Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991–2005

Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991–2005

Journal of Environmental Radioactivity 102 (2011) 302e310 Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal h...

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Journal of Environmental Radioactivity 102 (2011) 302e310

Contents lists available at ScienceDirect

Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991e2005 Shinji Oikawa a, *, Teruhisa Watabe a, Naohiko Inatomi b, Naohiko Isoyama a, Jun Misonoo a, Chiyoshi Suzuki a, Motokazu Nakahara a, Ryoichi Nakamura a, Shigemitsu Morizono a, Seiji Fujii a, Takeya Hara b, Katsutoshi Kido a a b

Marine Ecology Research Institute, Head Office, Research and Survey Group, Towa-Edogawabashi Bldg. 7F., 347 Yamabuki-cho, Shinjuku-ku, Tokyo 162-0801, Japan Marine Ecology Research Institute, Central Laboratory, 300 Iwawada, Onjuku-machi, Isumi-gun, Chiba 299-5105, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2010 Received in revised form 20 December 2010 Accepted 24 December 2010 Available online 12 January 2011

A radioactivity survey was launched in 1991 to determine the background levels of 239þ240Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the 240Pu/239Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of 239þ240Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the 239þ240Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of 239þ240Pu observed in the bottom water sample at sampling points having greater depth. The 240Pu/239Pu atom ratio in the seawater and sediment samples was higher than that of global fallout Pu, and comparable with the data in the other sea area around Japan which has likely been affected by close-in fallout Pu originating from the Pacific Proving Grounds. The 240Pu/239Pu atom ratio in bottom sediment samples decreased with sea depth. The land-originated Pu is not considered as the reason of the increasing 239þ240Pu concentration and also decreasing the 240Pu/239Pu atom ratio with sea depth, and further study is required to clarify it. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: 239þ240 Pu 240 Pu/239Pu atom ratio Seawater Sediment Reprocessing plant Radioactivity monitoring

1. Introduction The Japanese islands are surrounded by the Pacific Ocean, the Sea of Japan, the Sea of Okhotsk and the East China Sea, all of which differ from each other with respect to oceanographic characteristics such as current systems, expanse of water and other features. Fifty four nuclear power reactors have been established for generating electricity in Japan (JAIF, 2010) and nuclear fuel cycle facilities, including a commercial-scale spent nuclear fuel reprocessing plant, are close to being completed in Rokkasho in northern Japan. The reprocessing plant is currently being subjected to final precommission tests. The maximum reprocessing capacity of the plant is 800 t of U per year, corresponding to the amount of spent nuclear fuel produced by 40 reactors at 1 GW-class nuclear power stations, or 80% of the spent nuclear fuel produced annually in Japan (JNFL, 2009). In Japan, all nuclear power stations are constructed near the coast so that they can use seawater as a cooling medium. The

* Corresponding author. Tel.: þ81 3 5225 3051; fax: þ81 3 5225 3050. E-mail address: [email protected] (S. Oikawa). 0265-931X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2010.12.007

reprocessing plant is also located near the coast into which it releases low-level radioactive liquid waste under controlled conditions. Since the sea is an important resource and is used by people engaged in fisheries, continuous monitoring is necessary in order to ensure the safety of the marine environment and fishery resources, and also to prepare for the unexpected release of radioactivity into the sea. As a result of global fallout from atmospheric nuclear weapon testing, long-lived radionuclides, such as 90Sr, 137Cs and Pu isotopes were distributed worldwide. These anthropogenically produced radionuclides persist in the natural environment and are also important targets in environmental radioactivity monitoring, particularly in the vicinity of nuclear fuel reprocessing plants. A comprehensive understanding of background radionuclide levels is therefore necessary before full operation of the reprocessing plant, which is why our environmental radioactivity survey program was planned and initiated in 1991. Approximately 11 PBq of 239þ240Pu is considered to have been deposited on the oceans as global fallout from the 543 atmospheric nuclear weapon tests undertaken since 1945 (UNSCEAR, 2000). Numerous studies have been made to

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determine the levels and inventory of 239þ240Pu in the ocean. In recent years, not only 239þ240Pu concentrations but the 240 Pu/239Pu atom ratio has become increasingly important in monitoring programs and oceanographic investigations, because the atom ratio can be used as a fingerprinting tool for identifying the origins and behavior in the ocean (Buesseler, 1997; Kim et al., 2003, 2004; Muramatsu et al., 2001; Oikawa and Yamamoto, 2007; Yamada et al., 2006, 2007; Zheng and Yamada, 2004, 2005a, 2006). Misonoh et al. (2006) had already summarized and outlined the data on 239þ240Pu radioactivity concentrations in seawater and bottom sediment samples collected during 1991e2004 in this monitoring program still in progress. In this paper, the results of long-term observations of 239þ240Pu, 90Sr and 137Cs in seawater and bottom sediment samples collected from 1991 to 2005 in the ocean near the reprocessing plant were reported in detail. Emphasis was placed on describing the spatiotemporal distribution of Pu isotopes and 240Pu/239Pu atom ratio in the area of interest. 2. Materials and methods 2.1. Sampling

303

samples had Pu concentrations that were less than the detection limit of alpha-ray spectrometry, respectively.

2.3.

240

Pu/239Pu atom ratio measurement by SF-ICP-MS

After measuring radioactivity, Pu on the stainless steel disk was leached by immersion in diluted HF þ HNO3 solution for a few minutes. After evaporating to dryness, Pu in the residues was again purified by passing through an anion exchange resin column. Finally, eluate was evaporated to dryness, dissolved in 4 M CH3COOH, and then passed through an anion exchange resin column (Dowex 1-X8, 100e200 mesh, 2 ml volume). The eluate was evaporated to dryness and dissolved in approximately 10 ml of 1 M HNO3. A single collector, sector field ICP-MS (SF-ICP-MS, Finnigan ELEMENT 2, Thermo Electron, Inc., Germany) was used to measure Pu isotopes under the low resolution mode in order to achieve high instrument sensitivity and low background. An important procedure in determining Pu isotopes by SF-ICP-MS was in elimination of the interference of 238U1Hþ ion on 239Pu peak area (Kim et al., 2007; Pointurier et al., 2008; Yamamoto et al., 2002). The influence of U on the m/z 239 mass peak intensity was corrected by using a U standard solution. The U of 0.1 ng ml1 in the final solution was equivalent to approximately 5 fg ml1 of 239Pu. In addition, prior to use the 242Pu tracer solution, the interference and down-mass tailing of 242Pu tracer (including 242Pu itself and the other impurities) on m/z 239, 240 and 242 resulting from polyatomic ions of lead isotopes such as 204Pb (204Pb35Clþ, 204 Pb36Arþ), 206Pb (206Pb36Arþ) and 207Pb (207Pb35Clþ) was checked by the blank test. Details of the operating conditions were described in another paper (Yamamoto et al., 2008).

The radioactivity surveillance program was implemented in the sea area, Aomori Pacific coastal region, adjacent to the reprocessing plant, Rokkasho, Aomori Prefecture. Sixteen sampling points were selected for fixed point observation, as shown in Fig. 1 and Table 1. At each sampling point, a seawater sample was collected semiannually in MayeJune and October. A bottom sediment sample was collected at the same point semiannually (MayeJune and October) and annually (MayeJune), during 1991e1995 and 1996e2005, respectively. Seawater was taken from two layers, namely surface and bottom layer. Although it would be difficult to define “bottom water” strictly from the view point of oceanography, the term was used in the present study for the water which was obtained from 10 to 30 m above the sea bed just for convenience. The surface water was taken using an electric pump directly from the sea surface, while the bottom water was taken using a Van Dorn type large volume water bottles (Rigo, Co. Ltd., Tokyo, Japan) and/or Niskin-X sampling bottles water sampling system with multi arrays (General Oceanics, Inc., Florida, USA). Both samplers were equipped with a conductivity temperature depth profiler (CTD) system (SBE 9plus, Sea-Bird Electronics, Inc., Washington, USA) and single beam echo sounder (Simrad EA-600, Kongsberg Maritime GmbH, Germany) for the measurement of sea depth. A sample of 260 l of seawater was collected at each point, bottled in 13  20 l polypropylene bottles to which 20 ml of concentrated hydrochloric acid was added to maintain acidic conditions to preserve the samples. Bottom sediment samples were collected by a box-type sampler, which removed a 40 cm  40 cm area of sediment on the ocean floor without disturbing the sediments. The upper 3 cm of the sediments were taken for analysis and frozen immediately in a freezer on board. After the samples were transferred to the laboratory, they were dried at 105  C, ground using a mortar, passed through a sieve screen (<2 mm), mixed well, and then pulverized again to homogeneous powder in a top grinder.

2.2. Pu measurement by alpha-ray spectrometry (seawater and bottom sediment) Plutonium analysis was performed using 100 l seawater sample after an addition of Fe3þ carrier and a known amount of 242Pu tracer supplied by National Institute of Standards and Technology, USA as Standard Reference Material 4334G (approx. 0.2 ng per sample). After the Pu in the sample was separated by using Fe(OH)3 coprecipitation and an anion exchange technique, it was electrodeposited on a stainless steel disk. A 50 g aliquot of dried bottom sediment sample was used for extracting Pu with 8 M nitric acid with a several drops of H2O2 after the addition of a known amount of 242 Pu tracer (approx. 0.2 ng per sample). The Pu in the extracted fraction was purified using an anion exchange resin column and electrodeposited on a stainless steel disk as described above. An alpha-ray semiconductor detector (Model U-020-450-AS series, Ortec Ultra Si, Ortec Products Group, Ametk Inc., Oak Ridge, Tennessee, USA; 450 mm2 active areas) was used to measure 239þ240Pu radioactivity. When the detection limit of Pu was taken as three times the fluctuation inherent in counting, the minimum levels of Pu corresponded to 0.007 mBq L1 and 0.03 Bq kg1-dry for 160,000 s counting of seawater sample, and 80,000 s counting of bottom sediment sample, respectively. Mean chemical recovery of 242Pu tracer exceeded 80% both for 960 seawater samples and 320 bottom sediment samples, respectively. Of these, 87 and 12

Fig. 1. Map showing seawater and bottom sediment sampling points (1e16, total 16 points) off the spent nuclear fuel reprocessing plant (Rokkasho, Aomori Prefecture) in the North Pacific Ocean. (1) Sado, (2) Ajigasawa and (3) Tomari are the sampling locations in Yamada et al. (2007), and (4) Sagami Bay is the locations in Zheng and Yamada (2004).

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Table 1 Location of sampling point off the reprocessing plant, Rokkasho, Aomori Prefecture, Japan. Sampling point

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a b c

Positiona

Sea depthb (m)

N

E

40 300 40 300 40 300 40 450 40 450 40 450 40 540 40 540 40 540 40 540 41 000 41 020 41 020 41 160 41 160 41 260

141 450 141 550 142 050 141 300 141 450 142 000 141 300 141 450 142 000 142 100 141 300 141 450 142 000 141 350 142 000 141 400

Qualitative specification of the sea bed

70 100 280 50 110 300 170 300 640 960 330 520 960 610 1100 740

JIS notation and color namec

Sediment classification

7.5Y3/1, olive black 10Y3/1, olive black 2.5GY3/1, dark olive 10Y3/1, olive black 10Y3/1, olive black 2.5GY3/1, dark olive 2.5GY3/1, dark olive 2.5GY3/1, dark olive 7.5Y3/2, olive black 5Y4/2, grayish olive 2.5GY3/1, dark olive 7.5Y3/2, olive black 7.5Y3/2, olive black 7.5Y3/2, olive black 10Y3/2, olive black 7.5Y3/2, olive black

Medium sand, mud Medium sand, coarse sand Medium sand, coarse sand Medium sand, mud Coarse sand, meduim sand Medium sand, coarse sand Medium sand, coarse sand Mud, medium sand Mud Mud Medium sand, mud Mud Mud Mud, medium sand Mud Mud, medium sand

gray

gray gray gray

gray

Represented by WGS84 (World Geodetic System 84). Most recent data on MayeJune 2007 (a significant figure: 2 digit). Munsell notation and translated color names obtained from the Revised Standard Soil Color Charts, Research council for Agriculture, Forestry and Fisheries, JAPAN, 1967.

To confirm the accuracy of the determination, 239þ240Pu concentration and Pu/239Pu atomic ratio were analyzed for certified reference materials of IAEA-135 (Irish Sea sediment) and IAEA-SOIL-6. Analytical results agreed well with those reported by Muramatsu et al. (1999) and Miura et al. (2001), showing that the present procedure was acceptable and sufficient enough for accurate measurement of Pu isotope by SF-ICP-MS. The detection limit of Pu was approximately 1 fg ml1 in the final solution, which is comparable with that reported in the literature (Pointurier et al., 2008). When 50 g of dried sediment sample was used, the detection limit was 0.003 Bq kg1 and 0.010 Bq kg1 for 239Pu and 240Pu, respectively.

240

2.4. Successive radioanalytical procedures for determination of seawater samples

90

Sr and

137

Cs in

3. Results and discussion 3.1.

239þ240

Pu,

90

Sr and

137

Cs concentrations in seawater samples

The results of 239þ240Pu measurements for the surface waters obtained by alpha-ray spectrometry are shown in Fig. 2. The concentration of 239þ240Pu in the surface water was relatively uniform in the study area, and it appeared to decrease with time on the average in spite of an appreciably wide range of areal variation from 14% to 33% in terms of coefficient of variation (Supplement A). Excluding 239þ240Pu concentrations less than the detection limit, the

A volume of 50 l of seawater sample was used for the successive radiochemical analysis of 90Sr and 137Cs. Cesium-137 was separated with co-precipitation with ammonium phosphomolybdate (AMP), and then purified with a cation exchange column. After 137Cs was precipitated as Cs chloroplatinate, its radioactivity determined using a gas-flow type low background anti-coincidence beta counter (LBC-471Q, Aloka Co. Ltd. Japan). Strontium-90 was separated from the supernatant of the co-precipitation procedure with cation exchange resin columns. The radioactivity of 90Sr was determined by measuring beta-rays emitted from 90Y at equilibrium with 90Sr using the gas-flow type counter. Mean chemical recovery of both Sr and Cs exceeded 70%. The detection limit of 90Sr and 137Cs radioactivity, taken as sum of measurement result and three times of its counting statistics error, was 0.4 mBq L1 with 3600 s counting and 0.5 mBq L1 with 5400 s counting for a seawater sample of 50 L, respectively.

Fig. 2. The time course of the 239þ240Pu concentration in the surface waters obtained by the alpha-ray spectrometry. A thick broken line shows an exponential regression line with a half-life of 17.8 y excluding the data less than the detection limit.

Fig. 3. Time-dependent decrease in the radionuclide concentration in surface water samples for (a) 90Sr and (b) 137Cs. A thick broken line shows an exponential regression line with a half-life of 16.7 y and 16.1 y in (a) and (b), respectively, excluding the data less than the detection limit.

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apparent half-life was estimated to be 17.8 y. A similar timedependent decline was observed in the concentrations of 90Sr and 137 Cs in surface water (Fig. 3a and b) with the estimated apparent half-lives of 16.7 y and 16.1 y for 90Sr and 137Cs, respectively. However, the environmental half-lives of 17.8 y, 39.8 y and 34.8 y respectively for 239þ240Pu, 90Sr and 137Cs after eliminating physical decay rate showed more effective removal of Pu isotopes from the surface water than 90Sr and 137Cs. Miyao et al. (1998) reported that the apparent half-life of 137Cs in the surface water of the Japan Sea was estimated to be about 16 y for long time scale transport and no systematic temporal variation of 239þ240Pu was observed for past two decades. Hirose and Aoyama (2003) analyzed the geographical and temporal changes of 137Cs and 239þ240Pu concentrations observed in surface waters of the Pacific Ocean during the period from 1965 to 1998 and determined their apparent half-lives. The estimates of the apparent half-life was 8 y and 15 y respectively for 239þ240 Pu and 137Cs for the mid-latitude region of the western North Pacific Ocean and the apparent half-lives of 239þ240Pu were generally shorter than those of 137Cs in all the regions of the Pacific Ocean. They concluded that the biochemical processes could be attributed to shortening the apparent half-life of 239þ240Pu in addition to the physical and physical oceanographic processes. They also pointed out the possibility that the decrease rate of Pu in seawater could lessen with time due to continued mixing and the relatively slow particle removal. The longer apparent half-life derived for 239þ240Pu in the present study than that in the previous study may have resulted from the calculation using only more recent data. In the most recent five years (2001e2005), 239þ240Pu concentration in the surface water apparently reached the constant level ranging from 0.0028 to 0.010 mBq L1 with the average value of

Fig. 4. Spatial distributions of the most recent five years (2001e2005) mean concentrations of 239þ240Pu in surface water samples.

305

0.0053  0.0008 mBq L1 (see Fig. 2). A sampling point dependent Pu concentration averaged over these 5 y is shown in Fig. 4. Inatomi et al. (2000) proposed a method to judge from which current system, the Oyashio Current or the Tsugaru Current, the seawater samples collected off the Shimokita Peninsula originated from the relationship between their temperature and salinity. It could be concluded that the Aomori Pacific coastal region was directly affected by the Tsugaru Current originating from the Tsushima Current through the Japan Sea as a branch of the Kuroshio Current. They revealed that the seawater samples mostly originated from the Tsugaru Current and hence had similar concentrations both of 90Sr and 137Cs to those in that current system. However, they sometimes found the different water mass, which had properties characterizing by lower temperature, lower salinity and lower radioactivity concentration. Such water mass could be attributed to intermixture of two different current systems, namely the Oyashio Current and Tsugaru Current. Uniform distribution of 239þ240 Pu concentration as shown in Fig. 4 suggested that the surface water was solely under the effect of the Tsugaru Current prevailing in the sea area, if the ratio of 239þ240Pu to 90Sr or 137Cs was constant between both currents. The 239þ240Pu concentrations in surface water observed in the present study were as the same level as the previously reported values for the samples taken from 239þ240

Fig. 5. The time course of the 239þ240Pu concentration in bottom water samples for each of four groups into which samples were classified by depth at sampling. (a) samples taken from the layer between 50 and 200 m in depth, (b) those from the layer between 200 and 400 m, (c) those from the layer between 500 and 800 m, and (d) those from the layer deeper than 900 m.

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the same area, for instance, 0.0046e0.0088 mBq L1 for the samples collected during 1999e2000 (Kondo et al., 2003), and 0.0076e0.0078 mBq L1 for the samples collected during 1991e1993 (Yamada and Zheng, 2008). The 239þ240Pu concentration showed larger variations in bottom waters than in surface waters, reflecting the difference in the depth at which seawater samples were taken. It was not so easy to identify, therefore, any time-dependent trend in the values as a whole. When the bottom water samples were classified into some groups of subsamples by the depth at sampling, the fate of Pu isotopes in the corresponding layer could be made clear more easily. Fig. 5(a)e(d) showed the time course of 239þ240Pu concentration in bottom waters for samples taken from the layer of 50e150 m, 200e400 m, 500e800 m and >900 m, respectively. A quite slow decrease of the

239þ240 Pu concentrations was observed with the lapse of time in the shallower three sub-samples, whereas any marked change did not appear in the deepest sub-sample. Furthermore, the regression curve in the third sub-sample (500e800 m) had a significantly slower slope than those for the upper two layers (SmirnoveGrubbs test). The environmental half-lives of 239þ240Pu from the regression curves were calculated to be 22.5 y, 18.9 y and 61.2 y respectively for the layers of 50e150 m, 200e400 m and 500e800 m in depth. Therefore, the 239þ240Pu concentration decreased more gradually in deeper layer, and that in the deepest layer retained the constant level. Fig. 5(a)e(d) also show generally higher concentrations in the bottom water samples from deeper sampling points. It was reported that the 239þ240Pu concentration in the subsurface waters 500e1000 m in depth were higher than the surface waters in the

Fig. 6. Scatter diagrams of longitudinal 90Sr, 137Cs and 239þ240Pu concentrations survey from surface and bottom water samples during 1991e2005. (a) 90Sre137Cs in surface water samples, (b) 90Sre137Cs in bottom water samples, (c) 90Sre239þ240Pu in surface water samples, (d) 90Sre239þ240Pu in bottom water samples, (e) 137Cse239þ240Pu in surface water samples and (f) 137Cse239þ240Pu in bottom water samples.

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open sea far from the land (Hirose, 2009). The higher 239þ240Pu concentrations in the deeper waters observed in the present study might reflect a vertical distribution of the nuclides, if the behavior of Pu isotopes in the coast of the sea principally did not differ from that in the open sea. The relations between the concentrations of 239þ240Pu, 90Sr and 137 Cs for surface and bottom waters would make it possible to compare the behavior of these radionuclides in seawater. As shown in Fig. 6(a) and (b), a proportional relationship was found between 90 Sr and 137Cs concentrations both in the surface and bottom water with the respective estimates of the 137Cs/90Sr activity ratio being 1.4 and 1.5. The 137Cs/90Sr ratio obtained in the present study was comparable with the value of 1.4, which was obtained at various sites around Japan (<200 m) (Inatomi and Kido, 2006). The behavior of 90Sr and 137Cs was similar in both the surface and bottom waters in the sea. There was no clear relationship between 239þ240Pu and 90Sr concentration in surface water (Fig. 6c), nor was there between 239þ240 Pu and 137Cs concentration in it (Fig. 6e), as supposed by general understanding that the behavior of Pu was completely different from that of Cs and Sr in ocean. However, the 239þ240Pu concentration was clearly inversely proportional to both 90Sr (R ¼ 0.71) and 137Cs (R ¼ 0.54) concentrations in bottom water as shown in Fig. 6(d) and (f), respectively. This was due to a reverse relationship in the vertical distribution between 239þ240Pu concentration and others; namely the former increased with increasing depth as mentioned above, whereas the latter decreased with increasing depth. Such reverse distribution could be caused by the difference of downward transportation mechanism between 239þ240 Pu and 90Sr (or 137Cs). Nozaki and Alibo (2003) explained transportation of rare earth elements in the seawater column in terms of scavenging by partially dissolved detrital particles originated primarily from rivers, and assigned the same mechanism to downward transportation of Pu. However the Pu in the river runoff was known to be relatively small in amount and to be adsorbed on to soil particles of suspended materials so tightly that it could not be reached out to river water (Hirose and Sugimura, 1981; Hirose et al., 1990). Scavenging Pu with detrital particles would be one of the probable driving forces in the transportation of Pu, although fresh water inflow from Mabechi River (142 km in length and 2050 km2 of catchment area) and Oirase River (67 km in length and 820 km2 of catchment area) into the target sea area (Fig. 4) was not so much. Broecker and Peng (1982) reported that the depth profile of Pu was similar to those of the constituents such as nitrate (NO 3 ), phosphate (HPO2 4 ) and silicate (H4SiO4). However, the depth of the maximum concentration of Pu generally does not coincide with those of nutrients. Actually, Livingston et al. (2001) reported that the Pu maximum layer has moved downward, whereas the maximum depth of nutrients shows no trend. There should be some biological mechanism to participate in the downward transport of Pu isotopes. The Oyashio Current flows from the northern sea areas, joins the Tsugaru Current off the coast of the Shimokita Peninsula and supplies a great quantity of planktons to the sea area of interest. The fate of particles produced by plankton would participate in the vertical transportation of Pu in the coastal sea. There was thus likely to be some inorganic and biological mechanism governing the downward transport of 239þ240Pu, which brought about higher radioactivity concentration of 239þ240Pu with increasing depth in the bottom water. 3.2. The concentration of

307

data were classified into a set of sub-data according to the depth of sampling as in the case of the data on seawater sample, the timedependent change of 239þ240Pu concentration became visible more clearly as shown in Fig. 7(a)e(d). It seemed that the level of 239þ240 Pu concentration was kept nearly constant at all sampling points throughout the period of survey, although the level was relatively low at the shallow sampling points (Fig. 7a and b) and higher at the deeper sampling points (Fig. 7c and d). The concentrations of 239þ240Pu averaged over the survey period ranged from 0.48 Bq kg1-dry (point 6) to 4.4 Bq kg1-dry (point 16) with appreciably wide variation from 8.0% to 20% in terms of coefficient of variation as shown in Supplement C. The 239þ240Pu concentrations in the present study were as the same level as the previously reported values; w1e8 Bq kg1-dry in Sagami Bay (Zheng and Yamada, 2004) and 1.7e2 Bq kg1-dry in the Northwest Pacific Ocean (Zheng and Yamada, 2006). The sampling points 1 to 8 and 11 composing Fig. 7(a) and (b) were all shallower than 400 m and located on the continental shelf comparatively well developing north-eastward from Hachinohe to offshore the south of the Shimokita Peninsula. On the other hand, sampling points 9, 10 and 12 to 16 composing Fig. 7(c) and (d) were deeper than 500 m and located to the east of the Shimokita Peninsula, which had a steep side falling directly to the sea bed. The spatial distribution of the 239þ240Pu concentration in bottom

239þ240

Pu in bottom sediment samples

The 239þ240Pu concentration in bottom sediment samples varied widely among the sampling points even in the same year. When the

Fig. 7. The time-dependent change of 239þ240Pu concentration in bottom sediment samples. The data was classified into a set of sub-data according to the depth of sampling as in the case of the data on seawater (bottom water) sample.

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sediment samples averaged over the recent 5 y is shown in Fig. 8. The bottom sediment samples collected at the sampling points shallower than 400 m consisted mainly of sand having relatively large particle size, while those collected at the sampling point deeper than 400 m were composed of argillaceous mud having small particle size. The different of particle size may be one of cause of distribution of 239þ240Pu concentration in the sediment samples. 3.3. 240Pu/239Pu atom ratio observed in seawater and bottom sediment samples Table 2 shows the results of measurements of the 240Pu/239Pu atom ratio along with the 239þ240Pu concentration in seawater samples taken at the sampling point 9 in both seasons in 2004. The 240 Pu/239Pu atom ratio is a good indicator for identifying the source of Pu contamination and would become an issue of great interest in radioactivity monitoring program in the near future. Recent studies showed that 240Pu/239Pu atom ratio in seawater and bottom sediment samples off the coast of Japan was slightly higher than that of global fallout Pu (0.176  0.014) reported by Krey et al. (1976). Muramatsu et al. (2001) and Yamada et al. (2007) reported that this higher atom ratio was likely due to close-in fallout Pu originating from the Pacific Proving Grounds (PPGs). For example, the 240 Pu/239Pu atom ratio in soil from Bikini Island was reported to be 0.338  0.0051 (Komura et al., 1984). The 240Pu/239Pu atom ratio could be lowered by mixing in the sea with Pu isotopes which were deposited directly from the atmospheric environment and transported by rivers and winds through weathering processes from the land (Kim et al., 2003, 2004; Okubo et al., 2008; Yamada and

Table 2 239þ240 Pu concentrations and 240Pu/239Pu atom ratios in seawater samples (Sampling point 9) determined using SF-ICP-MS (2004). Year and perioda

Layer

Sampling depth (m)

239þ240

2004-F

Surface Bottom Surface Bottom

1 628 1 610

0.00598 0.0229 0.00497 0.0219

2004-L

Pub (mBq L1)    

240

Pu/239 Pu atom ratioc 0.00046 0.0012 0.00060 0.0007

0.245 0.249 0.243 0.235

   

0.044 0.029 0.065 0.014

a

F; First half of the year and L; Last half of the year. Obtained by isotope dilution method. Error shows one standard deviation for three time measurements. c Error shows one standard deviation for three acquisition replicates (total 300 passes scans). b

Nagaya, 2000; Yamada et al., 2007; Zheng and Yamada, 2004, 2005a, 2006). Yamada and Zheng (2008) reported that the mean value of 240Pu/239Pu atom ratio in surface waters from the western North Pacific Ocean and Japan Sea was 0.227  0.006 and that the contributions of the PPGs close-in fallout could be estimated as 33% on average and remaining 67% represented the contribution from the global fallout. They also summarized the 240Pu/239Pu atom ratio observed in the surface waters around the Japanese Islands and gave the values of 0.221  0.029, 0.221  0.019 and 0.235  0.023 along the Tsushima Current near the Sado Island in the sea of Japan, in the coast off Ajigasawa on the northeast of the Sea of Japan and in the coast off Tomari in the western North Pacific, respectively (Fig. 1). Furthermore, no temporal or spatial variations of 240 Pu/239Pu atom ratios in water column of the Sea of Japan were observed in 1984 and 1993 (Yamada and Zheng, 2010). The present data of 240Pu/239Pu atom ratio in the seawater samples were comparable with those reported so far and nearly identical irrespectively of the depth of sampling point, although the 239þ240Pu concentration was factors of magnitude higher in bottom water than in surface water. Table 3 shows the 240Pu/239Pu atom ratio as well as the concentration of 239þ240Pu for bottom sediment samples collected at 16 sampling points in 2004, and Fig. 9 schematically represents the spatial distribution of the 240Pu/239Pu atom ratio. In general, the 240 Pu/239Pu atom ratio was not always distributed uniformly and was dependent upon the depth of sampling point, namely, the

Table 3 239þ240 Pu concentrations and 240Pu/239Pu atom ratios in bottom sediment samples determined using SF-ICP-MS (2004).

Fig. 8. The spatial distributions of the 239þ240Pu concentration in bottom sediment samples averaged over the most recent five years (2001e2005).

Sampling point

Sea depth (m)

239þ240

Pua (Bq kg1-dry)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

71 107 283 48 112 313 169 311 655 962 326 524 957 597 1042 731

0.912 0.565 0.591 0.546 0.588 0.485 0.665 0.744 2.81 3.93 0.572 2.21 4.62 2.02 4.00 3.85

               

0.019 0.004 0.012 0.007 0.005 0.007 0.009 0.009 0.02 0.10 0.009 0.03 0.07 0.02 0.07 0.05

240

Pu/239Pu atom ratiob 0.248 0.235 0.230 0.246 0.240 0.228 0.242 0.224 0.221 0.218 0.233 0.221 0.221 0.228 0.219 0.221

               

0.001 0.004 0.007 0.003 0.003 0.006 0.001 0.005 0.002 0.0004 0.003 0.002 0.0003 0.001 0.001 0.001

a Obtained by isotope dilution method. Error shows one standard deviation for three time measurements. b Error shows one standard deviation for three acquisition replicates (total 300 passes scans).

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ratio in seawater samples of the Tsugaru Current, and also in the shallow sediment samples under the current, would therefore be basically equal to the values commonly observed in those in the seas around Japan. On the other hand, the coastal area adjacent to the Shimokita Peninsula is subject to inflows of land-source Pu from rivers, which generally has lower 240Pu/239Pu atom ratios than those in seawater. The land-source Pu would be readily scavenged both with inorganic particulates and with co-existing biogenic particulates produced by a large population of plankton in the study area. Ohtsuka et al. (2004) reported that the 240Pu/239Pu atom ratios (0.18  0.04) in agricultural fields near the sea of concern in this study were similar to that of the global fallout (0.176  0.014). Zheng and Yamada (2005b) reported that the 240Pu/239Pu atom ratios (0.186  0.016) in sediments collected from the Lake Obuchi, Rokkasho were also similar to that of global fallout. Ueda et al. (2009) reported that the 240Pu/239Pu atom ratio in sediments in brackish lakes adjacent to the sea ranged from 0.17 to 0.20, and were similar to the average value of global fallout (0.176  0.014). Those data show that Pu in land of Shimokita Peninsula is originated from global fallout and has lower 240Pu/239Pu atom ratio than that in the present sediment samples. The increasing 240Pu/239Pu atom ratio with sampling depth is not caused by the effect of land-originated Pu. Further study including analysis of Pu in seawater samples in the Oyashio Current, is required to elucidate the pattern of Pu concentration and 240 Pu/239Pu atom ratio shown in Fig. 10. 4. Conclusions

Fig. 9. The spatial distributions of 240Pu/239Pu atom ratio in the bottom sediment samples collected in the first half of the year, 2004.

lower 240Pu/239Pu atom ratios were observed in the samples at deeper sampling points. Fig. 10 shows Pu radioactivity concentrations in bottom sediment samples along with their 240Pu/239Pu atom ratio as a function of the depth of the sampling point. The higher concentration of 239þ240Pu was found in the sediment samples at deeper sampling points, and vice versa for 240Pu/239Pu atom ratio. As described before, the Tsugaru Current, which is a branch of the Tsushima Current originated from the Kuroshio Current, is a major current off the Shimokita Peninsula. The 240Pu/239Pu atom

An environmental survey of 239þ240Pu, 90Sr and 137Cs was conducted in the coastal sea adjacent to the Rokkasho Reprocessing Plant in Aomori Prefecture from 1991 to 2005 in order to determine their background levels before full operation of the plant. The mean 239þ240Pu concentration in surface water decreased slowly over time. The apparent half-life for 239þ240Pu was 17.8 y, which is comparable with those of 90Sr and 137Cs (16.7 y and 16.1 y). The slower decrease of the 239þ240Pu concentration was observed in the deeper water, and the 239þ240Pu concentration reached the constant level in the deepest layer (>900 m). The 240Pu/239Pu atom ratio ranged from 0.235 to 0.249 and from 0.218 to 0.248 respectively for seawater and bottom sediment samples, which could be regarded as the background levels in the area of interest. The concentration of 239þ240Pu in bottom sediments was lower at the shallower sampling points. The 240Pu/239Pu atom ratio in the sediments decreased with the sea depth at the sampling point. The reason of the variation of the 240 Pu/239Pu atom ratio and the 239þ240Pu concentration with sea depth is unknown, and further study is required to clarify it. Acknowledgements

Fig. 10. 239þ240Pu concentrations in bottom sediment samples along with their 240 Pu/239Pu atom ratios as a function of the depth of the sampling point.

This environmental radioactivity survey project is part of contract from the Ministry of Education, Culture, Sports, Science and Technology, Tokyo, Japan. The authors express appreciation to the captain and crew of the ships for sample collection. The authors gratefully acknowledge Mr. Katsuhiko Yoshida for his helpful advice and discussions, and Ms. Naoko Suzuki for her assistance with the research. Thanks are due to Messrs Hideo Matsuda, Gou Abe and Kenji Kaneko for their technical assistance with plutonium measurement. They also would like to thank Dr. S. Hisamatsu and the two anonymous reviewers for useful comments on the manuscript.

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