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Temporal and spatial variations of anthropogenic radionuclides in Japan Sea waters
ARTICLE IN PRESS
Deep-Sea Research II 50 (2003) 2701–2711
Temporal and spatial variations of anthropogenic radionuclides in Japan Sea waters Toshimichi Itoa,c,*, Pavel P. Povineca, Orihiko Togawaa,c, Katsumi Hiroseb a International Atomic Energy Agency—Marine Environment Laboratory, MC 98000, Monaco Japan Meteorological Agency—Meteorological Research Institute, Tukuba-shi, Ibaraki 305-0052, Japan c Marine Research Laboratory, Japan Atomic Energy Research Institute, Mutsu-shi, Aomori 035-0064, Japan b
Abstract 90
Sr, 137Cs and 239+240Pu data covering three decades were examined for temporal and spatial variations of their concentrations and inventories in waters of the Japan Sea/East Sea. The time trend of radionuclide concentrations in surface water showed a gradual decrease over the whole period and the effective half-lives of 90Sr, 137Cs and 239+240Pu were estimated to be about 15, 20 and 15 years, respectively. The averaged concentrations of 90Sr, 137Cs and 239+240Pu for the year 2000 were estimated to be 1.6, 2.8 Bq m3 and 6.6 mBq m3, respectively. 90Sr and 137Cs concentrations in the south of the Japan Sea were higher than the concentrations in the north, whereas 239+240Pu showed a reverse pattern. The radionuclide concentrations decreased with time in the surface layer and increased in the subsurface layer for 90Sr and 137Cs, or increased below the subsurface layer for 239+240Pu. The averaged inventories of 90Sr, 137Cs and 239+240 Pu in Japan Sea waters in the 1990s were estimated to be 1.8 , 3.1 kBq m2 and 63 Bq m2, respectively, and were 30–40% higher than in the Pacific Ocean. The area of high inventory intruded from the north of the Japan Sea into the south, and the low inventory region appeared in the central part of the Sea around the Yamato Rise. Distinctive changes and distributions of radionuclides found in this study are explained by the geographic and oceanographic characteristics of the Japan Sea. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction The Japan Sea/East Sea is a marginal sea of the western North Pacific Ocean with the fourth largest surface area (1.01 106 km2) in the world (Fig. 1). The mean depth of the Japan Sea is 1350 m, and the maximum depth is 3796 m (NAS, *Corresponding author. Present address: Japan Atomic Energy Research Institute, Tokar-mura, Naka-gun, Ibaraki 319-1195, Japan. Tel.: +81-29-282-5171; fax: +81-29-2826760. E-mail address: [email protected] (T. Ito).
2001). The bottom topography of the Japan Sea is bowl like and separated into three basins: the Japan Basin in the north, the Yamato Basin in the southeast, and the Tsushima Basin in the southwest. The Yamato Rise, with a maximum height above sea level of about 250 m, is in the center of these basins. The Japan Sea is almost enclosed, surrounded by Sakhalin Island, the Asian continent, the Japanese Islands, and isolated from the Pacific Ocean and adjacent marginal seas except four narrow and shallow straits—the Tsushima Strait, the Tsugaru Strait, the Soya Strait, and the Mamiya Strait—with sill depths shallower than
0967-0645/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0967-0645(03)00142-5
ARTICLE IN PRESS T. Ito et al. / Deep-Sea Research II 50 (2003) 2701–2711
2702 55
MY
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25 20 120
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Fig. 1. Map of the Japan Sea. ‘‘TS’’, ‘‘TG’’, ‘‘SY’’ and ‘‘MY’’ indicate the Tsushima Strait, the Tsugaru Strait, the Soya Strait, and the Mamiya Strait, respectively.
200 m. Seawater is carried from the Pacific Ocean through the East China Sea over the sill of the Tsushima Strait into the surface layer of the Japan Sea by the Tsushima Warm Current (TWC), a branch of the Kuroshio Current. In the Japan Sea, the TWC flows northeastward along the west coast of the Island of Honshu with many meanders and eddies (Isoda, 1994; Hase et al., 1999). A subarctic front is formed between the subtropical waters carried by the TWC and subarctic waters at around 40 N (Senjyu, 1999). Surface seawater exits the Japan Sea to the Pacific Ocean through the Tsugaru Strait (‘‘TG’’ in Fig. 1) and to the Sea of Okhotsk through the Soya Strait (‘‘SY’’ in Fig. 1). Because of the shallowness of the Straits, the exchange of seawater is limited to the surface layer. The deep water of the Japan Sea is known as the Japan Sea Proper Water (JSPW) (Uda, 1934) and characterized by relatively low temperatures, 0.0–1.0 C, relatively low and homogenous salinity, 34.04–34.10 PSU, and a high concentration of dissolved oxygen, 220–250 mmol kg1 (Harada and Tsunogai, 1986; Miyao et al., 1998). Senjyu and Sudo (1994) suggested that the upper portion of JSPW is formed by wintertime convection in the region west of 136 E between 40 and 43 N. Anthropogenic radionuclides in the Japan Sea have several sources. The main source is global
fallout delivered from atmospheric nuclear tests. An additional source of anthropogenic radionuclides is the Chernobyl accident in 1986 (UNSCEAR, 2000). Further, radioactive wastes were dumped in the Japan Sea by the Russian Federation and former Soviet Union. The IAEA (1999a) reported that about 444 TBq of liquid radioactive wastes and about 307 TBq of solid-state wastes had been disposed in the Japan Sea. Long-lived anthropogenic radionuclides, such as 90 Sr, 137Cs and 239+240Pu, have been widely used as radioactive traces to investigate transport processes in the Japan Sea, to study processes in the water column, scavenging and sedimentation processes, etc. (e.g., Hirose, 1997; Yamada et al., 1996; Miyao et al., 1998, 2000; Hirose et al., 1999; Ikeuchi et al., 1999; Hirose et al., 2001). The main purpose of this paper is to elucidate the temporal variations of concentrations and inventories of 90Sr, 137Cs and 239+240Pu in Japan Sea waters by examining the data extracted from the GLOMARD (IAEA, 2000) and HAM (Hirose and Aoyama, 2003) databases.
2. Data The data on 90Sr, 137Cs and 239+240Pu concentrations in seawater of the Japan Sea used in this study were extracted from the Global Marine Radioactivity Database (GLOMARD), constructed by International Atomic Energy Agency—Marine Environment Laboratory, Monaco (IAEA, 2000) and the Historical Artificial Radionuclides in the Pacific Ocean and its Marginal Seas database (HAM) established by the Japan Meteorological Agency—Meteorological Research Institute (Hirose and Aoyama, 2003); see also http://www.mrijma.go.jp. More recent data were also extracted from the literature (Kang et al., 1997; Kim et al., 1997; Yamada et al., 1996). Data were rejected if the uncertainty was three times greater than the value or no uncertainty was reported. Further, the data set for multi-layer sampling was not used in the inventory calculation if less than five layers were sampled from 0 to 2000 m or if depth intervals were more than 1000 m below 500 m depth.
ARTICLE IN PRESS T. Ito et al. / Deep-Sea Research II 50 (2003) 2701–2711
3. Results and discussion 3.1. Temporal variations of radionuclides and recent distributions Data on radionuclide concentrations in surface water (0–50 m) are plotted as time series in Fig. 2 with best-fitting curves. The concentrations of 90Sr and 137Cs in surface water before 1972 were decreasing faster than after 1976, except for the 137 Cs values in 1986–1987 which were affected by the Chernobyl accident. The effective half-lives of 90 Sr and 137Cs were estimated therefore from the
90
Sr (Bqm-3)
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10
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0.1 1960
1970
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Cs (Bqm-3)
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1990
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1970
1980 Year
1990
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239+240
Pu (mBqm-3)
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Fig. 2. Temporal variations of radionuclides in the surface water of the Japan Sea: (a) 90Sr, (b) 137Cs, (c) 239+240Pu.
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data after 1976, and they are about 15 and 20 years, respectively. Contrary to 90Sr and 137Cs, 239+240 Pu concentrations in surface seawater of the Japan Sea were scattered over a relatively wide range (Fig. 2c). The effective half-life was estimated to be about 15 years. As radionuclide data in surface water were collected from a large area of the Japan Sea during the 1990s, it was possible to investigate the geographic distributions of radionuclides in the Japan Sea. The contour plot of 90Sr concentrations in the 1990s is shown in Fig. 3a (the data were decay-corrected to 1 January 2000 using a half-life of 28.78 years; IAEA, 1999b). 90Sr concentrations in surface water were in the range of 0.4–3.3 Bq m3, and the averaged value was 1.8 Bq m3 in the 1990s. The distribution shows a rather complicated pattern. Generally, highconcentration areas appeared west of the Japanese coast and south of 40 N in the central Japan Sea, namely, in the region of the TWC. Fig. 3b shows the distribution of 137Cs in surface waters (data were decay-corrected to 1 January 2000 by using the 137Cs half-life of 30.14 year; IAEA, 1999b). The 137Cs concentrations were in the range of 1.0–4.4 (average 2.7) Bq m3. The areas of high 137 Cs concentrations were similar to areas of high 90 Sr concentrations. 239+240Pu concentrations were in the range 1.3–25.0 mBq m3 (Fig. 3c), with an averaged value of 7.9 mBq m3, and opposite to 90 Sr and 137Cs distributions, higher 239+240Pu concentrations were observed in the subarctic region. Deep convection driven by cooling and strong westerly winds occurring in the northwest Japan Sea in wintertime (Seung and Yoon, 1995), transports subsurface water with relatively low 90 Sr and 137Cs concentrations and high 239+240Pu concentrations to the sea surface. On the other hand, it is supposed that the process which keeps the relatively high 90Sr and 137Cs concentrations and a relatively low 239+240Pu concentration stable in the southern part of the Japan Sea, is horizontal transport from the Pacific Ocean, which is mainly controlled by the TWC. Pacific water containing relatively high 90Sr and 137Cs levels and low 239+240 Pu levels (Povinec et al., 2003) is transported into the Japan Sea by the TWC
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Fig. 3. Spatial distribution of radionuclides in surface water of the Japan Sea in the 1990s: (a) 90Sr, (b) 137Cs, (c) 239+240Pu. The data for 90Sr and 137Cs were decay-corrected to 1 January 2000.
(Kasamatsu and Inatomi, 1998), passing through the shallow Tsushima Strait, and is distributed in the south and southwest region of the Japan Sea. In the 1990s, the averaged concentrations of 90Sr, 137 Cs and 239+240Pu in surface water at about 30 N in the Pacific Ocean were about 1.6, 2.6 Bq m3 and 3.8 mBq m3, respectively (Hirose et al., 2001; Povinec et al., 2003; Ito et al.,
submitted), showing a good agreement with the radionuclide concentrations in the TWC region of the Japan Sea. 3.2. Radionuclide profiles The vertical distributions of 90Sr in the Japan Sea are shown in Fig. 4. In order to allow a clear
ARTICLE IN PRESS T. Ito et al. / Deep-Sea Research II 50 (2003) 2701–2711 90
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Fig. 4. Vertical distribution of curves.
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comparison of the temporal change in the distributions, the data (decay-corrected to 1 January 2000) in the 1960s, 1970s, 1980s and 1990s were plotted in the individual figures with the averaged profile curves given by weight-averaged data (Figs. 4a–d). Further, the averaged profile curves were combined in Fig. 4e. The figures are showing a typical vertical distribution of 90Sr in the water column—the maximum 90Sr value appearing at the surface and the concentration decreasing exponentially with depth. The averaged concentration of 90 Sr decreased from 8.2 to 1.7 Bq m3 in the surface layer and slightly increased in the middle layer (300–2000 m). There were no changes in the deep layer below 2000 m (Fig. 4e). At around 30 N in the NW Pacific Ocean, 90Sr concentrations at 0, 500, 1000 and 1500 m depths measured in samples collected during the ‘‘IAEA ‘97 Pacific Expedition’’ (Povinec et al., 2003) were about 1.8, 1.3, 0.3 and 0.1 Bq m3, respectively, and 90Sr was almost constant below 1500 m (decay-corrected to 1 January, 2000). On the other hand, 90Sr in the water column of the Japan Sea was estimated from the averaged profile curve of the 1990s to be 1.7,
1.3, 0.8 and 0.6 Bq m3 at the corresponding depths in the NW Pacific Ocean. 90Sr concentrations at depths of less than 500 m in the 1990s were similar to that observed in the NW Pacific Ocean, while the concentrations in deeper layers were higher in the Japan Sea. Fig. 5 shows the vertical distributions of 137Cs concentrations in the water column of the Japan Sea. The maximum 137Cs concentrations were at the surface and decreased exponentially with depth in the same way as 90Sr. With time, the 137Cs concentrations decreased from 11.6 to 2.8 Bq m3 in the surface layer (o500 m) and increased in the mid-depth layer (500–2000), and were almost constant in the deep layer (>2000 m) with a value of about 0.3 Bq m3 (Fig. 5e). The changes in 137Cs profiles were very similar to the changes in 90Sr profiles. The recent 137Cs concentrations obtained at around 30 N in the NW Pacific Ocean were about 2.6 Bq m3 at the surface, about 2.0 Bq m3 at 500 m, about 0.5 Bq m3 at 1000 m and about 0.2 Bq m3 below 1500 m (Povinec et al., 2003; decay-corrected to 1 January 2000). The corresponding 137Cs concentrations in the 1990s in the
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Fig. 5. Vertical distribution of curves.
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Cs in the water column of the Japan Sea in: (a) 1960s, (b) 1970s, (c) 1980s, (d) 1990s, (e) all fitting
Japan Sea were higher by 0.2, 1.0 and 0.6 Bq m3 at 500, 1000 and 1500 m, respectively, although the concentrations were almost the same in the surface layers. The 239+240Pu profiles are shown in Fig. 6. Minimum concentrations of 239+240Pu were found in the surface layer and maximum concentrations in the subsurface layer; the concentrations then gradually decreased with depth for all periods. From the 1970s to the 1990s, the averaged surface concentration decreased from 21.4 to 8.8 mBq m3. It is clearly seen that the peak depths became deeper, from about 100 to 800 m, and the concentrations increased from 30.8 to 38.3 mBq m3 with time from the 1970s. Further, in the 1990’s profiles, high concentrations of 239+240 Pu (>30 mBq m3) remained in wide depth ranges between the peak depth and near bottom. The peak depth found in the Japan Sea was similar to the peak depth in the NW Pacific Ocean; however, the concentrations in the Japan Sea were higher than in the NW Pacific Ocean at all depths (Povinec et al., 2003). The differences in concen-
trations were especially large below the peak depth. 3.3. Inventories of radionuclides in the water column Inventories of 90Sr, 137Cs and 239+240Pu from 1975 were calculated from the valid multi-layer data set to investigate the accumulation of radionuclides in the Japan Sea (the 90Sr and 137Cs data were decay-corrected to 1 January 2000). In order to compensate for the differences caused by the different total depths of the sites, the inventories were calculated for 0–2000 m water depth only. The temporal changes in the inventories of 90Sr, 137 Cs and 239+240Pu are shown in Figs. 7a–c. Here, four cumulative inventories (I5 ; 0–500 m, I10 ; 0– 1000 m, I15 ; 0–1500 m and I20 ; 0–2000 m) were plotted with their best-fitting lines. In the inventory plot for 90Sr (Fig. 7a), there is a tendency for I5 and I10 to decrease slightly with time and I15 and I20 to increase over the two decades. This may suggest that 90Sr has
ARTICLE IN PRESS T. Ito et al. / Deep-Sea Research II 50 (2003) 2701–2711 239+240
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4000
(d)
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Fig. 6. Vertical distribution of 239+240Pu in the water column of the Japan Sea in: (a) 1960s, (b) 1970s, (c) 1980s, (d) 1990s, (e) all fitting curves.
accumulated below 1000 m (1000–2000 m) as expected from the temporal change of the vertical profile shown in Fig. 4. The averaged I20 in the 1990s was estimated to be about 1.870.2 kBq m2 and increased by about 15% from 1977 onwards. As only one valid data set is available for the 1970s, it is difficult to make a precise comparison. The I5 of 137Cs had remained almost constant, and I10 ; I15 and I20 increased in value and rate gradually from 1977. This means that 137Cs had been accumulating below 500 m depth. The averaged I20 was 3.170.5 kBq m2 in the 1990s, showing an increase of about 15%, similar to 90Sr. I10 ; I15 and I20 showed relatively small values and ranges in 1985 while the inventories were widely scattered in 1993. After 1994, the inventories show smaller variations compared to 1993. It is inferred that the variation is relevant to a change in water ventilation in the deep layer of the Japan Sea. Gamo (1999) suggested that the deep water of the Japan Sea had been only weakly ventilated in recent decades. It has also been suggested (e.g., Kumamoto, 1998; Miyao et al., 2000) that after
1985 deep water was formed only intermittently in the Japan Sea. The cumulative inventories of 239+240Pu also increased with time. The cumulative inventories of 239+240 Pu were larger in the deeper layer, indicating that more 239+240Pu has accumulated in the deeper layers of the Japan Sea. In the 1990s, the averaged I20 reached 63 Bq m2, about twice the amount in 1979 (Fig. 7c). The I10 2I20 inventories are widely scattered in 1993 and relatively stable after 1994, similar to the variations of the 90Sr and the 137Cs inventories. The temporal variations in 90Sr and 137Cs inventories can be explained by inflow of higher activity Pacific waters into the Japan Sea by the Tsushima current which are further diluted by winter surface mixing in the Japan Sea. Therefore, waters of relatively low 90Sr and 137Cs concentrations flow out of the Japan Sea to the North Pacific; however, in bottom waters, due to vertical transport, higher 90Sr and 137Cs concentrations are accumulating. On the other hand, 239+240Pu concentrations in surface Pacific waters are
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Inventory of 90Sr (kBqm-2)
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Sea. Therefore, similar processes as suggested for the formation of 90Sr and 137Cs inventories also may control change of 239+240Pu inventories in the Japan Sea. The geographic distributions of I20 for 90Sr, 137 Cs and 239+240Pu in the 1990s are shown in Figs. 8a–c. The I20 of 90Sr and 137Cs showed very similar patterns. I20 was high in the north Japan Sea and decreased towards the south. The low I20 area appeared around the Yamato Rise (39–40 N and 134–136 E). Furthermore, a high I20 intrusion into the southern part of the Sea was found around 41 N and 137 E. Senjyu and Sudo (1994) indicated, from their analysis of hydrographic data on the Japan Sea, that wintertime convection caused the formation of the upper part of the JSPW (UJSPW) in the region west of 136 E between 40 and 43 N and its advection into the Yamato Basin. The intrusion of the high I20 (Figs. 8a and b) can be demonstrated by the behavior of the UJSPW explained above. In Fig. 8c, the low 239+240 Pu I20 area appeared around the Yamato Rise, similar to the 90Sr and 137Cs. The low I20 areas found in all figures may be a result of the effect of the bottom topography of the Yamato Rise. Unfortunately, the present data are insufficient to investigate this, and further research is needed to reveal the mechanisms of the formation of the low inventory area.
1980
1985
1990
1995
2000
Year
Fig. 7. Temporal variations of cumulative inventories in the water column of the Japan Sea: (a) 90Sr, (b) 137Cs, (c) 239+240Pu.
relatively low; however, they are higher in the East China Sea (Lee et al., 2003), and as the Tsushima current has a part in this Sea, 239+240Pu can be transported from the East China Sea to the Japan
The 90Sr, 137Cs and 239+240Pu concentration data extracted from IAEA-MEL’s GLOMARD and JMS-MRI’s HAM databases were examined for the temporal and spatial variations of radionuclide concentrations and inventories in waters of the Japan Sea. The radionuclide concentrations in surface water showed a decrease over the last 30 years. The effective half-lives of 90Sr, 137Cs and 239+240 Pu were estimated to be about 15, 20 and 15 years, respectively. In surface water, the averaged concentrations of 90Sr, 137Cs and 239+240Pu were for the year 2000 estimated to be 1.6, 2.8 Bq m3 and 6.6 mBq m3, respectively. There was a characteristic distribution indicating that 90Sr and 137Cs concentrations in the southern Japan
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Inventory of 90 Sr in seawater column of 0-2000m (kBqm -2 )
46
[Period:1991-2000, Ref: 2000.01.01]
Latitude (degree-N)
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44
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Inventory of 239+240 Pu in seawater column of 0-2000m (Bqm-2 ) [Period: 1991-2000]
Latitude (degree-N)
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Sea were higher than the concentrations in the north, whereas 239+240Pu showed the opposite pattern. Evidence of the vertical transport of radionuclides was found in the temporal variations in the distributions of radionuclides in the water column. The concentrations of radionuclides decreased in the surface layer and increased in the subsurface layer for 90Sr and 137Cs, or below the subsurface layer for 239+240Pu over time. In comparing the vertical distributions of radionuclide concentrations in the 1990s in the Japan Sea with those in the Pacific Ocean, the concentrations of 90Sr and 137Cs were almost the same level at depths above 500 m and higher below 500 m, while 239+240Pu was higher at all depths in the Japan Sea. Temporal variations of the inventories suggests that radionuclides had accumulated in the Japan Sea over time as a consequence of vertical transport. From the mid-1970s for 90Sr and 137Cs or the late 1970s for 239+240Pu, to the late 1990s, the inventories calculated down to 2000 m ðI20 Þ increased by about 15% for 90Sr and 137Cs and about 2-fold for 239+240Pu. The averaged I20 for 90 Sr, 137Cs and 239+240Pu was for the 1990s estimated to be 1.8, 3.1 kBq m2 and 63 Bq m2, respectively. These inventories were 30–40% higher than those estimated for the NW Pacific Ocean at about 30 N. In the spatial distribution of I20 ; two characteristic features were found. One was the high I20 intrusion from the north Japan Sea into the south, although this is not clear from the I20 map for 239+240Pu. The other was the low I20 region that appeared in the central Japan Sea around the Yamato Rise. It would appear that the north–south concentration gradient in surface water, the relatively rapid vertical transport and large accumulation in contrast with the Pacific Ocean, and the distinctive horizontal distributions of the inventories were caused by geographic and oceanographic factors specific to the Japan Sea—a surface-limited transport of radionuclides by the TWC passing the
34 128
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Fig. 8. Spatial distribution of I20 in the Japan Sea: (a) 90Sr, (b) 137 Cs, (c) 239+240Pu.
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Tsushima Strait, the bowl-like bottom topography and the formation of JSPW by deep convection in wintertime and its movement below the surface layer of the Japan Sea.
Acknowledgements This study was carried out in the framework of IAEA-MEL’s WOMARS project, which was supported by extra-budgetary funding from The Science and Technology Agency (STA) (now the Ministry of Education, Culture, Sports, Science and Technology) of Japan, and we would like to express our thanks to the STA for their valuable support. We would also like to acknowledge the kind help and valuable advice of Dr. M. Aoyama of the JMA-MRI on the HAM database. IAEA-MEL operates under a bilateral agreement between the IAEA and the Principality of Monaco.
References Gamo, T., 1999. Global warming may have slowed down the deep conveyor belt of a marginal sea of northwestern Pacific: Japan Sea. Geophysical Research Letters 26 (20), 3137–3140. Harada, K., Tsunogai, S., 1986. 226Ra in the Japan Sea and the residence time of the Japan Sea water. Earth and Planetary Science Letters 77, 236–244. Hase, H., Yoon, J.-H., Koterayama, W., 1999. The current structure of the Tsushima warm current along the Japanese coast. Journal of Oceanography 55, 217–235. Hirose, K., 1997. Complexation-scavenging of plutonium in the ocean. Radioprotection—Coll 32 (C2), 225–230. Hirose, K., Aoyama, M., 2003. 137Cs and 239,240Pu concentrations in surface waters of the Pacific. Deep-Sea Research II, this issue (doi:10.1016/S0967-0645(03)00141-3). Hirose, K., Amano, H., Baxter, M.S., Chaykovskaya, V.B., Hong, H.G., Isogai, K., Kim, C.K., Kim, S.H., Miyao, T., Morimoto, T., Nikitin, A., Oda, K., Pettersson, H.B.L., Povinec, P.P., Steto, Y., Tkalin, A., Togawa, O., Veletova, N.K., 1999. Anthropogenic radionuclides in seawater in the East Sea/Japan Sea: results of the first-stage Japanese– Korean–Russian expedition. Journal of Environmental Radioactivity 43, 1–13. Hirose, K., Aoyama, M., Miyao, T., Igarashi, Y., 2001. Plutonium in the western North Pacific. Journal of Radioanalytical and Nuclear Chemistry 248 (3), 771–776.
IAEA, 1999a. Inventory of radioactive waste disposals at sea. IAEA-TECDOC-1105, International Atomic Energy Agency, Vienna. IAEA, 1999b. IAEA AQCS Catalogue for reference materials and intercomparison Exercises 1998/1999, International Atomic Energy Agency, Vienna. IAEA, 2000. Global marine radioactivity database (GLOMARD). IAEA-TECDOC-1146, International Atomic Energy Agency, Vienna. Ikeuchi, Y., Amano, H., Aoyama, M., Berezhnov, V.I., Chaykovskaya, E., Chumichev, V.B., Chung, C.S., Gastaud, J., Hirose, K., Hong, G.H., Kim, C.K., Kim, S.H., Miyao, T., Morimoto, T., Nikitin, A., Oda, K., Pettersson, H.B.L., Povinec, P.P., Tkalin, A., Togawa, O., Veletova, N.K., 1999. Anthropogenic radionuclides in seawater of the Far Eastern Seas. The Science of the Total Environment 237, 238, 203–212. Isoda, Y., 1994. Warm eddies in the Japan sea. Kaiyo Monthly 10, 779–787 (in Japanese). Ito, T., Aramaki, T., Kitamura, T., Otosaka, S., Suzuki, T., Togawa, O., Kobayashi, T., Senjyu, T., Chaykovskayay, E.L., Karasev, E.V., Lishavskaya, T.S., Novichkov V.P., Tkalin A.V., Shcherbinin, A.F., Volkov, Yu.N., 2003. Anthropogenic Radionuclides in the Japan Sea: their distribution and transport processes. Journal of Environmental Radioactivity 68, 249–267. Kang, D.-J., Chung, C.S., Kim, S.K., Kim, K.-R., Hong, G.H., 1997. Distribution of 137Cs and 239,240Pu in the Surface waters of the East Sea (Sea of Japan). Marine Pollution Bulletin 35, 305–312. Kasamatsu, F., Inatomi, N., 1998. Effective environmental half-lives of 90Sr and 137Cs in the coastal seawater of Japan. Journal of Geophysical Research 103 (C1), 1209–1219. Kim, C.-K., Kim, C.-S., Yun, J.-Y., Kim, K.-H., 1997. Distribution of 3H, 137Cs and 239,240Pu in the surface seawater around Korea. Journal of Radioanalytical and Nuclear Chemistry 218, 33–40. Kumamoto, Y., Yoneda, M., Shibata, Y., Kume, H., Tanaka, A., Uehiro, T., Morita, M., Shitashima, K., 1998. Direct observation of the rapid turnover of the Japan Sea bottom water by means of AMS radiocarbon measurement. Geophysical Research Letters 25, 651–654. Lee, S.H., Gastaud, J., Povinec, P.P., Hong, G.H., Kim, S.H., Chung, C.S., Lee, K.-W., Pettersson, H., 2003. Distribution of plutonium and americium in the marginal seas of the northwest Pacific Ocean. Deep-Sea Research II, this issue (doi:10.1016/S0967-0645(03)00150-4). Miyao, T., Hirose, K., Aoyama, M., Igarashi, Y., 1998. Temporal variation of 137Cs and 239,240Pu in the Sea of Japan. Journal of Environmental Radioactivity 40, 239– 250. Miyao, T., Hirose, K., Aoyama, M., Igarashi, Y., 2000. Trace of the recent deep water formation in the Japan Sea deduced from historical 137Cs data. Geophysical Research Letters 27, 3731–3734. NAS, 2001. Chronological Scientific Tables (Rika nenpyo), National Astronomical Observatory, Tokyo (in Japanese).
ARTICLE IN PRESS T. Ito et al. / Deep-Sea Research II 50 (2003) 2701–2711 Povinec, P.P., Livingston, H.D., Shima, S., Aoyama, M., Gastaud, J., Goroncy, I., Hirose, K., Huynh-Ngoc, L., Ikeuchi, Y., Ito, T., La Rosa, J., Liong Wee Kwong, L., Lee, S.H., Moriya, H., Mulsow, S., Oregioni, B., Pettersson, H., Togawa, O., 2003. IAEA’97 expedition to the NW Pacific Ocean—Results of oceanographic and radionuclide investigations of the water column. Deep-Sea Research II, this issue (doi:10.1016/S0967-0645(03)00138-3). Senjyu, T., 1999. The Japan Sea Intermediate Water; its characteristics and circulation. Journal of Oceanography 55, 111–122. Senjyu, T., Sudo, H., 1994. The upper portion of the Japan Sea proper water: its source and circulation as deduced from isopycnal analysis. Journal of Oceanography 50, 663–690.
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Seung, Y.-H., Yoon, J.-H., 1995. Some feature of winter convection in the Japan Sea. Journal of Oceanography 51, 61–73. Uda, M., 1934. The results of simultaneous oceanographical investigations in the Japan Sea and its adjacent waters in May and June, 1932. Journal of Imperial Fishery Experimental Station 5, 57–190 (in Japanese). UNSCEAR, 2000. Sources and Effects of Ionizing Radiation. United Nations, UN, New York. Yamada, M., Aono, T., Hirano, S., 1996. 239+240Pu and 137Cs distributions in seawater from the Yamato Basin and the Tsushima Basin in the Japan Sea. Journal of Radioanalytical and Nuclear Chemistry 210 (1), 129–136.
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Temporal and spatial variations of anthropogenic radionuclides in Japan Sea waters Toshimichi Itoa,c,*, Pavel P. Povineca, Orihiko Togawaa,c, Katsumi Hiroseb a International Atomic Energy Agency—Marine Environment Laboratory, MC 98000, Monaco Japan Meteorological Agency—Meteorological Research Institute, Tukuba-shi, Ibaraki 305-0052, Japan c Marine Research Laboratory, Japan Atomic Energy Research Institute, Mutsu-shi, Aomori 035-0064, Japan b
Abstract 90
Sr, 137Cs and 239+240Pu data covering three decades were examined for temporal and spatial variations of their concentrations and inventories in waters of the Japan Sea/East Sea. The time trend of radionuclide concentrations in surface water showed a gradual decrease over the whole period and the effective half-lives of 90Sr, 137Cs and 239+240Pu were estimated to be about 15, 20 and 15 years, respectively. The averaged concentrations of 90Sr, 137Cs and 239+240Pu for the year 2000 were estimated to be 1.6, 2.8 Bq m3 and 6.6 mBq m3, respectively. 90Sr and 137Cs concentrations in the south of the Japan Sea were higher than the concentrations in the north, whereas 239+240Pu showed a reverse pattern. The radionuclide concentrations decreased with time in the surface layer and increased in the subsurface layer for 90Sr and 137Cs, or increased below the subsurface layer for 239+240Pu. The averaged inventories of 90Sr, 137Cs and 239+240 Pu in Japan Sea waters in the 1990s were estimated to be 1.8 , 3.1 kBq m2 and 63 Bq m2, respectively, and were 30–40% higher than in the Pacific Ocean. The area of high inventory intruded from the north of the Japan Sea into the south, and the low inventory region appeared in the central part of the Sea around the Yamato Rise. Distinctive changes and distributions of radionuclides found in this study are explained by the geographic and oceanographic characteristics of the Japan Sea. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction The Japan Sea/East Sea is a marginal sea of the western North Pacific Ocean with the fourth largest surface area (1.01 106 km2) in the world (Fig. 1). The mean depth of the Japan Sea is 1350 m, and the maximum depth is 3796 m (NAS, *Corresponding author. Present address: Japan Atomic Energy Research Institute, Tokar-mura, Naka-gun, Ibaraki 319-1195, Japan. Tel.: +81-29-282-5171; fax: +81-29-2826760. E-mail address: [email protected] (T. Ito).
2001). The bottom topography of the Japan Sea is bowl like and separated into three basins: the Japan Basin in the north, the Yamato Basin in the southeast, and the Tsushima Basin in the southwest. The Yamato Rise, with a maximum height above sea level of about 250 m, is in the center of these basins. The Japan Sea is almost enclosed, surrounded by Sakhalin Island, the Asian continent, the Japanese Islands, and isolated from the Pacific Ocean and adjacent marginal seas except four narrow and shallow straits—the Tsushima Strait, the Tsugaru Strait, the Soya Strait, and the Mamiya Strait—with sill depths shallower than
0967-0645/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0967-0645(03)00142-5
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2702 55
MY
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Fig. 1. Map of the Japan Sea. ‘‘TS’’, ‘‘TG’’, ‘‘SY’’ and ‘‘MY’’ indicate the Tsushima Strait, the Tsugaru Strait, the Soya Strait, and the Mamiya Strait, respectively.
200 m. Seawater is carried from the Pacific Ocean through the East China Sea over the sill of the Tsushima Strait into the surface layer of the Japan Sea by the Tsushima Warm Current (TWC), a branch of the Kuroshio Current. In the Japan Sea, the TWC flows northeastward along the west coast of the Island of Honshu with many meanders and eddies (Isoda, 1994; Hase et al., 1999). A subarctic front is formed between the subtropical waters carried by the TWC and subarctic waters at around 40 N (Senjyu, 1999). Surface seawater exits the Japan Sea to the Pacific Ocean through the Tsugaru Strait (‘‘TG’’ in Fig. 1) and to the Sea of Okhotsk through the Soya Strait (‘‘SY’’ in Fig. 1). Because of the shallowness of the Straits, the exchange of seawater is limited to the surface layer. The deep water of the Japan Sea is known as the Japan Sea Proper Water (JSPW) (Uda, 1934) and characterized by relatively low temperatures, 0.0–1.0 C, relatively low and homogenous salinity, 34.04–34.10 PSU, and a high concentration of dissolved oxygen, 220–250 mmol kg1 (Harada and Tsunogai, 1986; Miyao et al., 1998). Senjyu and Sudo (1994) suggested that the upper portion of JSPW is formed by wintertime convection in the region west of 136 E between 40 and 43 N. Anthropogenic radionuclides in the Japan Sea have several sources. The main source is global
fallout delivered from atmospheric nuclear tests. An additional source of anthropogenic radionuclides is the Chernobyl accident in 1986 (UNSCEAR, 2000). Further, radioactive wastes were dumped in the Japan Sea by the Russian Federation and former Soviet Union. The IAEA (1999a) reported that about 444 TBq of liquid radioactive wastes and about 307 TBq of solid-state wastes had been disposed in the Japan Sea. Long-lived anthropogenic radionuclides, such as 90 Sr, 137Cs and 239+240Pu, have been widely used as radioactive traces to investigate transport processes in the Japan Sea, to study processes in the water column, scavenging and sedimentation processes, etc. (e.g., Hirose, 1997; Yamada et al., 1996; Miyao et al., 1998, 2000; Hirose et al., 1999; Ikeuchi et al., 1999; Hirose et al., 2001). The main purpose of this paper is to elucidate the temporal variations of concentrations and inventories of 90Sr, 137Cs and 239+240Pu in Japan Sea waters by examining the data extracted from the GLOMARD (IAEA, 2000) and HAM (Hirose and Aoyama, 2003) databases.
2. Data The data on 90Sr, 137Cs and 239+240Pu concentrations in seawater of the Japan Sea used in this study were extracted from the Global Marine Radioactivity Database (GLOMARD), constructed by International Atomic Energy Agency—Marine Environment Laboratory, Monaco (IAEA, 2000) and the Historical Artificial Radionuclides in the Pacific Ocean and its Marginal Seas database (HAM) established by the Japan Meteorological Agency—Meteorological Research Institute (Hirose and Aoyama, 2003); see also http://www.mrijma.go.jp. More recent data were also extracted from the literature (Kang et al., 1997; Kim et al., 1997; Yamada et al., 1996). Data were rejected if the uncertainty was three times greater than the value or no uncertainty was reported. Further, the data set for multi-layer sampling was not used in the inventory calculation if less than five layers were sampled from 0 to 2000 m or if depth intervals were more than 1000 m below 500 m depth.
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3. Results and discussion 3.1. Temporal variations of radionuclides and recent distributions Data on radionuclide concentrations in surface water (0–50 m) are plotted as time series in Fig. 2 with best-fitting curves. The concentrations of 90Sr and 137Cs in surface water before 1972 were decreasing faster than after 1976, except for the 137 Cs values in 1986–1987 which were affected by the Chernobyl accident. The effective half-lives of 90 Sr and 137Cs were estimated therefore from the
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Fig. 2. Temporal variations of radionuclides in the surface water of the Japan Sea: (a) 90Sr, (b) 137Cs, (c) 239+240Pu.
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data after 1976, and they are about 15 and 20 years, respectively. Contrary to 90Sr and 137Cs, 239+240 Pu concentrations in surface seawater of the Japan Sea were scattered over a relatively wide range (Fig. 2c). The effective half-life was estimated to be about 15 years. As radionuclide data in surface water were collected from a large area of the Japan Sea during the 1990s, it was possible to investigate the geographic distributions of radionuclides in the Japan Sea. The contour plot of 90Sr concentrations in the 1990s is shown in Fig. 3a (the data were decay-corrected to 1 January 2000 using a half-life of 28.78 years; IAEA, 1999b). 90Sr concentrations in surface water were in the range of 0.4–3.3 Bq m3, and the averaged value was 1.8 Bq m3 in the 1990s. The distribution shows a rather complicated pattern. Generally, highconcentration areas appeared west of the Japanese coast and south of 40 N in the central Japan Sea, namely, in the region of the TWC. Fig. 3b shows the distribution of 137Cs in surface waters (data were decay-corrected to 1 January 2000 by using the 137Cs half-life of 30.14 year; IAEA, 1999b). The 137Cs concentrations were in the range of 1.0–4.4 (average 2.7) Bq m3. The areas of high 137 Cs concentrations were similar to areas of high 90 Sr concentrations. 239+240Pu concentrations were in the range 1.3–25.0 mBq m3 (Fig. 3c), with an averaged value of 7.9 mBq m3, and opposite to 90 Sr and 137Cs distributions, higher 239+240Pu concentrations were observed in the subarctic region. Deep convection driven by cooling and strong westerly winds occurring in the northwest Japan Sea in wintertime (Seung and Yoon, 1995), transports subsurface water with relatively low 90 Sr and 137Cs concentrations and high 239+240Pu concentrations to the sea surface. On the other hand, it is supposed that the process which keeps the relatively high 90Sr and 137Cs concentrations and a relatively low 239+240Pu concentration stable in the southern part of the Japan Sea, is horizontal transport from the Pacific Ocean, which is mainly controlled by the TWC. Pacific water containing relatively high 90Sr and 137Cs levels and low 239+240 Pu levels (Povinec et al., 2003) is transported into the Japan Sea by the TWC
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Fig. 3. Spatial distribution of radionuclides in surface water of the Japan Sea in the 1990s: (a) 90Sr, (b) 137Cs, (c) 239+240Pu. The data for 90Sr and 137Cs were decay-corrected to 1 January 2000.
(Kasamatsu and Inatomi, 1998), passing through the shallow Tsushima Strait, and is distributed in the south and southwest region of the Japan Sea. In the 1990s, the averaged concentrations of 90Sr, 137 Cs and 239+240Pu in surface water at about 30 N in the Pacific Ocean were about 1.6, 2.6 Bq m3 and 3.8 mBq m3, respectively (Hirose et al., 2001; Povinec et al., 2003; Ito et al.,
submitted), showing a good agreement with the radionuclide concentrations in the TWC region of the Japan Sea. 3.2. Radionuclide profiles The vertical distributions of 90Sr in the Japan Sea are shown in Fig. 4. In order to allow a clear
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comparison of the temporal change in the distributions, the data (decay-corrected to 1 January 2000) in the 1960s, 1970s, 1980s and 1990s were plotted in the individual figures with the averaged profile curves given by weight-averaged data (Figs. 4a–d). Further, the averaged profile curves were combined in Fig. 4e. The figures are showing a typical vertical distribution of 90Sr in the water column—the maximum 90Sr value appearing at the surface and the concentration decreasing exponentially with depth. The averaged concentration of 90 Sr decreased from 8.2 to 1.7 Bq m3 in the surface layer and slightly increased in the middle layer (300–2000 m). There were no changes in the deep layer below 2000 m (Fig. 4e). At around 30 N in the NW Pacific Ocean, 90Sr concentrations at 0, 500, 1000 and 1500 m depths measured in samples collected during the ‘‘IAEA ‘97 Pacific Expedition’’ (Povinec et al., 2003) were about 1.8, 1.3, 0.3 and 0.1 Bq m3, respectively, and 90Sr was almost constant below 1500 m (decay-corrected to 1 January, 2000). On the other hand, 90Sr in the water column of the Japan Sea was estimated from the averaged profile curve of the 1990s to be 1.7,
1.3, 0.8 and 0.6 Bq m3 at the corresponding depths in the NW Pacific Ocean. 90Sr concentrations at depths of less than 500 m in the 1990s were similar to that observed in the NW Pacific Ocean, while the concentrations in deeper layers were higher in the Japan Sea. Fig. 5 shows the vertical distributions of 137Cs concentrations in the water column of the Japan Sea. The maximum 137Cs concentrations were at the surface and decreased exponentially with depth in the same way as 90Sr. With time, the 137Cs concentrations decreased from 11.6 to 2.8 Bq m3 in the surface layer (o500 m) and increased in the mid-depth layer (500–2000), and were almost constant in the deep layer (>2000 m) with a value of about 0.3 Bq m3 (Fig. 5e). The changes in 137Cs profiles were very similar to the changes in 90Sr profiles. The recent 137Cs concentrations obtained at around 30 N in the NW Pacific Ocean were about 2.6 Bq m3 at the surface, about 2.0 Bq m3 at 500 m, about 0.5 Bq m3 at 1000 m and about 0.2 Bq m3 below 1500 m (Povinec et al., 2003; decay-corrected to 1 January 2000). The corresponding 137Cs concentrations in the 1990s in the
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Japan Sea were higher by 0.2, 1.0 and 0.6 Bq m3 at 500, 1000 and 1500 m, respectively, although the concentrations were almost the same in the surface layers. The 239+240Pu profiles are shown in Fig. 6. Minimum concentrations of 239+240Pu were found in the surface layer and maximum concentrations in the subsurface layer; the concentrations then gradually decreased with depth for all periods. From the 1970s to the 1990s, the averaged surface concentration decreased from 21.4 to 8.8 mBq m3. It is clearly seen that the peak depths became deeper, from about 100 to 800 m, and the concentrations increased from 30.8 to 38.3 mBq m3 with time from the 1970s. Further, in the 1990’s profiles, high concentrations of 239+240 Pu (>30 mBq m3) remained in wide depth ranges between the peak depth and near bottom. The peak depth found in the Japan Sea was similar to the peak depth in the NW Pacific Ocean; however, the concentrations in the Japan Sea were higher than in the NW Pacific Ocean at all depths (Povinec et al., 2003). The differences in concen-
trations were especially large below the peak depth. 3.3. Inventories of radionuclides in the water column Inventories of 90Sr, 137Cs and 239+240Pu from 1975 were calculated from the valid multi-layer data set to investigate the accumulation of radionuclides in the Japan Sea (the 90Sr and 137Cs data were decay-corrected to 1 January 2000). In order to compensate for the differences caused by the different total depths of the sites, the inventories were calculated for 0–2000 m water depth only. The temporal changes in the inventories of 90Sr, 137 Cs and 239+240Pu are shown in Figs. 7a–c. Here, four cumulative inventories (I5 ; 0–500 m, I10 ; 0– 1000 m, I15 ; 0–1500 m and I20 ; 0–2000 m) were plotted with their best-fitting lines. In the inventory plot for 90Sr (Fig. 7a), there is a tendency for I5 and I10 to decrease slightly with time and I15 and I20 to increase over the two decades. This may suggest that 90Sr has
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1991-2000
Fig. 6. Vertical distribution of 239+240Pu in the water column of the Japan Sea in: (a) 1960s, (b) 1970s, (c) 1980s, (d) 1990s, (e) all fitting curves.
accumulated below 1000 m (1000–2000 m) as expected from the temporal change of the vertical profile shown in Fig. 4. The averaged I20 in the 1990s was estimated to be about 1.870.2 kBq m2 and increased by about 15% from 1977 onwards. As only one valid data set is available for the 1970s, it is difficult to make a precise comparison. The I5 of 137Cs had remained almost constant, and I10 ; I15 and I20 increased in value and rate gradually from 1977. This means that 137Cs had been accumulating below 500 m depth. The averaged I20 was 3.170.5 kBq m2 in the 1990s, showing an increase of about 15%, similar to 90Sr. I10 ; I15 and I20 showed relatively small values and ranges in 1985 while the inventories were widely scattered in 1993. After 1994, the inventories show smaller variations compared to 1993. It is inferred that the variation is relevant to a change in water ventilation in the deep layer of the Japan Sea. Gamo (1999) suggested that the deep water of the Japan Sea had been only weakly ventilated in recent decades. It has also been suggested (e.g., Kumamoto, 1998; Miyao et al., 2000) that after
1985 deep water was formed only intermittently in the Japan Sea. The cumulative inventories of 239+240Pu also increased with time. The cumulative inventories of 239+240 Pu were larger in the deeper layer, indicating that more 239+240Pu has accumulated in the deeper layers of the Japan Sea. In the 1990s, the averaged I20 reached 63 Bq m2, about twice the amount in 1979 (Fig. 7c). The I10 2I20 inventories are widely scattered in 1993 and relatively stable after 1994, similar to the variations of the 90Sr and the 137Cs inventories. The temporal variations in 90Sr and 137Cs inventories can be explained by inflow of higher activity Pacific waters into the Japan Sea by the Tsushima current which are further diluted by winter surface mixing in the Japan Sea. Therefore, waters of relatively low 90Sr and 137Cs concentrations flow out of the Japan Sea to the North Pacific; however, in bottom waters, due to vertical transport, higher 90Sr and 137Cs concentrations are accumulating. On the other hand, 239+240Pu concentrations in surface Pacific waters are
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Inventory of 90Sr (kBqm-2)
2.5 I 5 (0-500) I10 (0-1000) I15 (0-1500) I20 (0-2000)
2.0
1.5
1.0
0.5
0.0 1975
1980
1985
(a)
1990
1995
2000
1990
1995
2000
Year
4.5 Inventory of 137Cs (kBqm-2)
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1975
1980
1985
(b)
Year
Inventory of
239+240
Pu (Bqm-2)
90
70
4. Conclusions
60 50 40 30 20 10 0 1975
(c)
I5 (0-500) I10 (0-1000) I15 (0-1500) I20 (0-2000)
80
Sea. Therefore, similar processes as suggested for the formation of 90Sr and 137Cs inventories also may control change of 239+240Pu inventories in the Japan Sea. The geographic distributions of I20 for 90Sr, 137 Cs and 239+240Pu in the 1990s are shown in Figs. 8a–c. The I20 of 90Sr and 137Cs showed very similar patterns. I20 was high in the north Japan Sea and decreased towards the south. The low I20 area appeared around the Yamato Rise (39–40 N and 134–136 E). Furthermore, a high I20 intrusion into the southern part of the Sea was found around 41 N and 137 E. Senjyu and Sudo (1994) indicated, from their analysis of hydrographic data on the Japan Sea, that wintertime convection caused the formation of the upper part of the JSPW (UJSPW) in the region west of 136 E between 40 and 43 N and its advection into the Yamato Basin. The intrusion of the high I20 (Figs. 8a and b) can be demonstrated by the behavior of the UJSPW explained above. In Fig. 8c, the low 239+240 Pu I20 area appeared around the Yamato Rise, similar to the 90Sr and 137Cs. The low I20 areas found in all figures may be a result of the effect of the bottom topography of the Yamato Rise. Unfortunately, the present data are insufficient to investigate this, and further research is needed to reveal the mechanisms of the formation of the low inventory area.
1980
1985
1990
1995
2000
Year
Fig. 7. Temporal variations of cumulative inventories in the water column of the Japan Sea: (a) 90Sr, (b) 137Cs, (c) 239+240Pu.
relatively low; however, they are higher in the East China Sea (Lee et al., 2003), and as the Tsushima current has a part in this Sea, 239+240Pu can be transported from the East China Sea to the Japan
The 90Sr, 137Cs and 239+240Pu concentration data extracted from IAEA-MEL’s GLOMARD and JMS-MRI’s HAM databases were examined for the temporal and spatial variations of radionuclide concentrations and inventories in waters of the Japan Sea. The radionuclide concentrations in surface water showed a decrease over the last 30 years. The effective half-lives of 90Sr, 137Cs and 239+240 Pu were estimated to be about 15, 20 and 15 years, respectively. In surface water, the averaged concentrations of 90Sr, 137Cs and 239+240Pu were for the year 2000 estimated to be 1.6, 2.8 Bq m3 and 6.6 mBq m3, respectively. There was a characteristic distribution indicating that 90Sr and 137Cs concentrations in the southern Japan
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Inventory of 90 Sr in seawater column of 0-2000m (kBqm -2 )
46
[Period:1991-2000, Ref: 2000.01.01]
Latitude (degree-N)
44 1.9 2.0
42 2.0
1.9 1.8 1.7 1.6
40
1.5
38
36
34 128
(a)
48
46
132
130
134 136 138 Longitude (degree-E)
140
142
140
142
140
142
Inventory of 137 Cs in seawater column of 0-2000m (kBqm-2 ) [Period: 1999-2000, Ref: 2000.01.01]
44
Latitude (degree-N)
3.6
42
3.6 3.4
3.2 3.0 2.8
40
3.4 3.2
3.0 2.6 2.8 2.4
38
36
34
128
130
132
134
(b)
136
138
Longitude (degree-E)
48
46
Inventory of 239+240 Pu in seawater column of 0-2000m (Bqm-2 ) [Period: 1991-2000]
Latitude (degree-N)
44
42
60
55
70
65
50
40
60
38
60
36
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Sea were higher than the concentrations in the north, whereas 239+240Pu showed the opposite pattern. Evidence of the vertical transport of radionuclides was found in the temporal variations in the distributions of radionuclides in the water column. The concentrations of radionuclides decreased in the surface layer and increased in the subsurface layer for 90Sr and 137Cs, or below the subsurface layer for 239+240Pu over time. In comparing the vertical distributions of radionuclide concentrations in the 1990s in the Japan Sea with those in the Pacific Ocean, the concentrations of 90Sr and 137Cs were almost the same level at depths above 500 m and higher below 500 m, while 239+240Pu was higher at all depths in the Japan Sea. Temporal variations of the inventories suggests that radionuclides had accumulated in the Japan Sea over time as a consequence of vertical transport. From the mid-1970s for 90Sr and 137Cs or the late 1970s for 239+240Pu, to the late 1990s, the inventories calculated down to 2000 m ðI20 Þ increased by about 15% for 90Sr and 137Cs and about 2-fold for 239+240Pu. The averaged I20 for 90 Sr, 137Cs and 239+240Pu was for the 1990s estimated to be 1.8, 3.1 kBq m2 and 63 Bq m2, respectively. These inventories were 30–40% higher than those estimated for the NW Pacific Ocean at about 30 N. In the spatial distribution of I20 ; two characteristic features were found. One was the high I20 intrusion from the north Japan Sea into the south, although this is not clear from the I20 map for 239+240Pu. The other was the low I20 region that appeared in the central Japan Sea around the Yamato Rise. It would appear that the north–south concentration gradient in surface water, the relatively rapid vertical transport and large accumulation in contrast with the Pacific Ocean, and the distinctive horizontal distributions of the inventories were caused by geographic and oceanographic factors specific to the Japan Sea—a surface-limited transport of radionuclides by the TWC passing the
34 128
(c)
130
132
134
136
138
Longitude (degree-E)
Fig. 8. Spatial distribution of I20 in the Japan Sea: (a) 90Sr, (b) 137 Cs, (c) 239+240Pu.
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Tsushima Strait, the bowl-like bottom topography and the formation of JSPW by deep convection in wintertime and its movement below the surface layer of the Japan Sea.
Acknowledgements This study was carried out in the framework of IAEA-MEL’s WOMARS project, which was supported by extra-budgetary funding from The Science and Technology Agency (STA) (now the Ministry of Education, Culture, Sports, Science and Technology) of Japan, and we would like to express our thanks to the STA for their valuable support. We would also like to acknowledge the kind help and valuable advice of Dr. M. Aoyama of the JMA-MRI on the HAM database. IAEA-MEL operates under a bilateral agreement between the IAEA and the Principality of Monaco.
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