Distribution and fate of 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

https://doi.org/10.1016/j.jenvrad.2018.06.025

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.

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

https://doi.org/10.3327/jnuce.22.2_21

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.

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

https://doi.org/10.1021/es503736d

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.

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 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.

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.

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. 410. Multiplying this ratio with net productivity of diatom, an amount of POI produced by phytoplankton in the environment may be roughly estimated.

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 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.

Dynamics of radionuclides and ecosystem response in the coastal area of Fukushima KS-21-23, KS-22-14

Takata, Hyoe*; Otsuki, Azusa*; Sato, Shun*; Otosaka, Shigeyoshi*; Toda, Ryoji*; Nishikawa, Jun*; Kenmochi, Hideyuki*; Ishikura, Mei*; Yamada, Momoka*; Shinkai, Yusuke*; et al.

no journal, , 

Research cruises were conducted by the Shinsei Maru from October 7 to October 17, 2021, and from October 2 to 13, 2022, in the coastal waters off Fukushima Prefecture. Those cruises aimed to monitor radionuclide distribution and to clarify the dynamics of radionuclides in the coastal water and the environmental response of ecosystems after the Fukushima Daiichi Nuclear Power Plant accident in March 2011.