RWH, (302-316) Encyclopedia of Sustainability Sci & Tech, Dr. Sangho Lee, Dr. Reeho Kim (pp 8688

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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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