Unformatted text preview: R
Radioactivity in the Marine
Environment
JORDI VIVES I BATLLE
Biosphere Impact Studies Unit, Belgian Nuclear
Research Centre, Mol, Belgium Article Outline
Glossary
Definition of the Subject and Its Importance
Introduction
Radionuclide Speciation in Seawater
Radionuclide Interactions In the Water Column
Radionuclides in Sediments
Behavior of Radionuclides in Estuaries
Cycling in the Foodweb
Radiological Implications of the Marine Environment
Conclusions
Future Directions
Bibliography
Glossary
Absorbed dose Quantity of energy imparted by ionizing radiation to unit mass of matter, such as tissue –
Units of Gray, 1 Gy = 1 J KgÀ1.
Activity concentration The activity of radionuclides
incorporated in a particular material per unit mass
or volume – Units of Bq kgÀ1, Bq mÀ3.
Allometric scaling The power relationship of average
body mass with the rates of many metabolic
processes.
Analogue radionuclide Radionuclide which has similar environmental behavior to another.
Bioavailability Fraction of a substance that can be
taken up by living organisms, depending both on chemical properties of the substance and the physiological status of the organism.
Bioindicator Biological species whose presence or
absence may be characteristic of environmental
conditions in a particular area of habitat.
Biokinetic studies Studies of the dynamic exchange of
substances between organisms and their surrounding environment.
Biological half-life The time that it takes for a substance incorporated in an organism to lose half of its
concentration by natural processes of elimination.
Bioturbation Stirring or mixing of sediment or soil
by organisms, resulting in the remobilization of
radionuclides.
Broken arrow An accidental event that involves the
loss of nuclear weapons or nuclear components but
which does not create the risk of nuclear war.
Colloid Substance dispersed as small particles in
water, typically 5–200 nm in diameter. Electrostatic
surface forces are the key factor in determining their
properties.
Concentration factor Equilibrium parameter quantifying the capacity of an organism to concentrate
a specific radioelement, expressed as the ratio of
activity concentration in an organism (Bq kgÀ1,
fresh weight) to activity concentration in water
(Bq mÀ3). Units of m3 kgÀ1.
Distribution coefficient (Kd) Equilibrium parameter
used to quantify the partition of a radioelement
between solid (soil or sediment) and liquid phases
(overlaying or interstitial water), expressed as the
ratio of activity concentration in solids (Bq kgÀ1) to
activity concentration in water (Bq mÀ3). Units of
m3 kgÀ1.
Dose conversion coefficient Dose rate received by an
organism exposed to radiation, relative to activity
concentration of the source. Units of Gy sÀ1 per
Bq kgÀ1 (or Bq m3). Robert A. Meyers (ed.), Encyclopedia of Sustainability Science and Technology, DOI 10.1007/978-1-4419-0851-3,
# Springer Science+Business Media, LLC 2012 8388 R Radioactivity in the Marine Environment Ecosystem The system formed by a biological community and its nonliving surroundings.
Foodweb A succession of organisms in an ecosystem
that is linked by predator–prey relationships, transferring mass and energy from one to another.
Global fallout Residual radiation hazard from the
atmospheric nuclear explosions of the 1940s to
1960s, with slowly declining global presence in the
planet up to the present day.
NORM Radionuclides from naturally occurring
radioactive materials released to the environment
by industrial processing, e.g., mineral extraction
and processing, oil and gas, phosphogypsum
by-products, and fertilizers.
OSPAR The Convention for the Protection of the
Marine Environment of the Northeast Atlantic, a
mechanism by which 15 Governments of the western coasts and catchments of Europe, together with
the European Community, cooperate to protect the
marine environment of the Northeast Atlantic.
Radiation weighting factor Factor representing the
relative effectiveness of different radiation types
relative to X- or gamma-rays, in producing biological effects of significance.
Remobilization The return of a radionuclide to circulation within an ecosystem, such as return to the
water column of radionuclides which had been
locked in sediments.
Scavenging Removal of substances from the water column by the action of settling particles and/or
organic matter.
Sediment mixing Mixing of sediment and water at the
interface between the two, by processes such as
bioturbation and water turbulence.
Speciation Partitioning of an element amongst defined
species in a chemical system.
Definition of the Subject and Its Importance
The sea is a complex system containing different components (water column, suspended particulates, colloids, sediments, and organic matter) and inhabited
by life forms at multiple scales (from plankton to
large mammals), undergoing complex interactions.
With the arrival of nuclear technology in the late
1940s, a variety of man-made radionuclides have
entered the marine environment, either as a result of military operations, industrial discharges, medical
releases, or nuclear accidents. This has resulted in
their widespread distribution, cycling across the sea
and uptake by biota, both locally (in the vicinity of
discharge points) and globally.
Studies of the different ways radioactive substances
interact with the marine environment fall into the
domain of marine radioecology. In the 1950s and
1960s, this discipline focused on studying how the
global fallout from nuclear weapons testing and satellite burn-up entered the oceans and was distributed
within the water column, reaching the seabed sediments and the food chain. Radionuclides of interest
were the relatively long-lived plutonium (Pu), americium (Am), radiocesium (Cs), and radiostrontium
(Sr), the fission products from nuclear detonations.
From the late 1970s to 1980s, more studies
explored the interaction mechanisms of radionuclides
with sediments, suspended particulates, and marine
colloids. A key focus for marine radioecology research
at that time was the input from European reprocessing
plants to the seas of the northern hemisphere.
Most significantly, the marine discharges from the
reprocessing plants at Sellafield, UK, and La Hague,
France resulted in enhanced radionuclide levels in the
Irish Sea and English Channel/North Sea respectively.
Fundamental to investigations at this time was to
understand the chemical speciation of the transuranic
radionuclides, predominantly plutonium and americium, and the mechanisms of how they concentrated
in sedimentary deposits and how they became
remobilized.
A key driver for this phase was the estimation of
radiation doses to humans arising from the consumption of contaminated marine foodstuffs. However,
beyond the purely radiological protection point of
view, substantial understanding was sought and gained
on the potential of radionuclides as tracers in the study
of marine and coastal processes, as well as understanding the transfer of radionuclides to the local communities of animals and plants.
The Chernobyl accident in 1986 resulted in a global
release of radionuclides. Although in the Irish and
North Seas the main influence continued to be the
input from reprocessing plants, the Baltic and Black
Seas became affected by the Chernobyl accident,
with 90Sr, 134Cs, 137Cs, and 239,240Pu entering these Radioactivity in the Marine Environment environments. Meanwhile, during the 1990s, Irish Sea
studies shifted focus to the 99Tc discharges arising from
the Enhanced Actinide Removal Plant (EARP) in
Sellafield, with 99Tc discharges reaching their peak
between1994 and 1996. As a result, levels of 99Tc in
organisms such as lobsters found off the coast of
Sellafield reached their highest levels in the late 1990s
and early 2000s. During this time, transuranium discharges decreased several orders of magnitude below
their peak discharge rates of the mid-1970s.
Before the Fukushima accident in Japan in 2011,
there were no more major releases of radioactivity into
the marine environment. In a period marked by concerns on long-term environmental sustainability came
the realization that radionuclides persist in the marine
environment for long periods of time. Research
addressed in more detail the interaction of radionuclides with individual organisms and foodwebs,
striving for a global understanding at a species and
ecosystem level. The traditionally anthropocentric
view of radiological protection was replaced by a
more ecocentric approach. There was a realization
that, even if humans are protected, the environment is
not necessarily protected because marine organisms
inhabit areas where humans cannot reach. This called
for the development of an international system
of radiological protection for the environment, and
new studies of the transfer of radionuclides to marine
biota.
Recently, dynamic situations such as accidental scenarios, decommissioning discharges and NORM
releases from offshore oil operations have attracted
scientific interest. These situations are characterized
by irregular discharge patterns, requiring dynamic
modeling of the transfer of radionuclides to marine
organisms. There is also interest in low-level ionizing
radiation effects to biota, and it is felt that Radioecology needs to be applied in a world of multiple
contaminants, exerting their combined stress on
interdependent species and entire ecosystems. A number of collaborative international projects are developing in this direction.
Radioactivity in the marine environment is here to
stay, and there is an onus to gain a scientific understanding of its implications, as part of a drive to preserve the quality of this environment for future
generations. R 8389 Introduction
Artificial radioactivity reaches the world’s oceans
from various sources: releases from nuclear power
plants and reprocessing facilities connected to coastal
areas directly or via waterways, the worldwide fallout
from nuclear weapons tests, dumping of waste to the
seafloor or accidental situations such as satellite burnup, “broken arrow” situations, or the sinking of nuclear
submarines.
The historical sources of radioactivity into the environment, detailing natural radioactivity, global fallout,
accidents, and NORM and discharges from the nuclear
industry to the marine environment have been abundantly described elsewhere [1–8], so they will only be
summarized briefly.
Historically, one of the most significant inputs with
relevance to the marine environment is the worldwide
fallout of nuclear weapons tests, resulting in the global
inventory of man-made radioactivity in the world’s
oceans [9, 10], to which one must add accidents involving space satellites [11, 12] and lost nuclear weapons
[2, 13–16]. The earliest weapons tests provided in fact
the first opportunities to investigate at close quarters
the impact of man-made radionuclides in the marine
ecosystem [17–19].
Nuclear accidents represent another source of
radioactivity to the marine environment. The Chernobyl accident affected indirectly several marine areas
[20–23]. The two major nuclear accidents in 1957,
Kyshtym and Windscale, also affected the marine environment to some degree, but they are not believed to
have been a major source of contamination to the
marine environment [6]. At the time of writing this
article (April 2011), it is too early to evaluate the
impact of the 2011 nuclear accident at the Fukushima
plant (Japan), though it is clear that this event resulted
in the direct release of radionuclides to sea, chiefly 131I
and 137Cs.
Routine operations from the nuclear industry have
been a source of radioactive contamination to sea since
the early 1950s. The most important contribution is
from the reprocessing of spent nuclear fuel in purposebuilt facilities discharging waste to sea. The importance
of the Sellafield site discharges to the Irish Sea
has resulted in numerous radioecological studies
being performed, examples of which are described R 8390 R Radioactivity in the Marine Environment here [24–28]. Another significant reprocessing site
which has been extensively studied from the marine
discharges point of view is the La Hague reprocessing
plant in France [29, 30]. In the Arctic Ocean, run-off
from contaminated sediment in the Ob and Yenisey
river system connected to the Mayak, Krasnoyarsk,
and Tomsk nuclear facilities and the Semipalatinsk
test site has also become a well-studied source of radionuclides [31].
Lastly, historical dumping of radioactive waste [32]
has been a controversial source of radioactivity release
into the world’s oceans. Between 1946 and 1982, packaged low-level radioactive waste (LLW) was dumped at
more than 50 sites in the northern part of the Atlantic
and Pacific Oceans. Beta-gamma emitters represented
the majority of the total radioactivity in that waste, but
alpha-emitters such as plutonium and americium were
also disposed of by this method [33].
By the above means, an extensive list of man-made
radionuclides has been introduced to the marine environment in varying amounts. This article cannot cover
every one of them. Due to their availability in the
environment, their radiological importance, and the
amount of environmental data available, the four
radionuclides to be considered further are 99Tc, 137Cs,
Pu (as the a-emitters 238Pu and 239+240Pu, as well as the
b-emitter 241Pu), and 241Am. These radionuclides can
be grouped broadly into two categories: those behaving
as dissolved in the water column (Tc and Cs) and those
with a strong affinity for suspended particulates and
sediments (Pu and Am).
In sea water, almost all of the naturally occurring
elements are present in variable quantities. The concentrations of the major constituents are found to be
directly proportional to the salinity of the water. The
more soluble Tc and Cs radionuclides follow the same
behavior, and so they are termed “conservative.” The
sediment-seeking Pu and Am are classed as
“nonconservative” radionuclides. The residence time
within the seabed for these non-conservative radioelements will generally be longer than the hydrological
transit time following release or remobilization from
the sediments.
After radionuclides have entered the water, physicochemical changes will occur, followed by dilution
and dispersion further afield by the action of oceanographic processes. However, it is also possible
for radionuclides to become accumulated in certain
parts of the marine environment, such as sediments,
through the processes of scavenging and particle deposition, as well as becoming biologically concentrated. A
conceptual representation of the key processes is given
in Fig. 1.
Whether radionuclides from a specific source are
associated with particles, colloids, or various chemical
species in the water depends on the type of source and
conditions prevailing in a particular environment
[7, 34]. For example, plutonium from global fallout is
mainly associated with submicron iron oxide particles;
in fallout from surface tests in the Marshall Islands,
plutonium was mainly attached to calcium hydroxide
particles [35]; Pu in effluents from nuclear reprocessing
plants are associated with particulate material or colloids [36].
Ultimately, the transport, distribution, and biological uptake of radionuclides in the marine environment
depends heavily on the physicochemical forms of
radionuclides (i.e., their speciation) in the discharged
effluents, as well as on the transformation processes
that occur after entering coastal waters. For this reason,
this article will cover in some detail radionuclide
speciation, a “hot topic” in the aquatic environment
where trace metals are concerned.
Trace element speciation in the marine environment is different from speciation under freshwater
conditions. This is due to varying features such as
salinity, pH, dissolved organic carbon and the size of
particulates in the water column, ranging from
nanocolloids to micron-sized particulates. Many
reviews on radionuclide speciation concentrate on particle size considerations, i.e., physical speciation. In this
entry, consideration will be given to the actual chemical
speciation, which refers to the molecular form of the
radionuclides in the marine environment.
Radionuclide Speciation in Seawater
Conservative radio elements, like 90Sr, 137Cs, and 99Tc,
have a relatively low (but measurable) capacity for
adsorption onto solid particles. They readily disperse
upon release to the marine environment, and therefore
can be used as tracers for oceanographic processes. Radioactivity in the Marine Environment R 8391 Advection Diffusion Suspended
particles
Sorption
Dissolved
Deposition fraction
Re-suspension Sorption Diffusion Seabed
sediment Radioactivity in the Marine Environment. Figure 1
Conceptual representation of the processes occurring when radionuclides enter the marine environment On the other hand, transuranium elements, like plutonium and americium, are not efficiently dispersed away
from the source point but, instead, are more efficiently
carried onto the seabed by scavenging particles, becoming incorporated onto sediment for protracted periods
of time. 241Am is a special case as additional quantities
will be formed by the decay of 241Pu if this radionuclide
is present.
The above picture is somewhat simplified, and
becomes more complicated by the fact that certain
radionuclides, such as plutonium, can have different
chemical forms, or species, capable of coexisting in
varying proportions in the water column. Some of
these species actually have low particle reactivity,
behaving conservatively, whereas others can have a high affinity for sediment and behave nonconservatively. In this section, the speciation of Pu and
241
Am is compared on one hand, and 99Tc and 137Cs
on the other, to illustrate how this phenomenon affects
their environmental behavior.
Plutonium and Americium Speciation
It is well known that, in aqueous solution, plutonium
can exist in the oxidation states +3, +4, +5, and +6 [37].
In the absence of complexing agents, these oxidation
states form the chemical species Pu3+ and Pu4+ (the
“reduced,” or Pu(III,IV) group) and PuO2+ and PuO22+
(the “oxidized,” or Pu(V,VI) group), respectively. These
two oxidation state groups of plutonium exhibit very R 8392 R Radioactivity in the Marine Environment different sediment sorption properties, with reduced
plutonium possessing an affinity for sediment approximately two orders of magnitude higher than oxidized
plutonium, relative to concentration in seawater.
Such species can be interconverted from one to
another by means of oxidation–reduction reactions,
coexisting in equilibrium in the aquatic environment
[9, 38–41]. Theoretical calculations show that in a pure
aqueous solution (without suspended particles or colloidal matter) PuO2+ is, by far, the predominant form
[42–45]. The insolubility of Pu(OH)4 is the limiting
factor of the net solubility of plutonium in oxic natural
waters, making Pu(V)O2+ the most stable oxidation
state [46].
Americium in aqueous solution can also exist in the
oxidation states +3, +4, +5, and +6 but the oxidation
state +4 is stable only in the presence of concentrated
H3PO4, K4P2O7 and fluoride solutions [47]. In the
absence of complexing agents, the oxidation states +3,
+5, and +6 can occur in the form of the hydrated ions
Am3+, AmO2+, and AmO22+. However, AmO2+ and
AmO22+ are quickly reduced to Am3+. Hence, the
chemical speciation of americium, as well as its environmental behavior, is less complex than that of plutonium and Am3+, likely to be in the form of
a hydroxide, is the predominant species of americium
in seawater [48].
Understanding the environmental behavior of plutonium in seawater requires consideration of the complex interactions of Pu (III,IV) and Pu (V,VI) within
a heterogeneous water column where two distinct solid
phases (particulate and colloidal) coexist, as well as
interactions with seabed sediments. The situation is
fully described elsewhere [49, 50]. Environ...
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