distribution of137cs and239,240pu in the sediment of the seto inland sea
TRANSCRIPT
Journal o f Radioanalytical and Nuclear Chemistry, Articles, Vol. 138, No. 1 (1990) 153- (64
D I S T R I B U T I O N O F 137CS AND 2 3 9 , 2 4 0 p u IN T H E S E D I M E N T O F T H E S E T O
I N L A N D SEA
KIYOSHI NAKAMURA AND YUTAKA NAGAYA
Division of Marine Roadioecology, National Institute of Radiological Sciences, 3609, Isozaki,
Nakaminato lbaraki 311-12, Japan
(Received June 13, 1989)
The contents and distribution patterns of 137Cs and 239,240pu in the
sediments of the Seto Inland Sea were determined. In most of the sediment columns,
approximately uniform concentrations were found to a depth of around 20 cm,
probably due to biotarbation. Total amounts of i37Cs in the sediment column are
considerably lower than the global fallout inputs. It is suggested that a significant
portion of 137Cs input was removed from the Seto Inland Sea to the open ocean.
Large excess inventories of 239,240pu over fallout input were observed in the
investigated sediment columns.
INTRODUC'TION
The Seto Inland Sea is the largest inland sea in Japan. It has an area of about 20,000 km 2 and a
mean water depth of about 40 m (1).
Recent intense industrialization made this region one of the most polluted coastal seas in Japan.
The distribution of pollutants in this sea have been widely investigated to determine the present levels of
pollution, especially for substances originated from manufacturing industries.
As opposed to these pollutants, 137Cs and 239,24~ are believed to originate only from global
fallout and are not supplied by the manufactur ing industries. So, by us ing these nuclides as a
geochemical tracer, useful information concerning the behavior of materials in this sea area will be
obtained.
In this report, the contents and distribution patterns of 137Cs and 239,240Pu in the sediments of
the Seto Inland Se a are determined to find out if there exists any distinctive characteristics w h ch
accumulate pollutants.
Elsevier Sequoia S. A., Lausar. e A kaddmiai Kiadt, Bud- ~e st
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF f a 7 Cs AND 2 a 9,24o Pu
Hiroshima Bay Aki Nada
Japan S e a , ~ ' / ~ .<-.,./ /r A i<
131 132 133 134 135"E
Hiuehi Nada Bingo Nada
....-Osaka Bay
Harima Nada
34"N
~ K i i $uido
33
Figure 1. Sampling Stations
MATERIALS AND METHODS
Samples discussed in this report are eight sea sediment cores and one estuarine sediment core.
Two surface sea water samples were also analyzed for the calculation of the radionuclide inventories.
All samples were obtained on cruises of R/V Toyoshio-Maru, Hiroshima University from 1983
to 1986, by using water pumping systems and gravity corers of J.C. Burke's type. The locations of
the sampling stations are shown in Figure 1.
The cores were sectioned into 1-6 cm intervals and radiochemical analyses were carried out for
each section, ff a sample volume was insufficient for analysis, some serial sections were combined and
used as one sample. Twenty to sixty grams of dried sediments were used for analysis. After leaching
with hot 8N nitric acid, the solution was treated by an anion exchange method for separating Pu and
Cs. Plutonium was purified by repeated anion exchange processes and finally electrodeposited onto a
stainless Steel disc. About 30 mBq of 242pu (US-NBS standard solution) was used for each sample as
plutonium yield monitor and plutonium isotopes were identified by alpha ray spectrometry, in which
242pu and 236Pu were used as alpha ray energy standards.
Cesium was precipitated by the use of ammonium molybdophosphate from the Cs effluent of
the anion exchange processes, purified by means of a cation exchange method, and precipitated in the
form of cesium chloroplatinate. Chemical recovery of 137Cs was determined by the gravimetric method
(40 mg of Cs+; cesium chloride-HC1 solution as a carrier), and the beta radioactivity was measured
with a low-background gas-flow counter.
For sea water analysis, i00 liters of unfiltered surface sea waters were used. Plutonium and
cesium were sequentially separated from every sample by co-precipitation with ferric hydroxide and
154
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 1 a7 Cs AND 2a9,24o Pu
ammonium molybdophosphate, respectively. The purification and measuring procedures of both
radionuclides were the same for the sediment samples. Chemical setrarations and purification
procedures are described in our previous reports (2 - 4).
RESULTS
The analytical data for surface sea waters and sediment cores are shown in Tables 1 and 2,
respectively. The radioactive decay of 137Cs is corrected to the dates of sampling. In these Tables,
precision criterion (standard deviation) is estimated from counting statistics only.
The vertical distribution profiles of 137Cs and 239,240pu in the sediment at Aid Nada and Kii
Suido are illustrated in Figures 2 and 3, respectively.
The Japan Chemical Analysis Center has measured 90Sr and 137Cs in surface sea water from 11
coastal areas in Japan and the level of 137Cs ranged from 300-520 mBq/100 1 in 1984, 300-520
mBq/100 1 in 1985, and 330-780 mBq/100 1 in 1986 (5). Our 137Cs results were well within the
range of these values and the Seto Inland Sea does not seem to be the only region with a 137Cs content.
For all the sediment investigated, the levels of 137Cs and 239,240pu content were less for the Seto
Inland Sea than those of the Tokyo Bay sediment (4).
DISCUSSION
Distribution of 137Cs and 239,240pu in the sediment
Approximately uniform contents were found to a depth of around 20 cm for each radionuclide
except the Osaka Bay sediment. In the Osaka Bay sediment, the uniform layer was somewhat thinner
than in other regions.
Table 1 Results of analysis of surface sea water
Station Sampling z:9. a4Opu '=~Cs Location
(Water Depth) Date (mBq/ I00~) (mBq/ 100~) Pu/Cs
Harima Nada 34 ~ 18.5 " N 11/I I / " ~6
(36m) 134" 27.0 " E
Aki Nada 34" 06.0 "N I I / I0 / '86
(37m) 132 ~ 42.0 " E
0.7 + 0.2 310 +_. 20 0.0022
1.3 + 0.2 440 + 110 0.0030
155
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF t37 Cs AND 239,240 Pu
Table 2 Results of analysis of sediment cores
Depth 239'240pu 137Cs Pu/Cs (cm) (mBq/kg.dry) (mBq/kg.dry)
Ki i Suido (34~ 134~ 0 - 2 730 • 24 4260 • 410 0.17 4 - 6 615 • 20 4890 • 440 0.13 8 -10 637 • 21 4480 • 410 0.14
12 -14 637 �9 20 4330 • 370 0.15 16 -18 681 • 23 4260 • 370 0.16 20 -22 437 • 18 2480 • 260 0.18 24 -26 163 �9 10 1630 • 300 0 . i 0 28 -30 120 • 8 1150 ~ 260 0 . I0 32 -34 24 • 3 220 • 150 0.12 36 -38 14 • 3 440 • 220 0.03
Water Depth 70.5 m, Date 11/13/ '84
Osaka Bay (34~ 135~ 0 - 6 385 • 27 1900 • 150 0.20 6 - 8 335 • 21 1820 • 150 0.19 8 -10 241 �9 17 1650 • 150 0.15
i0 -12 252 • 18 1400 • 140 0 . 1 8 12 -14 267 ~ 13 1390 • 140 0.19 14 -16 89 • 7 970 * 130 0.09 16 -18 48 • 6 520 • 70 0.09 18 -20 52 • 5 220 • 150 0.23 20 -22 59 • 5 670 • 220 0.09 22 -24 56 • 4 260 • 150 0.21 24 -26 36 • 4 0 • 190 - 26 -28 4 • 2 370 • 220 0.01 28 -30 16 • 5 190 • 300 0.08 30 -32 25 • 3 260 • 300 0.10 32 -34 83 • 6 370 • 150 0.22 34 -36 29 • 5 70 �9 150 0.40 36 -38 44 • 3 150 �9 300 0.30 38 -40 7 • 2 40 • 330 0.20
Water Depth 36 m, Date 11/14/ '84
Bingo Nada (34~ 133~ 0 - 3 344 • 13 3000 • 190 0.11 3 - 6 389 ~ 26 2330 • 190 0.17 6 - 9 381 ~ 19 2850 • 220 0.13 9 -12 378 • 11 2890 • 190 0.13
12 -15 437 m 15 4150 • 260 0 . I I 15 -18 378 • 19 2890 • 190 0.13 18 -21 352 • 15 2560 • 190 0.14 21 -24 178 • 19 3040 • 220 0.06 24 -27 126 ~ 7 2410 • 190 0.05 27 -30 174 • I i - 30 -33 100 • 7 1700 • 190 0.06 33 -36 38 • 4 1370 • 220 0.03 36 -39 i00 • 7 1520 ~ 260 0.07 39 -42 30 • 19 1410 • 220 0.02
Water Depth 29 m, Date 11/14/ '86
156
Table 2. (cont'd)
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF ta t Cs AND :39,240 Pu
Depth 239'240pu 137Cs Pu/Cs (cm) (mBq/kg.dry) (mBq/kg.dry)
Hiuchi Nada (34~ 133~ 0 - 4 693 ~ 33 3070 • 270 0.23 4 - 6 874 ~ 52 3300 • 280 0.26 6 - 8 837 • 52 3090 • 270 0.27 8 - I0 652 • 48 3540 ~ 300 0.18
I0 -12 652 • 37 3080 • 280 0.21 12 -14 711 • 37 2900 • 260 0.25 14 -16 756 • 44 3610 • 290 0.21 16 -18 763 • 52 3030 • 270 0.25 18 -20 656 • 41 2640 ~ 260 0.25 20 -22 674 • 48 3340 • 290 0.20 22 -24 270 • 19 2000 • 250 0.14 24 -26 256 • 15 1640 s 220 0.16 26 -28 126 • 11 1360 • 210 0.09 28 -30 122 • 7 1220 • 190 0 . I0 30 -32 74 • 7 1120 • 190" 0.07 32 -34 67 • 7 860 • 170 0.08 34 -36 26 • 4 620 • 160 0.04 36 -38 26 • 4 560 • 160 0.05 38 -40 33 • 7 860 • 180 0.04 40 -42 26 • 4 330 • 150 0.08
Water Depth 27 m, Daze 11/18/'83
Aki Nada (34~ 132~ 0 - 3 630 • 52 2150 • 150 0.29 3 - 6 674 ~ 26 2370 • 190 0.28 6 - 9 674 • 37 2190 • 190 0.31 9 -12 911 • 67 2560 • 190 0.36
12 -15 933 • 56 2410 • 150 0.39 15 -18 741 ~ 44 2300 • 190 0.32 18 -21 378 i 26 1260 • 150 0.30 21 -24 119 ~ 30 i000 • 150 0.11 24 -27 48 • 4 560 • 110 0.09 27 -30 33 • 4 410 • 110 0.08 30 -33 167 * 15 700 • 150 0.24 33 -36 119 • 15 440 • 150 0.27
Water Depth 37 m, Date 11/12/ '86
K; NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 1 a 7 Cs AND 2 3 9,z,* o Pu
Table Z (cont'd}
Depth 239'240pu 137Cs Pu/Cs (cm) (mBq/kg.dry) (mBq/kg.dry)
Hiroshima Bay A (33~ 132~ 0 - 4 2462 • 6130 • 480 0.40 4 - 6 1310 • 88 5420 • 370 0.24 6 - 8 821 • 53 5720 • 360 0.14 8 -10 1596 • 97 5410 • 340 0.30
10 -12 1311 • 76 6670 �9 400 0.20 12 -14 1249 • 73 6530 • 430 0.19 14 -16 1244 • 60 6490 • 380 0.19 16 -18 1152 • 61 6120 • 410 0.19 18 -20 901 • 47 4500 • 320 0.20 20 -22 536 • 29 4630 • 330 0.12 22 -24 741 • 41 3940 • 310 0.19 24 -26 512 • 30 3530 r 290 0.15 26 -28 431 • 33 3050 • 260 0.14 28 -30 260 • 26 2740 • 270 0.09 30 -32 179 • 17 2190 • 240 0~08 32 -34 228 • 3 1660 • 210 0.14 34 -36 159 • 14 970 • 200 0.16 36 -38 65 • 11 410 i 180 0.16 38 -40 21 • 7 410 • 190 0.05 40 -42 11 • 13 360 • 190 0.03 42 -44 0 • 9 290 • 170
Water Depth 40 m, Date 11/18/'83
Hiroshima Bay B (34~ 132~ 0 - 3 841 • 26 2370 • 190 0.35 3 - 6 889 • 30 3260 • 190 0.27 6 - 9 1144 • 37 2700 • 190 0.42 9 -12 1115 • 33 2000 • 520 0.56
12 -15 963 • 33 2370 • 150 0.41 15 -18 330 • 19 1480 • 150 0.22 18 -21 93 • 15 700 • 110 0.13 21 -24 304 �9 44 960 • 150 0.32 24 -27 59 �9 7 780 • 190 0.08 27 -30 44 • 4 960 • 150 0.05 30 -33 122 �9 11 740 • 190 0.16 33 -36 156 • 11 960 • 190 0.16
Water Depth 30 m, Date 11/13/'86
t 5 8
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 1 a 7 Cs AND 2 a 9,2 4o Pu
Tab~2~ont'd)
Depth 239'240pu 137Cs Pu/Cs ( c m ) (mBq/kg.dry) (mBq/kg.dry)
Hiroshima Bay C (34~ 132~ 2 - 4 993 • 33 5740 • 330 0.17 6 - 8 996 �9 41 5780 • 410 0.17
10 -12 996 • 63 5670 • 560 0.18 14 -16 1170 • 63 6220 • 370 0.19 18 -20 815 • 56 5670 • 370 0.14 22 -24 307 • 52 3440 • 300 0.09 26 -28 141 • 11 2740 • 330 0.05 30 -32 67 • 7 2630 • 300 0.03 34 -36 67 • 7 1930 • 260 0.03
Water Depth 33 m, Date 6/12/'85
Hiroshima Bay D ( r iver mouth) (34~ 132~ 0 - 3 344 • 22 8190 • 480 0.04 3 - 6 311 • 15 8110 • 330 0.04 6 - 9 285 • 11 7300 • 300 0.04 9 -12 315 • 15 8070 • 300 0.04
12 -15 3~ • 15 8300 • 330 0.04 15 -18 315 • 7 8040 • 370 0.04 18 -21 274 • 15 7700 • 440 0.04 21 -24 444 • 22 10590 • 300 0.04 24 -27 285 • 15 8960 • 300 0.03 27 -30 393 • 19 9480 • 370 0.04 30 -33 344 • 15 7930 • 330 0.04 33 -36 374 • 19 7960 • 410 0.05 36 -39 404 • 19 7480 • 370 0.05 39 -42 352 • 19 8190 • 370 0.04
Water Depth I0.5 m, Date 11/13/'86
This mixed surface layer is supposed to be due to-sediment particle mixing by bioturbation after
deposition. ' In the Seto Inland Sea, almost all the area could be bottom trawl fishing ground, so
physical mixing by fishing devices is also probable (6). However, since no irregular distribution
pattern was obtained in all the sediment cores investigated in this work, the effect of this artificial
physical mixing can hardly be distinguished.
In the case of fallout radionuclides, their deposition patterns show maxima from 1962 to 1964
and the annual variation of 137Cs and 90Sr in surface sea water corresponded closely to those of dry
deposition. Accordingly, in such a shallow coastal area, fallout radionuclides reach the bottom with
very short time lag and subsurface maxima is supposed to be recorded in the sediment. But there are
only a few cases in which fallout deposition patterns were well preserved in the vertical profile of a
coastal sediment column. Our sediment data does not show any historical distribution prof'fle and it
could be assumed that the rate of sedimentation is small relative to the biological mixing depth in the
Seto Inland Sea (7, 8).
There are many reports that show the penetration patterns of fallout radionuelides into deep
layers does not coincide with particle mixing rates calculated from 210pb dating (9-13).
159
�9 K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF t3v Cs AND 239'240 Pu
Concentration (mBq/kg.dry)
0 1000 2000 3000 Pu/Cs
0.29
0.28
0.31
10 " J 0.36
I 0.0 ] 0.33
20 0.3O Q.
13 0.12
0.09
0.08
30 0.23
0.26
~ Pu
40 l I J cs
Figure 2. Distribution profile of 137Cs and 239,240pu in the sediment at Aki Nada
Heavy metal pollution in the sediments were studied at several areas of the Seto Inland Sea by
Matsumoto et al. (14) and Hoshika et al. (15-18), using a 21~ technique to determine the
accumulation rates of sediment. According to their results, the history of anthropogenic loads of
copper and zinc was traced in the sediment, and the effects of regulation of pollutant discharge in the
1970's was also found in some cores.
Tanabe et al. (19) determined the distribution of PCB in the sediment of Hiuchi Nada and found
the subsurface maxima at a depth of about 10 cm indicating the history of PCB production from 1954
to 1972. But their sampling station was in the most polluted area and very near industrial factories, so
that biological activity may be poor and there remains the possibility that biological reworking of the
sediment particles was not enough to affect the sediment accumulation pattern.
160
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 137 Cs AND 2$9,24(11~
o
J~
o .
13
Concent ra t ion (mBq/kg.dry)
Pu lCs 1000 2000 3000 4000 5000 I ! I I I
0.13
10
20
30
40
�89 VIA
Y/A
YA I
0.14
0.15
0.16
0,18
1 0.10
Pu
I los
i 0.10
0 . t 2
0.03
Figure 3. Distribution profile of 137Cs and 239,240pu in the sediment at Kii Suido
The fact that distribution profiles of 137Cs and 239,240pu differ from those of heavy metal and
PCB in the same area imply other mixing mechanism of sediment particles after deposition on the sea
bottom. One attractive answer is that the vertical transport of fallout radionuclides by benthic fauna has
preferentially occured in the coastal regions and in the open ocean, as mentioned in our previous reports
(17, 20) as well as in others (9, I1),
Distribution of 137Cs and 239,2a0.pu. in the Estuarine Sediment Core
The sampling station D for Hiroshima Bay is located at the mouth of the Ohta river, the ~argest
river entering this bay.
The mean Pu/Cs ratio of this sediment is 0.04 and the value is several times lower than the
mean ratios o f marine sediments of 3 stations of Hiroshima Bay. This seems to indicate that the main
161
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF a ~ 7 Cs AND 239 ~24 o Pu
origin of this sediment is river sediment particles and that those Pu/Cs ratios were kept in estuarine
condition.
Uniform contents were found down to a depth of 42 cm, showing the heavy physical and
biological mixing. There are many artificial factors affecting the sediment of this area; for example,
sewage discharge from the city with a large population, reclamation work on the shore, an oyster
culturing field, etc., so that the dominant factor for such mixing could not be specified.
Inventory balance
To evaluate the role of sediment as a sink for radionuclides in the sea, inventories (total
amounts) of 137Cs and 239,240pu in each sediment column were calculated. The results are shown in
Table 3.
There seems to exist significant regional differences in inventories of 239,240pu and 137Cs. Considerable differences were also found in the Pu/Cs ratios among marine sediment columns. For the
discussion of these facts, detailed information on local meteorological and topographical conditions are
required as well as oceanographical data.
The fallout inputs were also calculated from data on the global distribution of fallout to compare
with the inventory data (20). The results of calculation for 30 ~ N - 40 ~ N latitude band are 2,400
MBq/krn2 for 137Cs and 40 MBq/km 2 for 239,240pu in 1983.
The large excess inventories (maximum 200% at Kii Suido) of 239,240pu over fallout inputs
were observed except for the Osaka Bay and Bingo Nada sediments. Considering the geographical
features o1~ the Seto Inland Sea, this excess 239,240Pu seems to be supplied from the surrounding land
rather than by horizontal movement from other sea region. These excess inventories are also reported
for other coastal sediments (21, 22).
For all the sediment columns analyzed, total amounts of 137Cs are considerably less than the
global fallout inputs. The contribution of 137Cs in sea water should be taken into account for the
inventory balance of 137Cs, while the contribution of 239,240pu in sea water is negligibly small. From
the sea water data of Table 1, total amounts of 137Cs in the water column are calculated to be 260
MBq/km 2 for Kii Suido station, where the water depth is greatest (70.5 m) and 137Cs content in the
sediment was the largest among the stations. Therefore, Kii Suido station contains 1,090 MBq/km 2 of
137Cs in total (water column + sediment column) and this value corresponds to only 46% of total
fallout input.
Furthermore, because there seems to be no significant fractionation between 239,240pu and
137Cs of fallout origin in the top layers of the soil and freshwater environment (23-25), it could be
assumed that the excess 137Cs over fallout input was supplied to the coastal area in the same manner as
239,240Pu. Therefore, if 200% of the excess 137Cs in the fallout input were assumed to have been
supplied to Kii Suido station in the same manner as 239,240pu it is calculated that only 15% of the total
137Cs supplied is retained in this region. As the 137Cs concentration data from only two samples were
used, there may remain some uncertainty in this calculation as to whether the sea water data is
representative, because the concentration of 137Cs in coastal water shows a wide variation. Recent data
162
K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 137 Cs AND 239,240 Pu
Table 3 137 Cs and = 39,240 Pu inventories in sediment columns
Water z39'24~ ~37Cs Station Pu/Cs
Depth (~q/km z) (blBq/km z)
Kii Sudo 70.5m I19 830 0.14
Osaka Bay 36m 17 I00 O. 17
Bingo Nada 29m 20 200 O. I0
Hiuchi Nada 27m 104 560 O. 18
Aki Nada 37m 63 210 0.30
Riroshima Bay A 40m llO 600 O. 19
Hiroshima Bay B" 30m 75 240 O. 31
Hiroshima Bay C 33m " 52 390 O. 13
Hiroshima Bay D tO. 5m > 39 > 960 0.04
(River Mouth)
on 137Cs concentration in sea water from coastal are'as in Japan, including port areas, bay, and open
ocean coast, ranged 300-780 mBq/100 1 from 1984 to 1986 (5).
If we use these values, the results of the calculations show that Kii Suido Station contains 14-
19% of the total input, and it is shown that our sea water data is representative enough for the
calculation of the 137Cs inventory balance.
It is suggested that a significant portion of the 137Cs input was removed from the Seto Inland
Sea to the open ocean. On the other hand, 239,240pu was retained in the sediment of the region.
We wish to express out thanks to Dr. H. Tsubota and other scientists participating on the cruises for.their
cooperation in sample collection.
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K. NAKAMURA, Y. NAGAYA: DISTRIBUTION OF 13 ~ Cs AND 2 3 9,z't o Pu
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