Preprints
https://doi.org/10.5194/bg-2020-491
https://doi.org/10.5194/bg-2020-491

  08 Feb 2021

08 Feb 2021

Review status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Impact of typhoons on particulate and dissolved 137Cs activities in seawater off the Fukushima Prefecture: results from the SOSO 5 Rivers cruise (October 2014)

Michio Aoyama1,2, Sabine Charmasson3, Yasunori Hamajima4, and Celine Duffa3 Michio Aoyama et al.
  • 1Centre for Research in Environmental Dynamics, Univ. of Tsukuba, 1-1-1 Ten-noudai, Tsukuba 305-8572, Japan
  • 2Institute of Environmental Radioactivity, Fukushima Univ. Kanayagawa 1, Fukushima 960-1296, Japan
  • 3Institute de Radioprotection et de Sûreté nucléaire (IRSN), PSE-ENV, LRTA, BP3, 13115 Saint Paul lez Durance, France
  • 4The Institute of Nature and Environmental Technology, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan

Abstract. Cruise SoSo 5 Rivers took place during October 2014 off the coast of Fukushima Prefecture shortly after the passage of two typhoons. Detection of dissolved 134Cs and 137Cs in all samples reflected contamination caused by accidental releases of radiocaesium from the Fukushima Dai-ichi Nuclear power plant (FNPP1) accident. The dissolved activities were generally higher at coastal sites and decreased with distance from shore, and they were higher in the surface than in the bottom water. The tendency of 137Cs activities to decrease with distance from the coast reflected mixing of coastal water and open-ocean water of which 137Cs activity concentration was ~1.5 Bq m−3. At stations very close to the coast, we observed high particulate 137Cs activity concentration that exceeded dissolved 137Cs activity concentration. 137Cs activities were generally 1–2 orders of magnitudes lower in organic particles than in dissolved form, and the ratios of 137Cs activity concentration in organic particles to 137Cs activity concentration in dissolved form ranged from 0.01 ± 0.00 to 0.12 ± 0.01. The ratio of 137Cs to 134Cs activity concentrations in organic particles did not change with distance from shore or with 137Cs activity concentration and generally remained around 1, even in samples collected far from the coast. This pattern indicated that the organic particles had come from rivers or a source very close to the coast. The 137Cs / 134Cs activity ratio in dissolved form north of FNPP1 region was estimated to be 1.074 ± 0.015, a ratio that is in good agreement with the 137Cs / 134Cs activity ratio in the core of Unit 1 of the FNPP1 while the 137Cs / 134Cs activity ratio at Tomioka port which located south of FNPP1 was 0.998 ± 0.017. Therefore we can conclude the source of radiocaesium in seawater in the coastal region north of FNPP1 was deposited radiocaesium released from the core of Unit 1 of FNPP1, while the source of radiocaesium observed in the coastal region south of FNPP1 was a mixture of deposited radiocaesium released from the core of Unit 2 and the core of Unit 1 of FNPP1. During September–October of each year, the typhoon season in Japan, the 137Cs activity concentration generally increased at Ukedo port, Tomioka port, FNPP1, and Iwasawa beach, and showed a good relationship with the 7-day modified antecedent precipitation index (API) while there is less correlation between the modified API and 137Cs activity concentration near the outlet of canal from unit 5 and 6 of FNPP1 to the sea.

Michio Aoyama et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2020-491', Anonymous Referee #1, 08 Mar 2021
    • AC1: 'Reply on RC1', Michio Aoyama, 21 Apr 2021
  • RC2: 'Comment on bg-2020-491', Anonymous Referee #2, 09 Mar 2021
    • AC2: 'Reply on RC2', Michio Aoyama, 21 Apr 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2020-491', Anonymous Referee #1, 08 Mar 2021
    • AC1: 'Reply on RC1', Michio Aoyama, 21 Apr 2021
  • RC2: 'Comment on bg-2020-491', Anonymous Referee #2, 09 Mar 2021
    • AC2: 'Reply on RC2', Michio Aoyama, 21 Apr 2021

Michio Aoyama et al.

Data sets

Dataset of 134Cs and 137Cs activity concentration concentrations in dissolved for, all particles and organic form of particles obtained by SoSo 5 rivers cruise in 2014 M. Aoyama, S. Charmasson, Y. Hamajima, and C. Duffa https://doi.org/10.34355/CRiED.U.TSUKUBA.00030

Dataset of 134Cs and 137Cs activity concentrations in dissolved form and, all particles at Tomioka, Fukushima in August 2014 M. Aoyama, S. Charmasson, Y. Hamajima, and C. Duffa https://doi.org/10.34355/CRiED.U.TSUKUBA.00031

Dataset of time series of radiocaesium activity concentrations at Tomioka, Fukushima during the period from 10 June 2014 to 24 April 2019 M. Aoyama, S. Charmasson, Y. Hamajima, and C. Duffa https://doi.org/10.34355/CRiED.U.TSUKUBA.00032

Michio Aoyama et al.

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Short summary
Our results showed that the dissolved radiocaesium activities were generally higher at coastal sites and decreased with distance from shore. The portion of particulate 137Cs to the total 137Cs ranged from 0.13 to 0.96. The ratio of 137Cs to 134Cs activity in organic particles did not change with distance from shore and generally remained around 1. The 137Cs / 134Cs activity ratio in seawater was estimated to be 1.074 ± 0.015 which was in good agreement with the ratio of 1.06 in core unit 1.
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