Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident

Accepted Manuscript Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident Wu ...

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Accepted Manuscript Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident Wu Men, Jian Zheng, Hai Wang, Youyi Ni, Yuichiro Kumamoto, Masatoshi Yamada, Shigeo Uchida PII:

S0269-7491(18)33413-4

DOI:

https://doi.org/10.1016/j.envpol.2018.12.007

Reference:

ENPO 11947

To appear in:

Environmental Pollution

Received Date: 25 July 2018 Revised Date:

3 December 2018

Accepted Date: 3 December 2018

Please cite this article as: Men, W., Zheng, J., Wang, H., Ni, Y., Kumamoto, Y., Yamada, M., Uchida, S., Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident, Environmental Pollution (2019), doi: https://doi.org/10.1016/ j.envpol.2018.12.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power

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Plant site within two months after the severe nuclear accident

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Wu Mena,b, Jian Zhenga,*, Hai Wanga,c, Youyi Nia, Yuichiro Kumamotod

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Masatoshi Yamadae, Shigeo Uchidaa,

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a

National Institute of Radiological Sciences, National Institutes for Quantum and

Radiological Science and Technology, 491 Anagawa, Inage, Chiba 263-8555, Japan b

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Laboratory of Marine Isotopic Technology and Environmental Risk Assessment, Third Institute of Oceanography, State Oceanic Administration,

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184 Daxue Road, Xiamen 361005, China c

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School of Nuclear Science and Technology, University of South China, Hengyang 421001, China

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Marine-Earth Science and Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan,

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Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan

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Research and Development Center for Global Change, Japan Agency for

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______________________________________________________________________

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* Corresponding author. E-mail: [email protected] 1

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Abstract The marine environment is complex, and it is desirable to have measurements for

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seawater samples collected at the early stage after the Fukushima Daiichi Nuclear

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Power Plant (FDNPP) accident to determine the impact of Fukushima-derived

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radionuclides on this environment. Here Pu isotopes in seawater collected 33-163 km

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from the FDNPP site at the very early stage after the accident were determined (May

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2011, within two months after the accident). The distribution and temporal variation

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of

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concentrations (from 0.81±0.16 to 11.18±1.28 mBq/m3) and

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(from 0.216±0.032 to 0.308±0.036) in these seawater samples were within the

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corresponding background ranges before the accident, and this suggested that

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Fukushima-derived Pu isotopes, if any, were in too limited amount to be distinguished

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from the background level in the seawater. The analysis of Pu isotopic composition

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indicated that the major sources of Pu in the seawater after the accident were still

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global fallout and the Pacific Proving Ground close-in fallout. The contribution

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analysis showed that the contributions of the Pacific Proving Ground close-in fallout

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in the water column of the study area ranged from 26% to 77% with the average being

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48%.

Pu and

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Pu were studied. The results indicated that both

239+240

Pu activity

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Pu/239Pu atom ratios

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atom ratio

0.3

240Pu/239Pu

0.4

0.2

May 2011

Before FDNPP accident

Pacific Proving Grounds (0.30-0.36)

Global fallout (0.176±0.014) 0.1 0

47 48 49

5

10

15 239+240Pu

20 (mBq/m3)

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Graphical abstract Keywords:239+240Pu; 240Pu/239Pu atom ratio; Fukushima accident; seawater; ICP-MS 2

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A capsule that summarizes the main finding of the work

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The analysis of Pu isotopes in seawater samples within two months after the

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Fukushima accident indicated the accident-derived Pu contamination was negligible.

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1. Introduction

Plutonium isotopes are a health concern due to their radiotoxicity and chemical

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toxicity. After the FDNPP accident, several studies confirmed that the FDNPP-derived

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Pu isotopes had been released into the terrestrial environment due to atmospheric

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deposition (Zheng et al., 2012a; MEXT 2011; Imanaka et al., 2012; Yamamoto et al.,

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2012; Lujaniene et al., 2012a, b) and the 239+240Pu amount was estimated to be 1.0-2.4

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GBq (Zheng et al., 2013). Since the marine environment received not only the

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atmospheric deposition but also the direct discharge of highly contaminated liquid

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wastes as well as the runoff input, it was unavoidable for the FDNPP derived Pu

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isotopes entering into the marine environment. It also caused concerns due especially

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to seafood safety, and studies that focused on marine sediments sampled within (Bu et

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al., 2013; 2014a; 2015; Wendel et al., 2017), and outside (Wendel et al., 2017; Zheng

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et al.,2012b; Bu et al., 2014b) a 30 km zone from the FDNPP site indicated negligible

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Pu contamination from the accident. For seawater, several studies have been

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conducted for samples collected within the 30 km zone from the FDNPP site in

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October 2014 (Casacuberta et al., 2017) and outside that zone in August 2011

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(Sakaguchi et al., 2012), August 2012 (Hain et al., 2017), January, May and July 2013

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(Bu et al., 2014c; 2015b; Men et al., 2018), and October 2014 (Casacuberta et al.,

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2017). However, the results obtained in these studies were inadequate to verify

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whether the FDNPP accident raised marine Pu contamination due to the delayed

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sampling period. The temporal variations of radio-cesium concentrations in seawater

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ACCEPTED MANUSCRIPT within 30-90km from the FDNPP site over the period March 2011 to February 2016

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are shown in Fig. 1 (Takata et al., 2017). It was obvious that the Fukushima-derived

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cesium decreased sharply with the time after undergoing marine processes such as

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water mass dilution, mixing and transportation in the coastal environment. Similarly,

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the FDNNP-derived Pu activities or atom ratios could have been decreased with time

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by these marine processes after being released into the marine environment. The

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sampling times in the reported seawater Pu studies were ~4 to ~41 months after the

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accident, and hence the resulting Pu signals might not be definitive. By contrast, the

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seawater samples collected in the early stage of the accident are much more promising

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to illustrate the contaminations of Pu isotopes. In this study,

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concentrations and

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months after the FDNPP accident were measured to provide the evidence for the

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contamination extent of Fukushima-derived Pu isotopes in the marine environment.

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239+240

Pu activity

Pu/239Pu atom ratios of seawater samples collected within two

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2. Methodology

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2.1. Study area and seawater sampling

Seawater samples were collected during two oceanographic expeditions

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YK11-E02 and NT11-E01, which were done in May 2011 by the Japan Agency for

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Marine-Earth Science and Technology (JAMSTEC). Activity concentrations of

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radiocesium and radioiodine in seawater samples from these cruises have been

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published (Oikawa et al., 2013). JAMSTEC also collected replicate seawater samples

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as spares and shared them with us. The map with the sampling station is shown in Fig.

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2.

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2.2. Analytical procedure for 239Pu and 240Pu analysis 4

ACCEPTED MANUSCRIPT All the seawater samples were filtered using a 0.20 µm pore size cartridge filter to

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remove suspended particles and were acidified to pH~2 by adding HNO3 for storage

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and transport to prevent Pu isotopes from adsorbing onto the wall of container. Pu

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isotopes were analyzed using a method which involved Fe(OH)2 primary

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co-precipitation, CaF2/LaF3 secondary co-precipitation, extraction chromatographic

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(TEVA, UTEVA, and DGA resins) separations and SF-ICP-MS detection (Men et al.,

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2018). Briefly, Pu isotopes in the pre-filtered seawater was reduced by TiCl3 and

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co-precipitated by Fe(OH)2 at pH = 8.8 - 9.0. Concentrated HNO3 was used to

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dissolve the Fe(OH)2 co-precipitate and 0.57 pg of 242Pu spike (CRM 130, plutonium

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spike assay and isotopic standard, New Brunswick Laboratory, USA) was added as a

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yield tracer. Ca2+, La3+ and concentrated HF were added to form the CaF2/LaF3

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co-precipitate. Then the CaF2/LaF3 precipitate was dissolved in 3 M HNO3 with the

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addition of H3BO3. The Pu (III) in the solution was adjusted to Pu (IV) by the addition

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of NaNO2 and heated at 40 °C for 0.5 h. The sample solution was loaded onto a

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TEVA resin cartridge, and the TEVA resin cartridge was washed with 3 M HNO3 to

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remove Ca, Fe, and rare earth elements, followed by 1 M HNO3 to remove U, Pb, and

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Tl, and 9 M HCl to remove Th and Bi. Before the elution of Pu, an UTEVA+DGA

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resin cartridge was connected under the TEVA resin cartridge. Then 3 M HNO3−0.1

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M ascorbic acid−0.02 M Fe2+ was employed to reduce Pu (IV) to Pu (III) and elute Pu

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(III) from TEVA resin to the DGA resin. The DGA resin cartridge was rinsed by 0.1

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M HNO3 to remove U, Tl, Pb, and Fe. Next, the plutonium on the DGA resin was

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eluted by 0.5 M HCl−0.1 M NH2OH·HCl. The eluted sample solution was evaporated

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to dryness at 250 oC and dissolved with aqua regia. After heating the dissolved sample

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solution to dryness at 200 oC, 1 mL of ultrapure HNO3 was added and heated to near

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dryness at 200 °C. Finally, the sample was dissolved in 0.7 mL 4% HNO3 and

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ACCEPTED MANUSCRIPT detected by a highly sensitive APEX/SF-ICP-MS analytical system. For each batch of

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seawater samples, the operation blanks were determined simultaneously. The Pu

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recovery was ~70% - ~90%. Detection limits of this method for 239Pu and 240Pu were

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both 0.08 fg/mL, corresponding to 0.01 mBq/m3 for 239Pu and 0.05 mBq/m3 for 240Pu

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(Men et al., 2018). A serial of IAEA-443(seawater standard reference material of

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IAEA) spiked seawater samples were used to illustrate the validation of the method.

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The results of 240Pu/239Pu atom ratios ranged from 0.225 ± 0.009 to 0.241 ± 0.040,

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which agreed well with the certificate values of 0.229 ± 0.006 within the range of

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error. In order to accurately measure

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atom ratios, some samples of different layers collected at the same station were

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combined into one sample. Ten stations were involved in this kind of combination and

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their detailed information is given in the column marked “Depth” in Table 1.

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Pu/239Pu

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Pu activity concentrations and

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3. Results and Discussion

The results of the activity concentrations of 239+240Pu and 240Pu/239Pu atom ratios as

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well as the relevant sampling information are listed in Table 1. The activities of

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239+240

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ranged from 0.216±0.032 - 0.308±0.036, with an average of 0.254 ±0.023 (average

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uncertainty 9.2%). The data of 137Cs activities provided by Takahata et al. (2018) and

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Kumamoto et al. were listed in Table 1. The extremely high values of

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(13.7-13800 Bq/m3) demonstrated that they were FDNPP-derived and the waterbody

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in the coastal areas near the FDNPP was highly polluted by the FDNPP accident

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(Aoyama et al., 2016a,b; Buesseler et al., 2017).

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Pu ranged from 0.81±0.16 - 11.18±1.28 mBq/m3. The

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Pu/239Pu atom ratios

137

Cs activity

149 150

3.1. Vertical distribution of

239+240

Pu in the water column of the coastal area off the 6

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FDNPP site Under simple dispersion conditions, the activities of radionuclides generally

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decrease with distance from their sources. For example, for the natural occurring

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radium isotopes, the river and the sediment are their sources, they always decreased

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with the increasing distance off the estuary horizontally or off the sediment vertically

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(Moore, 2000; Liu et al., 2010; Men et al., 2011). This is a general feature that can be

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utilized to distinguish the sources. In addition, comparing the distributions of target

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nuclides with those of other nuclides which have known sources is another way. The vertical distributions of

239+240

Pu and

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Cs activities are shown in Fig. 3. 137

Except for station A, it shows that the distribution pattern of

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water column had obviously higher 137Cs activities in surface water than in deep water,

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which suggested that

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Unlike the distribution of 137Cs, the 239+240Pu activities in bottom water layer were all

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obviously higher than the intermediate layer. There were two patterns for the vertical

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distribution of Pu. One was that

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bottom layers than the intermediate layer, as shown by Stations A, S-1 and S-2. The

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other one was that

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Station S-3. In addition, for the stations with combined samples Stations 1, 3 and B

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(Table 1), the 239+240Pu activities in the bottom water layer were obviously higher than

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those of upper layers. Plutonium is a typical particle-reactive element. The boundary

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scavenging effect makes continental sediments relatively enriched in Pu. During this

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process, Pu is transported downward into the sediments with sinking biogenic

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particles and partially released into near-bottom nepheloid layer through the repeated

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resuspensions of fine particles (Lindahl et al., 2010; Hirose, 2009; Zheng and Yamada,

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2006; Yamada and Aono, 2002; 2003; 2006). Therefore, this kind of geochemical

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137

Cs activities in the

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Cs was transported from the surface water to the deep water.

Pu had higher activities at the surface and

Pu activities increased with increasing depth, as shown by

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239+240

239+240

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behavior of Pu isotopes resulted in the distributions in this work. The difference of

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vertical distributions between Pu isotopes and

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from different sources, i.e. Pu isotopes were from sediment and

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surface water. Fig. 4 shows the distribution of

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bottom water as well as

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well negatively correlate with the increasing distance off the FDNPP both in surface

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water and bottom seawater. It implied that there was complicated mixing and

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transport system in the study area as 137Cs was mainly controlled by water mixing and

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current transport in the ocean. As for

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distance off the FDNPP. Except for the complicated mixing and transport system, the

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sediment property and the source were the other influence factors for the distributions

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of 239+240Pu.

239+240

Pu and

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137

137

Cs was from the

Cs activities in the

Cs activities did not

Pu, it had also no correlativity with the

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Cs distribution in surface water.

3.2. The impact of Pu isotopes on the seawater

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Cs might suggest that they came

137

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The variation of 239+240Pu activities is the most direct indicator for a new source of 239+240

Pu isotopes. Before the FDNPP accident,

Pu activities in seawater of the

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Northwest Pacific and its marginal seas changed with a large variation (i.e. not

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detected -32.0 mBq/m3) (Bertine et al., 1986; Kim et al., 2004; Norisuye et al., 2005;

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Yamada et al., 2006; Yamada and Zheng, 2008; 2010; 2011; Oikawa et al., 2015;

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Oikawa et al., 2011; Wu et al., 2017). Our study found that the

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0.81-11.18 mBq/m3 in the sea area 33-163 km from the FDNPP site within 60 days

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after the accident. The

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accident. One reasonable explanation for this was that contribution of the

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FDNPP-derived Pu was covered by the background in the seawater and the amount

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released from the FDNPP accident was too small to change the Pu activity level in the

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239+240

239+240

Pu activity was

Pu activities were still within the background before the

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seawater.

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The

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Pu/239Pu atom ratio is another indicator for a new source. The

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Pu/239Pu

atom ratio has been well characterized for various sources and applied widely as a

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fingerprint to identify radioactive contamination sources (Zheng and Yamada, 2006;

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Yamada and Zheng, 2012). The worldwide integrated global fallout sourced Pu is

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characterized by

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(Krey et al., 1976). Additionally, the Pacific Proving Ground (PPG) close-in fallout is

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another source with the

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shown in Fig. 5, the PPG sourced Pu is transported mainly by the North Equatorial

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Current and Kuroshio Current to the Northwest Pacific and its marginal seas (Bu et al.,

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2014b; 2015; Wu et al., 2014; Zheng and Yamada, 2004), which includes the study

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area of this work, and the

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global fallout and PPG close-in fallout (Bertine et al., 1986; Kim et al., 2004;

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Norisuye et al., 2005; Yamada et al., 2006; Yamada and Zheng, 2008; 2010; 2011;

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Oikawa et al., 2015; Oikawa et al., 2011; Wu et al., 2017; Zheng et al., 2012b). The

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240

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Pacific (Bertine et al., 1986; Yamada et al., 2006; Buesseler, 1997), from 0.18 to 0.33

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in the marginal seas including the Yellow Sea, Japan Sea, Tsushima Strait and the

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coastal area of Japan (Kim et al., 2004; Norisuye et al., 2005; Yamada et al., 2006;

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Yamada and Zheng, 2008; 2010; 2011; Oikawa et al., 2015; Oikawa et al., 2011),

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from 0.184 to 0.263 in the marginal sea including the South China Sea, East China

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Sea and Kuroshio current area (Yamada et al., 2006; Yamada and Zheng, 2011; Wu et

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al., 2017). In this study, the highest atom ratio of 0.308 was close to the PPG value.

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However, the wide range of the PPG ratios (0.30-0.36) and the overlap with the atom

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ratio range of the FDNPP reactors (0.320-0.356) (Nishihara et al., 2012) prevent us

Pu/239Pu ratios of 0.17-0.19 with the average of 0.176±0.014

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Pu/239Pu atom ratio of >0.30 (Lindahl et al., 2012). As

Pu/239Pu atom ratio in this sea area falls between that of

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AC C

EP

Pu/239Pu atom ratio was reported to vary from 0.178 to 0.26 in the Northwest

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from stating that this is evidence of Fukushima-derived Pu in seawater. The same as

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for the

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also in the background range before the FDNPP accident, suggesting that the amount

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of FDNPP-derived Pu was too small to increase the 240Pu/239Pu atom ratio level in the

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seawater.

Pu activity, the

More reasonably, the

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Pu/239Pu atom ratio in this study (0.216 - 0.308) was

239+240

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239+240

Pu/239Pu atom ratios in this study

Pu activities and

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should be compared with the coastal area of Japan to eliminate any possible regional

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difference. Oikawa et al. (2015) reported

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240

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coastal areas during 3 years (2008-2010) just before the FDNPP accident, which could

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be used as a much more specific background level for Pu isotopes off the FDNPP site.

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Values for

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(2008-2010) and this study were plotted in Fig. 6. It was obvious that neither 239+240Pu

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activities nor

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FDNPP accident. Therefore, it can be concluded conservatively that there were no

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notable amounts of Pu isotopes released into the marine environment from the

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accident. This conclusion has been supported by previously reported findings (Bu et

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al., 2013; 2014a; 2015; Wendel et al., 2017; Zheng et al.,2012b; Bu et al., 2014b;

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Casacuberta et al., 2017; Sakaguchi et al., 2012; Hain et al., 2017; Bu et al., 2014c;

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2015b; Men et al., 2018).

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Pu activities (2.8-32.0 mBq/m3) and

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Pu/239Pu atom ratios (0.173-0.322) in surface and bottom seawater layers of Japan

239+240

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Pu/239Pu atom ratios from Oikawa et al. (2015)

Pu activities and

Pu/239Pu atom ratios exceeded the corresponding ranges before the

EP

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239+240

The relationship between

239+240

Pu and

137

Cs activities is shown in Fig.7. It shows

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that there was no correlation between them. 239+240Pu/137Cs activity ratios ranged from

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9.91 × 10-8-3.00 × 10-4 (Table 1). Table S1 (in Supplementary data) shows the

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239+240

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litter, soil and black substances) and the seawater in the North Pacific before the

Pu/137Cs activity ratios of reactor units of FDNPP, terrestrial samples (including

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ACCEPTED MANUSCRIPT 239+240

Pu/137Cs activity ratios between

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FDNPP accident. The large differences of

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reactor units of the FDNPP and seawater samples could be attributed to the different

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sources.

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FDNPP accident, while the FDNPP accident-derived 239+240Pu was too limited to raise

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the seawater

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global fallout and PPG. Unlike the Cs which is highly volatile, Pu is nonvolatile. In

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addition, Cs is dissolved in the seawater while Pu is easily adsorbed on the

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suspending particles. As we know, the radionuclides released from the FNNPP

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through the explosion to the atmosphere and the direct discharge of radioactive liquids

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as well as uncontrolled leaking of the heavily contaminated coolant water (Buesseler

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et al., 2017; Kaeriyama, 2017). Therefore, no matter what way that the radionuclides

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were released into the marine environment by the FDNPP accident, the fractionation

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between

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239+240

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derived Pu.

239+240

137

Cs were not from the same source.

239+240

Pu was mainly from

Pu and

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239+240

137

Cs was unavoidable. Due to the unconservative property of

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Pu/137Cs activity ratios, it cannot be an indicator for tracing the Fukushima

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3.3. Temporal variation of

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FDNPP accident

239+240

AC C

EP

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269

239+240

Cs was from the

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Pu activity level after the accident;

137

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Pu and

The temporal variation of

Pu activities and

239+240

240

Pu/239Pu atom ratios after the

Pu activities and

240

Pu/239Pu atom ratios in

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seawater within 100km from the FDNPP site after the accident (May 2011-August

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2014) were plotted as a box plot in Fig. 8. Unlike Fukushima-derived cesium, which

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decreased sharply with time after entering into the coastal environment (30-90km off

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FDNPP site) (Fig. 1),

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Most of the data every year were all below 10 mBq/m3 and the median values were all

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below 5 mBq/m3. The maximum

239+240

Pu had no obvious interannual variation in the seawater.

239+240

Pu activity (15.30 mBq/m3) observed in

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October 2014 (Casacuberta et al., 2017) did not exceed the background level before

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the FDNPP accident (ND-32.0 mBq/m3). As for

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fell between that of PPG close-in fallout and global fallout, within the background

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level before the FDNPP accident (0.173-0.322) (Oikawa et al., 2015). There was also

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no obvious interannual variation except for August 2012, which had lower 240Pu/239Pu

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atom ratios. From these temporal variations of both

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atom ratio, it is obvious that the release of Pu into the marine environment from

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FDNPP even at the early stage of the accident was negligible.

3.4. Contribution of the PPG close-in fallout

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239+240

Pu activity and

240

Pu/239Pu

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Pu/239Pu atom ratios, all the data

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240

As the abovementioned work proved that the amount of Pu isotopes released into

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the marine environment from the FDNPP accident was too small to be distinguished

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from the background level in the seawater, the major sources of Pu were still global

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fallout and PPG close-in fallout. The contributions of global fallout and PPG close-in

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fallout Pu were estimated using the two end-member mixing model proposed by Krey

291

et al. (1976).

EP =

.

(1)

.

AC C

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286

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where R refers to 240Pu/239Pu ratio, subscripts P, G and S refer the PPG close-in fallout,

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and the global stratospheric fallout and the sample, respectively. The value 3.674 is

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the ratio of the specific activity of 240Pu to 239Pu, by which the atom ratio is converted

296

to the activity ratio. The contributions of the PPG close-in fallout of our study ranged

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from 26% to 77%, with a mean of 48% (Supplementary data, Table S2). Buesseler

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(1997) suggested that fallout Pu derived from the PPG would be preferentially

299

removed from the water column, compared with global stratospheric fallout Pu which 12

ACCEPTED MANUSCRIPT is more soluble. However, up to 77% of the PPG close-in fallout Pu was still retained

301

in the seawater columns. In our previous study (Men et al., 2018), there was also a

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much higher 240Pu/239Pu atom ratio (0.322) with the high contribution of PPG close-in

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fallout Pu of 84% in Northwest Pacific 905 km from the FDNPP site. This might

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suggest that the PPG is still an important source even more than 60 years after the

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1950s (Wu et al., 2014) and the North Equatorial Current systems continuously

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transport the remobilized Pu from the Marshall Islands area to the Northwest Pacific

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Ocean.

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Acknowledgements

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We thank JAMSTEC for providing us the spare samples collected during the

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coastal monitoring done by the Japanese Government in 2011. This work was

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supported by the Grant of Fukushima Prefecture related to Research and Development

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in Radiological Sciences, the Interdisciplinary Project on Environmental Transfer of

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Radionuclides, the Agency for Natural Resources and Energy, the Ministry of

315

Economy, Trade and Industry (METI), Japan, and the JSPS KAKENHI (Grant

316

number JP17k00537). W. Men thanks the China Scholarship Council for financial

317

support to carry out research abroad. We sincerely thank for the patience and valuable

318

suggestions of the reviewers which improves this work a lot.

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Supplementary data

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Supplementary table. Table S1: 239+240Pu/137Cs activity ratios of different items. Table

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S2: Contribution of the PPG close-in fallout.

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141°24.0′

37°39.9′

42

2

141°23.9′

37°35.0′

37

3

141°24.1′

37°30.0′

34

4 5 6 7 8 9 10

141°23.9′ 141°23.9′ 141°24.0′ 141°24.0′ 141°24.1′ 141°15.0′ 141°04.9′

37°22.9′ 37°15.9′ 37°11.9′ 37°05.9′ 36°59.9′ 37°00.0′ 37°00.0′

33 37 41 48 57 51 47

A

141°05.0′

37°45.0′

37

B

141°15.0′

37°42.4′

37

S-1

141°05.0′

37°48.3′

43

S-2

141°15.1′

37°48.2′

47

Sampling time

2+61 114 119 2+64 122 3+71 136 4+76 85+160 172 132 82 3 14 20 3+30 50 3 16 20 20 4 30

Cruise YK11-E02 2011/5/3 17.1 2011/5/3 8.5 2011/5/5 8.9 2011/5/3 17.0 2011/5/3 8.6 2011/5/5 17.1 2011/5/3 8.5 2011/5/5 17.7 2011/5/3 16.3 2011/5/5 8.9 2011/5/7 9.0 2011/5/5 8.9 2011/5/3 7.9 2011/5/3 9.0 2011/5/3 9.1 2011/5/5 17.5 2011/5/5 8.7 2011/5/7 9.0 2011/5/7 8.6 2011/5/7 8.8 2011/5/3 8.4 2011/5/5 8.7 2011/5/5 9.0

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1

Depth* m

Volume L

240

Pu/239Pu atom ratio

0.261±0.050 0.308±0.036 0.270±0.034 / 0.256±0.036 / 0.260±0.032 0.256±0.040 0.262±0.041 / 0.235±0.031 0.289±0.038 0.269±0.049 0.216±0.032 0.234±0.038 / 0.271±0.035 0.279±0.040 0.245±0.028 0.222±0.018 0.278±0.038 0.233±0.040 0.243±0.030

20

239+240

SC

Distance from the site km

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Latitude N

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Longitude E

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Station

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Table 1 239+240Pu, 137Cs activity concentrations and 240Pu/239Pu atom ratios in seawater collected 30-163 km from the FDNPP site during May 2011 Pu activity mBq/m3

1.58±0.30 3.29±0.42 6.15±0.79 1.00±0.23 3.60±0.52 1.12±0.20 3.64±0.47 1.46±0.23 2.48±0.42 1.84±0.38 2.52±0.34 3.88±0.54 7.02±1.30 2.98±0.42 6.74±1.08 0.81±0.16 3.69±0.49 4.30±0.63 2.47±0.29 8.67±0.70 5.77±0.84 4.30±0.72 3.42±0.43

137

Cs activity Bq/m3

2010±80 27.4±1.0 20.3±0.8 178±10 25.3±1.0 136±10 25.6±1.1 505±29 19.7±0.5 14.0±0.6 24.0±0.9 444±37 1470±70 1490±80 1620±90 8170±270 8880±390** 13800±500** 5140±220** 10300±400** 3380±150 7670±340** 7190±290

239+240

Pu/137Cs activity ratio 7.88×10-7 1.19×10-4 3.00×10-4 5.61×10-6 1.41×10-4 8.28×10-6 1.05×10-4 2.89×10-6 1.25×10-4 1.34×10-4 6.32×10-5 8.76×10-6 4.71×10-6 2.00×10-6 4.16×10-6 9.91×10-8 4.16×10-7 3.12×10-7 4.81×10-7 8.42×10-7 1.71×10-6 5.61×10-7 4.76×10-7

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Distance from the site km

S-3

141°19.0′

36°56.5′

59

S-4

141°05.0′

36°56.5′

54

2 3 4 5 8

142°00.0′ 141°59.8′ 142°29.7′ 142°29.8′ 141°30.0′

38°00.1′ 37°20.0′ 37°19.7′ 36°40.1′ 36°00.0′

107 86 130 155 163

Depth* m

Sampling time

47 3 84 163 3 54

Volume L

2011/5/5 8.6 2011/5/3 8.2 2011/5/3 9.0 2011/5/3 8.6 2011/5/5 8.9 2011/5/5 9.0 Cruise NT11-E01 2011/5/12 20 2011/5/10 10 2011/5/10 20 2011/5/11 20 2011/5/11 20

240

Pu/239Pu atom ratio

3+101 100 3+99 3+100 3+100

239+240

Pu activity mBq/m3

RI PT

Latitude N

0.269±0.049 0.241±0.034 0.218±0.028 0.230±0.024 0.262±0.029 /

SC

Longitude E

M AN U

Station

/ 0.231±0.041 / / /

137

Cs activity Bq/m3

5.76±1.06 2.57±0.36 2.89±0.36 6.29±0.70 11.18±1.28 1.81±0.52

4710±210 4450±180** 37.1±1.3 30.2±1.2 811±43** 585±31

0.97±0.20 1.09±0.19 0.95±0.25 1.13±0.23 0.85±0.18

36.1±1.1 15.0±0.7 97.5±2.6 2.63±0.11 1.56±0.07

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* “number + number” means the seawater sample at the depth of the corresponding number (m) were combined together, for example, “2+61” means the samples at the depth of 2 m and 61 m were combined. ** Takahata et al. (2018) “/” no data

21

239+240

Pu/137Cs activity ratio 1.22×10-6 5.78×10-7 7.73×10-5 2.07×10-4 1.38×10-5 3.09×10-6

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Fig. 1 Temporal variations of radio-cesium concentrations in seawater within 30-90km from the FDNPP site over the period March 2011 to

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February 2016, cited from Takata et al. (2017). Fig. 2 Map showing sampling stations where seawater was collected.

Fig. 3 Vertical distributions of 239+240Pu and 137Cs activity concentrations at Stations A, S-1, S-2 and S-3.

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Fig. 4 The distribution of 239+240Pu and 137Cs activity concentrations in the surface and bottom water layers. Fig. 5 Distribution of 240Pu/239Pu atom ratio in water columns in the Northwest Pacific and its marginal sea before the FDNPP accident (Bertine

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et al., 1986; Kim et al., 2004; Norisuye et al., 2005; Yamada et al., 2006; Yamada and Zheng, 2008; 2010; 2011; Oikawa et al., 2015; Oikawa et al., 2011; Wu et al., 2017; Zheng et al., 2012b; Buesseler, 1997 ).

Fig. 6 239+240Pu activities vs 240Pu/239Pu atom ratio in seawater off the coast of the FDNPP site. Data compiled from Oikawa et al. (2015) and this study.

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Fig. 7 The relationship between 239+240Pu and 137Cs activity concentrations.

Fig. 8 Box plot showing the time evolution of the 239+240Pu activities and 240Pu/239Pu atom ratios in the seawater. The data (within 100 km from the FDNPP site) were plotted: May 2011 (this study), August 2012 (Bu et al., 2014a), May 2013 (Hain et al., 2017) and October 2014

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(Casacuberta et al., 2017). The box plot displays the distribution of data based on the five-number summary: minimum (MIN), first

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22

Fig. 1

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Fig. 2 24

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15

15

A

S-1 1400 1600 137Cs (Bq/m3)

1800

25 4000

8000 12000 137Cs (Bq/m3)

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2 0

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10

10

40

20

80

30

40

25

4

6

8

10

160

S-3 200 6000 (Bq/m3)

137Cs

Fig. 3

2

120

S-2 50 3000

16000

0

(mBq/m3)

0

Depth(m)

10

8

M AN U

5

6

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(mBq/m3) 6 8

239+240Pu

239+240Pu

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10

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239+240Pu

239+240Pu

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9000

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2500 (Bq/m3)

137Cs

5000

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Fig. 4

26

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Fig. 5

27

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Fig. 6 28

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Fig. 7

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Fig. 8

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Highlights Plutonium isotopes in seawater at the very early stage of Fukushima accident were determined.

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Pu activities and atom ratios of the seawater samples 33-163 km off FDNPP site were provided.

Fukushima-derived plutonium isotopes were too limited to be distinguished from the background.

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Contributions of the PPG Pu in the water column of the study area ranged from 26 % to 77 %.