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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

 Percentile:100(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

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:11 Percentile:45.16(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

11 (Records 1-11 displayed on this page)
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