Journal of Nuclear and Radiochemical Sciences, Vol. 10, No.1, pp. R7-R16, 2009

Plutonium in the Ocean Environment: Its Distributions and Behavior
Katsumi Hirose*
Meteorological Research Institute, Geochemical Research Department, Nagamine1-1, Tsukuba, Ibaraki 3050052, Japan
Received: December 19, 2009
Marine environments have been extensively contaminated by plutonium as a result of global fallout due to atmospheric nuclear-weapons testing. Knowledge of the levels and behavior of plutonium in marine environments isnecessary to assess the radiological and ecological effects of plutonium. Such analytical techniques as radiochemical analysis, α-spectrometry, and mass spectrometry have been developed to analyze the plutonium in seawaterover the past five decades. Because of complex chemical properties (e.g. high reactivity to particles), plutonium inthe ocean exhibits more complicated behavior than other long-lived anthropogenic radionuclides, such as 137Cs. In
the present study, I reviewed the research history of plutonium in the ocean, including spatial and temporal changesof plutonium levels and distributions, and its oceanographic behavior.

1. Introduction
The first global contamination by plutonium, which occurredfollowing nuclear explosions at Aramogode, Hiroshima, andNagasaki in August 1945, was recorded in the ice core of Arctic
Canada.1 Plutonium released from atmospheric nuclear tests,
serious nuclear reactor accidents, and other sources are almost
globally detectable in the atmosphere, ocean, and terrestrial
environment. Most originated from atmospheric nuclear-weapons testing in the early 1960s.2-4 The cumulative deposition ofbomb-derived 239,240Pu on the Earth’s surface was estimated tobe 11 × 1015 Bq, whereas the cumulative depositions of 137Cs
and 90 Sr, typical long-lived fission products, were 948 × 1015
and 622 × 1015 Bq.5 In 1964, 1.3 × 1015 Bq of 238Pu was injected
into the upper atmosphere of the southern hemisphere as aresult of a US satellite burnup.6 Additionally, local and regional
contamination of plutonium in the environment have resulted
from nuclear waste discharge from the nuclear fuel reprocessing plants in the UK and France and from ocean dumping of
nuclear wastes. For example, the Sellafield reprocessing plantin the UK released 0.68 × 1015 Bq of 239,240Pu into the Irish Sea.7
In the mid-1950s, a substantial amount of plutonium was
injected into the western North Pacific surface waters by closein fallout from the US nuclear explosions conducted at the
Pacific Proving Grounds in the Marshall Islands8,9 ; the estimated total burden of close-in fallout plutonium was 3.8 to 6.3
× 1015 Bq. The 1986 Chernobyl nuclear reactor accident
resulted in atmospheric release of 0.06 × 1015 Bq of 239,240Pu;
and ten days after the explosion, 38 × 1015 of 137Cs and 8 × 1015
Bq of 90Sr were released into the environment.10 The effect of
the Chernobyl fallout on the ocean was very limited, sincemost of the Chernobyl plutonium contaminated the land near
Chernobyl; the Chernobyl-derived contribution of plutonium
deposition in the Baltic Sea was estimated to be 10% of thetotal plutonium deposition.11 Although no information is available on the environmental effects of accidental plutonium inthe ocean, the potential sources of plutonium in marine environments are nuclear weapons accidents.12 First, in 1965, an
attack aircraft loaded with one B43 nuclear weapon rolled off
the deck of the USS Ticonderoga, and the nuclear weapon wasnever found. Second, a former USSR missile submarine with*Corresponding author. E-mail: hirose45073@mail2.accsnet.
ne.jp

three nuclear-armed ballistic missiles sank 750 miles northwestof the island of Oahu.
The major sinks of plutonium released in the environment
are the ocean and land. Knowledge of the levels and behaviorof plutonium in marine environments is important for assessing
the ecological effects of plutonium, due to high public concern
about the chemical and radiotoxicity of plutonium. Since thefirst report of plutonium concentrations in seawater was published in 1964,13 many researchers have determined plutonium
concentrations in the ocean and studied oceanographic behavior of plutonium. The data of oceanic plutonium concentrations have been compiled in a database.14,15
Plutonium in the ocean is transported by physical and biogeochemical processes. During the past four decades, plutonium concentrations in the surface waters of the open ocean
have decreased with sea-area-dependent residence time, whichis generally shorter than that of the corresponding surface
137
Cs.16,17 Plutonium, which is a typical particle-reactive radionuclide in contrast to 137Cs, moves vertically with sinking biogenic particles18,19 and is regenerated in deep waters as a resultof microbial decomposition of particles. Biogeochemical models have reproduced the vertical profile of plutonium with surface minimum, mid-depth maximum, and decrease withdepth.20-22 However, plutonium behavior in oceanic water ismore complicated because of such physical processes as advection and upwelling.23 For example, the observation that theplutonium maximum layer in the mid-latitude region of theNorth Pacific deepened with time9 has been explained by asimple one-dimensional biogeochemical model 20 ; however,biogeochemical processes alone cannot be used to explain adecrease of inventory in the corresponding site, where advection may play a significant role. The North Pacific deep waters(more than 2000 m depth) contain a significant amount of plutonium,8 the input processes of which are still unknown.
Plutonium in the environment consists of a number of different isotopes (238Pu, 239Pu, 240Pu, and 241Pu): the most abundantare 239Pu, which has radioactive half-life of 24,110 years; and240
Pu, which has radioactive half-life of 6,563 years. Although
they have lower atom concentrations, 238Pu (with a radioactive
half-life of 87.7 years) and 241Pu (with a radioactive half-life of
14.1 years) are also important isotopes of plutonium. All plutonium isotopes except 241Pu are α-emitters. The α particle
energy of 238Pu is 5.49 MeV; that of 239Pu is 5.10 MeV; and thatof 240Pu is 5.15 MeV. The β-particle energy of 241Pu is 5.2 keV.

© 2009 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/27/2009

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J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009

Therefore, α-spectrometry is effective in determining plutonium concentrations in seawater. However, the sum of 239Pu
and 240Pu activities has been used as a measure of plutonium
concentration because the separation of 239Pu and 240Pu is difficult using α-spectrometry, due to their similar energy range of
α particles. Plutonium isotope ratios depend on their sources;
for example, the average global fallout 240Pu/239Pu atom ratio is0.18,24-27 whereas an elevated ratio was reported for fallout from
US nuclear tests in the 1950s.28,29 The plutonium isotope signature is therefore a useful tool for identifying the sources of pluton iu m a nd for cla r i f yi ng t hei r b ehavior i n ma r i neenvironments.
In this study, I review the research history of plutonium inmarine environments, including distributions of plutonium inthe ocean, long-term trends of plutonium concentrations, plutonium inventories in the water column, speciation, and isotope
ratios.
2. Plutonium analysis
I simply reviewed methods of analyzing plutonium in seawater. Concentrations of plutonium in seawater are usually
extremely low (<50 mBq m-3) except in highly contaminatedareas, and most of its isotopes emit α particles. Therefore, plutonium contents have been determined by α spectrometry,
Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and
Accelerator Mass Spectrometry (AMS), after longer radiochemical processes (e.g., preconcentration from large-volume
samples, radiochemical separation and purification, and electrodeposition).
2.1. Sampling. Large volumes (50 to 500 L) of seawater are
required for the assay of plutonium and americium because of
the extremely low plutonium concentrations in open ocean
water. Surface water samples of 100 to 500 L were usually collected with submergible pumping systems on board. Deepwater samples were collected with a 100 L GoFlo sampler or
CTD-rosette sampler. All water samples were filtered througha fine membrane filter (Millipore HA, 0.45µm pore size)
immediately after sampling.
2.2 Preconcentration of Pu. Coprecipitation is generally
used for extracting trace amounts of Pu from large volumes ofseawater, and this process is usually performed on board. In
the early stage, Pu was coprecipitated with ferric hydroxide or
bismuth phosphate.30,31 However, low and erratic recoveries oftrace Pu occurred in the environmental samples. A proposed
simple concentration method included the partial precipitationof (Mg, Ca) hydroxides/carbonates by adding small amounts of
sodium hydroxide.32,33 Recently, coprecipitation with MnO2
was proposed as a method of preconcentrating Pu in seawater.34,35 First, after the seawater sample was acidified, 242Pu wasadded as a yield tracer, and KMnO4 was added to change thechemical form of Pu to a dissolved form (Pu(VI)). Next, after
NaOH and MnCl2 were added to the solution, Pu coprecipitatedwith the bulky manganese dioxide at pH 9. Finally, the settled
MnO2 precipitation was collected. Another method of preconcentrating Pu36 involved acidifying a 200 L seawater sample topH 2 with 12 M HCl (200 mL). Ferric chloride (2 g), a known
amount of tracer (242Pu), and K 2S2O5 (100 g) were then added,
and the solution was stirred for 1 h, so that all the Pu species inthe solution was reduced to Pu(III). Finally, Pu was coprecipitated with ferric hydroxide at pH 10 by adding diluted NaOH
solution (0.5 to 1 M).
2.3. Radiochemical separation. Relatively large amounts
of precipitates were obtained at the preconcentration stage.
Although precipitate reduction is usually required, a simple
separation method using anion exchange resin with a large column was proposed.32,33 In order to remove large amounts of
MnO2 or Ca(OH) 2 and Mg(OH) 2 , additional coprecipitation
was performed.34 The MnO2 precipitates were separated andHirosedissolved in 2 M HCl with excess NH2OH HCl to reduce it toMn(II). To scavenge Pu with ferric hydroxide, 50 mg of Fe(III)
was added in the solution and oxidized from Pu(III) to Pu(IV)with NaNO2 . Ammonium hydroxide was added to form the
Fe(OH)3 precipitate at a pH of 8 to 9. The solution was briefly
boiled, and then the pH in the solution was adjusted to 6 or 7,
in which residual amounts of MnO2 changed to Mn(II). The
Fe(OH)3 precipitates were separated from the supernatant solution by centrifugation. The precipitate was dissolved with 14
M HNO3. Finally, the acid concentration of the solution was
adjusted to 8 M HNO3. The solution was passed through theanion exchange column (Dowex 1-X8, NO3 form), in which Pu
and Th were adsorbed onto resin, whereas Am, U, and Fe
remained in the effluent. The column was washed with 8M
HNO3, and the Th fraction was eluted with 10 M HCl. The Pu
fraction was eluted with 9 M HCl solution containing 0.1 M
NH4I. The chemical recoveries were 40 to 60%.
Recently, new types of extraction chromatographic resin
have been proposed for radiochemical separation and purification of actinides instead of anion exchange resin. 37 TRU
Resin™ (comprised of 0.75 M solution bifunctional organophosphorus extractant octylphenyl-N, N-di-isobutyl carbamoylphosphine oxide in tri-n-butyl phosphate immobilized oninert porous polymeric resin) was used for separating
actinides.34,35,37-39 Kim et al46 developed analytical techniques
for determining Pu in seawater using sector field ICP-MS (SFICP-MS), with a sequential flow injection system including a
TEVA-Spec (Eichrom Industries) column for the radiochemical separation of Pu.
2.4. Radiochemical separation of plutonium in particulate matter. Filtered samples (100 to 1000 L) were digested
with 14 M HNO3 and 12 M HCl in an appropriate beaker on a
hot plate.33 Next, 242Pu was added as a tracer of the chemicalyield. The solution was evaporated to dryness on a hot plate.
The residue was then dissolved with 8 M HNO3 and NaNO2
was added on a hot plate. After the filtration of the filter paper,
the solution was passed through an anion exchange column.
The column was successively washed with 20 mL of 8 M HNO3
and 50 mL of 9 M HCl. The Pu fraction was eluted with 100
mL of 1.2 M HCl including 1 mL of 30% H2O2. The chemicalyield exceeded 90%.
2.5. Electrodeposition. Pu samples are usually electroplated onto a stainless steel disk for α counting. The disk
diameter depends on the active surface area of the detectors.
In this study, electrodeposition was performed using an electrolysis apparatus with an electrodeposition cell consisting of a
Teflon cylinder, a platinum cathode electrode, and a stainlesssteel disk anode. Pu was electrodeposited under two conditions (aqueous sulfuric acid media40 and ethanol media33). To
establish homogeneous plating onto the disk, the disk surface
had to be cleaned by appropriate washing processes.
2.6. α-spectrometry. The α-spectrometer consisted of several vacuum chambers, including a solid-state detector, a pulseheight analyzer, and a computer system. The detector, which
was a silicone surface barrier type (PIPS, energy resolution<25 keV full width at half maximum (FWHM), counting efficiency: 15 to 25%), had an active surface area of 450 to 600
mm2 and a minimum depletion thickness of 100 µm. The vacuum condition in the chamber was reduced to less than 200
mTorr using a vacuum pump, and the counting time was morethan 80000 s.
2.7. Mass spectrometry. Thermal Ionization Mass
Spectrometry (TIMS), 41- 43 Resonance Ionization Mass
Spectrometry (RIMS),44 ICP-MS, and AMS45 have been used
for determining plutonium isotopes. The recent development
of SF-ICP-MS allowed us to collect more data of 239Pu concentrations and 240 Pu/ 239 Pu atom ratios in marine samples.46-54
Since ICP-MS detection limits of plutonium were in the subfemtogram range, I could reduce the sample volumes of several

Plutonium in the Ocean Environment: Its Distributions and Behavior

J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009

R9

Figure 1. Map of the Pacific Ocean.

tens of liters. In order to obtain high-quality data of plutonium
isotopes, however, it was necessary to develop simpler purification techniques to remove interference elements. The MS techniques, which provided information on 240Pu/ 239Pu atom ratios
in the seawater samples, were useful tools for identifying
sources of plutonium in marine environments.
3. Plutonium in the Ocean: Distribution and Behavior
3.1. Surface 239,240Pu concentrations. Although 239,240 Pu
concentrations in surface waters were determined in the
1960s,13,55 the first geographical distribution of surface 239,240Pu
in the Pacific (Figure. 1) in the early 1970s was depicted by
Miyake and Sugimura,56 who found higher 239,240Pu concentrations in the mid-latitude region of the North Pacific. The geographical distribution of surface 239,240 Pu was governed by
global fallout from the atmospheric nuclear testing conductedin the early 1960s. In the early 1980s, a similar geographicalpattern of surface 239,240Pu was observed,57 although the latitudinal gradient of surface 239,240 Pu decreased, compared withthat in the early 1970s. In the early 1990s, a geographic distribution of surface 239,240Pu in the Atlantic, Indian, and Pacific
Oceans ranging from 0.6 to 11.5 mBq m-3 was observed,58 withhigher concentration in the western North Pacific and North
Atlantic, and a low concentration in the eastern North Pacific.
In the 2000s, the 239,240 Pu concentrations in the surface
waters of the world ocean were evaluated from comprehensive
dataset based on the IAEA-MEL research program ofWorldwide Marine Radioactivity Studies15,60,61 and the HAM
database constructed by Aoyama and Hirose.14 The highest
239,240
Pu concentration in the surface waters (>100 mBq m-3)
occurred in the Irish Sea, in which the effect of the Sellafild
discharge7 continued. A relatively high 239,240Pu concentration
was observed in the surface waters (10 to 50 mBq m-3) in thenorthern North Atlantic, the Barents Sea, the North Sea, theMediterranean, and the English Channel. The 239,240Pu concentrations in the surface waters of the Pacific, Atlantic, andIndian Oceans ranged from 1 to 10 mBq m-3; however, in the
Antarctic Ocean, the concentration was less than 0.5 mBq m-3.
Plutonium data in the surface water of the South Pacific werestill scant. Therefore, in 2003, the high-density longitudinal
distribution of surface 239,240 Pu along 32.5°S in the South
Pacific was depicted, 59 with 239,240 Pu concentrations ranging
from 0.5 to 4.1 mBq m-3, which indicated a spatial distribution
governed by oceanographic processes.
Temporal changes of surface 239,240 Pu in the Pacific havebeen investigated.16,17,62 Surface 239,240Pu decreased with time inall the oceans, due to the continuous decrease of fallout input
after the end of the Chinese atmospheric nuclear test in 1980
and the subsequent decreasing stratospheric fallout.4 The
239,240
Pu concentration in the surface water has decreased exponentially since 1970, although decrease rates depend on the sea
area. The decrease rates of the surface 239,240Pu concentrationsin the ocean are generally faster than that of 137Cs, due to biogeochemical processes.
The 239,240 Pu concentrations in the surface waters of the
Pacific in 2000 were estimated to range from 0.3 to 2.7 mBqm-3, with a spatial variation larger than that of 137Cs.63 Higher
surface 239,2 40 Pu concentrations appeared in the subarctic
Pacific and the eastern equatorial Pacific. Higher surface
239,240
Pu concentrations in the subarctic Pacific and the Sea ofJapan could be explained by rapid recycling of scavenged
239,240
Pu due to large vertical mixing in winter,64,65 whereas surface 239,240 Pu in the eastern equatorial Pacific was possibly
maintained at a higher level by equatorial upwelling.66 The
lower values in the mid-latitude region of the eastern North
Pacific were consistent with the higher decrease rate of 137Cs in
the mid-latitude region of the eastern North Pacific. Estimatedsurface 239,240Pu concentrations in the subtropical region of thewestern North Pacific were higher than those in the subtropicalregion of the eastern North Pacific. Furthermore, the difference between the surface 239,240Pu concentration of the mid-latitude region of the Northern Hemisphere and that of theSouthern Hemisphere was small, although its causes are still
unknown. Therefore, in addition to the physical processes, thebiogeochemical process is a significant factor controlling thecurrent geographic distribution of the 239,240Pu concentration in
the surface waters of the Pacific,8,9,18,20,62 considering the relative short residence time of surface 239,240Pu.17
3.2. Vertical profiles of 239,240Pu in the ocean. The firstR10 J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009

Hirose239,240

Vertical Profile of 239 Pu concentration in the central south Pacific(mBq m-3)

0

2

4

6

8

10

0

1000

depth (m)

2000

3000

4000

5000

6000

7000
Vertical Profile of 239,240 Pu concentration in the western North Pacific(mBq m-3)

0

10

20

30

40

50

0

1000

depth (m)comprehensive data on vertical distributions of
Pu in open
ocean water columns were reported for GEOSECS stations,8,67
although a preliminary measurement of deep 239,240Pu had been
carried out by Bowen et al 30 and Miyake and Sugimura. 56
Figure 2 depicts vertical profiles of 239,240 Pu observed in thesubtropical regions of the North and South Pacific.68,69 All vertical profiles of 239,240Pu in the ocean were characterized by atypical distribution pattern with surface minimum, subsurface
maximum, and gradual decrease with increasing depth.8,9,16,70-75
Maximum 2 39, 2 4 0 Pu concentrations in the water column
appeared at depths of 500 to 1000 m.69 The shape of 239,240Pu
profiles depended on the sea area: a rather broad peak of
239,240
Pu maximum was observed in the Sea of Japan 23,76-80 andin the Mediterranean Sea.81 Furthermore, although a surface
minimum of 239,240Pu was observed, definite profiles of 239,240Pu
with a subsurface maximum (as in the Pacific) were notobserved in the Arctic Atlantic Ocean.82,83 The broad profilesof 239,240Pu in water columns may be explained by deep convection due to meteorological and oceanographic conditions
related to each sea. The 239,240Pu maximum of the South Pacificwas smaller than that of the North Pacific.68,84 This pattern ofvertical 239,240Pu profiles and their temporal change was demonstrated by particle-scavenging processes, which involved theremoval of 239,240Pu by sinking particles following the regeneration of dissolved 239,240Pu due to the biological degradation ofparticles.18,20
Temporal change of the 239,240Pu profile in the eastern North
Pacific at the same site (31°23’N, 150°02’W) as the GEOSECS
observation in 1973 was examined.85 The 239,240Pu maximum
in the mid-latitude region of the North Pacific moved to a
deeper layer over time. The peak 239,240Pu concentration at a
depth of 400 m markedly decreased over a 28-year period,
whereas the 239,240Pu concentrations in deep waters (>1000 m
depth) revealed no significant difference from GEOSECS data.
Similar changes in the vertical distribution of 239,240 Pu wereobserved in the western North Pacific mid-latitude region.9
These findings suggested that the 239,240 Pu concentrations in
mid-depth (200 to 800 m) in the mid-latitude region of theNorth Pacific decreased over the past three decades. Thischange could not be explained by a simple biogeochemical process including only vertical transport, e.g. particle scavenging,
because no marked increase of the 239,240 Pu concentrations
occurred in deeper water layers (>1000 m). Thus, it was likelythat most of the 239,240Pu at mid-depth in the mid-latitude regionof the North Pacific was transported by horizontal advection.
On the other hand, when two sites in the equatorial North
Pacific, location: 7°59’N, 164°54’E (Hirose et al 85) and 5°53’N,
170°01’W (GEOSECS, 1973), were compared, no apparentchange in the vertical 239,240Pu profile in the depth layers of thesurface to 1000 m occurred over the past three decades.85 A
similar tendency was observed in the subtropical western North
Pacific.9,69 However a significant difference of 239,240Pu concentrations in deep waters (>1500 m depth) occurred between twosites: deep 239,240Pu concentrations in the station near the Pacific
Proving Ground nuclear test sites (11°35’N, 165°23’E) werehigher than those in the eastern station (GEOSECS site).
3.3. Deep 239,240 Pu. The North Pacific deep waters contained substantial amounts of 239,240Pu,8,71,85 whereas plutonium
concentrations in the South Pacific deep waters were 1 mBq
m-3,68 or one order of magnitude lower than that in the North
Pacific. The Japan and Bonin trench waters (>6000 m depth)in the western North Pacific contained marked amounts of
239,2 4 0
Pu.74,86 The introduction of 239,2 4 0 Pu in deep waters
resulted in an increased interest in deep plutonium and how
processes were related to the vertical transport of 239,240Pu in
deep layers. Livingston et al9 suggested that the 239,240Pu concentrations in deep water (more than 2000 m depth) weremaintained at a nearly constant level during the past two
decades. In order to clarify the present feature of deep 239,240Pu,

2000

3000

4000

5000

6000

Figure 2. Vertical profiles of plutonium concentrations in the Northand South Pacific subtropical regions. The plots used data cited in
Povinec et al (Ref.69) and Hirose et al (Ref.68).

latitudinal distributions of deep 239,240Pu at two different layers,
upper (2000 to 3500 m) and lower (4000 to 6000 m), along
165°E and 180° longitudes were analyzed.85 The latitudinal
distribution of the 239,240 Pu concentrations in the upper deep
layer along 165°E longitude revealed two peaks, one in the
mid-latitude region (30 to 40°N) corresponding to the latitudinal pattern of global fallout, and another in the south subtropical region (10 to 15°N), corresponding to close-in fallout due tothe Pacific Ground nuclear test sites. The lower 239,240Pu concentrations in the upper deep layer occurred in the north subtropical (15 to 30°N) and equatorial (0 to 10°N) regions. The
239,240
Pu concentrations in the lower deep layer were lower thanthose in the upper deep layer. For the latitudinal distribution of
the 239,240Pu in deep water along 180° longitude (Figure. 3), a
clear structure of the latitudinal distribution of the 239,240Pu concentrations was observed in the upper deep layer, with a highPlutonium in the Ocean Environment: Its Distributions and Behavior

A
25

20

15

10

5

239,240

Pu concentrations in deep waters (mBq m-3)

30

0

40S

20

0

20

40

60N

20

40

60N

Latitude239,240

Pu concentrations in deep waters (mBq m-3)

40
35

B

30
25
20
15
10
5
0
40S

20

0

LatitudeFigure 3. Latitudinal distributions of 239,240Pu concentrations in deep
waters along 180° in the Pacific. Depth range: A. 2000 to 3500 m, B.
4000 to 6000 m.

concentration in the North Pacific and a low concentration in
the South Pacific. However, higher 239,240Pu concentrations in
the North Pacific subtropical region were lower than those in
the corresponding latitude along 165°E. The latitudinal distribution of the 239,240 Pu concentrations in the lower deep layer
was similar to that in the upper deep waters except for a different structure with a pronounced peak in the mid-latitude region(30° to 40°N); specifically, the 239,240Pu concentrations in thelower deep layer in the mid-latitude region were greater thanthose in the upper deep layer.
These findings suggest that the geographical structure ofdeep 239,240Pu concentrations with a larger latitudinal gradientthan that of surface 239,240Pu exists in the Pacific: the high-concentration area is located in the western North Pacific and the
low-concentration area is the South Pacific. The geographicaldistributions of deep 239,240Pu suggest little contribution of vertical transport of 239,240Pu and its mineralization in the North
Pacific deep waters (>2000 m). Model study21 suggests lesscontribution of biogeochemical vertical transport to deep239,240
Pu (>2000 m depth), although four decades have passed
since surface injection of 239,240Pu. These findings suggest thatsome fallout 239,240Pu-bearing particles were rapidly transported
to the deep layers and dissolved in deep waters. Therefore, it islikely that the geographical distributions of 239,240 Pu in deep
waters reflect spatial distributions of global fallout and close-in

J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009 R11fallout. Furthermore, detection of temporal change in deep239,240
Pu might provide important information on the thermohaline circulation of deep water in the Pacific.
3.4. Residence times of surface Pu. The residence time ofplutonium in surface waters is an important factor for assessing
the long-term ecological and radiological effects of plutonium,and for obtaining oceanographic information using plutonium
as a tracer. The half-residence time (HRT) of plutonium in thesurface layer was calculated from vertical flux data of a sediment trap experiment.19 Hirose et al62 estimated the HRT of
surface 239,240 Pu from the temporal change of 239,240 Pu/137Cs
ratios in the surface water of the western North Pacific. Since
the residence times of plutonium in surface waters depend on
the sea area, temporal changes of surface plutonium in theocean were examined for different sea areas, whose division
took into account the oceanographic and input conditions.16,17
The 239,240Pu concentrations in the surface waters of each sea
area decreased exponentially during the past three decades.
With the assumption that the surface 239,240 Pu exponentiallydecreased with time, curve fitting was carried out to determine
the apparent residence time of 239,240Pu in the surface waters.
In the mid-latitude region of the western North Pacific, theapparent residence time of surface 239,240Pu was calculated to be
7.5 years, which was the same order of magnitude as previous
estimates.16 The apparent residence time of surface 239,240 Pu
was longer than the residence time of 239,240Pu (four years), due
to particle scavenging as determined by sediment traps19 and
239,240
Pu/137Cs ratios.62 This result suggested recycling processes of 239,240 Pu, such as vertical mixing and upwelling ofdeep waters with higher 239,240 Pu concentrations, and supply
due to horizontal advection.
In the western North Pacific (10 to 20°N), surface 239,240 Pu
gradually decreased during the same period.83 However, theapparent decrease rate of surface 239,240Pu was slower than thatin the mid-latitudes. The apparent HRT of surface 239,240Pu wascalculated to be 15 years. The longer apparent HRT of surface
239,240
Pu in the subtropical North Pacific may be attributable toless particle scavenging due to low productivity in the oligotrophic ocean, in addition to recycling processes. In the equatorial Pacific and subarctic Pacific, a longer HRT of plutonium insurface waters occurred, although higher scavenging of 239,240Pu
than in the oligotrophic ocean due to relatively high productivity was expected.17 A relatively long HRT in the equatorial
region was estimated for surface 137Cs.17,87 A longer HRT of
239,240
Pu in these sea areas may be explained by such physicalprocesses as deep ventilation in the subarctic and equatorial
upwelling. It must be noted that for surface 239,240 Pu in theequatorial Pacific as well as subtropical North Pacific, there
may be a contribution of outflow from potential sources of239,240
Pu at the Marshall Islands nuclear explosion sites.5
In the Mediterranean, the mean residence time of plutonium
in the surface waters was estimated to be 15 years from thetemporal change of surface 239,240Pu concentrations.88
3.5. Chemical speciation of plutonium. The chemistry ofplutonium in natural waters is generally complicated because
plutonium exhibits four valences (III, IV, V, and VI) and forms
complexes with inorganic and organic ligands.89-91 Choppin
and Morgenstern92 reviewed the chemical behavior of plutonium in an aqueous environment. The most important chemical property of plutonium in the marine environment is its
oxidation state, because many chemical processes (e.g. solubility, hydrolysis, and complexation) are largely affected byvalence. The oxidation state of plutonium affects its biogeochemical processes in the marine environments and radiochemical analyses of water samples (e.g. concentration andseparation). The oxidation of plutonium depends on the pH of
the solution: lower oxidation states are stabilized in acidic solution, whereas higher oxidation states are more stable in alkaline
solution. The reactivity of Pu(IV) and Pu(III) is generallyR12 J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009

coincided with those in global fallout, although lower
240
Pu/ 239Pu atom ratios (<0.05) due to the French nuclear tests
were observed in the Mururoa lagoon waters.
Although data of 238Pu/ 239,240 Pu activity ratios in the open
ocean waters are limited, lower and higher 238Pu/239,240Pu ratios
than those in global fallout were observed in the subtropicalwestern North Pacific,69 and higher 238Pu/ 239,240Pu ratios wereobserved in the eastern equatorial Pacific.58
A β-emitter with a half-life of 14.1 years, 241Pu is an important isotope because it is a parent nuclide of 241Am, an α-emitter with a half-life of 433 years. Livingston et al106 found that atypical value of the 241Pu/239,240Pu activity ratio in global fallout
samples was 13 to 14.
3.7. Inventory of 239,240Pu in the water column. The watercolumn inventories of 239,240Pu in the Pacific were evaluated for
the GEOSECS stations in the early 1970s.8 The 239,240Pu inventory in the water columns of the open ocean was calculated byinterpolating the 239,240 Pu concentration measured at each
depth.9 A pronounced feature of latitudinal distribution of the239,240
Pu inventory (Figure. 4) was markedly high 239,240 Pu
inventories in the western North Pacific, compared with that of
the latitudinal distribution of global fallout 239,240 Pu.107 The
higher water column inventories of 239,240 Pu in the western
North Pacific (100 to 130 Bq m-2) 69,74 continued in the 1990s.9
This finding suggested that a significant amount of the 239,240Pu
inventory in the water column in the western North Pacificoriginated from sources other than global fallout, such as closein fallout, consistent with the fact that close-in fallout fromtesting at the Pacific Proving Grounds is the most importantsource of plutonium in the North Pacific, according to a plutonium isotope study, 41 and that most of the close-in fallout
239,240
Pu still remains in the western subtropical North Pacific.
It is probable that the present water column inventories of239,240
Pu are controlled by horizontal advection and bottom
topography of the North Pacific instead of deposition due toglobal and close-in fallout, because the major deposition of
239,240
Pu originating from atmospheric nuclear weapon testing
occurred in the 1950s and early 1960s. It is also possible thatthe apparent inventories of plutonium depend on depth.
Therefore, in order to gain a better understanding of surface
processes including biogeochemical processes and surface
input from unknown sources such as radioactive waste dumping by the former USSR,80 2300 m-depth inventories of 239,240Pu
were introduced to clarify the characteristics of the spatial distribution of the 239,240Pu water column inventories.9,108 The 2300

Pu inventory in the waters column (Bq m-2)

160

239,240

greater than that of Pu(V) and Pu(VI) because Pu(V) exists as
PuO2+ and Pu(VI) exists as PuO22+. However, it is difficult forPu(IV) in natural aquatic conditions to form complexes withorganic ligands such as humic acids because the free Pu(IV)ion concentration is extremely low as a result of strong hydrolysis.
Speciation studies33,93-95 revealed that the dominant speciesof Pu dissolved in seawater is in oxidized form (V and/or VI).
Orlandini et al96 and Choppin97 demonstrated that Pu(V) dominates in the oxidized fraction and that Pu(VI) is unstable innatural waters. In most natural systems, the significant oxidation states are Pu(IV) and Pu(V), while Pu(III) and Pu(VI) are
generally insignificant. Taking into account hydrolysis andcomplexation with carbonate species, PuO2+ is a dominant species of Pu(V) under the conditions of artificial seawater,whereas Pu(IV) exists as hydroxo complexes (Pu(OH) 4).92
In order to clarify the behavior of plutonium in the ocean, it
is important to gain a better understanding of the interaction
between plutonium and marine particulate matter. The portionof particulate 239,240 Pu to total 239,240 Pu was less than 10% in
su r face wat er s, i nclud i ng t he Nor t h Paci f ic a nd
Mediterranean, 33,88,98-100 whereas it was less than 2% in the
Pacific deep waters.100 Colloidal plutonium (<1 kD) and its
isotope ratios were determined by cross-flow ultrafiltration(CFF) following TIMS measurements,43 in which the majorityof plutonium was found in the low molecular weight fraction.
Sequential leaching experiments for particulate matter101 suggested that a major chemical form of particulate 239,240Pu wasan organic complex of Pu(IV), which was strongly supportedby the relationship between particulate plutonium and ligand.102
3.6. Plutonium isotope ratios. Plutonium isotope ratios inseawater are useful tools for determining the sources of plutonium because these ratios depend on nuclear weapons design,
test yields of nuclear explosions, and discharge from nuclear
fuel processing plants. The 2 4 0 Pu / 2 39 Pu atom ratio and238
Pu/ 239,240Pu activity ratio in fallout that were affected from akind of nuclear explosions and the SNAP9A satellite burnup
were variable.4 Recent development of ICP-MS techniques
allowed us to determine plutonium isotopic ratios (240Pu/239Pu)
in environmental samples with high precision.38,46,50-52,54 The
global fallout average 240Pu/239Pu atom ratio was 0.18, based on
aerosols, soil samples, and ice core data.24-27 However, a different nuclear test series could be characterized by either higher
or lower ratios. For example, generally lower 240Pu/239Pu ratios
characterized the fallout from Nagasaki (average value of
0.042), the Nevada test site (average value of 0.035), and the
Semipalatinsk test site (average value of 0.036) 41,103-105; however, elevated 240Pu/239Pu ratios (0.21 to 0.36) were measured insoil samples from Bikini Atoll.28,29
The 240 Pu/ 239 Pu atom ratios in water samples were determined in the North Pacific and marginal seas.41,48,50-52,85 The
240
Pu/239Pu ratios in the North Pacific waters ranged from 0.164
to 0.28, and thus were in the range of the 240Pu/ 239Pu ratio of
global fallout (0.18) and that of close-in fallout (0.3). The
240
Pu/ 239Pu ratios in the shallower layer of the North Pacifictended to be lower than that in deep waters.41 The recent
240
Pu/ 239Pu atom ratios in deep waters were 0.25, supporting
the results of Buesseler.41 Higher 240Pu/ 239Pu atom ratios wereobserved in the surface waters of the sea areas around theJapanese Islands, including the Sea of Japan.47,48,50-52 The elevated 240Pu/239Pu ratios, compared with the ratio of global fallout, resulted in close-in fallout plutonium from the Pacific
Proving Ground nuclear testing. However, significant low
240
Pu/ 239 Pu atom ratios occurred in shallower layers of theequatorial North Pacific (7°59’N, 165°03’E), which may indicate southward movement of subsurface waters including
global fallout 239,240Pu.85 It must be noted that the 240Pu/ 239Pu
atom ratios in the western North Atlantic coastal waters43 andFrench Polynesia deep water (500 to 1000 m depth; 15°S) 84

Hirose

140
120
100
80
60
40
20
0
50S

0

50N

LatitudeFigure 4. Latitudinal distribution of 239,240Pu water column inventories in the Pacific. Bar shows the latitudinal distribution of global
fallout 239,240Pu (Ref.107) and open circle shows the 239,240Pu water column inventory.

Plutonium in the Ocean Environment: Its Distributions and Behavior
239,240m
Pu inventories in the Sikoku Basin of the western North
Pacific (~70 Bq m-2 ) 85,108 seemed to be greater than those in
mid-latitudes of the central North Pacific (35 to 53 Bq m-2) andof the same order of magnitude as in the sea area near Bikini
and Enewetak Atolls observed in 1997 (61 and 74 Bq m-2).9The 239,240Pu inventory in the mid-latitude region of the western North Pacific decreased, in comparison with that in
GEOSECS.9,69 A major part of the decrease of the 239,240 Pu
inventory in the mid-latitude region was attributable to thedeficient upper layer (0 to 1000 m depth). A decrease of inventory in the water columns of the North Pacific mid-latitude
region was observed for 137 Cs.10 9 However, no temporaldecrease of the 239,240Pu inventories was observed in the subtropical and equatorial North Pacific water columns.9
The fallout plutonium originating from the Pacific Proving
Ground testing (Bravo test) was characterized by a lower
238
Pu/ 239,240 Pu activity ratio (0.001) and a higher 240 Pu/ 239Pu
atom ratio (0.33).103 If plutonium in the water columns of thewestern North Pacific consisted of only two components (i.e.
global fallout and close-in fallout (Bravo test)), 239,240Pu inventories derived from individual fallout could be estimated fromplutonium isotope data based on a simple mixing model. The
239,240
Pu water column inventory originating from global fallout
for the use of 238Pu/239,240Pu activity ratios can be estimated by
IWG,1 = (IWT,2 - IWT,1 RB )/( RG - RB),

(1)

where IWT,1 is the total inventory of 239,240Pu in the water column, IWG,1 is the total inventory of 239,240Pu from global fallout,
I WT,2 is the total inventory of 238Pu in the water column, andI WG,2 is the total inventory of 238Pu from global fallout. The
value of RG is the 238Pu/ 239,240Pu activity ratio in global fallout(0.029), and that of R B is the 238Pu/ 239,240Pu activity ratio in
close-in fallout from US nuclear explosions (0.001). As anotherestimation of close-in fallout plutonium, the 239,240Pu concentration derived from the Bikini nuclear explosion is calculated
from 240Pu/240Pu atom ratios by
AB = AO (1 + λR RB)/[1 + λR RB + (1 + λR RG)Q]and
Q = (RO – RB)/(RG – RO),(2)

where AB is the Bikini-derived 239,240Pu activity concentration,
AO is the observed 239,240Pu activity concentration, λR is the ratioof the radioactive decay constant of 239Pu to 240 Pu, R B is the240
Pu/ 239Pu atom ratio of the Bikini water sample, R G is the240
Pu/ 239Pu atom ratio of Bikini global fallout water sample,
and RO is the 240Pu/239Pu atom ratio of the observed water sample. The Bikini-derived 239,240Pu inventory was calculated fromthe calculated Bikini-derived 239,240Pu concentrations at each
depth. The results suggest that a significant amount of 239,240Pu
derived from close-in fallout spread over the subtropical regionof the western North Pacific,37-61 and close-in fallout 239,240Pu
accounted for 37–61 % of the current 239,240Pu inventory. A gapof the 239,240Pu inventory between the subtropical and equatorial
North Pacific was resulted from the close-in fallout, since mostof the close-in fallout due to the Pacific Proving ground nuclear
explosion occurred in the northern sea area of Bikini andEnewetak Atolls.110
3.8. Local contamination of plutonium. Nuclear treatmentsites; e.g. Bikini (11°35’N, 165°23’E), Enewetak (11°30’N,
162°15’E), and Mururoa Atolls (21°50’S, 138°50’W) (Figure1); produced hot spots of plutonium-contaminated areas in themarine environment. High 239,240Pu concentrations in lagoon
waters (more than 1 Bq m-3 in Bikini in 1982, 0.54 Bq m-3 in
Enewetak in 1994,111-113 and more than 0.1 Bq m-3 in Mururoa
in 1991114-116) were observed, although no information is available on the effects on the outflow of local 239,240 Pu contaminated waters to the open ocean. The 239,240Pu concentrations forJ. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009 R13
the Mururoa lagoon waters decreased, with an apparent HRTof nine years.
4. Concluding remarks
Knowledge provided by plutonium studies in the ocean during the past four decades is summarized as follows.
1. α-spectrometry is still the major technique for determining plutonium in seawater, although mass spectrometric techniques such as ICP-MS are predicted to be a powerful tool.
2. The present geographical distribution of surface 239,240Pu
is governed by oceanographic processes rather than input processes. The 239,240Pu concentration in surface waters revealed
temporal variations, decreasing exponentially in mid-latitudes
and the subtropical regions of the Pacific. However, decrease
rates depended on the sea areas, whereas no decrease tendencyof the equatorial Pacific surface 239,240 Pu has been detected
since 1985.
3. The vertical profiles of 239,240Pu in the water columns of
the Pacific exhibited a typical pattern with a surface minimum,
mid-depth maximum, and gradual decrease with increasing
depth. The temporal variation of the 239,240Pu vertical profiles
depended on the sea areas. In the mid-latitude region of theNorth Pacific, the 239,240Pu maximum layer deepened with time,
and the corresponding 239,240Pu concentrations decreased; however, no change of the 239,240Pu vertical profiles was observed in
the subtropical and equatorial North Pacific.
4. The 239,240 Pu concentrations in deep waters (>2000 m
depth) of the Pacific had a spatial structure that seemed toreflect input patterns of global fallout and close-in fallout. Thisfinding suggests that temporal variation of deep 239,240Pu may
be a tool for determining deep-water circulation because of its
smaller biogeochemical contribution, although little temporalvariability of deep 239,240Pu may occur on a time scale of several
decades.
5. The plutonium isotope signature, which is useful for identifying sources, suggests that a large amount of 239,240Pu was
injected into the subtropical western North Pacific as close-in
fallout of the Pacific Proving Ground nuclear testing in theearly 1950s, and spread due to advection and diffusion in the
North Pacific over the subsequent five decades.
6. A large latitudinal gradient of current 239,240Pu inventories
was observed in the Pacific, with a high inventory in the western North Pacific and a low inventory in the South Pacific.
Individual 239,240Pu inventories from global fallout and close-in
fallout were estimated using plutonium isotope ratios; theclose-in fallout 239,240 Pu still accounted for 37 to 61% of the239,240
Pu water column inventory in the subtropical North
Pacific.
7. Chemical speciation studies revealed that the major dissolved plutonium species was inorganic complexes of Pu(V),whereas particulate plutonium was an organic complex ofPu(IV) with ligands in particulate matter.
The development of analytical tools has resulted in increasing information about distributions and behavior of plutonium
in the ocean. However, less data exists on 239,240Pu concentrations in the ocean than on 137Cs.14 Plutonium is a unique transient tracer of biogeochemical processes in the marine
environment. In order to gain a better understanding of thebiogeochemical behavior of 239,240 Pu in the ocean, we needmuch more 239,240Pu concentration data in seawater, especiallyin deep waters. Thus, further development of analytical techniques, including improvement of measurement system sensitivity to reduce sample volumes of less than 10L andsimplification of separation and purification processes, isrequired.
Acknowledgements. The author thanks Prof. N. Momoshima(Kyushu Univ.) for the opportunity to review related oceanic

R14 J. Nucl. Radiochem. Sci., Vol. 10, No. 1, 2009
research on plutonium. This study was supported by the
Min ist r y of Education, Cult ure, Spor ts, Science a nd
Technology funds for radioactivity surveys in Japan.
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