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Temporal variations of 90Sr and 137Cs concentrations in Japanese coastal surface seawater and sediments from 1974 to 1998
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Deep-Sea Research II 50 (2003) 2713–2726
Temporal variations of 90Sr and 137Cs concentrations in Japanese coastal surface seawater and sediments from 1974 to 1998 Yoshihiro Ikeuchi* Japan Chemical Analysis Center, 295-3, San-no-cho, Inage, Chiba 263-0002, Japan Received 1 November 2001; received in revised form 31 January 2003
Abstract 90
Sr and 137Cs concentrations were determined in surface water and bottom sediments collected at 11 sites offshore from Japan during the period 1974–1998, to investigate their temporal variations and behaviour in the coastal marine environment. The concentrations of 90Sr and 137Cs in surface water have decreased with time since 1974. After the period of atmospheric nuclear weapons tests, the mean residence times of 90Sr and 137Cs were about 41 and 51 years, respectively. The 137Cs/90Sr activity ratios in coastal seawater during the atmospheric nuclear weapons tests (up until 1980) were lower than those after the tests due to the inflow of 90Sr in river water. A sharp increase in 137Cs levels was observed in airborne dust, in precipitation on the Japanese islands, and in coastal surface seawater in 1986 following the Chernobyl accident. However, the 137Cs levels in surface water returned to pre-1986 levels quickly, indicating rapid removal of Cs from the surface to deeper water. Concentrations of 90Sr in sediments were generally much lower than those for 137Cs, reflecting the more effective scavenging of Cs from the water column. In Ca-rich sediments, consisting of corals and shells, higher 90Sr levels and 90Sr/137Cs activity ratios were found, reflecting higher accumulation of Sr than Cs in marine organisms. Higher accumulation of 90Sr than 137Cs was also found in seaweed (gulfweed and wakame). r 2003 Elsevier Ltd. All rights reserved.
1. Introduction Large amounts of 90Sr and 137Cs have been deposited globally due to fallout from the atmospheric nuclear weapons tests conducted by the USA, USSR, UK, France and China from 1945 to 1980, and later due to the Chernobyl nuclear power plant accident in the former USSR in 1986. Thereby, large amounts of 90Sr and 137Cs were introduced into the coastal marine environment *Tel.: +81-43-424-8661; fax: +81-43-423-5326. E-mail address: [email protected] (Y. Ikeuchi).
via atmospheric fallout and river discharge. It is therefore important to investigate the behaviour and fate of these radionuclides in order to assess the degree of contamination in coastal areas. These areas are influenced both by the ocean and the adjacent terrestrial environment, and their relative degree of influence is unique to the individual site. Seawater and seabed sediments were collected annually by prefectural government institutes from 1974 to 1998 at 11 Japanese sites from the north to the south: Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Aichi, Osaka,
0967-0645/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0967-0645(03)00143-7
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45
Hokkaido Aomori 40
Sea of Japan North latitude
Fukushima Tsushima Current
Niigata Aichi
35
Oyashio Current
Kanagawa
Yamaguchi Osaka Fukuoka Kuroshio Current
Kagoshima 30
Okinawa 25 130
135
140
145
East longitude
Fig. 1. Sampling sites (o) of seawater and seabed sediment.
Yamaguchi, Fukuoka, Kagoshima and Okinawa (Fig. 1). In addition, airborne-dust, precipitation, river water and marine organisms also were collected in some prefectures. Subsequent analyses of 90Sr and 137 Cs were performed at the Japan Chemical Analysis Center (JCAC). The resulting data have since been archived at the Science and Technology Agency (STA) (Reports to STA, 1974-1998). In this paper, the temporal variation of 90Sr and 137 Cs concentrations in Japanese coastal surface seawater over the 25-year period is described. Also discussed is the behaviour of 90Sr and 137Cs in coastal waters by comparing the results of 90Sr and 137 Cs analyses in seabed sediments, airborne-dust, precipitation, river water, and marine organisms, as well as the impact of the Chernobyl accident on Japanese coastal waters.
2. Sampling and analytical procedures 2.1. Seawater More than 40 l of seawater was collected by boat from each sampling site once a year. Sampling was performed in July or August, periods of low precipitation, in order to avoid dilution with
rainwater. All the seawater samples were acidified on site by adding about 80 ml of concentrated HCl and then sent to JCAC for analysis. The concentrations of 90Sr and 137Cs were determined using well-established radiochemical analytical methods (STA, 1976, 1983). About 40 kg of seawater was weighed accurately using a balance and transferred to a 65-l vessel. Fifty mg of Cs carrier was added to the vessel and 400 g of Na2CO3 was added while stirring the sample. The pH of the sample solution was adjusted to 9.6 by adding sodium carbonate gradually to separate Ca and Sr from Mg. The supernatant solution at the Ca–Sr carbonate precipitation phase was used for analysis of Cs. Sr in the carbonate precipitation was separated from Ca by using fuming HNO3. Ba and Ra were removed as chromates by precipitation at pH 5–5.5. The amount of Sr in seawater before analysis was calculated using salinity data. After scavenging of 90Y, SrCO3 was recovered and weighed to obtain the chemical yield. After 2 weeks, the 90Y was separated from 90Sr by the 90Y milking method, and the 90Y activity was measured using a low background gas flow GM beta-ray counter. The Cs in the supernatant was separated as Cs3PO4 12MoO3 in 5% HCl solution. The Cs3PO4 12MoO3 was dissolved by NaOH, and 87 Rb impurities were removed by the cation exchange method. After adding H2PtCl6 solution, Cs2PtCl6 was recovered and weighed to obtain the chemical yield and the beta-activity of 137Cs was measured using a low background gas flow GM beta-ray counter. Unfortunately, analysis of 134Cs was not performed since it was not included in the analytical program supported by the STA. The efficiency of beta-activity measurements by a GM beta-ray counter was determined using 90Sr and 137Cs international standard solutions calibrated by the Electro-technical Laboratory in Japan (total uncertainty 3.3% and 2.5% for 90Sr and 137Cs, respectively, at 95% confidence level). The detection limits (95% confidence level) for both 90Sr and 137Cs were about 0.5 mBq/l for seawater and about 0.4 Bq/kg for seabed sediment. The JCAC has regularly participated in intercomparison exercises organized by the WHO, IAEA and EML, and their results were consistent with recommended values.
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The salinity in seawater was determined at prefectural government institutes using a titration method for Cl (STA, 1983) or a salinometer, which measures electric conductivity using, e.g., TS-DIGI-AUTO to evaluate the effects of the dilution of seawater with river water or rainwater. 2.2. Seabed sediment About 4 kg of upper seabed sediments (surface layer of about 5 cm) were collected using a grab sampler at the same sampling sites as seawater at water depths of mainly 5–20 m (Fig. 2). After drying at 105 C, the sediments were sent to the JCAC for analysis. The concentrations of 90Sr and 137Cs in seabed sediments were determined using previously reported radiochemical analytical methods (STA, 1976, 1983). After 100 g of seabed sediment were weighed, 100 mg of Sr carrier and 50 mg of Cs carrier were added to the sample. Sr and Cs were extracted by nitric acid, and Sr was then separated as carbonate and oxalate precipitation. Sr in the oxalate was separated from Ca by fuming HNO3.
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The Cs in the supernatant solution at the carbonate precipitation stage was separated as Cs3PO4 12MoO3 in 5% HCl solution. Subsequent separation and activity counting steps were the same as for seawater samples. 90 Sr and 137Cs in airborne dust (104 m3 filtered air/3-months), monthly precipitation (0.5 m2 surface area), river water (100 l) and marine organism (1 kg wet weight) were also determined. The radiochemical separation and activity counting steps for these samples were similar to those used in the analysis of seabed sediment. The chemical yield using the described radiochemical analytical methods is expected to be more than 70% for 90Sr and 137Cs.
3. Results and discussion 3.1. 90Sr and 137Cs activity concentrations in coastal seawater—their temporal variations The temporal variations of 90Sr and 137Cs concentrations in seawater at the 11 coastal sites are shown in Figs. 3 and 4, respectively.
0
Water depths (m)
10
20
30
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
40
50
60
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 2. Water depths at sampling sites of seabed sediments.
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Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
10 9
Activity concentration n
8 7 6 5 4 3 2 1
Chernobyl accident Atmospheric nuclear weapon tests
After nuclear weapon tests
After Chernobyl accident
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 3.
90
Sr activity levels in coastal seawater at 11 sites in Japan.
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
14
Activity concentration mBq/l
12 10 8 6 4 2 0
Atmospheric nuclear weapon tests
Chernobyl accident
After nuclear weapon tests
After Chernobyl Accident
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 4.
137
Cs activity levels in coastal seawater at 11 sites in Japan.
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(‰ )
40 35 30
Salinity
25 20 Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
15 10 5 0
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 5. Salinity in the analysed coastal seawater at 11 sites in Japan.
The salinity in seawater off Hokkaido, Aomori, Fukushima, Kanagawa, Niigata, Yamaguchi, Fukuoka, Kagoshima and Okinawa ranged from 21.7% to 36.3% (Fig. 5) with an average of 32.2%. However, the salinity in seawater off Aichi and Osaka ranged from 6.7% to 31.4% (Fig. 5) with an average of 21.1%, which indicated a higher dilution with river water at these two sampling sites. The highest concentrations of 90Sr and 137Cs, observed in the 1970s, are due to the influence of 11 Chinese atmospheric nuclear weapons tests carried out from 1974 to 1980 (Norris et al., 1994). During this period, the annual average 90Sr concentrations adjacent to Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Yamaguchi, Fukuoka, Kagoshima and Okinawa (with the exception of Aichi and Osaka, where the effect of river water inflow was apparent) were between 4.3 and 5.7 mBq/l. The annual average 137Cs concentrations ranged between 5.2 and 7.2 mBq/l. After the testing period (1981–1998, with the exception of 1986) the 90Sr and 137Cs concentrations at the same nine sampling sites ranged between 2.5 and 2.8 mBq/l and 3.4 and 4.0 mBq/l, respectively.
During the period of nuclear weapons tests, slightly higher 90Sr and 137Cs concentrations were found in northern Japan compared to southwestern Japan (Figs. 3 and 4). This tendency may be due to the latitudinal distribution of 90Sr and 137Cs deposition, as the maximum 90Sr deposition rates in soil (in 1963 and early 1964) were found at latitudes 40–50 N (List et al., 1965). The cumulative 90Sr deposition, based on analyses of soils collected during 1965–1967 (Meyer et al., 1968), was about 3 GBq/km2 in Hokkaido, about 2.2 GBq/km2 in Honshu and Kyusyu, and about 1.5 GBq/km2 in Okinawa. The average concentration of 90Sr in precipitation in prefectures bordering the Sea of Japan from April to June in 1981, immediately after large-scale nuclear tests in China in 1980, was 10.2 MBq/km2 at 37 500 –39 430 , 8.6 MBq/km2 at 35 280 –35 310 and 7.5 MBq/km2 at 32 480 –33 300 (Reports to STA, 1974-1998). After the atmospheric nuclear weapons tests, the concentrations of 90Sr and 137Cs in coastal surface seawater showed only small geographical differences. The average concentrations of 90Sr and 137Cs and the activity ratios of 137Cs/90Sr at 9 of the
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16
2.5 Chernobyl accident
12
Atmospheric nuclear weapon tests
after Chernobyl accident
after nuclear weapon tests
2 137
10
90
Cs/ Sr
1.5
137
Cs
8 y = 7.017e-0.0404x
6
1
R2 = 0.8786
Activity ratio
Activity concentration mBq/l
14
4 0.5
90
Sr
2
Atmospheric nuclear weapon tests by China
y = 6.0386e-0.0519x R2 = 0.9832 0
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 6. Average
90
Sr and
137
Cs activity levels in coastal seawater at nine sites.
sampling sites (excluding Aichi and Osaka) are shown in Fig. 6. The ratios of 137Cs/90Sr ranged from 1.11 to 1.29, with an average of 1.17 during the Chinese atmospheric nuclear weapons test period (1974–1980), from 1.25 to 1.34 with an average of 1.29 after the Chinese atmospheric nuclear tests and before the Chernobyl accident (1981–1985), and from 1.24 to 1.67 with an average of 1.47 after the Chernobyl accident (1987–1998). In general, these results indicate lower 137Cs/90Sr ratios during nuclear weapons tests than those after the tests. The concentrations of 90Sr and 137Cs and the activity ratios of 137Cs/90Sr in seawater off Osaka, where low salinity has been observed, are shown in Fig. 7. The average ratio of 137Cs/90Sr during the Chinese atmospheric nuclear weapons tests is 0.58; the ratio is 0.57 for 1981–1985 and 0.83 for 1987– 1998. The concentrations of 90Sr and 137Cs in river water in Osaka are shown in Fig. 8. The average 90 Sr and 137Cs concentrations during the Chinese atmospheric nuclear weapons tests are 9.2 and 0.66 mBq/l, respectively. The average ratio of
137
Cs/90Sr in Osaka during the Chinese atmospheric nuclear weapons tests is 0.072; the ratio is 0.067 for 1981–1985 and 0.12 for 1987–1998. These results indicate that because 90Sr is released faster than 137Cs from soil to river water, more 90 Sr than 137Cs was flowing into the sea, especially during the nuclear weapons test period. The 90Sr and 137Cs concentrations of Osaka river water, seawater off Osaka and surface water in the Western Northwest Pacific Ocean (Nagaya and Nakamura, 1981) in 1978 (at the end of the Chinese atmospheric nuclear weapons test period) at similar latitudes, are shown in Table 1. When Osaka river water is mixed with surface water in the Pacific Ocean, the 90Sr and 137Cs concentrations in seawater off Osaka could be estimated using the following equation: C3 ¼ C2 ðS3 =S2 Þ þ C1 ð1 S3 =S2 Þ; 90
137
ð1Þ
where C1= Sr or Cs concentrations in Osaka river water, C2=90Sr or 137Cs concentrations in Pacific Ocean water, C3=calculated 90Sr or 137Cs concentrations in seawater off Osaka, S2=salinity
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10
2.5
8 7 6
2 90
137
Sr 137
90
Cs/ Sr
1.5
Cs
5 4
1
Activity ratio
Activity concentration mBq/l
9
3 2 1
Atmospheric nuclear weapon tests
Chernobyl accident
after nuclear
0.5
after Chernobyl
0
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 7.
14
Activity concentration mBq/l
12
90
Sr and
137
Cs activity levels in coastal seawater off Osaka.
Atmospheric nuclear weapon tests
after nuclear
90
weapon tests
Sr
Chernobyl accident after Chernobyl accident
10 8 6 4 137
2
Cs
0 74 74 75 75 76 76 77 77 78 78 79 79 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87 88 88 89 89 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97 98 98 Year
Fig. 8.
90
Sr and
137
Cs activity levels in river water in Osaka.
in Pacific Ocean water, S3=salinity in seawater off Osaka. According to this calculation, the concentrations of 90Sr and 137Cs are 5.670.20 and 5.570.22 mBq/l in seawater off Osaka, respec-
tively. However, the measured concentrations of Sr and 137Cs in seawater off Osaka are 7.870.52 and 4.870.44 mBq/l, respectively. These results indicate a 20–35% excess of 90Sr. It can further be
90
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2720 Table 1 90 Sr and
137
Cs concentrations in river water, coastal seawater and the Pacific Ocean at almost the same latitude in 1978
Sample
North latitude
East latitude
Sampling date
90
137
Salinity (%)
River water in Osaka Seawater off Osaka Surface water in the Northwest Pacific Oceana
34 440 5800 34 380 3100 34 180 3000
135 340 3400 135 240 1000 141 580 3600
1978.07.21 1978.08.25 1978.03.03
10.470.44 7.870.52 3.770.22
0.7470.11 4.870.44 7.470.30
— 24.75 34.72
a
Sr (mBq/l)
Cs (mBq/l)
After Nagaya and Nakamura (1981).
Table 2 Effective half-lives of
90
Sr and
137
Cs in Japanese coastal surface seawaters during 1981–1998
Sampling site
Effective half-lives of 90 Sr (year)
Correlation coefficient, R2
Effective half-lives of 137 Cs, [A] (year)
Correlation coefficient, R2
Effective halflives of 137Cs normalized by salinity [B] (year)
Correlation coefficient, R2
[A]/[B]
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa Average Standard deviation
14.1 12.8 10.6 13.4 16.0 16.8 11.2 14.3 18.0 14.7 14.5 14.2 2.2
0.855 0.868 0.887 0.920 0.923 0.833 0.903 0.926 0.688 0.842 0.768
16.5 19.2 19.5 12.3 19.1 75.3 26.6 15.1 17.8 14.4 18.5 23.1 17.7a
0.718 0.655 0.475 0.799 0.705 0.035 0.208 0.858 0.699 0.637 0.540
14.9 17.0 18.6 12.8 15.3 18.1 17.1 15.5 16.9 13.8 16.3 16.0 1.8
0.636 0.511 0.551 0.706 0.697 0.598 0.617 0.787 0.670 0.717 0.665
1.11 1.13 1.05 0.96 1.25 4.16 1.50 0.97 1.05 1.04 1.13
a
Average7standard deviation except Aichi and Osaka is 16.972.5.
concluded that this 90Sr excess, which is mainly derived from river water outflow before 1978, has remained in surface seawater off Osaka. 3.2. Residence times of surface seawater
90
Sr and
137
Cs in coastal
The apparent effective half-lives of 90Sr and Cs calculated for all the 11 sampling sites for the period following the atmospheric nuclear weapons tests (1981–1998), but excluding the influence of the Chernobyl accident in 1986, are shown in Table 2. The concentrations of 137Cs in coastal seawater are diluted by river water, especially at Aichi (Fig. 7). Therefore all 137Cs concentration data were
137
normalized to their salinity (Table 2). After this normalization for Aichi, the correlation coefficient R2 changed from 0.035 to 0.60 and the apparent effective half-life of 137Cs changed from 75.3 to 18.1 years, which is similar to the other 10 sampling sites. Because the inflow of 137Cs from river water to seawater is negligible due to the relatively low concentration of 137Cs (Fig. 8), this normalization is an effective method, especially for seawater in which salinity varies significantly (Fig. 9). The obtained normalized apparent effective half-lives for 137Cs (mean 16 years) show good agreement with data obtained from surface waters in the open sea of the Sea of Japan (mean half-life 16 years, Miyao et al., 1998).
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7
35
Salinity
137
Cs/salinity
30
5
25
4
20
3
15
Ratio of
137
Activity concentration
137
Cs (mBq/l)
Cs
6
2
y = 3.664e-0.0211x R2 = 0.1736
1 137
(
y = 2.2533e-0.0397x R2 = 0.7271
Cs/Salinity) *10
0
10
5
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year
Fig. 9.
90
137
Cs activity levels and salinity in coastal seawater off Aichi.
The respective mean residence times (TM ) of Sr and 137Cs are given by
1=TM ¼ ln ð2Þ=TEF ln ð2Þ=TR ;
ð2Þ
where TEF is the apparent effective half-lives of both 90Sr and 137Cs, and TR is their radioactivedecay half-lives. The calculated TM of 90Sr and 137 Cs at 11 sampling sites ranged from 24.2 to 69.2 years and from 32.2 to 70.6 years with means of 41 and 51 years, respectively. These calculated TM ’s are likely to be slightly overestimated since additional inputs to seawater from fallout and river discharges after the atmospheric nuclear weapons test period are not considered. 3.3. The impact of the Chernobyl accident on Japanese coastal waters Clear concentration peaks of 137Cs were observed at all sampling sites at the time of the Chernobyl accident (Fig. 4). However, no significant increase of 90Sr concentrations was found (Fig. 3). The 137Cs/90Sr activity ratios in 1986 were 1.9, 1.8, 1.7, 1.9, 1.7, 1.6, 1.5 1.3 and 1.3 in seawater
offshore from Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Yamaguchi, Fukuoka, Kagoshima and Okinawa prefectures, respectively. In general, the ratios in northern Japan are somewhat higher than the global fallout ratio of 1.6 (UNSCEAR, 1993). The 137Cs data are corrected for 134 Cs contribution using Eqs. (3) and (4) shown below. The results can be explained by the fact that larger amounts of 137Cs were transported from Chernobyl (located at 51 N, 30 E) by the prevailing winds to northern Japan than to southern Japan (Aoyama et al., 1986, 1987). 90 Sr and 137Cs concentrations in coastal seawater at nine of the sampling sites have gradually decreased with time since 1974, except for the year 1986 when the Chernobyl accident resulted in a sharp increase of 137Cs (Fig. 6). By regression analysis of the 90Sr and 137Cs data (Fig. 6), the effect of the Chernobyl release can be estimated for 1986 data. The measured mean 90Sr and 137Cs concentrations were 3.4 and 6.0 mBq/l (5.6 mBq/l after correction for 134Cs contribution (Eqs. (3) and (4)), and the nuclear weapons fallout derived 137 Cs mean concentrations then amounts to 4.2 mBq/l. However, the increase of 90Sr due to
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the Chernobyl accident is not statistically significant. Based on release data from the Chernobyl accident (137Cs/90Sr activity ratio 4.7; IAEA, 1986), only minor additions of 90Sr to the nuclear weapons fallout derived 90Sr can be expected. In addition, Sr was much less volatilized than Cs and thus less abundant in tropospheric transport (UNSCEAR, 1988). This is also confirmed by the analysis of airborne dust and precipitation, where the average activity ratios of 137Cs/90Sr were 60 and 61, respectively, in 12 of the prefectures in Japan from April to June 1986: REFF C134 þ C137Cherno ¼ CMEA CEST ;
ð3Þ
C134 =C137Cherno ¼ C134REL EXP½ðln ð2Þ=T134 ÞT1 = C137REL ;
ð4Þ
where C134=mean 134Cs concentration (mBq/l) in seawater due to the Chernobyl accident at the date of measurement (9 months after the Chernobyl accident), C137Cherno=mean 137Cs concentration (mBq/l) in seawater due to the Chernobyl accident, REFF=counting efficiency ratio of 134Cs/137Cs for GM beta-ray (0.7), CMEA=measured mean 137Cs concentration in 1986 (6.01 mBq/l), CEST=estimated nuclear weapon-derived mean 137Cs concentration in 1986 (4.15 mBq/l), C134REL=released amount of 134Cs due to the Chernobyl accident (1.9 1016 Bq; IAEA, 1986), C137REL=released amount of 137Cs due to the Chernobyl accident (3.77 1016 Bq; IAEA, 1986), T134 =radioactivedecay half-life of 134Cs (2.06 year), T1 =date of measurement (nine months after the Chernobyl accident). The average concentration of 137Cs in precipitation in 32 prefectures, which include the previously mentioned 12 prefectures, from April to June in 1986 was 104 Bq/m2, which is about 160 times the corresponding average concentration observed in 1985. The Chernobyl-derived average 137Cs concentration in coastal surface seawater collected in July–August 1986 was 1.46 mBq/l (=1.46 Bq/m3), which indicates fast removal of Cs from the surface to deeper water. For example, for a total water depth of 10 m, the water column would have received about 100 Bq/m2 of 137Cs that corresponds to 10 Bq/m3, far higher than observed in surface water. This fast removal also has been
observed in surface water in the open sea of the Sea of Japan (Miyao et al., 1998). 3.4. 90Sr and 137Cs activity concentrations in seabed sediment and correlation with those in seawater The concentrations of 90Sr and 137Cs in seabed sediments collected at the same 11 sampling sites as seawater are shown in Figs. 10 and 11, respectively. It was found that the 137Cs values were higher than 90Sr at all the sites and in all years. The 90Sr data at all sampling sites showed a slight temporal change, with concentrations ranging from o0.4 to 1.2 Bq/kg. The 137Cs concentrations over 25 years off Aomori were the highest of the 11 sites and ranged from 4.6 to 10 Bq/kg. Those off Okinawa were the lowest and ranged from o0.4 to 0.93 Bq/kg. These ranges of concentrations in seabed sediments may reflect the differences in the nature of seabed sediments for the accumulation of 137Cs. It should be noted that the annually observed concentrations may not represent only depositing sediments as the samples were collected using a conventional grab sampler. This could be the reason why no significant increase of 137Cs was detected in sediment in 1986 at any site. For deep-sea samples (several thousand meters) collected in the Far Eastern Seas in 1995, the ratios of 137Cs/90Sr in surface and bottom waters, showed significant variation (Ikeuchi et al., 1999). The ratios in surface waters ranged from 1.5 to 1.9, with an average of 1.7, and those in deep waters ranged from 1.3 to 3.0 with an average of 2.3. The enrichment of 137Cs detected in bottom seawater compared to surface seawater may be due to the accumulation of Cs in phytoplankton (Wahlgren and Marshall, 1975). The ratios in seabed sediments at the same sites ranged from 3.3 to 12 with an average of 7.0 (Pettersson et al., 1999). The results show that enrichment of 137Cs against 90Sr in seabed sediments is occurring, which is in accord with data from the literature which show a higher distribution coefficient (Kd ) for Cs than for Sr. Since the 90Sr levels in coastal seabed sediments and in deep seas are lower or much lower than the concentrations of 137Cs, a likely conclusion is that
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1.2
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
Activity concentration Bq/kg
1.1 1 0. 9 0.8 0. 7 0.6
83 84 85
86 87 88 Year
Fig. 10.
90
89 90 91
92 93 94
95 96 97
Site
Sr activity levels in seabed sediment at 11 sites.
12.4
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
10.4 Activity concentration Bq/kg
98
Niigata Hokkaido
80 81 82
Osaka
77 78 79
Kanagawa
74 75 76
Fukuoka
0.4Bq/kg is detection limit
Okinawa
0.4
a
0. 5
8.4
6.4
4.4
2.4
0.4 0.4Bq/kg is 74 detection limit
75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Fig. 11.
137
Cs activity levels in seabed sediment at 11 sites.
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0.6
3.5
Atmospheric nuclear weapon tests
0.5
Chernobyl accident
after nuclear weapon tests
3 after Chernobyl accident
Activity concentration Bq/kg
90
Sr
2.5
2 0.3
137
0.2
90
Cs/ Sr
1.5
Activity ratio
0.4
137
Cs
1
0.1
0.5
0
0 75
76
77
78
79
80
81
82
83
83
84
84
85
85
86
86
87
87
88
88
Year 90
Fig. 12.
Sr and
137
Cs activity levels in gulfweed off Aomori.
0.4 Atmosperic nuclear weapon tests
0.35
after nuclear weapon tests
Chernobyl accident after Chernobyl accident
0.3 Activity concentration
137
90
Cs/ Sr
0.25 137
Cs
0.2 90
Sr
0.15 0.1 0.05 0 74
75
76
77
78
79
80
Fig. 13.
81
90
82
Sr and
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
137
Cs activity levels in wakame off Niigata.
Sr is more effectively removed from seawater by marine organisms than Cs. Results of 90Sr and 137 Cs analysis in gulfweed off Aomori and wakame off Niigata (Figs. 12 and 13), show a higher uptake
of 90Sr than 137Cs, especially during the period of atmospheric nuclear weapons tests. It is reported that Sr/Ca ratios in corals are generally higher compared to those in seawater (Sato and Ohde,
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2001). 137Cs/90Sr in seabed sediment in 1975, 1976 and 1980 off Okinawa, which mainly consists of white Ca-rich sand, probably derived from corals and shells, were 2.3, 2.4 and 2.1, respectively. In contrast, at the other 10 sites, the sediments are less Ca-rich and the 137Cs/90Sr activity ratios are generally higher (from 3.1 to 30, average 15.3 (Figs. 10 and 11)).
4. Conclusions The temporal variations of 90Sr and 137Cs concentrations in coastal surface seawater and seabed sediments offshore from the Japanese islands observed at 11 sampling sites during the 25 years from 1974 to 1998 indicate the following: (1) The 137Cs/90Sr activity ratios in coastal seawater during the nuclear test period were lower than the global fallout ratio due to the inflow of large amounts of 90Sr from river water. (2) After the atmospheric nuclear weapons test period, the concentrations of 90Sr and 137Cs in coastal seawater have gradually decreased with mean residence times of 41 years for 90Sr and 51 years for 137Cs at 11 sites. (3) In 1986, following the Chernobyl accident, a sharp increase in 137Cs levels was observed in airborne dust, in precipitation on the Japanese islands and in coastal surface seawater. However, in coastal waters, the 137Cs levels in surface water returned to pre-1986 levels quickly, indicating fast removal of Cs from the surface to deeper water. As expected, the levels of 90Sr were not significantly changed. (4) With the exception of sediments off Okinawa, much lower 90Sr than 137Cs concentrations were found in seabed sediments at all sites, showing the more effective scavenging of Cs from the water column. Okinawa sediments consist of Ca-rich sand, probably derived from corals and shells, thereby reflecting a higher uptake of Sr than Cs from the water column by marine organisms. Results of analysis of
2725
gulfweed and wakame also show higher accumulation of 90Sr than 137Cs confirming more effective removal of Sr than Cs by marine organisms.
Acknowledgements The author thanks the staff of the local government institutes of the 11 prefectures who collected the samples, and the colleagues of the JCAC who assisted in the analysis of the samples. He would also like to express his gratitude to Prof. K. Fuwa (Japan Chemical Analysis Center), Prof. P.P. Povinec (International Atomic Energy Agency-Marine Environmental Laboratory) and Prof. A. Masuda (University of Tokyo) for their kind support and suggestions. The described research work was funded by the Science and Technology Agency of Japan.
References Aoyama, M., Hirose, K., Suzuki, Y., Inoue, H., Sugimura, Y., 1986. High level radioactive nuclides in Japan in May. Nature 321, 819–820. Aoyama, M., Hirose, K., Sugimura, Y., 1987. Deposition of gamma-emitting nuclides in japan after the reactor-accident at chernobyl. Journal of Radioanalytical and Nuclear Chemistry Articles 116, 291–306. IAEA, 1986. Summary report on the post-accident review meeting on the Chernobyl accident. Safety Series No. 75INSAG-1 33-34. 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. List, R.J., Machta, L., Alexander L.T., Allen, J.S., Meyer, M.W., Valassis, V.T., Hardy Jr., E.P., 1965. Strontium-90 on the Earth’s Surface. In: Klement Jr., A.W. (Ed.), Radioactive Fallout from Nuclear Weapons Tests, Germantown MD., November 3–6. AEC Symposium Series, No.5 (CONF-765), 1964, pp. 359–368. Meyer, M.W., Allen, J.S., Alexander, L.T., Hardy, E., 1968. Strontium-90 on the Earth’s Surface. Summary and Interpretation of a World-Wide Soil Sampling Program:
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1961–1967 Results. USAEC Report TID-24341, Health and Safety Laboratory. 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. Nagaya, Y., Nakamura, K., 1981. Artificial radionuclides in the Western Northwest Pacific (I) 90Sr and 137Cs in the deep waters. Journal of the Oceanographical Society of Japan 37, 135–144. Norris, R.S., Burrows, A.S., Fieldhouse, R.W., 1994. British, French and Chinese Nuclear Weapons. Nuclear Weapons Databook, Vol. V. Natural Resources Defense Council (NRDC), Washington, DC, 1994. Pettersson, H.B.L., Amano, H., Berezhnov, V.I., Chaykovskaya, E., Chumichev, V.B., Chung, C.S., Gastaud, J., Hirose, K., Hong, G.H., Kim, C.K., Kim, S.H., Lee, S.H., Morimoto, T., Nikitin, A., Oda, K., Povinec, P.P., Suzuki, E., Tkalin, A., Togawa, O., Veletova, N.K., Volkov, Y., Yoshida, K., 1999. Anthropogenic radionuclides in sediments in the NW Pacific Ocean and its marginal seas: results of the 1994–1995 Japanese–Korean– Russian expeditions. The Science of the Total Environment 237/238, 213–224.
Reports to STA, 1974–1998. The results of 90Sr and 137Cs analysed by Japan Chemical Analysis Center in environmental samples in Japan. Science and Technology Agency of Japan (in Japanese). STA, 1976. Analytical method of radioactive cesium, Radioactivity Measurement Series No.3. Science and Technology Agency of Japan, Japan (in Japanese). STA, 1983. Analytical method of radioactive strontium, Radioactivity Measurement Series No.2. Science and Technology Agency of Japan, Japan (in Japanese). Sato, Y., Ohde, S., 2001. A Sr/Ca thermometer in coral aragonite. Chikyukagaku (Geochemistry) 35, 1–11. UNSCEAR, 1988. Sources and effects of ionizing radiation. United Nations Scientific Committee on the effects of atomic radiation. Report to the General Assembly, United Nations, New York. UNSCEAR, 1993. Sources and effects of ionizing radiation. United Nations Scientific Committee on the effects of atomic radiation. Report to the General Assembly, United Nations, New York. Wahlgren, M.A., Marshall, J.M., 1975. The behavior of plutonium and other long-lived radionuclides in lake Michigan. Proceedings of the Symposium on Impacts of Nuclear Releases into the Aquatic Environment. IAEA, Vienna, pp. 227–243.
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Temporal variations of 90Sr and 137Cs concentrations in Japanese coastal surface seawater and sediments from 1974 to 1998 Yoshihiro Ikeuchi* Japan Chemical Analysis Center, 295-3, San-no-cho, Inage, Chiba 263-0002, Japan Received 1 November 2001; received in revised form 31 January 2003
Abstract 90
Sr and 137Cs concentrations were determined in surface water and bottom sediments collected at 11 sites offshore from Japan during the period 1974–1998, to investigate their temporal variations and behaviour in the coastal marine environment. The concentrations of 90Sr and 137Cs in surface water have decreased with time since 1974. After the period of atmospheric nuclear weapons tests, the mean residence times of 90Sr and 137Cs were about 41 and 51 years, respectively. The 137Cs/90Sr activity ratios in coastal seawater during the atmospheric nuclear weapons tests (up until 1980) were lower than those after the tests due to the inflow of 90Sr in river water. A sharp increase in 137Cs levels was observed in airborne dust, in precipitation on the Japanese islands, and in coastal surface seawater in 1986 following the Chernobyl accident. However, the 137Cs levels in surface water returned to pre-1986 levels quickly, indicating rapid removal of Cs from the surface to deeper water. Concentrations of 90Sr in sediments were generally much lower than those for 137Cs, reflecting the more effective scavenging of Cs from the water column. In Ca-rich sediments, consisting of corals and shells, higher 90Sr levels and 90Sr/137Cs activity ratios were found, reflecting higher accumulation of Sr than Cs in marine organisms. Higher accumulation of 90Sr than 137Cs was also found in seaweed (gulfweed and wakame). r 2003 Elsevier Ltd. All rights reserved.
1. Introduction Large amounts of 90Sr and 137Cs have been deposited globally due to fallout from the atmospheric nuclear weapons tests conducted by the USA, USSR, UK, France and China from 1945 to 1980, and later due to the Chernobyl nuclear power plant accident in the former USSR in 1986. Thereby, large amounts of 90Sr and 137Cs were introduced into the coastal marine environment *Tel.: +81-43-424-8661; fax: +81-43-423-5326. E-mail address: [email protected] (Y. Ikeuchi).
via atmospheric fallout and river discharge. It is therefore important to investigate the behaviour and fate of these radionuclides in order to assess the degree of contamination in coastal areas. These areas are influenced both by the ocean and the adjacent terrestrial environment, and their relative degree of influence is unique to the individual site. Seawater and seabed sediments were collected annually by prefectural government institutes from 1974 to 1998 at 11 Japanese sites from the north to the south: Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Aichi, Osaka,
0967-0645/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0967-0645(03)00143-7
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2714
45
Hokkaido Aomori 40
Sea of Japan North latitude
Fukushima Tsushima Current
Niigata Aichi
35
Oyashio Current
Kanagawa
Yamaguchi Osaka Fukuoka Kuroshio Current
Kagoshima 30
Okinawa 25 130
135
140
145
East longitude
Fig. 1. Sampling sites (o) of seawater and seabed sediment.
Yamaguchi, Fukuoka, Kagoshima and Okinawa (Fig. 1). In addition, airborne-dust, precipitation, river water and marine organisms also were collected in some prefectures. Subsequent analyses of 90Sr and 137 Cs were performed at the Japan Chemical Analysis Center (JCAC). The resulting data have since been archived at the Science and Technology Agency (STA) (Reports to STA, 1974-1998). In this paper, the temporal variation of 90Sr and 137 Cs concentrations in Japanese coastal surface seawater over the 25-year period is described. Also discussed is the behaviour of 90Sr and 137Cs in coastal waters by comparing the results of 90Sr and 137 Cs analyses in seabed sediments, airborne-dust, precipitation, river water, and marine organisms, as well as the impact of the Chernobyl accident on Japanese coastal waters.
2. Sampling and analytical procedures 2.1. Seawater More than 40 l of seawater was collected by boat from each sampling site once a year. Sampling was performed in July or August, periods of low precipitation, in order to avoid dilution with
rainwater. All the seawater samples were acidified on site by adding about 80 ml of concentrated HCl and then sent to JCAC for analysis. The concentrations of 90Sr and 137Cs were determined using well-established radiochemical analytical methods (STA, 1976, 1983). About 40 kg of seawater was weighed accurately using a balance and transferred to a 65-l vessel. Fifty mg of Cs carrier was added to the vessel and 400 g of Na2CO3 was added while stirring the sample. The pH of the sample solution was adjusted to 9.6 by adding sodium carbonate gradually to separate Ca and Sr from Mg. The supernatant solution at the Ca–Sr carbonate precipitation phase was used for analysis of Cs. Sr in the carbonate precipitation was separated from Ca by using fuming HNO3. Ba and Ra were removed as chromates by precipitation at pH 5–5.5. The amount of Sr in seawater before analysis was calculated using salinity data. After scavenging of 90Y, SrCO3 was recovered and weighed to obtain the chemical yield. After 2 weeks, the 90Y was separated from 90Sr by the 90Y milking method, and the 90Y activity was measured using a low background gas flow GM beta-ray counter. The Cs in the supernatant was separated as Cs3PO4 12MoO3 in 5% HCl solution. The Cs3PO4 12MoO3 was dissolved by NaOH, and 87 Rb impurities were removed by the cation exchange method. After adding H2PtCl6 solution, Cs2PtCl6 was recovered and weighed to obtain the chemical yield and the beta-activity of 137Cs was measured using a low background gas flow GM beta-ray counter. Unfortunately, analysis of 134Cs was not performed since it was not included in the analytical program supported by the STA. The efficiency of beta-activity measurements by a GM beta-ray counter was determined using 90Sr and 137Cs international standard solutions calibrated by the Electro-technical Laboratory in Japan (total uncertainty 3.3% and 2.5% for 90Sr and 137Cs, respectively, at 95% confidence level). The detection limits (95% confidence level) for both 90Sr and 137Cs were about 0.5 mBq/l for seawater and about 0.4 Bq/kg for seabed sediment. The JCAC has regularly participated in intercomparison exercises organized by the WHO, IAEA and EML, and their results were consistent with recommended values.
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The salinity in seawater was determined at prefectural government institutes using a titration method for Cl (STA, 1983) or a salinometer, which measures electric conductivity using, e.g., TS-DIGI-AUTO to evaluate the effects of the dilution of seawater with river water or rainwater. 2.2. Seabed sediment About 4 kg of upper seabed sediments (surface layer of about 5 cm) were collected using a grab sampler at the same sampling sites as seawater at water depths of mainly 5–20 m (Fig. 2). After drying at 105 C, the sediments were sent to the JCAC for analysis. The concentrations of 90Sr and 137Cs in seabed sediments were determined using previously reported radiochemical analytical methods (STA, 1976, 1983). After 100 g of seabed sediment were weighed, 100 mg of Sr carrier and 50 mg of Cs carrier were added to the sample. Sr and Cs were extracted by nitric acid, and Sr was then separated as carbonate and oxalate precipitation. Sr in the oxalate was separated from Ca by fuming HNO3.
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The Cs in the supernatant solution at the carbonate precipitation stage was separated as Cs3PO4 12MoO3 in 5% HCl solution. Subsequent separation and activity counting steps were the same as for seawater samples. 90 Sr and 137Cs in airborne dust (104 m3 filtered air/3-months), monthly precipitation (0.5 m2 surface area), river water (100 l) and marine organism (1 kg wet weight) were also determined. The radiochemical separation and activity counting steps for these samples were similar to those used in the analysis of seabed sediment. The chemical yield using the described radiochemical analytical methods is expected to be more than 70% for 90Sr and 137Cs.
3. Results and discussion 3.1. 90Sr and 137Cs activity concentrations in coastal seawater—their temporal variations The temporal variations of 90Sr and 137Cs concentrations in seawater at the 11 coastal sites are shown in Figs. 3 and 4, respectively.
0
Water depths (m)
10
20
30
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
40
50
60
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 2. Water depths at sampling sites of seabed sediments.
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Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
10 9
Activity concentration n
8 7 6 5 4 3 2 1
Chernobyl accident Atmospheric nuclear weapon tests
After nuclear weapon tests
After Chernobyl accident
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 3.
90
Sr activity levels in coastal seawater at 11 sites in Japan.
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
14
Activity concentration mBq/l
12 10 8 6 4 2 0
Atmospheric nuclear weapon tests
Chernobyl accident
After nuclear weapon tests
After Chernobyl Accident
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 4.
137
Cs activity levels in coastal seawater at 11 sites in Japan.
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(‰ )
40 35 30
Salinity
25 20 Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
15 10 5 0
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 5. Salinity in the analysed coastal seawater at 11 sites in Japan.
The salinity in seawater off Hokkaido, Aomori, Fukushima, Kanagawa, Niigata, Yamaguchi, Fukuoka, Kagoshima and Okinawa ranged from 21.7% to 36.3% (Fig. 5) with an average of 32.2%. However, the salinity in seawater off Aichi and Osaka ranged from 6.7% to 31.4% (Fig. 5) with an average of 21.1%, which indicated a higher dilution with river water at these two sampling sites. The highest concentrations of 90Sr and 137Cs, observed in the 1970s, are due to the influence of 11 Chinese atmospheric nuclear weapons tests carried out from 1974 to 1980 (Norris et al., 1994). During this period, the annual average 90Sr concentrations adjacent to Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Yamaguchi, Fukuoka, Kagoshima and Okinawa (with the exception of Aichi and Osaka, where the effect of river water inflow was apparent) were between 4.3 and 5.7 mBq/l. The annual average 137Cs concentrations ranged between 5.2 and 7.2 mBq/l. After the testing period (1981–1998, with the exception of 1986) the 90Sr and 137Cs concentrations at the same nine sampling sites ranged between 2.5 and 2.8 mBq/l and 3.4 and 4.0 mBq/l, respectively.
During the period of nuclear weapons tests, slightly higher 90Sr and 137Cs concentrations were found in northern Japan compared to southwestern Japan (Figs. 3 and 4). This tendency may be due to the latitudinal distribution of 90Sr and 137Cs deposition, as the maximum 90Sr deposition rates in soil (in 1963 and early 1964) were found at latitudes 40–50 N (List et al., 1965). The cumulative 90Sr deposition, based on analyses of soils collected during 1965–1967 (Meyer et al., 1968), was about 3 GBq/km2 in Hokkaido, about 2.2 GBq/km2 in Honshu and Kyusyu, and about 1.5 GBq/km2 in Okinawa. The average concentration of 90Sr in precipitation in prefectures bordering the Sea of Japan from April to June in 1981, immediately after large-scale nuclear tests in China in 1980, was 10.2 MBq/km2 at 37 500 –39 430 , 8.6 MBq/km2 at 35 280 –35 310 and 7.5 MBq/km2 at 32 480 –33 300 (Reports to STA, 1974-1998). After the atmospheric nuclear weapons tests, the concentrations of 90Sr and 137Cs in coastal surface seawater showed only small geographical differences. The average concentrations of 90Sr and 137Cs and the activity ratios of 137Cs/90Sr at 9 of the
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16
2.5 Chernobyl accident
12
Atmospheric nuclear weapon tests
after Chernobyl accident
after nuclear weapon tests
2 137
10
90
Cs/ Sr
1.5
137
Cs
8 y = 7.017e-0.0404x
6
1
R2 = 0.8786
Activity ratio
Activity concentration mBq/l
14
4 0.5
90
Sr
2
Atmospheric nuclear weapon tests by China
y = 6.0386e-0.0519x R2 = 0.9832 0
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 6. Average
90
Sr and
137
Cs activity levels in coastal seawater at nine sites.
sampling sites (excluding Aichi and Osaka) are shown in Fig. 6. The ratios of 137Cs/90Sr ranged from 1.11 to 1.29, with an average of 1.17 during the Chinese atmospheric nuclear weapons test period (1974–1980), from 1.25 to 1.34 with an average of 1.29 after the Chinese atmospheric nuclear tests and before the Chernobyl accident (1981–1985), and from 1.24 to 1.67 with an average of 1.47 after the Chernobyl accident (1987–1998). In general, these results indicate lower 137Cs/90Sr ratios during nuclear weapons tests than those after the tests. The concentrations of 90Sr and 137Cs and the activity ratios of 137Cs/90Sr in seawater off Osaka, where low salinity has been observed, are shown in Fig. 7. The average ratio of 137Cs/90Sr during the Chinese atmospheric nuclear weapons tests is 0.58; the ratio is 0.57 for 1981–1985 and 0.83 for 1987– 1998. The concentrations of 90Sr and 137Cs in river water in Osaka are shown in Fig. 8. The average 90 Sr and 137Cs concentrations during the Chinese atmospheric nuclear weapons tests are 9.2 and 0.66 mBq/l, respectively. The average ratio of
137
Cs/90Sr in Osaka during the Chinese atmospheric nuclear weapons tests is 0.072; the ratio is 0.067 for 1981–1985 and 0.12 for 1987–1998. These results indicate that because 90Sr is released faster than 137Cs from soil to river water, more 90 Sr than 137Cs was flowing into the sea, especially during the nuclear weapons test period. The 90Sr and 137Cs concentrations of Osaka river water, seawater off Osaka and surface water in the Western Northwest Pacific Ocean (Nagaya and Nakamura, 1981) in 1978 (at the end of the Chinese atmospheric nuclear weapons test period) at similar latitudes, are shown in Table 1. When Osaka river water is mixed with surface water in the Pacific Ocean, the 90Sr and 137Cs concentrations in seawater off Osaka could be estimated using the following equation: C3 ¼ C2 ðS3 =S2 Þ þ C1 ð1 S3 =S2 Þ; 90
137
ð1Þ
where C1= Sr or Cs concentrations in Osaka river water, C2=90Sr or 137Cs concentrations in Pacific Ocean water, C3=calculated 90Sr or 137Cs concentrations in seawater off Osaka, S2=salinity
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10
2.5
8 7 6
2 90
137
Sr 137
90
Cs/ Sr
1.5
Cs
5 4
1
Activity ratio
Activity concentration mBq/l
9
3 2 1
Atmospheric nuclear weapon tests
Chernobyl accident
after nuclear
0.5
after Chernobyl
0
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year Fig. 7.
14
Activity concentration mBq/l
12
90
Sr and
137
Cs activity levels in coastal seawater off Osaka.
Atmospheric nuclear weapon tests
after nuclear
90
weapon tests
Sr
Chernobyl accident after Chernobyl accident
10 8 6 4 137
2
Cs
0 74 74 75 75 76 76 77 77 78 78 79 79 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87 88 88 89 89 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97 98 98 Year
Fig. 8.
90
Sr and
137
Cs activity levels in river water in Osaka.
in Pacific Ocean water, S3=salinity in seawater off Osaka. According to this calculation, the concentrations of 90Sr and 137Cs are 5.670.20 and 5.570.22 mBq/l in seawater off Osaka, respec-
tively. However, the measured concentrations of Sr and 137Cs in seawater off Osaka are 7.870.52 and 4.870.44 mBq/l, respectively. These results indicate a 20–35% excess of 90Sr. It can further be
90
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2720 Table 1 90 Sr and
137
Cs concentrations in river water, coastal seawater and the Pacific Ocean at almost the same latitude in 1978
Sample
North latitude
East latitude
Sampling date
90
137
Salinity (%)
River water in Osaka Seawater off Osaka Surface water in the Northwest Pacific Oceana
34 440 5800 34 380 3100 34 180 3000
135 340 3400 135 240 1000 141 580 3600
1978.07.21 1978.08.25 1978.03.03
10.470.44 7.870.52 3.770.22
0.7470.11 4.870.44 7.470.30
— 24.75 34.72
a
Sr (mBq/l)
Cs (mBq/l)
After Nagaya and Nakamura (1981).
Table 2 Effective half-lives of
90
Sr and
137
Cs in Japanese coastal surface seawaters during 1981–1998
Sampling site
Effective half-lives of 90 Sr (year)
Correlation coefficient, R2
Effective half-lives of 137 Cs, [A] (year)
Correlation coefficient, R2
Effective halflives of 137Cs normalized by salinity [B] (year)
Correlation coefficient, R2
[A]/[B]
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa Average Standard deviation
14.1 12.8 10.6 13.4 16.0 16.8 11.2 14.3 18.0 14.7 14.5 14.2 2.2
0.855 0.868 0.887 0.920 0.923 0.833 0.903 0.926 0.688 0.842 0.768
16.5 19.2 19.5 12.3 19.1 75.3 26.6 15.1 17.8 14.4 18.5 23.1 17.7a
0.718 0.655 0.475 0.799 0.705 0.035 0.208 0.858 0.699 0.637 0.540
14.9 17.0 18.6 12.8 15.3 18.1 17.1 15.5 16.9 13.8 16.3 16.0 1.8
0.636 0.511 0.551 0.706 0.697 0.598 0.617 0.787 0.670 0.717 0.665
1.11 1.13 1.05 0.96 1.25 4.16 1.50 0.97 1.05 1.04 1.13
a
Average7standard deviation except Aichi and Osaka is 16.972.5.
concluded that this 90Sr excess, which is mainly derived from river water outflow before 1978, has remained in surface seawater off Osaka. 3.2. Residence times of surface seawater
90
Sr and
137
Cs in coastal
The apparent effective half-lives of 90Sr and Cs calculated for all the 11 sampling sites for the period following the atmospheric nuclear weapons tests (1981–1998), but excluding the influence of the Chernobyl accident in 1986, are shown in Table 2. The concentrations of 137Cs in coastal seawater are diluted by river water, especially at Aichi (Fig. 7). Therefore all 137Cs concentration data were
137
normalized to their salinity (Table 2). After this normalization for Aichi, the correlation coefficient R2 changed from 0.035 to 0.60 and the apparent effective half-life of 137Cs changed from 75.3 to 18.1 years, which is similar to the other 10 sampling sites. Because the inflow of 137Cs from river water to seawater is negligible due to the relatively low concentration of 137Cs (Fig. 8), this normalization is an effective method, especially for seawater in which salinity varies significantly (Fig. 9). The obtained normalized apparent effective half-lives for 137Cs (mean 16 years) show good agreement with data obtained from surface waters in the open sea of the Sea of Japan (mean half-life 16 years, Miyao et al., 1998).
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7
35
Salinity
137
Cs/salinity
30
5
25
4
20
3
15
Ratio of
137
Activity concentration
137
Cs (mBq/l)
Cs
6
2
y = 3.664e-0.0211x R2 = 0.1736
1 137
(
y = 2.2533e-0.0397x R2 = 0.7271
Cs/Salinity) *10
0
10
5
0 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Year
Fig. 9.
90
137
Cs activity levels and salinity in coastal seawater off Aichi.
The respective mean residence times (TM ) of Sr and 137Cs are given by
1=TM ¼ ln ð2Þ=TEF ln ð2Þ=TR ;
ð2Þ
where TEF is the apparent effective half-lives of both 90Sr and 137Cs, and TR is their radioactivedecay half-lives. The calculated TM of 90Sr and 137 Cs at 11 sampling sites ranged from 24.2 to 69.2 years and from 32.2 to 70.6 years with means of 41 and 51 years, respectively. These calculated TM ’s are likely to be slightly overestimated since additional inputs to seawater from fallout and river discharges after the atmospheric nuclear weapons test period are not considered. 3.3. The impact of the Chernobyl accident on Japanese coastal waters Clear concentration peaks of 137Cs were observed at all sampling sites at the time of the Chernobyl accident (Fig. 4). However, no significant increase of 90Sr concentrations was found (Fig. 3). The 137Cs/90Sr activity ratios in 1986 were 1.9, 1.8, 1.7, 1.9, 1.7, 1.6, 1.5 1.3 and 1.3 in seawater
offshore from Hokkaido, Aomori, Niigata, Fukushima, Kanagawa, Yamaguchi, Fukuoka, Kagoshima and Okinawa prefectures, respectively. In general, the ratios in northern Japan are somewhat higher than the global fallout ratio of 1.6 (UNSCEAR, 1993). The 137Cs data are corrected for 134 Cs contribution using Eqs. (3) and (4) shown below. The results can be explained by the fact that larger amounts of 137Cs were transported from Chernobyl (located at 51 N, 30 E) by the prevailing winds to northern Japan than to southern Japan (Aoyama et al., 1986, 1987). 90 Sr and 137Cs concentrations in coastal seawater at nine of the sampling sites have gradually decreased with time since 1974, except for the year 1986 when the Chernobyl accident resulted in a sharp increase of 137Cs (Fig. 6). By regression analysis of the 90Sr and 137Cs data (Fig. 6), the effect of the Chernobyl release can be estimated for 1986 data. The measured mean 90Sr and 137Cs concentrations were 3.4 and 6.0 mBq/l (5.6 mBq/l after correction for 134Cs contribution (Eqs. (3) and (4)), and the nuclear weapons fallout derived 137 Cs mean concentrations then amounts to 4.2 mBq/l. However, the increase of 90Sr due to
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the Chernobyl accident is not statistically significant. Based on release data from the Chernobyl accident (137Cs/90Sr activity ratio 4.7; IAEA, 1986), only minor additions of 90Sr to the nuclear weapons fallout derived 90Sr can be expected. In addition, Sr was much less volatilized than Cs and thus less abundant in tropospheric transport (UNSCEAR, 1988). This is also confirmed by the analysis of airborne dust and precipitation, where the average activity ratios of 137Cs/90Sr were 60 and 61, respectively, in 12 of the prefectures in Japan from April to June 1986: REFF C134 þ C137Cherno ¼ CMEA CEST ;
ð3Þ
C134 =C137Cherno ¼ C134REL EXP½ðln ð2Þ=T134 ÞT1 = C137REL ;
ð4Þ
where C134=mean 134Cs concentration (mBq/l) in seawater due to the Chernobyl accident at the date of measurement (9 months after the Chernobyl accident), C137Cherno=mean 137Cs concentration (mBq/l) in seawater due to the Chernobyl accident, REFF=counting efficiency ratio of 134Cs/137Cs for GM beta-ray (0.7), CMEA=measured mean 137Cs concentration in 1986 (6.01 mBq/l), CEST=estimated nuclear weapon-derived mean 137Cs concentration in 1986 (4.15 mBq/l), C134REL=released amount of 134Cs due to the Chernobyl accident (1.9 1016 Bq; IAEA, 1986), C137REL=released amount of 137Cs due to the Chernobyl accident (3.77 1016 Bq; IAEA, 1986), T134 =radioactivedecay half-life of 134Cs (2.06 year), T1 =date of measurement (nine months after the Chernobyl accident). The average concentration of 137Cs in precipitation in 32 prefectures, which include the previously mentioned 12 prefectures, from April to June in 1986 was 104 Bq/m2, which is about 160 times the corresponding average concentration observed in 1985. The Chernobyl-derived average 137Cs concentration in coastal surface seawater collected in July–August 1986 was 1.46 mBq/l (=1.46 Bq/m3), which indicates fast removal of Cs from the surface to deeper water. For example, for a total water depth of 10 m, the water column would have received about 100 Bq/m2 of 137Cs that corresponds to 10 Bq/m3, far higher than observed in surface water. This fast removal also has been
observed in surface water in the open sea of the Sea of Japan (Miyao et al., 1998). 3.4. 90Sr and 137Cs activity concentrations in seabed sediment and correlation with those in seawater The concentrations of 90Sr and 137Cs in seabed sediments collected at the same 11 sampling sites as seawater are shown in Figs. 10 and 11, respectively. It was found that the 137Cs values were higher than 90Sr at all the sites and in all years. The 90Sr data at all sampling sites showed a slight temporal change, with concentrations ranging from o0.4 to 1.2 Bq/kg. The 137Cs concentrations over 25 years off Aomori were the highest of the 11 sites and ranged from 4.6 to 10 Bq/kg. Those off Okinawa were the lowest and ranged from o0.4 to 0.93 Bq/kg. These ranges of concentrations in seabed sediments may reflect the differences in the nature of seabed sediments for the accumulation of 137Cs. It should be noted that the annually observed concentrations may not represent only depositing sediments as the samples were collected using a conventional grab sampler. This could be the reason why no significant increase of 137Cs was detected in sediment in 1986 at any site. For deep-sea samples (several thousand meters) collected in the Far Eastern Seas in 1995, the ratios of 137Cs/90Sr in surface and bottom waters, showed significant variation (Ikeuchi et al., 1999). The ratios in surface waters ranged from 1.5 to 1.9, with an average of 1.7, and those in deep waters ranged from 1.3 to 3.0 with an average of 2.3. The enrichment of 137Cs detected in bottom seawater compared to surface seawater may be due to the accumulation of Cs in phytoplankton (Wahlgren and Marshall, 1975). The ratios in seabed sediments at the same sites ranged from 3.3 to 12 with an average of 7.0 (Pettersson et al., 1999). The results show that enrichment of 137Cs against 90Sr in seabed sediments is occurring, which is in accord with data from the literature which show a higher distribution coefficient (Kd ) for Cs than for Sr. Since the 90Sr levels in coastal seabed sediments and in deep seas are lower or much lower than the concentrations of 137Cs, a likely conclusion is that
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1.2
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
Activity concentration Bq/kg
1.1 1 0. 9 0.8 0. 7 0.6
83 84 85
86 87 88 Year
Fig. 10.
90
89 90 91
92 93 94
95 96 97
Site
Sr activity levels in seabed sediment at 11 sites.
12.4
Hokkaido Aomori Niigata Fukushima Kanagawa Aichi Osaka Yamaguchi Fukuoka Kagoshima Okinawa
10.4 Activity concentration Bq/kg
98
Niigata Hokkaido
80 81 82
Osaka
77 78 79
Kanagawa
74 75 76
Fukuoka
0.4Bq/kg is detection limit
Okinawa
0.4
a
0. 5
8.4
6.4
4.4
2.4
0.4 0.4Bq/kg is 74 detection limit
75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Fig. 11.
137
Cs activity levels in seabed sediment at 11 sites.
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0.6
3.5
Atmospheric nuclear weapon tests
0.5
Chernobyl accident
after nuclear weapon tests
3 after Chernobyl accident
Activity concentration Bq/kg
90
Sr
2.5
2 0.3
137
0.2
90
Cs/ Sr
1.5
Activity ratio
0.4
137
Cs
1
0.1
0.5
0
0 75
76
77
78
79
80
81
82
83
83
84
84
85
85
86
86
87
87
88
88
Year 90
Fig. 12.
Sr and
137
Cs activity levels in gulfweed off Aomori.
0.4 Atmosperic nuclear weapon tests
0.35
after nuclear weapon tests
Chernobyl accident after Chernobyl accident
0.3 Activity concentration
137
90
Cs/ Sr
0.25 137
Cs
0.2 90
Sr
0.15 0.1 0.05 0 74
75
76
77
78
79
80
Fig. 13.
81
90
82
Sr and
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
137
Cs activity levels in wakame off Niigata.
Sr is more effectively removed from seawater by marine organisms than Cs. Results of 90Sr and 137 Cs analysis in gulfweed off Aomori and wakame off Niigata (Figs. 12 and 13), show a higher uptake
of 90Sr than 137Cs, especially during the period of atmospheric nuclear weapons tests. It is reported that Sr/Ca ratios in corals are generally higher compared to those in seawater (Sato and Ohde,
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2001). 137Cs/90Sr in seabed sediment in 1975, 1976 and 1980 off Okinawa, which mainly consists of white Ca-rich sand, probably derived from corals and shells, were 2.3, 2.4 and 2.1, respectively. In contrast, at the other 10 sites, the sediments are less Ca-rich and the 137Cs/90Sr activity ratios are generally higher (from 3.1 to 30, average 15.3 (Figs. 10 and 11)).
4. Conclusions The temporal variations of 90Sr and 137Cs concentrations in coastal surface seawater and seabed sediments offshore from the Japanese islands observed at 11 sampling sites during the 25 years from 1974 to 1998 indicate the following: (1) The 137Cs/90Sr activity ratios in coastal seawater during the nuclear test period were lower than the global fallout ratio due to the inflow of large amounts of 90Sr from river water. (2) After the atmospheric nuclear weapons test period, the concentrations of 90Sr and 137Cs in coastal seawater have gradually decreased with mean residence times of 41 years for 90Sr and 51 years for 137Cs at 11 sites. (3) In 1986, following the Chernobyl accident, a sharp increase in 137Cs levels was observed in airborne dust, in precipitation on the Japanese islands and in coastal surface seawater. However, in coastal waters, the 137Cs levels in surface water returned to pre-1986 levels quickly, indicating fast removal of Cs from the surface to deeper water. As expected, the levels of 90Sr were not significantly changed. (4) With the exception of sediments off Okinawa, much lower 90Sr than 137Cs concentrations were found in seabed sediments at all sites, showing the more effective scavenging of Cs from the water column. Okinawa sediments consist of Ca-rich sand, probably derived from corals and shells, thereby reflecting a higher uptake of Sr than Cs from the water column by marine organisms. Results of analysis of
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gulfweed and wakame also show higher accumulation of 90Sr than 137Cs confirming more effective removal of Sr than Cs by marine organisms.
Acknowledgements The author thanks the staff of the local government institutes of the 11 prefectures who collected the samples, and the colleagues of the JCAC who assisted in the analysis of the samples. He would also like to express his gratitude to Prof. K. Fuwa (Japan Chemical Analysis Center), Prof. P.P. Povinec (International Atomic Energy Agency-Marine Environmental Laboratory) and Prof. A. Masuda (University of Tokyo) for their kind support and suggestions. The described research work was funded by the Science and Technology Agency of Japan.
References Aoyama, M., Hirose, K., Suzuki, Y., Inoue, H., Sugimura, Y., 1986. High level radioactive nuclides in Japan in May. Nature 321, 819–820. Aoyama, M., Hirose, K., Sugimura, Y., 1987. Deposition of gamma-emitting nuclides in japan after the reactor-accident at chernobyl. Journal of Radioanalytical and Nuclear Chemistry Articles 116, 291–306. IAEA, 1986. Summary report on the post-accident review meeting on the Chernobyl accident. Safety Series No. 75INSAG-1 33-34. 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. List, R.J., Machta, L., Alexander L.T., Allen, J.S., Meyer, M.W., Valassis, V.T., Hardy Jr., E.P., 1965. Strontium-90 on the Earth’s Surface. In: Klement Jr., A.W. (Ed.), Radioactive Fallout from Nuclear Weapons Tests, Germantown MD., November 3–6. AEC Symposium Series, No.5 (CONF-765), 1964, pp. 359–368. Meyer, M.W., Allen, J.S., Alexander, L.T., Hardy, E., 1968. Strontium-90 on the Earth’s Surface. Summary and Interpretation of a World-Wide Soil Sampling Program:
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Y. Ikeuchi / Deep-Sea Research II 50 (2003) 2713–2726
1961–1967 Results. USAEC Report TID-24341, Health and Safety Laboratory. 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. Nagaya, Y., Nakamura, K., 1981. Artificial radionuclides in the Western Northwest Pacific (I) 90Sr and 137Cs in the deep waters. Journal of the Oceanographical Society of Japan 37, 135–144. Norris, R.S., Burrows, A.S., Fieldhouse, R.W., 1994. British, French and Chinese Nuclear Weapons. Nuclear Weapons Databook, Vol. V. Natural Resources Defense Council (NRDC), Washington, DC, 1994. Pettersson, H.B.L., Amano, H., Berezhnov, V.I., Chaykovskaya, E., Chumichev, V.B., Chung, C.S., Gastaud, J., Hirose, K., Hong, G.H., Kim, C.K., Kim, S.H., Lee, S.H., Morimoto, T., Nikitin, A., Oda, K., Povinec, P.P., Suzuki, E., Tkalin, A., Togawa, O., Veletova, N.K., Volkov, Y., Yoshida, K., 1999. Anthropogenic radionuclides in sediments in the NW Pacific Ocean and its marginal seas: results of the 1994–1995 Japanese–Korean– Russian expeditions. The Science of the Total Environment 237/238, 213–224.
Reports to STA, 1974–1998. The results of 90Sr and 137Cs analysed by Japan Chemical Analysis Center in environmental samples in Japan. Science and Technology Agency of Japan (in Japanese). STA, 1976. Analytical method of radioactive cesium, Radioactivity Measurement Series No.3. Science and Technology Agency of Japan, Japan (in Japanese). STA, 1983. Analytical method of radioactive strontium, Radioactivity Measurement Series No.2. Science and Technology Agency of Japan, Japan (in Japanese). Sato, Y., Ohde, S., 2001. A Sr/Ca thermometer in coral aragonite. Chikyukagaku (Geochemistry) 35, 1–11. UNSCEAR, 1988. Sources and effects of ionizing radiation. United Nations Scientific Committee on the effects of atomic radiation. Report to the General Assembly, United Nations, New York. UNSCEAR, 1993. Sources and effects of ionizing radiation. United Nations Scientific Committee on the effects of atomic radiation. Report to the General Assembly, United Nations, New York. Wahlgren, M.A., Marshall, J.M., 1975. The behavior of plutonium and other long-lived radionuclides in lake Michigan. Proceedings of the Symposium on Impacts of Nuclear Releases into the Aquatic Environment. IAEA, Vienna, pp. 227–243.