seasonal variations in 228ra226ra ratio within

8/2/2019 Seasonal Variations in 228Ra226Ra Ratio Within

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Seasonal variations in 228Ra/226Ra ratio withincoastal waters of the Sea of Japan: implications

for water circulation patterns in coastal areas

M. Inoue*, K. Tanaka, S. Watanabe, H. Kofuji 1,

M. Yamamoto, K. Komura

Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology,

Kanazawa University, Nomi, Ishikawa 923-1224, Japan

Received 14 December 2005; received in revised form 13 April 2006; accepted 17 April 2006

Available online 8 June 2006

Abstract

In this study, low-background g-spectrometry was used to determine the 228Ra/226Ra ratio of 131

coastal water samples from various environments around Honshu Island, Japan (mainly around Noto

Peninsula) at 1e3 month intervals from April 2003 until September 2005. Spatial variation in228Ra/226Ra ratios was also assessed by analyzing 34 coastal water samples from five areas within the

Sea of Japan during May and June 2004.

The 228Ra/226Ra ratio of coastal water from all sites around Noto Peninsula shows seasonal variation, with

minimum values during summer (228Ra/226Ra 0.7) and maximum values during autumnewinter

(228Ra/226Ra 1.7e2). This seasonal variation is similar to that recorded for coastal water between Tsushima

Strait and Noto Peninsula. The measured lateral variation in 228Ra/226Ra ratios within coastal water between

Tsushima Strait and Noto Peninsula is only minor (0.5e0.7; MayeJune 2004). Coastal waters from two other

sites (Pacific shore and Tsugaru Strait, north Honshu) show no clear seasonal variation in228

Ra/226

Ra ratio.These measured variations in 228Ra/226Ra ratio, especially the temporal variations, have important

implications for seasonal changes in patterns of coastal water circulation within the Sea of Japan.

2006 Elsevier Ltd. All rights reserved.

Keywords: 228Ra/226Ra ratio; Low-background g-spectrometry; Coastal water circulation; Seasonal variation; Sea of Japan

* Corresponding author. Tel.: 81 761 51 4440; fax: 81 761 51 5528.

E-mail address: [email protected] (M. Inoue).1

Present address: Japan Marine Science Foundation, Mutsu Marine Laboratory, Minato 4-24, Mutsu, Aomori035-0064, Japan.

0265-931X/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jenvrad.2006.04.001

Journal of Environmental Radioactivity 89 (2006) 138e149www.elsevier.com/locate/jenvrad
mailto:[email protected]://www.elsevier.com/locate/jenvradhttp://www.elsevier.com/locate/jenvradmailto:[email protected]


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1. Introduction

The Sea of Japan is one of the largest marginal seas of the western Pacific Ocean, and is

surrounded by the Eurasian continent and the Japanese Islands (Gamo and Horibe, 1983). It

is widely believed that the circulation patterns of seawater within the Sea of Japan, especiallyin coastal waters, are complex and vary seasonally with changes in the water mass distribution,

water transportation, and vertical mixing of ocean water (Toba et al., 1982; Hase et al., 1999).

Circulation of coastal water has important implications for environmental change and the

transport of biomass, nutrients, and pollutants released by human accidents; consequently, it

is important to attain a detailed knowledge of the circulation of coastal water in the Sea of

Japan.

The radium isotope content of seawater is thought to reflect diffusion from coastal and shal-

low shelf sediments (Moore, 1969). The short half-life of 228Ra (5.75 years) means that the228Ra/226Ra activity ratio is a tracer of water that has been in contact with sediments (often

from coastal sources) (Kaufman et al., 1973; Moore et al., 1980); however, for analyses of

low-level activities of 228Ra and 226Ra, more than several hundred litres (200e1000 L) of

seawater is usually required. Measurements of 228Ra/226Ra ratio have been used to infer

wide-ranging water circulation in the Sea of Japan (Okubo, 1980). The 228Ra/226Ra ratio of

an annual seaweed (Sargasso) collected locally (radius of area:


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2.2. Chemical treatment

For CWNT and CWHN samples, barium (480 mg) and iron (800 mg) carriers and SO42 (100 mL of

5% Na2SO4 solution) were added to coprecipitate radium with BaSO4 (Rutgers van der Loeff and Moore,

1999). For most analyzed samples, lesser Ra-contaminated Ba-carrier (w1.5 mBq/g-Ba for 226Ra; less

than detectable for 228Ra) was used when compared to that used in a previous study (w2.5 mBq/g-Ba

for 226Ra; Inoue et al., 2005). Fe(OH)3 was then deposited by re-adjustment to pH 7 and gathered togetherwith BaSO4. For CWSJ samples,

137Cs was quantitatively collected from supernatant by coprecipitation

with ammonium phosphomolybdate (AMP); results for 137Cs are presented elsewhere (Tanaka et al., sub-

mitted for publication). After evaporating to dryness, the BaSO4 and Fe(OH)3 (and AMP) fractions were

mixed and compressed to a diameter of 19 or 17 mm for g-counting. The reagent blank from the Ba-

carrier corresponds to 2e3% for 226Ra; the chemical yield of radium was 79% of the yield of BaSO4(mean value in this study). The 226Ra and 228Ra activities in water samples were corrected for these values

of contaminants and yield.

In seven samples from the OHsite (OH-405-410), radium was coprecipitated with BaSO4 by Ba-carrier

(138e276 mg) following the addition of CO32 and pre-concentrated radium with CaSO4, in turn following

the dissolution of CaCO3. In other samples (OH-411-509), only 460 mg of Ba-carrier was directly added

to water samples. The BaSO4 was collected for g-counting by filtration.

2.3. g-Spectrometry

The g-spectrometry of all water samples was performed using large-volume well-type Ge-detectors

with detection efficiencies of 73, 70, 65 and 52% relative to a 7.6 cm diameter 7.6 cm height NaI

(Tl) detector except for water from the OHsite, which was analyzed using a 34% planar-type Ge-detector.

All of these detectors were specially designed for low-background counting and equipped at Ogoya Un-

derground Laboratory, Japan (Hamajima and Komura, 2004; Komura and Hamajima, 2004). For each

sample, counting usually lasted 2e3 days. The 226Ra and 228Ra activities were evaluated from g-ray peaks

of 214Pb (295 and 352 keV) and 228Ac (338 and 911 keV), respectively. To calibrate the 226Ra and 228Ra

activities of coastal water samples, mock-up samples were prepared with approximately the same chem-

ical composition as coastal water samples by using uranium standard issued from New Brunswick Labo-

ratory, USA (NBL-42-1) in radioactive equilibrium state throughout the uranium series nuclides and KCl.

Sea of Japan

OH

IS

NI

OM

SM FK

MT

HK

500 km

HG

SZ

TK

TY

MN

AT

50 km

500 km

Tsushima

coast

al

branch

Kuros

hio

Tsushima St.

Tsugaru

St.

Pacific Ocean

a b

East China

Sea

Noto Pen.

40N

30

40N

30

130 140E 130 140E

Fig. 1. (a) Map showing the main coastal water circulation systems of the Sea of Japan and the East China Sea and (b)

sampling locations for CWHN samples (sites HG, MT, OM, and OH), CWSJ samples (areas SM, FK, IS, NI, and HK),

and CWNT samples (sites AT, MN, SZ, TK, and TY) (inset map). Solid circles (CWNT and CWHN samples) indicate

sites of samples used in the study of seasonal variations in 228Ra/226Ra ratio.

140 M. Inoue et al. / J. Environ. Radioactivity 89 (2006) 138e149


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226Ra activity was evaluated from peak ratios of coastal water, mock-up samples, and 228Ra using a detec-

tion efficiency curve obtained from the mock-up samples.

3. Results and discussion

228Ra and 226Ra activities and 228Ra/226Ra ratio for CWNT, CWHN, and CWSJ samples are

provided in Tables 1, 2, and 3, respectively. The precision of the analyses, based on 1s of count-

ing statistics, was 3e6% for 226Ra and 3 to


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Table 1226Ra and 228activities and 228Ra/226Ra ratio of CWNT samples

Site Sample Sampling date

(m.d.y)

Salinity

(PSU)

226Ra

(mBq/L)

228Ra

(mBq/L)

228Ra/226Ra

ratio

AT AT-305 5/20/2003 n.a. 1.45 0.08 2.19 0.15 1.51 0.14

AT-307 7/1/2003 n.a. 1.60 0.08 1.59 0.13 0.99 0.10

AT-309 9/18/2003 n.a. 1.72 0.08 2.88 0.16 1.68 0.12

AT-311 11/18/2003 n.a. 2.16 0.08 3.63 0.16 1.68 0.10

AT-402 2/11/2004 n.a. 2.06 0.07 3.70 0.16 1.80 0.10

AT-403 3/10/2004 n.a. 2.53 0.09 3.52 0.19 1.39 0.09

AT-405 5/11/2004 10.65 0.87 0.04 0.82 0.07 0.94 0.09

AT-406 6/30/2004 n.a. 1.46 0.06 1.36 0.10 0.93 0.08

AT-408 8/10/2004 n.a. 1.70 0.07 2.10 0.13 1.23 0.09

AT-409 9/10/2004 n.a. 2.12 0.09 3.61 0.18 1.70 0.11

AT-410 10/12/2004 23.78 1.05 0.04 1.88 0.09 1.78 0.10

AT-412 12/1/2004 32.83 1.80 0.11 4.78 0.29 2.66 0.23AT-504 4/20/2005 33.86 0.91 0.04 1.06 0.07 1.16 0.10

AT-506 6/23/2005 33.01 1.47 0.03 1.57 0.06 1.07 0.05

AT-507 7/28/2005 26.64 1.53 0.09 3.18 0.23 2.09 0.19

AT-509 9/15/2005 n.a. 2.66 0.07 4.61 0.16 1.74 0.08

MN MN-305 5/20/2003 n.a. 2.05 0.09 2.51 0.15 1.22 0.09

MN-308 8/5/2003 n.a. 1.18 0.08 0.79 0.11 0.67 0.10

MN-309 9/19/2003 n.a. 2.11 0.09 3.41 0.18 1.62 0.11

MN-311 11/19/2003 n.a. 1.98 0.08 3.31 0.17 1.67 0.11

MN-402 2/11/2004 n.a. 2.02 0.08 2.99 0.14 1.48 0.09

MN-403 3/10/2004 n.a. 2.56 0.10 2.89 0.17 1.13 0.08

MN-405 5/12/2004 33.76 1.97 0.08 1.72 0.13 0.87 0.08MN-406 6/30/2004 n.a. 1.70 0.06 1.63 0.10 0.96 0.07

MN-408 8/10/2004 33.80 1.65 0.07 2.02 0.11 1.22 0.08

MN-409 9/10/2004 33.28 2.31 0.07 3.84 0.16 1.66 0.09

MN-410 10/13/2004 33.27 1.35 0.07 2.50 0.16 1.85 0.15

MN-412 12/1/2004 32.89 1.92 0.08 2.95 0.15 1.54 0.10

MN-504 4/20/2005 33.50 1.14 0.07 1.44 0.13 1.27 0.14

MN-506 6/24/2005 34.23 1.60 0.06 1.61 0.09 1.01 0.07

MN-507 7/29/2005 33.42 1.99 0.07 3.49 0.13 1.75 0.09

MN-509 9/13/2005 32.02 2.67 0.12 5.33 0.26 1.99 0.13

SZ SZ-305 5/20/2003 n.a. 1.95 0.09 2.32 0.16 1.19 0.10

SZ-306 6/30/2003 n.a. 2.62 0.16 1.49 0.21 0.57 0.09SZ-308 8/5/2003 n.a. 1.24 0.06 1.02 0.09 0.82 0.09

SZ-309 9/19/2003 n.a. 2.00 0.10 3.16 0.19 1.58 0.12

SZ-311 11/19/2003 n.a. 2.33 0.10 3.33 0.18 1.43 0.10

SZ-402 2/10/2004 n.a. 1.88 0.05 2.69 0.11 1.43 0.07

SZ-403 3/9/2004 n.a. 2.28 0.09 2.50 0.16 1.10 0.08

SZ-405 5/12/2004 33.46 2.13 0.06 1.93 0.10 0.91 0.05

SZ-406 6/30/2004 n.a. 1.76 0.06 1.42 0.08 0.81 0.05

SZ-408 8/10/2004 34.03 1.83 0.07 1.97 0.13 1.08 0.09

SZ-409 9/10/2004 33.57 2.73 0.06 3.88 0.12 1.42 0.06

SZ-410 10/13/2004 n.a. 1.88 0.06 2.42 0.11 1.29 0.07

SZ-412 12/1/2004 33.11 1.67 0.09 2.69 0.16 1.61 0.13

SZ-504 4/20/2005 33.49 2.22 0.07 2.28 0.12 1.03 0.06SZ-506 6/24/2005 34.31 1.77 0.06 1.28 0.08 0.72 0.05

SZ-507 7/28/2005 n.a. 1.28 0.06 1.26 0.10 0.99 0.09

SZ-509 9/13/2005 32.65 2.44 0.09 3.63 0.18 1.49 0.09



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3.2. Possible mechanism of coastal water circulation

The vertical distribution of226Ra and 228Ra activities and 228Ra/226Ra ratio for CWSJ sam-

ples is shown in Fig. 4. The 226Ra activity for CWSJ samples does not show marked vertical

variation (Fig. 4a). In contrast, for deep sites (!250 m depth) the 228Ra activity and228Ra/226Ra ratio of bottom waters are notably lower than those of shallow sites (250 m)

(Fig. 4b and c); these trends are consistent with those of potential temperature (Fig. 4d). In

coastal areas, increased activities above the thermocline appear to reflect shallow water advec-

tion in an offshore direction, while lower activities below the thermocline are possibly indica-

tive of shorter residence times. The boundary layer (thermocline at w250 m depth; Kasamatsu

et al., 1994) disturbs the exchange of radium between shallow and deep water.From the SMto HKareas, the 228Ra activity and 228Ra/226Ra ratio of surface water markedly

increased from 0.5 to 2 mBq/L and from 0.5 to 1.1, respectively. These variations may reflect

differences in the water circulation system between the western (SM, FK and IS areas;

Table 1 (continued)


(m.d.y)

Salinity

(PSU)

226Ra

(mBq/L)

228Ra

(mBq/L)

228Ra/226Ra

ratio

TK TK-306 6/30/2003 n.a. 1.32 0.06 1.07 0.10 0.81 0.09

TK-308 8/5/2003 n.a. 1.42 0.08 1.15 0.12 0.81 0.09TK-309 9/19/2003 n.a. 1.92 0.08 3.40 0.18 1.77 0.12

TK-311 11/18/2003 n.a. 1.54 0.07 2.92 0.14 1.90 0.13

TK-402 2/10/2004 n.a. 1.82 0.05 2.44 0.11 1.34 0.07

TK-403 3/9/2004 n.a. 2.20 0.06 2.58 0.13 1.17 0.07

TK-405 5/11/2004 28.13 2.02 0.05 2.08 0.07 1.03 0.04

TK-406 6/30/2004 n.a. 1.99 0.08 1.56 0.12 0.79 0.07

TK-408 8/10/2004 33.01 1.76 0.06 2.15 0.10 1.22 0.07

TK-409 9/10/2004 33.49 2.15 0.09 2.82 0.17 1.31 0.09

TK-410 10/13/2004 32.97 1.98 0.09 2.76 0.17 1.39 0.11

TK-412 12/1/2004 n.a. 2.09 0.09 3.15 0.15 1.51 0.10

TK-504 4/20/2005 28.42 1.35 0.06 1.39 0.10 1.03 0.09

TK-506 6/23/2005 34.15 1.86 0.08 1.63 0.15 0.87 0.09

TK-507 7/28/2005 31.98 2.12 0.09 2.34 0.14 1.10 0.08

TK-509 9/13/2005 32.59 2.49 0.09 4.06 0.19 1.63 0.09

TY TY-305 5/20/2003 n.a. 1.53 0.07 2.16 0.15 1.41 0.12

TY-307 7/1/2003 n.a. 1.47 0.07 2.01 0.12 1.37 0.11

TY308 8/4/2003 n.a. 1.15 0.07 0.94 0.11 0.82 0.11

TY-309 9/18/2003 n.a. 1.46 0.07 2.34 0.13 1.60 0.12

TY-311 11/18/2003 n.a. 2.24 0.09 3.98 0.19 1.77 0.11

TY-402 2/10/2004 n.a. 1.84 0.06 2.41 0.12 1.30 0.08

TY-403 3/9/2004 n.a. 1.94 0.07 2.36 0.12 1.21 0.07

TY-405 5/11/2004 25.17 2.12 0.07 2.20 0.11 1.04 0.06

TY-406 6/30/2004 n.a. 1.17 0.04 1.21 0.08 1.03 0.08TY-408 8/10/2004 29.68 1.59 0.07 1.66 0.12 1.04 0.09

TY-410 10/13/2004 n.a. 2.80 0.10 3.60 0.20 1.28 0.09

TY-412 12/2/2004 n.a. 2.48 0.11 3.12 0.21 1.26 0.10

TY-504 4/19/2005 n.a. 2.29 0.09 1.69 0.14 0.74 0.07

TY-506 6/24/2005 27.33 1.27 0.04 0.84 0.06 0.67 0.05

TY-507 7/28/2005 28.82 2.23 0.08 4.85 0.19 2.17 0.12

TY-509 9/14/2005 19.14 3.03 0.11 4.20 0.20 1.38 0.08

n.a. denotes not analyzed.

143M. Inoue et al. / J. Environ. Radioactivity 89 (2006) 138e149


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Table 2226Ra and 228Ra activities and 228Ra/226Ra ratio of CWHN samples


(m.d.y)

Salinity

(PSU)

226Ra

(mBq/L)

228Ra

(mBq/L)

228Ra/226Ra

ratio

HG HG-304 4/22/2003 n.a. 1.64 0.09 2.92 0.18 1.79 0.15

HG-305 5/23/2003 n.a. 1.77 0.08 2.21 0.14 1.25 0.10

HG-306 6/27/2003 n.a. 1.66 0.08 1.54 0.15 0.93 0.10HG-307 7/29/2003 n.a. 1.42 0.07 1.86 0.13 1.31 0.11

HG-309 9/1/2003 n.a. 1.74 0.08 2.10 0.15 1.21 0.10

HG-310 10/16/2003 n.a. 2.15 0.10 2.98 0.18 1.38 0.10

HG-406-1 6/5/2004 n.a. 1.91 0.08 1.24 0.11 0.65 0.06

HG-406-2 6/30/2004 n.a. 1.82 0.06 1.33 0.08 0.73 0.05

HG-408 8/14/2004 34.00 1.83 0.08 2.46 0.17 1.35 0.11

HG-409 9/24/2004 n.a. 2.28 0.09 3.70 0.18 1.62 0.10

HG-411 11/7/2004 33.59 2.12 0.08 2.21 0.15 1.04 0.08

HG-503 3/27/2005 n.a. 1.24 0.05 1.41 0.09 1.14 0.09

HG-505-1 5/17/2005 n.a. 1.32 0.07 1.15 0.10 0.87 0.09

HG-505-2 5/24/2005 n.a. 1.70 0.06 1.37 0.09 081 0.06

HG-508 8/5/2005 33.28 1.76 0.09 3.48 0.18 1.98 0.14

MT MT-408 8/5/2004 n.a. 1.92 0.08 3.53 0.17 1.84 0.11

MT-409 9/12/2004 n.a. 4.36 0.07 5.57 0.12 1.28 0.04

MT-410 10/17/2004 17.67 2.34 0.07 2.69 0.13 1.15 0.06

MT-501 1/14/2005 23.33 1.86 0.07 2.27 0.12 1.22 0.08

MT-502 2/28/2005 19.43 1.52 0.05 1.66 0.08 1.09 0.06

MT-505 5/2/2005 25.70 1.88 0.08 1.53 0.13 0.81 0.08

MT-506-1 6/6/2005 30.03 1.67 0.07 1.46 0.10 0.88 0.07

MT-506-2 6/30/2005 34.23 1.78 0.05 1.76 0.08 0.99 0.06

MT-508 8/30/2005 28.16 2.18 0.13 3.81 0.29 1.74 0.17

OM OM-409 9/21/2004 n.a. 2.01 0.09 1.21 0.10 0.60 0.06

OM-411 11/18/2004 33.84 1.43 0.07 0.38 0.06 0.26 0.05

OM-412 12/26/2004 33.73 1.76 0.06 0.94 0.07 0.53 0.04

OM-501 1/21/2005 33.96 1.67 0.06 0.59 0.07 0.35 0.04

OM-502 2/1/2005 33.95 1.22 0.06 0.71 0.08 0.58 0.07

OM-503 3/20/2005 n.a. 1.60 0.07 0.56 0.06 0.35 0.04

OM-504 4/23/2005 32.36 1.85 0.07 0.78 0.08 0.42 0.05

OM-505 5/22/2005 n.a. 1.33 0.07 0.63 0.07 0.47 0.06

OM-506 6/26/2005 n.a. 1.09 0.06 0.73 0.09 0.67 0.09

OM-507 7/30/2005 n.a. 2.30 0.08 1.15 0.11 0.50 0.05

OM-508 8/17/2005 n.a. 1.58 0.08 1.05 0.09 0.66 0.07

OH OH-405a 5/30/2004 33.45 n.a. n.a. 1.27 0.20

OH-406a 6/25/2004 33.84 n.a. n.a. 1.05 0.19

OH-407a 7/10/2004 33.29 n.a. n.a. 1.17 0.15

OH-408-1a 8/5/2004 33.67 n.a. n.a. 1.16 0.15

OH-408-2a 8/29/2004 34.00 n.a. n.a. 1.21 0.30

OH-409a 9/26/2004 33.82 n.a. n.a. 1.32 0.29

OH-410a 10/30/2004 33.95 n.a. n.a. 1.35 0.40

OH-411 11/20/2004 33.84 1.22 0.05 1.69 0.11 1.38 0.11

OH-412 12/15/2004 33.96 1.16 0.05 1.22 0.11 1.05 0.11

OH-501 1/15/2005 n.a. 1.55 0.06 2.17 0.12 1.40 0.09

OH-504 4/10/2005 33.62 0.77 0.03 1.14 0.07 1.49 0.11

OH-505 5/22/2005 33.60 1.02 0.05 1.51 0.11 1.48 0.13

OH-506 6/19/2005 33.84 1.43 0.05 1.75 0.10 1.22 0.08

OH-508 8/2/2005 33.74 1.05 0.04 1.18 0.09 1.12 0.09

OH-509 9/17/2005 33.70 1.27 0.05 1.83 0.11 1.44 0.10

n.a. denotes not analyzed.a Unknown yield of BaSO4.


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0.5e1.1 mBq/L and 0.5e0.7) and eastern sides (NI and HK areas; 1.5e2 mBq/L and 0.7e1.1)

of Noto Peninsula. Such differences may include the diffusion of radium from coastal sedi-

ments, the mixing ratio of offshore water of the Sea of Japan, and the effect of the coastal

branch current from the East China Sea, which appears to influence the measured seasonal

variation in 228Ra/226Ra ratio for CWNT samples. Although variations in 228Ra activity and228Ra/226Ra ratio in coastal water from SM to IS may indicate differences in the local environ-

ments, including the effect of coastal sediment, the mixing of the 228Ra source during autumne

winter seems to overcome these local differences.

The measured seasonal variation in228

Ra/226

Ra ratio for CWNT samples can be explainedby horizontal water migration rather than vertical circulation of water within coastal areas. For

example, the 228Ra activity and 228Ra/226Ra ratio of surface water from the East China Sea

changed markedly from 12 to 0.5 mBq/L and from 3.7 to 0.5, respectively, along a transect

Table 3226Ra and 228Ra activities and 228Ra/226Ra ratop pf CWSJ samples

Area Site Sample Sampling date

(m.d.y)

Depth

(m)

Salinity

(PSU)

226Ra

(mBq/L)

228Ra

(mBq/L)

228Ra/226Ra

ratio

HK HK2 HK2-405S 5/17/2004 1 34.02 2.40 0.11 1.79 0.17 0.74 0.08HK2-405B 5/17/2004 383 34.07 2.28 0.07 0.41 0.07 0.18 0.03

HK3 HK3-405S 5/17/2004 1 34.02 2.16 0.08 1.58 0.12 0.73 0.06

HK3-405B 5/17/2004 440 34.07 2.32 0.09 0.42 0.07 0.18 0.03

HK4 HK4-405S 5/17/2004 1 33.82 1.96 0.07 2.03 0.10 1.04 0.06

HK4-405B 5/17/2004 335 34.06 2.10 0.08 0.74 0.10 0.35 0.05

NI NI1 NI1-405S 5/27/2004 1 33.00 1.72 0.10 1.49 0.13 0.87 0.09

NI1-405B 5/27/2004 340 34.07 2.36 0.07 0.71 0.07 0.30 0.03

NI2 NI2-405S 5/26/2004 1 33.24 1.88 0.07 1.90 0.11 1.01 0.07

NI2-405B 5/26/2004 460 34.07 2.80 0.09 0.70 0.09 0.25 0.03

NI4 NI4-405S 5/25/2004 1 33.46 1.65 0.09 1.79 0.13 1.08 0.10

NI4-405B 5/25/2004 220 34.18 1.81 0.08 1.82 0.27 1.01 0.15

IS IS2 IS2-405S 5/30/2004 1 34.28 1.77 0.08 1.37 0.10 0.77 0.07

IS2-405B 5/30/2004 175 34.35 1.32 0.07 1.13 0.10 0.86 0.09

IS3 IS3-405S 5/30/2004 1 34.33 1.52 0.07 1.08 0.09 0.71 0.07

IS3-405B 5/30/2004 155 34.55 1.08 0.06 0.86 0.08 0.79 0.09

IS4 IS4-405S 5/30/2004 1 34.38 1.41 0.07 0.96 0.09 0.68 0.07

IS4-405B 5/30/2004 105 34.31 1.42 0.08 0.64 0.08 0.45 0.06

FK FKE2 FKE2-405S 5/31/2004 1 34.41 1.45 0.07 1.04 0.08 0.72 0.06

FKE2-405B 5/31/2004 240 34.07 2.17 0.08 0.70 0.08 0.32 0.04

FKE3 FKE3-405S 5/31/2004 1 34.39 1.91 0.08 1.01 0.09 0.53 0.05

FKE3-405B 5/31/2004 110 34.68 1.72 0.08 1.22 0.20 0.71 0.12

FKW1 FKE1-406S 6/1/2004 1 34.32 1.14 0.08 0.63 0.10 0.56 0.10FKW1-406B 6/1/2004 115 34.54 1.49 0.06 0.96 0.20 0.65 0.14

FKW3 FKW3-406S 6/1/2004 1 34.11 1.28 0.06 0.66 0.07 0.52 0.06

FKW3-406B 6/1/2004 200 34.09 1.86 0.08 1.18 0.25 0.63 0.14

FKW4 FKW4-406S 6/1/2004 1 34.33 1.36 0.08 0.74 0.09 0.55 0.07

FKW4-406B 6/1/2004 115 34.58 2.66 0.12 1.97 0.17 0.74 0.07

SM SM2 SM2-406S 6/2/2004 1 33.53 1.11 0.07 0.69 0.08 0.62 0.08

SM2-406B 6/2/2004 70 34.40 1.62 0.10 1.16 0.33 0.72 0.21

SM3 SM3-406S 6/2/2004 1 33.94 1.07 0.07 0.49 0.07 0.45 0.07

SM3-406B 6/2/2004 90 34.45 1.51 0.07 0.82 0.08 0.54 0.06

SM4 SM4-406S 6/2/2004 1 33.87 1.36 0.07 0.92 0.09 0.67 0.07

SM4-406B 6/2/2004 85 34.44 1.25 0.07 0.74 0.08 0.59 0.07



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from China to the Japanese Islands (Nozaki et al., 1989). One plausible explanation for thesetrends is that seasonal variations in the 228Ra/226Ra ratio of coastal water from the MT site to

Noto Peninsula, including the HG site, are mainly governed by the mixing ratio of 228Ra-rich

water of the East China Sea prior to passing through Tsushima Strait (Kawabe, 1982; Morimoto

226Ra(mBq/L

)

228Ra(mBq/L)

228Ra/226Raratio

0

0.5

1

1.5

2

2.5

3

AT

MN

SZ

TK

TY

mean

a

0

1

2

3

4

5

b

0

0.5

1

1.5

2

2.5

c

3 3 36 6 69 9 912 12

2003 2004 2005

Fig. 2. Temporal variations in (a) 226Ra activity, (b) 228Ra activity, and (c) 228Ra/226Ra ratio of coastal surface water

from around Noto Peninsula (CWNT samples), as measured from May 2003 until September 2005.



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0.5

1

1.5

2

HG

MT

CWNT

a

228Ra/226Raratio

0

0.5

1

1.5

2

OMOH

CWNT

b

3 3 36 6 69 9 912 12

2003 2004 2005

228Ra/226Raratio

Fig. 3. Comparison of temporal variations in 228Ra/226Ra ratio between the mean value for CWNT samples and coastal

surface water around Honshu Island from (a) HG and MT sites and (b) OM and OH sites (CWHN samples).

0.5 1 1.5 2 2.5

0

100

200

300

400

500

HK

NI

IS

FK

SM

a

depth(m)

226Ra (mBq/L)

0 0.5 1 1.5 2

b

228Ra (mBq/L)

c

228Ra/226Ra ratio

0 0.2 0.4 0.6 0.8 1 0 5 10 15 20

d

SM3

IS2

FKE2

NI2HK3

temperature (C)

Fig. 4. Vertical distributions of (a) 226Ra activity, (b) 228Ra activity, (c) 228Ra/226Ra ratio, and (d) potential temperature

of surface and bottom water (70e460 m depth) from sites HK, NI, IS, FK, and SM (CWSJ samples; May 17eJune 2,

2004).



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and Yanagi, 2001). This mixing ratio is controlled by seasonal changes in water circulation of

the East China Sea. As evident in Fig. 3a, there was no time lag apparent in the temporal change

in 228Ra/226Ra ratio between CWNT and water from the MTsite (overw400 km distance); this

may reflect the 1 month migration time of the coastal branch from the MT site to Noto Pen-

insula (current velocity is !15 cm/s).Further clarification of the circulation of the coastal branch within the Sea of Japan (e.g.,

current path and velocity, and the source of radium) will be facilitated by high-resolution tem-

poral and spatial measurements of the 228Ra/226Ra ratio of coastal water collected from the Sea

of Japan and the East China Sea.

4. Summary and conclusions

In this study, 131 coastal water samples were collected from various environments around

Honshu Island, Japan (mainly around Noto Peninsula), at 1e3 month intervals from April

2003 until September 2005. Temporal variation in measured228

Ra/226

Ra ratio within the sam-ples was determined via low-background g-spectrometry. Thirty-four coastal water samples

were also obtained from the Sea of Japan during May and June 2004 to study spatial variations

in the 228Ra/226Ra ratio.

The 228Ra/226Ra ratio of coastal water from Noto Peninsula showed similar patterns of sea-

sonal variation in samples from varied coastal environments, with minimum values during sum-

mer (228Ra/226Ra 0.7) and maximum values during autumnewinter (228Ra/226Ra 1.7e2).

Similar variations in 228Ra/226Ra ratio were also found in coastal water between Tsushima

Strait and Noto Peninsula, as well as a solitary island located w50 km from Noto Peninsula.

The measured 228Ra/226Ra ratio of coastal water from two other sites on the Pacific shore

and within Tsugaru Strait showed no clear seasonal variation.The 228Ra activity and 228Ra/226Ra ratio of surface water were notably lower in areas west of

Noto Peninsula (0.5e1.1 mBq/L and 0.5e0.7) than areas east of the peninsula (1.5e2 mBq/L

and 0.7e1.1); these variations may reflect differences in the water circulation system to the east

and west of Noto Peninsula.

Coastal areas from Tsushima Strait to Noto Peninsula are surrounded by a single seasonal

water circulation system. During autumnewinter, coastal water enriched with radium isotopes

(especially 228Ra) preferentially migrates to these areas.

Acknowledgements

We are indebted to Dr. J. Misonou (Marine Ecology Research Institute) for providing CWSJ

samples. We thank Dr. A. Suzuki (Shizuoka Prefectural Environmental Radiation Monitoring

Center) and Dr. T. Nakashima for their assistance during sampling. This work was partly sup-

ported by the Environment Radiation Department, Ishikawa Prefectural Institute of Public

Health and Environmental Science.

The authors would like to thank Dr. S. Sheppard and two anonymous reviewers for construc-

tive comments on this manuscript.

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