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Appearances of Fukushima Daiichi Nuclear Power Plant-derived 137Cs in coastal waters around Japan: Results from marine monitoring off nuclear power plants and facilities, 1983–2016 Hyoe Takata, Masashi Kusakabe, Naohiko Inatomi, and Takahito Ikenoue Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03956 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Environmental Science & Technology

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Appearances of Fukushima Daiichi Nuclear Power Plant-derived 137Cs in coastal

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waters around Japan: Results from marine monitoring off nuclear power plants and

3

facilities, 1983–2016

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Hyoe Takata*, Masashi Kusakabe, Naohiko Inatomi, Takahito Ikenoue

6

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Central Laboratory, Marine Ecology Research Institute, 300 Iwawada, Onjuku-machi,

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Isumi-gun, Chiba 299-5105, Japan

9 137

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ABSTRACT: Monitoring of

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1983 and 2016 yielded new insights into the sources and transport of Fukushima

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Nuclear Power Plant (FDNPP)-derived 137Cs, particularly along the west coast of Japan.

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Before the FDNPP accident (1983–2010), the activity concentrations of

14

from fallout, were decreasing exponentially. Effective

15

seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011)

16

activity concentrations in seawater off Fukushima and neighboring prefectures

17

immediately increased. Since May/June 2011,

18

been declining, and they are now approaching pre-accident levels. Along the west coast

19

of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity

Cs in seawater in coastal areas around Japan between

137

137

Cs, mainly

Cs half-lives in surface 137

Cs

137

Cs activity concentrations there have

1 ACS Paragon Plus Environment


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concentrations started increasing by 2013, with earlier (May/June 2011) increases at

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some sites due to airborne transport and fallout. The inventory of

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(in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 ×

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1014 Bq, meaning that 0.44 × 1014 Bq of FDNPP-derived

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estimated global fallout

25

137

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accounts for approximately 0.2% of the total

27

from the accident.

137

137

Cs in the Japan

Cs was added to the

137

Cs inventory in 2016 (0.53 × 1014 Bq). The net increase of

Cs inventory in the Japan Sea through the addition of FDNPP-derived

137

Cs

137

Cs flux from the plant to the ocean


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1. INTRODUCTION

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All of the nuclear power plants and nuclear reprocessing facilities in Japan are

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located on the coast. The integrity of the marine environment with respect to

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radioactivity is a great concern to the public, especially the safety of fishery products.

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Since 1984, the Marine Ecology Research Institute (Chiba, Japan) has been monitoring

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levels of radioactive materials in the waters off nuclear power plants and nuclear

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reprocessing facilities under contract with the Science and Technology Agency (AST,

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1983–2001), the Ministry of Education, Culture, Sports, Science and Technology

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(MEXT, 2001–2013), and the Secretariat of the Nuclear Regulation Authority of Japan

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(NRA, 2013–present). Targets of the monitoring include seawater, bottom sediment,

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and fish.

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Since right after the accident at the Fukushima Daiichi Nuclear Power Plant

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(FDNPP), there has been additional monitoring in the waters off Fukushima and

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nearby prefectures. Although the results from this latter monitoring have been

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documented in several scientific papers1,2 among others, results from the former have

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been published only in the project reports (in Japanese) and a few scientific papers3.

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This paper has two goals: (1) to describe the spatiotemporal variation of

137

Cs

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activity concentrations in seawater in the coastal areas off nuclear power plants in

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Japan from 1983 to 2010, and (2) to evaluate the impacts of the FDNPP accident in



47

these coastal waters, with emphasis on the delayed increase of

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concentrations from 2013 to 2016 in areas remote from the accident site.

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137

Cs activity

49

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2. MATERIALS AND METHODS

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2.1. Oceanographic setting of sampling sites. Monitoring was carried out at 15

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sampling sites adjacent to nuclear power plants, including one station near the

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Higashi-dori Nuclear Power Plant still under construction (Figure 1). Four major ocean

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currents flow around the Japanese Islands, possibly affecting the spatial distributions of

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man-made radionuclides such as radiocesium: the Tsugaru Warm Current, the Oyashio,

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the Kuroshio, and the Tsushima Warm Current (TWC). Our sampling sites can be

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grouped into three areas on the basis of the related current systems as follows (Figure

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1).

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Area I: Aomori (HG), Miyagi (MI), Fukushima-daiichi (FSN), Fukushima-daini

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(FSS), and Ibaraki (IB) sites. Oceanographic conditions in this area are variable

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because of the coexistence of the Tsugaru Warm Current, the Oyashio, and the

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Kuroshio.

63 64 65

Area II: Shizuoka (SZ), Ehime (EH), Saga (SG), and Kagoshima (KG) sites. The Kuroshio and/or TWC predominate in this area in all seasons. Area III: Hokkaido (HK), Niigata (NI), Ishikawa (IS), Fukui-daiichi (FKE),


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Fukui-daini (FKW), and Shimane (SM) sites. These sites are located in the Japan Sea,

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where the horizontal migration of water masses is controlled by the TWC flowing from

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the North Pacific through the East China Sea over the sill of the Tsushima Strait.

69 70

2.2. Sampling. Each site has four sampling stations, where seawater was collected

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annually from just below the surface (0–1-m depth interval) and bottom layers (10–30

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m above the seafloor) during May–June (Table S1, Supporting Information). Note that

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the locations of most of the sampling stations during the early stage of the project (i.e.,

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1983–1987) were slightly different from those from 1988 to the present. Detailed

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sampling positions are given in Table S2 of the Supporting Information.

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Surface water samples were taken directly from just below the surface using an

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electric pump, a Van Dorn-type large volume water bottle (Rigo Co. Ltd., Tokyo,

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Japan) or a Niskin-X bottle water sampling system with multiple sampling bottles

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(General Oceanics, Inc., Miami, FL, USA). Bottom water was collected using Van

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Dorn or Niskin-X samplers. Each bottom-water sampler was equipped with a

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conductivity-temperature-depth (CTD) profiler system (SBE 9plus, Sea-Bird

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Electronics, Inc., Bellevue, WA, USA). From both surface and bottom seawater

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samples, a total of 100 L was distributed into five 20-L aliquots in polypropylene

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bottles to which were added 40 mL of 6 M HCl to maintain acidic conditions.

85



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2.3. Determination of radiocesium. From each seawater sample, half (50 L) was

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used for radiocesium measurement. Cesium was separated by co-precipitation with

88

ammonium phosphomolybdate (AMP) and then purified with a cation exchange

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column. After cesium was precipitated as cesium chloroplatinate, its radioactivity was

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determined using a gas-flow type low-background anti-coincidence beta counter

91

(LBC-471Q, Aloka Co. Ltd. Japan). This counter has high efficiency for detecting the

92

beta ray from radiocesium, but it does not discriminate

93

concentrations of

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virtually zero, even in 1986 when the Chernobyl Power Plant accident occurred. It has

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been confirmed that

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(right after the Chernobyl accident) and 1987 by analyzing those samples with a

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gamma ray spectrometer in 1987, so the pre-accident radiocesium activity

98

concentration based on the beta counter represents the activity concentration of 137Cs.

134

134

Cs from

137

Cs. The activity

Cs in seawater before the FDNPP accident (1983–2010) were

134

Cs was not detected in samples collected in the years of 1986

99

The same method using AMP precipitation and a beta counter was also used for

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samples taken after the FDNPP accident in 2011, except for one surface water sample

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collected each year at station 1 at each site (total, 15 samples per year). The reported

102

activity concentrations for this period, therefore, are the sum of 134Cs and 137Cs activity

103

concentrations (see Table S2 for details).

104

The detection limits for radiocesium were set to three times the value of the

105

counting statistics error; the minimum detectable activities were expected to be 1.0

106

mBq/L for counting times of thousands of seconds. The activity concentrations of


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radiocesium in the water samples were decay-corrected to the sampling date.

108 109

2.4. Gamma-ray spectrometry. Because

134

Cs and

137

Cs were detected in

110

seawater samples collected immediately after the FDNPP accident1, radiocesium in

111

surface seawater samples from each site in 2011 and in all samples collected from

112

2012 to 2016 was measured by gamma-ray spectrometry.

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Cesium in 50-L seawater sample was co-precipitated with AMP after adjusting the

114

sample pH to around 1 with hydrochloric or nitric acid. The activity of radiocesium

115

(134Cs and 137Cs) in AMP was measured using coaxial-type Ge detectors. The detection

116

limits for

117

error; the minimum detectable activities of

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mBq/L for counting times of tens of thousands of seconds. The activity concentrations

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of radiocesium in the water samples were decay-corrected to the sampling date.

134

Cs and

137

Cs were set to three times the value of the counting statistics 134

Cs and

137

Cs were expected to be 1.0

120

121

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3. RESULTS AND DISCUSSION

3.1.

137

Cs distributions before the FDNPP accident. Monitoring data were used 137

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to determine the spatiotemporal distributions of

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(Figure 2 and Table S1). The origin of most

125

Japan Sea before the FDNPP accident can be traced to the global fallout from nuclear

137

Cs in the three sampling areas

Cs in the North Pacific Ocean and the



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weapons testing carried out in the 1950s and 1960s. The distribution and influence of

127

FDNPP-derived radiocesium in the three sampling areas are clearly different because

128

of the current systems described in section 2.1. We therefore separately evaluated the

129

distributions of 137Cs in seawater for each area.

130 131

3.1.1. Surface waters. In the two decades following the end of atmospheric 137

132

nuclear weapons testing,

133

Pacific decreased to about 10 mBq/L by the early 1980s4. During 1983–1985, before

134

the Chernobyl NPP accident (1986),

135

study ranged from 3.3 to 5.6 mBq/L with an average of 4.1 mBq/L. In 1986,

136

concentrations in the surface waters increased to as much as 10 mBq/L in the northern

137

area (Site NI) of Area III 2–3 months after the Chernobyl NPP accident (Figure 2), yet

138

134

139

concentrations returned to 3.3–4.8 mBq/L, which was almost the same as in 1983–

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1985, suggesting that the impact of the Chernobyl accident on the surface seawater in

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those areas was insignificant compared to that from the global fallout from nuclear

142

weapons testing. After 1987, the following year after the Chernobyl accident,

143

concentrations in the surface waters decreased to a maximum of 1.9 mBq/L in 2010

144

(mean, 1.5 mBq/L).

Cs activity concentrations in surface waters in the North

137

Cs activity concentrations over all areas in this

Cs was not detected in any samples. In the following year, 1987, the

137

Cs

137

Cs activity

137

Cs

145 146

3.1.2. Bottom waters. Because the bottom waters were collected 10–30 m above


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the seafloor, their sampling depths ranged from a few tens to several hundreds of

148

meters. Thus,

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along with the sampling depths.

137

Cs activity concentrations in bottom waters are plotted in Figure 2

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In Area I from 1983 to 2010, 137Cs activity concentrations in bottom water at sites

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MI, FSN, FSS, and IB were almost the same as those in surface water because of

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vigorous vertical mixing in these shallow waters (35–160 m). At site HG, however,

153

137

154

(0.49–1.6 mBq/L) compared with those in the surface waters (1.1–2.5 mBq/L) because

155

the bottom samples were collected from greater depths (440–660 m).

Cs activity concentrations in bottom water from 2003 to 2010 were relatively low

156

In Area II, 137Cs levels in bottom water from shallow sampling depths ( 0.90 in Table 1) suggest



210

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that these are reasonable estimates of the effective half-lives.

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The effective 137Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. The

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average effective surface-water half-life of 137Cs in the three areas ranged from 15.8 to

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17.6 y. The overall effective half-life in surface waters for all sites was estimated as

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16.6 ± 0.7 y (mean ± s.d.). This value is within the range of 16–19 y calculated for the

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western North Pacific4 and the Japan Sea7–9. In this study, however, the values were

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slightly higher in Area II (17.6 ± 0.7 y) than in Area I (15.8 ± 0.2 y), probably because

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of the weaker dilution effect from mixing of surface water with

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water in Area II, the Kuroshio area, than in Area I.

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The effective half-life of

137

137

Cs-spears deep

Cs in bottom waters ranged from 14.3 to 23.4 y.

220

Distinctively high effective half-lives of 22.2 y and 23.4 y were found at sites HK and

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NI, respectively, in the northern part of the Japan Sea; these longer half-lives were

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likely due to sluggish exchange of deep water. As for the other sites, the effective

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half-lives were variable within the range of 14.3–17.6 y, comparable to the range for

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the corresponding surface waters, probably indicating that radiocesium was well mixed

225

vertically throughout the water column in those areas.

226 227

3.2.

137

Cs distribution after the FDNPP accident. The total amount of

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FDNPP-derived radiocesium released directly into seawater within one month after the

229

accident was estimated to be about 3 PBq10, although another study reported an

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estimated direct discharge of 27 PBq 11. Of the 137Cs released to the atmosphere (15–20


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PBq), 12–15 PBq was deposited on the surface of the North Pacific Ocean12. The

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detailed behavior of FDNPP-derived radiocesium in seawater around the FDNPP

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corresponding to our Area I has been reported1,13–15. In this section, we focus on the

234

fate and effect of FDNPP-derived radiocesium on the other coastal areas in Japan,

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especially Areas II and III. As noted in section 2.3, values for radiocesium in most

236

seawater samples collected during May–June 2011 are reported as the sum of 134Cs and

237

137

Cs (134+137Cs) in Figure 2.

238 239

3.2.1. Surface waters. As a result of atmospheric input and direct release of 137

240

radionuclides to the coastal waters off Fukushima and neighboring prefectures,

241

activity concentrations in seawater near the FDNPP have exceeded 1 × 107 mBq/L

242

during the period of late March–early April 201110,11. Our monitoring showed

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FDNPP-derived radiocesium (134+137Cs) activity concentrations increasing in surface

244

seawater in Area I in May–June 2011, ranging from 27 to 510 mBq/L with a mean of

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190 mBq/L. The

246

activity concentrations ranged from 1.9 to 5.0 mBq/L with a mean of 2.7 mBq/L,

247

approaching the level for this area before the FDNPP accident (range, 1.1–1.8 mBq/L;

248

mean, 1.5 mBq/L; n = 20).

249

In Areas II and III,

137

Cs activity concentration then declined with time. In 2016,

137

Cs

Cs

137

Cs activity concentrations were relatively high (2.1–5.2

250

mBq/L) at station 1 of sites SZ, HK, and NI in May–June 2011, compared with the

251

corresponding values in 2010 (1.4–1.5 mBq/L). In the following year, however, the



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252

137

253

activity concentrations in 2012 (range, 1.0–2.2 mBq/L; mean, 1.7 mBq/L; n = 40) were

254

similar to those in 2010 (range, 1.3–1.9 mBq/L; mean, 1.6 mBq/L; n = 40).

Cs activity concentrations decreased to pre-accident levels at all sites (Figure 2). The

255

It is worth noting that the 137Cs activity concentrations increased steadily from 2013

256

to 2016 in Areas II and III. In particular, in the Japan Sea (i.e. Area III), the annual

257

mean of the 137Cs activity concentrations in surface waters increased from 2013 (range,

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1.6–2.2 mBq/L; mean, 1.9 mBq/L; n = 24) to 2016 (range, 1.9–2.5 mBq/L; mean, 2.2

259

mBq/L; n = 24) (Figure S2). It is interesting that the

260

Areas II and III, remote from the accident site, have been increasing while the input to

261

the marine environment from the FDNPP dropped by orders of magnitude10,14. In 2015

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and 2016, another study detected 134Cs from the FDNPP accident along the coast of the

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Japan Sea16. The monitoring results in this study indicate that the

264

concentration started increasing in the Japan Sea in 2013 at the latest (Figure 2).

265

137

Cs activity concentrations in

As shown in section 3.1.1., the activity concentration of

137

Cs activity

137

Cs in surface waters

266

had been decreasing exponentially with time over two decades beginning in the 1980s.

267

Taking into account this decrease, we attempted to evaluate the net increase of

268

after the FDNPP accident at each site as follows. We generated a time series of the

269

ratios between the observed

270

background 137Cs (i.e. bomb-derived 137Cs) activity concentration (Csbkgd) based on the

271

regression line for each site (Figure 3). The strong correlation coefficients obtained (r

272

> 0.90) suggest that the estimated

137

137

Cs

Cs activity concentration (Csobs) and the estimated

137

Cs activity concentrations at each site were


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273

reasonable for 1983–2010 (except for the years of the Chernobyl NPP accident, 1986

274

and 1987); the activity concentration ratios were approximately equal to 1.0 throughout

275

that time.

276

Immediately after the FDNPP accident in 2011, the activity ratios in Area I

277

increased above 1.0, as confirmed by a one-tailed Student’s t test. This suggests the

278

addition of

279

radionuclides were also atmospherically transported northwestward to the northern

280

areas of the Japan Sea in late March of 201117. The ratios then decreased with time as a

281

result of water mixing and advection as follows: In Areas II and III, the activity

282

concentration changes led to an increase in the ratios in 2011, returning to pre-accident

283

levels (average, 1.1) in the following year (Figure 3). A similar temporal change was

284

observed in 1987, the year after the Chernobyl NPP accident (Figure 2). This pattern

285

suggests that airborne transport of FDNPP-derived

286

minimal impact on these areas. The migration of

287

not to affect those two areas immediately after the accident because the Kuroshio

288

forms a southern boundary for Area I for the transport of radiocesium. The main body

289

of contaminated waters therefore moved east along the boundary formed by the

290

Kuroshio18.

137

Cs from the FDNPP accident via atmospheric transport because the

137

137

Cs from the accident had a

Cs with water currents appeared

291

Since 2013, the ratios in Areas II and III have been gradually increasing (Figure 3).

292

In the Japan Sea (Area III) the ratios in 2011 were 1.35–1.73 at site HK (in the

293

northern Japan Sea), 1.40–1.75 at site FKE (mid-way in the Japan Sea), and 1.30–1.85



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at site SM (southern Japan Sea). Most of the ratios in those areas, however, fell below

295

the threshold for outliers (below the red lines in Figure 3) in 2012. However, statistical

296

analysis of the ratios using the data from 1988 to 2016 suggests that most of the ratios

297

from 2013 to 2016 in Areas II and III were again outliers at a significance level of

298

99.6%. Thus, there has been an addition of 137Cs in Areas II and III. The average ratio

299

in Areas II and III reached about 1.7 in 2016.

300 301

3.2.2. Bottom waters. As shown in section 3.1.2, the activity concentrations of

302

137

303

the activity concentrations in samples from shallow sampling depths ( 400 m) were lower than those in the surface waters. In Area

306

II, bottom-water activity concentrations at all sites showed distributions and levels

307

similar to the corresponding surface water (Figure 2), except for station 3 at site SZ,

308

where the bottom sample was collected at 314 m and its activity concentration was

309

significantly higher than that of surface water in 2013. Kumamoto et al. (2014) have

310

shown that subtropical mode water has been playing an important role in the

311

southward transport of FDNPP-derived 134Cs19. The high 134Cs concentration in bottom

312

water would be due to intrusion of mode water to a depth of about 300 m, as evidenced

313

by its sigma-theta values of about 25.2, which is a typical density for mode water.

314

However, this was the only station with a relatively high 134Cs concentration in bottom

Cs in bottom water before the accident depended on the sampling depths. In Area I,


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water, so further evaluation is necessary.

316

In Area III, located in the Japan Sea, there was a temporal increase from 2011 to

317

2016 in 137Cs activity concentrations in bottom water collected at depths ≤ 220 m or at

318

temperatures ≥ 10 °C, which are the general properties of the water mass in the TWC20.

319

The activity concentrations in waters collected at greater depths (>220 m) before the

320

accident (2005–2010) ranged from 0.97 to 2.1 mBq/L with a mean of 1.6 mBq/L (n =

321

63) and after the accident (2011–2016) from 0.68 to 2.0 mBq/L with a mean of 1.4

322

mBq/L (n = 64). These two datasets, from before and after the accident, reveal that

323

FDNPP-derived

324

depths greater than 220 m.

137

Cs has not yet appeared in the bottom water of the Japan Sea at

325 326

3.2.3. Transport mechanism for FDNPP-derived 137Cs to the Japan Sea. The

327

increase in Csobs/Csbkgd ratios after the accident in Areas II and III means that these

328

areas received additional

329

FDNPP. A simple approach suggests two likely pathways for the additional

330

airborne transport, and migration with seawater via coastal currents around Japan (i.e.

331

migration of the contaminated water to Area III in the Japan Sea passing through sites

332

SZ, EH, KG, and SG of Area II). As mentioned in section 3.2.1., the transport of 137Cs

333

is much faster by air than by water, and yet there is no evidence of 137Cs deposition in

334

the Japan Sea immediately after the accident, so the former pathway can be ruled out.

335

Intuitively, it seems unlikely that the latter pathway affected the increase in Areas II

137

Cs after the accident, most likely originating from the 137

Cs:



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336

and III because the Kuroshio forms a southern boundary to Area I, limiting the further

337

southward transport of radiocesium because contaminated waters move eastward along

338

the Kuroshio18. At the same time,

339

reduced through mixing with Kuroshio water. Even though it is possible for 137Cs to be

340

directly advected from the accident location to southern areas, for example by hugging

341

the coast and moving past site SZ, this pathway was presumably negligible because the

342

137

343

2010 (Figure 2).

344

137

Cs activity concentrations in this water would be

Cs activity concentrations at this site in 2012 were the same as or lower than those in

It appears that the trend of increasing annual mean

137

Cs activity concentration

345

ratios during 2013–2016 was a common feature in the water column from 0 to 220 m

346

at the coastal sites in the Japan Sea (HK, NI, FKE, FKW, and SM). This feature might

347

result from oceanographic factors specific to the Japan Sea—specifically, a

348

surface-limited transport of the radionuclide by the TWC, which passes through the

349

Tsushima Strait.

350

The Kuroshio forms by a merger of water currents between 10°N and 35°N in the

351

North Pacific Ocean, and its circulation is clockwise21. Migration of FDNPP-derived

352

radiocesium into the Japan Sea might occur along two routes. The first is entrainment

353

of subsurface FDNPP-derived radiocesium into the surface waters of Area II through

354

vertical mixing21. The formation of mode waters below the surface leads to a

355

southward flow that crosses the Kuroshio, is quickly entrained back into the Kuroshio

356

farther upstream, and thus is returned to Area II and then to the west coast of Japan via


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357

the TWC, thereby raising the levels of FDNPP-derived radiocesium in Area III (Japan

358

Sea) after a relatively short time-lag. The second possible route would require that a

359

portion of the FDNPP-derived radiocesium was carried eastward with the Kuroshio

360

(flowing away from Japan), eventually being transported westward by the return flow

361

of the current13,18,19,21,22. This route would take far too long for the FDNPP-derived

362

radiocesium to show up on the west coast of Japan by 2013. However, waters with

363

FDNPP-derived radiocesium on this roundabout route might eventually appear in the

364

Japan Sea after several years. If so, this process would explain the continuous increase

365

in 137Cs in the Japan Sea, along with the increase from radiocesium following the first

366

route. Our results imply that radiocesium released from the FDNPP into the North

367

Pacific Ocean was transported not only eastward along with the surface currents but

368

also southward through the formation and subduction of mode water after the accident.

369 370

3.3. Flux of

137

Cs to the Japan Sea. The flux of

137

Cs to the Japan Sea via the

371

TWC was calculated using the annual average water flow rate into the sea through the

372

Tsushima Strait (2.5 × 106 m3/s)

373

concentrations in the surface and bottom waters at sites SG and KG in Area III. These

374

two sites were chosen because they are close to the Tsushima Strait entrance (Figure 1).

375

Thus, they can provide initial

376

flux to the Japan Sea.

377

137

23

and the annual average of

137

Cs activity

Cs activity concentrations for the calculation of

137

Cs

The annual observed 137Cs fluxes (Fluxobs) from 2010 to 2016 are listed in Table 2,



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378

together with the background flux (Fluxbkgd) calculated using Csbkgd and the inflow

379

water volume of the TWC and the model fitted by Equation (1); this model is

380

applicable when there was no addition of 137Cs from any accident after 2010. In 2010,

381

Fluxobs was calculated as 1.3 × 1014 Bq/y, similar to Fluxbkgd (1.3 × 1014 Bq/y). In 2011,

382

immediately after the FDNPP accident, Fluxobs increased to 1.4 × 1014 Bq and then

383

decreased to 1.3 × 1014 Bq in 2012. After that, however, Fluxobs showed a temporally

384

increasing trend. In contrast, Fluxbkgd displayed a decline with time from 2010 (1.3 ×

385

1014 Bq) to 2016 (1.0 × 1014 Bq). The differences between Fluxobs and Fluxbkgd are

386

assumed to be the flux of the FDNPP-derived

387

increased from 0.2 × 1014 Bq/y to 0.7 × 1014 Bq/y from 2013 to 2016.

137

Cs (Fluxadd) (Table 2). Fluxadd

388 389

3.4. Inventory of 137Cs in the TWC in the Japan Sea. The inventory of 137Cs in 137

390

the TWC in the Japan Sea can be estimated by multiplying the concentration of

391

in seawater by the water volume. The water volume was estimated from the surface

392

area of the TWC and its thickness (220 m). Although the area of the Japan Sea covered

393

by the TWC ranges between 1.0 and 3.0 × 1011 m2 seasonally24, the surface area during

394

the sampling period (May–June) was about 2.0 × 1011 m2. Thus, the water volume was

395

calculated to be 4.4 × 1013 m3. The inventories of

396

before (2010) and after the accident (2011–2016) were then obtained by multiplying

397

the annual mean 137Cs concentrations in Area III (at depths ≤ 220 m) by the estimated

398

water volume (Table 2).

137

Cs

Cs in the TWC in the Japan Sea


Page 21 of 36


399

Before the FDNPP accident, the observed inventory of

137

Cs (Invobs) was calculated

400

to be 0.70 × 1014 Bq. In May–June 2011, two months after the accident, Invobs showed

401

a slight increase (0.88 × 1014 Bq), and fell back the following year to the same level as

402

before the accident. However, Invobs again started to increase and reached 0.97 × 1014

403

Bq in May–June 2016. The estimated background inventory of 137Cs (Invbkgd = Csbkgd ×

404

TWC water volume) was assumed to continue the decreasing trend from before the

405

accident. The value in 2016 was assumed to be 0.53 × 1014 Bq. As with the 137Cs flux,

406

the differences between Invobs and Invbkgd after the accident reflect the addition of

407

FDNPP-derived 137Cs. Thus, the 137Cs inventory derived from the accident (Invadd) was

408

calculated to be 0.22 × 1014 Bq in 2011. In the following year (2012), Invadd seemed to

409

fall to the same level as before the accident because of the dilution effect of seawater,

410

as in 1985–1987 before and after the Chernobyl NPP accident. However, after 2012,

411

Invadd increased linearly to 0.44 × 1014 Bq by 2016, meaning that the amount of

412

in the Japan Sea increased by 80% through the addition of FDNPP-derived

413

radioecologically minimal (a few mBq/L in activity concentration) yet statistically

414

significant value.

415

137

137

Cs

Cs; a

Invadd in 2011, 1–2 months after the FDNPP accident, could be due to air-borne 137

416

transport of FDNPP-derived

417

calculated to be approximately 0.1% of the total amount of

418

transported into the marine environment after the accident: 12–15 × 1015 Bq11. In

419

contrast, the continuous increase of Invadd after its decrease in 2012 is attributable to

Cs into the Japan Sea. Therefore, Invadd in 2011 was 137

Cs atmospherically



Page 22 of 36

420

horizontal transport from the North Pacific Ocean via the Tsushima Strait, which is

421

controlled mainly by the TWC. Invadd in 2016 (0.44 × 1014 Bq) was about 0.2% of the

422

total 137Cs flux (19–24 × 1015 Bq) from FDNPP to the ocean15.

423

When the mean residence time (Invobs/Fluxobs) was calculated to be 6.7 months

424

(range: 5.8–7.5 months) for 2013–2016, the Fluxobs in the surface water of the TWC in

425

the Japan Sea affect quickly the increase of Invobs of the Japan Sea. In addition, the

426

calculated residence time in this study is comparable to that for the TWC (a few

427

months)25. This short residence time might be due to the high flow rate of the TWC

428

(around 7 cm/s) in the Japan Sea26. In contrast, the residence time of

429

water (>220 m) before the FDNPP accident has been estimated at 144–192 y7. These

430

results indicate that most of the surface water with relatively high

431

concentrations rarely mixes with the deep water in the Japan Sea and exits the Japan

432

Sea to the Pacific Ocean and the Sea of Okhotsk through the Tsugaru and Soya straits

433

at the north end of the Japan Sea.

137

Cs in deep

137

Cs activity

434

435

■ ASSOCIATED CONTENT

436

Supporting Information

437

The Supporting Information is available free of charge on the ACS Publications


Page 23 of 36


438

website.

439

Supplementary Table (Table S2: EXCEL).

440

441

■ AUTHOR INFORMATION

442

Corresponding Author

443

*Phone: +81-470-68-5111; fax: +81-470-68-5115; e-mail: [email protected].

444

Notes

445

The authors declare no competing financial interest.

446



Page 24 of 36

447

■ACKNOWLEDGEMENTS

448

We are grateful for helpful comments on the manuscript from four anonymous

449

reviewers. We thank the staff of the Marine Ecology Research Institute (Chiba, Japan)

450

for helpful discussions and technical assistance. We also thank the Japan Chemical

451

Analysis Center for their analysis of radiocesium in seawater. The marine

452

environmental radioactivity survey is a research project contracted from the Japanese

453

Ministry of Education, Culture, Sports, Science and Technology (May 2011–March

454

2013) and the Secretariat of the Nuclear Regulation Authority (April 2013–present).

455

456

■REFERENCES

457

(1) Oikawa, S.; Takata, H.; Watabe, T.; Suzuki, C.; Nakahara, M.; Misonoo, J. Long 90

137

458

term temporal changes of

459

marine organism samples. From the Chernobyl accident to immediately after

460

the Fukushima accident. Bunseki Kagaku 2013, 62, 455–474 (in Japanese

461

with English abstract).

Sr and

Cs in seawater, bottom sediment and

462

(2) Kusakabe, M.; Oikawa, S.; Takata, H.; Misonoo, J. Spatiotemporal distributions of

463

Fukushima derived radionuclides in nearby marine surface sediments.

464

Biogeosciences 2013, 10, 5019–5030.


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(3) Watabe, T.; Oikawa, S.; Isoyama, N.; Suzuki, C.; Misonoo, J.; Morizono, S. 137

466

Spatiotemporal distribution of

467

in the decades before the disaster at the Fukushima Daiichi Nuclear Power

468

Plant in 2011. Sci. Total Environ. 2013, 463–464, 913–921.

Cs in the sea surrounding Japanese Islands

469

(4) Povinec, P. P.; Aarkrog, A.; Buesseler, K. O.; Delfanti, R.; Hirose, K.; Hong, G.

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Pacific and Indian Oceans e WOMARS results. J. Environ. Radioact. 2005,

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90

Sr,

137

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Pu concentration surface water time series in the

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E. V.; Novichkov, V. P.; Shcherbinin, A. F.; Tkalin, A. V.; Volkov, Y. N.

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recent observations and the temporal change of concentrations. J. Nucl. Sci.

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(6) Gamo, T. Global warming may have slowed down the deep conveyor belt of a

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137

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(8) Ikeuchi, S. Temporal variations of 90Sr and 137Cs concentrations in Japanese coastal



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surface seawater and sediments from 1974 to 1998. Deep-Sea Res. Pt. II

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2003, 50, 2713–2726.

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(9) Ito, T.; Povinec, P. P.; Togawa, O.; Hirose, K. Temporal and spatial variations of

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anthropogenic radionuclides in Japan Sea waters. Deep-Sea Res. Pt. II 2003,

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50, 2701–2726.

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(10) Tsumune, D.; Tsubono, T.; Aoyama, M.; Uematsu, M.; Misumi, K.; Maeda, Y.; 137

492

Yoshida, Y.; Hayami, H. One-year, regional-scale simulation of

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radioactivity in the ocean following the Fukushima Dai-ichi Nuclear Power

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Plant accident. Biogeosciences 2013, 10, 5601–5617.

Cs

495

(11) Bailly du Bois P.; Laguionie, P.; Boust, D.; Korsakissok, I.; Didier, D.; Fiévet, B.

496

Estimation of marine source-term following Fukushima Dai-ichi accident. J.

497

Environ. Radioact. 2012, 114, 2–9.

498

(12) Aoyama, M.; Kajino, M.; Tanaka, T. Y.; Sekiyama, T. T.; Tsumune, D.; Tsubono,

499

T.; Hamajima, Y.; Inomata, Y.; Gamo, T. 134Cs and 137Cs in the North Pacific

500

Ocean derived from the TEPCO Fukushima Dai-ichi Nuclear Power Plant

501

accident, Japan in March 2011. Part Two: estimation of

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inventories in the North Pacific Ocean. J. Oceanogr. 2016, 72, 67–76.

134

Cs and

137

Cs

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(13) Kaeriyama, H.; Ambe, D.; Shimizu, Y.; Fujimoto, K.; Ono, T.; Yonezaki, S.; Kato,

504

Y.; Matsunaga, H.; Minami, H.; Nakatsuka, S.; Watanabe, T. Direct

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observation of

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central North Pacific after the Fukushima Dai-ichi nuclear power plant

134

Cs and

137

Cs in surface seawater in the western and


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accident. Biogeosciences 2013, 10, 4287–4295. (14) Takata, H.; Kusakabe, M.; Inatomi, N. The contribution of sources to the sustained 137

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elevated inventory of

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Fukushima Dai-ichi Nuclear Power Station accident. Environ. Sci. Technol.

511

2016, 50, 6957–6963.

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Cs in offshore waters east of Japan after the

(15) Aoyama, M.; Hamajima, Y.; Hult, M.; Uematsu, M.; Oka, E.; Tsumune, D.; 134

137

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Kumamoto, Y.

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March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident,

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Japan. Part one: surface pathway and vertical distributions. J. Oceanogr.

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2016, 72, 53–65.

Cs and

Cs in the North Pacific Ocean derived from the

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(16) Aoyama, M.; Hamajima, Y.; Inomata, Y.; Oka, E. Recirculation of FNPP1-derived

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radiocaesium observed in winter 2015/2016 in coastal regions of Japan. Appl.

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Radiat. Isot. 2017, 126, 83–87.

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(17) Yoshida, N.; Takahashi, Y. Land-surface contamination by radionuclides from the

521

Fukushima Daiichi Nuclear Power Plant Accident, ELEMENT 2012, 8, 201–

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206.

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(18) Buesseler, K. O. Fukushima-derived radionuclides in the ocean and biota off Japan. Proc. Natl. Acad. Sci. USA. 2012, 109, 5984–5988.

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(19) Kumamoto, Y.; Aoyama, M.; Hamajima, Y.; Aono, T.; Kouketsu, S.; Murata, A.;

526

Kawano, T. Southward spreading of the Fukushima-derived radiocesium

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across the Kuroshio extension in the North Pacific. Sci. Rep. 2014, 4, 4276–



528 529 530

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4285. (20) Moriyasu, S. The Tsushima Current. In Kuroshio and Its Physical Aspects; H. Stommel, H., Yoshida, K., Eds.; Univ. of Tokyo Press, 1972; pp 353–369.

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(21) Kumamoto, Y.; Aoyama, M.; Hamajima, Y.; Nagai, H.; Yamagata, T.; Kawai, Y.;

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Oka, E.; Yamaguchi, A.; Imai, K.; Murata, A. Fukushima-derived

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radiocesium in the western North Pacific in 2014. Journal of Radioanalytical

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and Nuclear Chemistry 2017, 311, 1209–1217.

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(22) Kaeriyama, H.; Shimizu, Y.; Ambe, D.; Masujima, M.; Shigenobu, Y.; Fujimoto,

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K.; Ono, T.; Nishiuchi, K.; Taneda, T.; Kurogi, H.; Setou, T.; Sugisaki, H.;

537

Ichikawa, T.; Hidaka, K.; Hiroe, Y.; Kusaka, A.; Kodama, T.; Kuriyama, M.;

538

Morita, H.; Nakata, K.; Morinaga, K.; Morita, T.; Watanabe, T. Southwest

539

intrusion of

540

power plant accident in the western north Pacific. Environ. Sci. Technol.

541

2014, 48, 3120–3127.

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Cs and

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Cs derived from the Fukushima Dai-ichi nuclear

542

(23) Fukudome, K. I.; Yoon, J. H.; Ostrovskii, A.; Takikawa, T.; Han, I. S. Seasonal

543

volume transport variation in the Tsushima Warm Current through the

544

Tsushima Straits from 10 years of ADCP observations. J. Oceanogr. 2010, 66,

545

539–551.

546

(24) Shigeoka, H. Temperature indices of the Polar Front in the Japan Sea using 439

547

MOVE/MRI.COM-WNP, Weather service bulletin 2010, 77, 109–118 (in

548

Japanese).


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549

(25) Inoue, M.; Tanaka, K.; Kofuji, H.; Nakano, Y.; Komura, K. Seasonal variation in 228

Ra/226Ra ratio of coastal water within the Sea of Japan: implications

550

the

551

for the origin and circulation patterns of the Tsushima Coastal Branch

552

Current. Mar. Chem. 2007, 107, 559–568.

553 554

(26) Hase, H.; Yoon, J. -H.; Koterayama, W. The current structure of the Tsushima warm current along the Japanese coast. J. Oceanogr. 1999, 55, 217–235.

555 556



Page 30 of 36

557

Figure captions

558

Figure 1. Study areas and sampling sites in coastal Japan. Sites in Area I (green

559

symbols):

560

Fukushima-1), Fukushima-daini (FSS, Fukushima-2), and Ibaraki (IB). Area

561

II (yellow symbols): Shizuoka (SZ), Ehime (EH), Saga (SG), and Kagoshima

562

(KG). Area III (blue symbols): Hokkaido (HK), Niigata (NI), Ishikawa (IS),

563

Fukui-daiichi (FKE, Fukui-1), Fukui-daini (FKW, Fukui-2), and Shimane

564

(SM). Black circles indicate nuclear power plants. Red circle indicates

565

Fukushima Daiichi Nuclear Power Plant (FDNPP).

566

Figure 2.

137

Aomori

(HG),

Miyagi

(MI),

Fukushima-daiichi

(FSN,

Cs activity concentrations in surface seawater at all sites. Vertical dashed

567

lines represent the boundary between before (1983–2010) and after (2011–

568

2016) the FDNPP accident.

569

Figure 3. Activity ratios between observed

137

Cs (Csobs) and estimated background

570

137

571

the boundary between before (1983–2010) and after (2011–2016) the FDNPP

572

accident. Dashed red lines indicate the threshold value for outliers. Note the

573

logarithmic y-axis scale for Area I. The values for the red lines at each site

574

were obtained as follows: We calculated the ratios between the measured

575

137

576

outliers was calculated as three times the standard deviation of these ratios.

577

The values associated with the red lines are listed in Table S3.

Cs (Csbkgd) in surface seawater at all sites. Vertical dashed lines represent

Cs concentrations and those estimated using Equation (1). The threshold for


Page 31 of 36


578

Table 1 Effective half-lives of 137Cs (years) in surface and bottom waters. Bottom water

Surface water Site

Effective half-life (y)

Correlation coefficient (r)

Effective half-life (y)

Correlation coefficient (r)

—

—

—

—

MI

16.0

0.94

14.3

0.93

Fukushima Dai-ichi and Dai-ni (FSN+FSS)

15.6

0.95

14.8

0.95

Ibaraki

15.8

0.94

15.2

0.94

Aomori (HG)a

Area I

(IB)

Mean ± SD

Area II

15.8 ± 0.2

Shizuoka (SZ)

18.1

0.95

16.4

0.65

Ehime (EH)

17.0

0.90

15.8

0.95

Saga (SG)

17.1

0.96

15.1

0.95

Kagoshima (KG)

18.4

0.94

16.1

0.95

Mean ± SD

Area III

17.6 ± 0.7 15.6

0.96

22.2

0.86

Niigata (NI)

16.2

0.95

23.4

0.72

Ishikawa (IS)

15.6

0.95

16.1

0.93

Fukui Dai-ichi and Daini (FKW+FKE)

17.1

0.96

17.6

0.82

Shimane (SM)

17.0

0.96

16.7

0.94

16.3± 0.7 All sites

a

15.8 ± 0.6

Hokkaido (HK)

Mean ± SD

579

14.8 ± 0.5

16.6

19.2 ± 3.4 0.99

15.3

0.80

Half-lives not calculated because of the scarcity of data from before the FDNPP accident.



580 581 582 583 584 585 586 587 588

Table 2 Annual observed 137Cs fluxes (Fluxobs) into the Japan Sea via the Tsushima Warm Current (TWC) using 137Cs data for surface and bottom waters at sites SG and KG in Area III, background 137Cs fluxes estimated by the equation (1) (Fluxbkgd), net fluxes of FDNPP-derived 137Cs (Fluxadd = Fluxobs – Fluxbkgd), annual observed 137Cs inventories (Invobs), background 137Cs inventories estimated by the fitted model (Invbkgd), and the Fukushima-derived 137 Cs inventories of the TWC in the Japan Sea (Invadd = Invobs – Invbkgd) from 2010 to 2016. Flux (×1014 Bq/y)

Sampling year

589 590 591

Page 32 of 36

Inventory (×1014 Bq)

Fluxobs

Fluxbkgd

Fluxadd

Invobs

Invbkgd

Invadd

2010

1.3

1.3

—a

0.70

0.70

—

2011*

1.4

1.3

0.1

0.88

0.66

0.22

2012

1.3

1.2

0.1

0.75

0.66

0.09

2013

1.4

1.2

0.2

0.84

0.62

0.22

2014

1.7

1.1

0.6

0.88

0.62

0.26

2015

1.9

1.1

0.8

0.92

0.57

0.35

2016

1.7

1.0

0.7

0.97

0.53

0.44

*Data for 137Cs activity concentration in surface seawater at station 1 at each site were used for the calculations. For more detail, see Table S4 in the Supporting Information. a —, Calculated values became negative.

592 593 594


Page 33 of 36


45˚ Hokkai do

Aomori

40˚

Japan Sea

Miyagi

Ishikawa

35˚

Ehime

Fukushima-1 Fukushima-2 Ibaraki

Fukui-1 Fukui-2

Shimane Saga

Niigata

Shizuoka

Kagoshima

30˚

595

25˚ 120˚

125˚

130˚

135˚

140˚

145˚

150˚

596 597 598 599

Fig 1

600



Page 34 of 36

10

Sampling depth (m) 1.0 0.5 10

HK

1.0 0.5 10

NI

0 25 50 75 100 125 150 175 ≧200

134

Cs 137 Cs 134+137

1.0 0.5 10

Cs

IS

1.0 0.5 10

HK

FKE+FKW

HG

1.0 SM 0.5 0 5 198 198

10 0 199

5 199

5 200

0 200

Year

MI

5 201 NI IS FKE+FKW 0 201

FSN FSS IB

EH KG

10

Radio Cs (mBq/L)

0.5 10

SZ

Radio Cs (mBq/L)

SZ

SG

1.0

HG

0.1 100

SM

1.0

100

10 1.0 0.1 1000

MI

100 10 1.0

FSN+FSS

0.1 1000 100 1.0 0.5 10

10

EH 1.0

IB

0.1 80 19

1.0 0.5 10

1.0

85 19

90 19

95 19

00 20

05 20

10 20

5 201

Year

SG

KG

0.5

5 0 5 0 5 0 5 0 198 198 199 199 200 200 201 201

Year

601

Fig 2


Page 35 of 36


3 2

HK

Activity ratio(Csobs /Csbkgd)

1 0 3 2

NI

1 0 3 2

IS

1 0 3 2

FKE+FKW

HK

1 0 3 2

100

1 0 19

80

HG

SM

10

MI

1

MI 19

85

19

90

19

95

00 00 5 01 0 01 5 2 2 2 20 IS Year FKE+FKW

FSN FSS

NI IB

SM

0.1 1000 100 10

SZ

SG

FSN+FSS

1 0.1 100

EH KG

Activity ratio(Csobs /Cs bkgd)

3 2

1

SZ

0.1 80

1

19

0 3 2

IB

10

4

85 990 995 000 005 010 015 19 1 1 2 2 2 2 Year

EH

1 0 3 2

SG

1 0 3 2

KG

1 0 80 985 990 995 000 005 010 015 020 2 2 2 2 2 1 1 1 19 Year

602 603

Fig. 3



Page 36 of 36

604


Appearances of Fukushima Daiichi Nuclear Power Plant-derived

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