $$^{137}$$Cs load was subject to the inter-annual variations in rainfalls, and decreased gradually over a long period of time due to a decrease in $$^{137}$$Cs concentration in SS.

Journal Articles

Demonstrative experiments on the migration of radiocesium from buried soil contaminated by the accident at Fukushima Daiichi Nuclear Power Station

Yamaguchi, Tetsuji; Shimada, Taro; Ishibashi, Makoto*; Akagi, Yosuke*; Kurosawa, Mitsuru*; Matsubara, Akiyoshi*; Matsuda, Yuki*; Sato, Shigeyoshi*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 22(2), p.21 - 27, 2015/12

It is predictable from previous studies that radiocesium hardly migrate into surrounding soils and groundwater from soils contaminated by the Fukushima Daiichi Nuclear Power Plant accident if they are buried and covered with indigenous soils. This study demonstrated the prediction by performing in-situ migration experiments over a year in a public park in Miho, Ibaraki prefecture and in two public parks in Misato, Saitama prefecture. Contaminated soils were buried at a depth range of 0.3 - 1.0 m or at 0.3 - 1.3 m and covered with indigenous soil layer of 0.3 m, and were sprinkled with water to accelerate the radiocesium migration. Migration of radiocesium was not observed from radiometric analyses of boring cores and soil water samples. Laboratory column and sorption experiments revealed that the radiocesium hardly leach out of the soil and even if they leach out from the contaminated soil, radiocesium is sorbed on surrounding soils and hardly migrate through the soli layer. Simulation of Cs-137 migration for 100 years by an advection-diffusion model showed that Cs-137 hardly migrate and decay out in the contaminated soil.

Journal Articles

A Passive collection system for whole size fractions in river suspended solids

Matsunaga, Takeshi; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Takeuchi, Erina; Tsuzuki, Katsunori; Nishimura, Shusaku; Koarashi, Jun; Otosaka, Shigeyoshi; Sato, Tsutomu*; Nagao, Seiya*

Journal of Radioanalytical and Nuclear Chemistry, 303(2), p.1291 - 1295, 2015/02

 Times Cited Count:3 Percentile:32.14(Chemistry, Analytical)

An innovative, yet simple method for the passive collection of radioactive materials in river water has been developed and validated. The method employes large filter vessels, containing multiple cartridge filters. River water is led to the system naturally using a drop of the riverbed by hose from upstream. This method makes long-term, unmanned monitoring possible. In addition to regular radioactivity analyses, this method provides an opportunity for the characterization of suspended materials based on its ample collection quantities (more than several tens of grams). This method may also be applicable to sediment-bound chemicals.

Journal Articles

Vertical and lateral transport of particulate radiocesium off Fukushima

Otosaka, Shigeyoshi; Nakanishi, Takahiro; Suzuki, Takashi; Sato, Yuhi; Narita, Hisashi*

Environmental Science & Technology, 48(21), p.12595 - 12602, 2014/11

 Times Cited Count:20 Percentile:60.12(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 $$^{137}$$Cs decreased over time with seasonal fluctuation. The $$^{137}$$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.

Journal Articles

Determination of total iodine concentration in aquatic environments using cathodic stripping voltammetry combined with sodium hypochlorite (NaClO) oxidation

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$$^{circ}$$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 $$mu$$mol g$$^{-1}$$. 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.

Oral presentation

Distribution and transport processes of iodine-129 in seabed sediment off Fukushima

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 $$^{129}$$I concentration in the sediments was measured with accelerator mass spectrometry. $$^{129}$$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 $$^{129}$$I, considered to originates from the accident of Fukushima Daiichi NPP, over $$^{137}$$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.

Oral presentation

Experimental study of iodine uptake by phytoplankton

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.

Oral presentation

Experimental study on dissolution of iodine from coastal sediments

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.

Oral presentation

Sinking fluxes of radiocesium off Fukushima

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 $$^{137}$$Cs was the highest in September 2011 (98 mBq/m$$^{2}$$/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.

Oral presentation

Accumulation of iodine into phytoplankton in the coastal region in the North Pacific

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$$times$$10$$^-5$$. Multiplying this ratio with net productivity of diatom, an amount of POI produced by phytoplankton in the environment may be roughly estimated.

Oral presentation

Transport processes of particulate materials near the seabed off Fukushima inferred from $$^{129}$$I/$$^{137}$$Cs ratio in sediment

Otosaka, 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 $$^{129}$$I and $$^{137}$$Cs concentrations in seabed sediment collected from August 2011 to January 2013 at totally 24 stations. $$^{129}$$I and $$^{137}$$Cs concentrations in the sediment surface (0$$sim$$1 cm layer) were 0.04$$sim$$0.45 mBq/kg and 12$$sim$$230 Bq/kg, respectively. The $$^{129}$$I/$$^{137}$$Cs ratio generally increased with time and the increasing tendency was remarkable in the shelf break region (bottom depth: 200$$sim$$400 m). These results indicate that (1) particles with higher $$^{129}$$I and $$^{137}$$Cs ratio settled to the bottom layer after 2012, and (2) such "recently settled" particles were accumulated in the shelf break region. The $$^{129}$$I/$$^{137}$$Cs in seabed sediment would be a helpful indicator to trace the fate of particulate radionuclides from coast to the open ocean.

Oral presentation

Preferential accumulation of particulate radioiodine in shelf-edge sediment off Fukushima, Japan

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 $$^{129}$$I in seabed sediment off Fukushima. Concentrations of $$^{129}$$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 $$^{129}$$I in the sediment surface slightly increased in the shelf-edge region (bottom depth: 200$$sim$$400 m), and such a trend was not observed for radiocesium. The preferential accumulation of $$^{129}$$I in the shelf-edge sediment was explained by lateral transport of sedimentary $$^{129}$$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 $$^{129}$$I to the offshore.

Oral presentation

Tracking the fate of $$^{129}$$I near the seafloor off Fukushima

Otosaka, Shigeyoshi*; Sato, Yuhi*; Suzuki, Takashi; Kuwabara, Jun; Nakanishi, Takahiro

no journal, , 

no abstracts in English

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