photosynthetic production of organic matter and in other biochemical processes, but it will be highly diluted by the large amount of ordinary water molecules available for these processes in the ocean.

Recent Advances in Ocean Science

Because of recent advancements in marine science, we are far better equipped today to address questions of disposal of radioactive materials in the ocean than we were 20 years ago. For example, the GEOSECS (Geochemical Ocean Sections Study) project sponsored by the National Science Foundation used radiotracerstritium, radon, radium, and carbon-14-to better understand the dynamics of chemical and physical processes in major ocean basins. In so doing, much information was gained regarding these nuclides.

The tritium story illustrates the global impact of human actions. Natural tritium from cosmic ray-induced reactions in the upper atmosphere yields a steadystate global inventory, estimated to be in the range of 3 to 7 kilograms. Atmospheric tests of nuclear weapons prior to 1963 added 100 kilograms of tritium to this global inventory, increasing tritium's inventory by a factor of 15 to 30. No evidence exists of any ecological impact resulting from such an increase, but oceanographers have utilized this alteration of the environment to learn about oceanic water transport rates. For example, GEOSECS has been able to trace the bomb-produced tritium in the upper 500 meters of the Atlantic Ocean and to depths greater than 4,000 meters in the North Atlantic (Figure IV-5).

Radon has been used to determine vertical ocean movement. Because of the short half-life of its most abundant isotope (3.8 days), radon gives information about transport processes that occur within time scales of up to two weeks. Radon measurements from GEOSECS revealed that the spatial extent of upward mixing waters above the seafloor varies greatly. In one Pacific Ocean location, the waters 400 meters above the seafloor had radon concentrations indicating recent contact with the seafloor.'

The use by GEOSECS of carbon-14 has helped in understanding the pathways of organic and inorganic carbon through the ocean. As with tritium, significant quantities of carbon-14 were produced by atmospheric tests of nuclear weapons. All of the radioactive carbon has long since decayed in such fossil fuels as petroleum, coal, and natural gas. Thus, the burning of fossil fuels reduces the ratio of carbon-14 to stable carbon-12 in the atmosphere. One of the main lessons from carbon14 is that while it provides information about the rates of organic production cycles and deep circulation in the sea, the reading of this "radiochemical clock" is not a simple matter. It requires a complete understanding of the pathways and processes involving organic and inorganic carbon in the ocean.

Use of these tracers has revealed two interesting points. First, the ocean, particularly the deep ocean, is a more dynamic environment than most oceanographers would have believed a decade ago. Second, the chemical nature of radionuclides, not the fact that they are radioactive, is a more important factor in determining their fates in the ocean. This latter point appears to be ignored by the practice of reporting and monitoring radioactive wastes placed into the ocean solely in terms of alpha and beta/gamma radioactivity. From that information, one cannot know the half-life of the radiation and hence the time scale of its importance nor can one make estimates about a nuclide's geochemical, physical, or biological cycling and transport in the sea.

Many of the anthropogenic radionuclides are metals such as strontium, cesium, iron, and zinc. Most naturally occurring metals are at very low concentrations in the marine environment and are affected to varying degrees by biological processes, hydrographic or oceanic water mass variations, and by sediment

release. Chemical studies of marine metals show that their chemistry is influenced by the formation of complexes with inorganic and organic substances in seawater, by their adsorption onto particles suspended in the ocean, and, in some cases, by the solubility of solid phases. Such chemical reactions influence the metal's availability for incorporation by marine organisms," and determine the conditions under which some metals can be toxic to marine organisms.

The mixing of water (and hence of radionuclides) appears to be both more complex and more dynamic than was believed 20 years ago. The Gulf Stream and other major currents are continuously throwing off rings and eddies, some more than 100 miles in diameter. Other types of eddies have been found in the central ocean basin and at varying depths. These phenomena have proved to be far more common than had been first anticipated. Because rings and eddies may be a major oceanic stirring mechanism, their passage through a radioactive waste dumpsite could influence the transport of radionuclides to a substantially greater degree than would be predicted based on average rates of oceanic mixing and advection.8

Hydrothermal vents are the most recent example that the ocean is still capable of providing new, important, and unexpected discoveries. These vents, or locations where hot water is streaming out of rocks in the seafloor, occur along the mid-ocean ridge in the eastern equatorial and the north Pacific Ocean. Recent chemical studies reveal that the hot water from these vents is seawater seeping along cracks in the rocks of the mid-ocean ridge that becomes heated by local geothermal heat fluxes. Chemical reactions occur between the seawater and the hot rocks, so that the water's chemical composition is considerably altered. Geochemical processes associated with these vents are of considerable importance to understanding the cycling of some chemicals in the sea.

The vents also support a heretofore unknown biological ecosystem at depths of 2 to 3 kilometers beneath the sea surface, in permanent darkness. The hydrothermal vent ecosystem is supported by both geothermal heat and chemical reduction. In vent communities, chemosynthetic bacteria occupy the same primary role as photosynthetic plants in surface communities. Because individual vents are episodic, there must exist a means for colonizing new vents on the order of a few decades. Thus, hydrothermal vents have implications for the disposal of radioactive wastes, because radionuclides could interact with the vent communities.

Anthropogenic Versus
Natural Radioactivity

One of the most striking statistics is that there is now about a thousand times more natural radioactivi

ty in the ocean than there is anthropogenic radioactivity, including the relatively large amounts of radioactivity that have entered the ocean from nuclear weapons tests. By comparison, the amount of radioactivity that would be added to the ocean, even if it were to become the final repository for all of the world's low-level radioactive waste, is relatively small. However, providing a comparison between the human input of radionuclides to the marine environment and natural levels of radioactivity is not as straightforward as one might think. The comparison should consider the characteristics of the particular marine environment, the sources of the anthropogenic and natural radioactivity and the behavior of specific nuclides in the sea. The total volume of the ocean is not available to assimilate anthropogenic radionuclides on reasonable time scales. Even for the radionuclides produced during atmospheric testing of nuclear weapons, which were widely dispersed in the atmosphere before finding their way to the Earth's surface, there was a strong hemispherical and latitudinal variation in the deposition of fallout on the continents and the ocean. The middle latitudes of the northern hemisphere received most of the fallout. Furthermore, the ocean is not well-mixed vertically on time scales of centuries at most latitudes. (See the distribution of tritium in Figure IV-5 from the nuclear weapons test.) Only at the northern latitudes has mixing extended from the surface layer to the deep water; elsewhere the tritium is concentrated in the surface layer.

Most methods that are being considered for the disposal of radioactive wastes in the marine environment would result in the designation of a particular location or disposal site for the material to be placed. If radionuclides escape from containers and become dispersed, they will initially affect that region of the ocean. On time scales of decades to possibly centuries, we can expect that the waters within a given ocean basin, say the western North Atlantic, will mix within that basin but not throughout the entire ocean.

Various disposal scenarios have been formulated and played against different kinds of ocean mixing models. Although the ocean has a large capacity to accommodate additional radioactivity without an appreciable increase in background radiation, its capacity is finite, particularly on a regional scale or for time scales of decades. The ocean's capacity for a waste also depends on whether the waste remains in the water column or becomes associated with the surface sediments on the seafloor. Efforts should be made to refine our assessments of the behavior of anthropogenic radionuclides in the sea by obtaining a inore complete understanding of their marine chemistry. Only when radioactive wastes have been characterized in terms of specific nuclides can we hope to make a meaningful assessment of their effect on the environment.

Mechanisms for Human Exposure to Radioactivity in the Sea

The ocean is an inherently dispersive medium and generally more remote from humans than continental nuclear waste repositories. Nevertheless, humans can be exposed to marine radionuclides through direct exposure from contact with seawater, marine organisms, and sediments, from ingestion of marine food sources, or from possible contamination of a marine resource that might be utilized in the future, such as manganese nodules.

Direct exposure-through swimming, boating, marine transportation, or aerosol particles produced in the ocean by spray-appears infinitesimally small. With a few exceptions, the disposal methods that have been used in the past and that are likely to be used in the future place the waste away from the coastal zone where humans have the greatest contact with the sea. When the United States deposited containerized radioactive wastes in the ocean, they were usually placed below the main thermocline at depths greater than 2,000 meters. An exception to this practice was 4,008 cannisters placed on the continental shelf off Boston. A potentially more significant exposure to humans could result from long-term discharges to coastal waters of radionuclides from nuclear facilities such as fuel reprocessing plants or electrical power plants.

The largest part of the radiation dose that the public receives from nuclear material in the ocean is from radionuclides in seafood. The technique that has been used to establish safe limits for the disposal of radioactive wastes in the marine environment is known as the critical pathway analysis. (Examples are presented in Appendix E, Case Histories.) This technique identifies pathways to humans from particular radionuclides and pinpoints the segment of the population that will receive the greatest exposure to that nuclide. Table IV-6 provides examples of the critical pathway and the population group for eight nuclear facilities.

Summary

The fate of radionuclides in the ocean is determined in part by such oceanic processes as dispersion, advection, incorporation into marine organisms, and deposition in sediments. With these processes, there can be substantial chemical transformations that alter the chemical behavior of the nuclide in the ocean.

Besides understanding these processes, we must also know the quantities disposed of specific radionuclides and the type of radiation they emit. Although much knowledge has been gained recently about marine radionuclide behavior, further information is needed

in such areas as the deep ocean environment and its biological communities and marine chemical cycles.

A comparison of anthropogenic and natural radioactivity in the marine environment indicates that present human activities are not increasing the level of total radioactivity in the ocean. The ocean has, however, a limited capacity to accommodate anthropogenic radionuclides of specific species and in specific regions for time periods of decades to centuries. This limited capacity is especially important for the surface sediments on the seafloor and for the upper mixed layer of the ocean. The continental shelves are especially poor regions for radioactive waste disposal, because these are regions of generally high biological activity, are close to human activity, and are in the upper layer of the ocean.

The most likely link between marine radioactivity and humans is through seafood. There have been a few instances in which a critical path assessment has indicated that exposure to coastal sediment or detritus could be significant.

'Friedlander, G., J.W. Kennedy, and J.M. Miller. 1964. Nuclear and Radiochemistry. John Wiley & Sons, New York, 585 p.

'Burton, J.D. 1975. Radioactive Nuclides in the Marine Environment. In J.P. Ripley and G. Skirrow (editors), Chemical Oceanography, Academic Press, Inc., London, p. 91-191.

Bender, Michael, Wallace Broecker, Vivian Gornitz, Ursula Middel. Robert Kay, Shine-Soon Sun, and Pierre Biscaye. 1971. Geochemistry of Three Cores from the East Pacific Rise. Earth and Planetary Science Letters 12(1971):425-433.

4 Park, P. Kilho, Dana R. Kester, Iver W. Duedall, and Bostwick H. Ketchum. 1983. Requirements for Radioactive Waste Management in the Ocean. In Park et al. (editors), Wastes in the Ocean. Volume 3. John Wiley & Sons, New York, p. 481-505.

'Sarmiento, J.L. 1976. The Relationship between Vertical Eddy Diffusion and Buoyancy Gradient in the Deep Sea. Earth and Planetary Science Letters 32(1976): 357-370.

• For instance, see:

Boyle, E.A., F.R. Sclater, and J.M. Edmond. 1977. The Distribution of Dissolved Copper in the Pacific. Earth and Planetary Science Letters 37(1977):38-54.

Klinhammer, G.D., and M.L. Bender. 1980. The Distribution of Manganese in the Pacific Ocean. Earth and Planetary Science Letters 46(1980):361-384.

Bruland, K.W. 1980. Oceanographic Distributions of Cadmium, Zinc, Nickel, and Copper in the North Pacific. Earth and Planetary Science Letters 47(1980):176-198.

Zuehlke, R.W., and D.R. Kester. 1983. Ultraviolent Spectroscopic Determination of the Stability Constants for Copper Carbonate and Bicarbonate Complexes up to the Ionic Strength of Seawater. Marine Chemistry 13:202-226.

For instance, see:

Backus, R. et al. 1981. Gulf Stream Cold Core Rings: Their Physics, Chemistry, and Biology. Science 212: 1,091-1,100.

Schink, D. et. al. 1983. Multidisciplinary Program to Study Warm Core Rings. Transactions of the American Geophysical Union 63(44):834-835. 'Park, P. Kilho, Dana R. Kester, Iver W. Duedall, and Bostwick H. Ketchum. 1983. Radioactive Wastes and the Ocean: An Overview. In Park et al. (editors), Wastes in the Ocean. Volume 3. John Wiley & Sons, New York, p. 3-46.

Chapter III explained that this Nation presently follows a land-based radioactive waste disposal policy. Nevertheless, several developments are stimulating continued examination of possibilities for resumption of ocean disposal of radioactive materials.

Disposal of Defueled, Decommissioned

Nuclear Submarines'

The U.S. Navy has about 120 nuclear submarines now in operation, 100 of which will be taken out of service in the next 20 to 30 years at a rate of three to five a year. Five already have been decommissioned and await permanent disposal.2

The Navy presented three disposal options for these defueled craft in a Draft Environmental Impact Statement (DEIS) dated December 1982: bury the radioactive hull section and reactor at existing governmental land disposal sites; drop the entire defueled submarine onto a predetermined part of the ocean floor off a U.S. coast; or continue protective storage at an inactive ship facility until permanent disposal is decided.'

If land disposal is the choice, the submarine's reactor compartments would be buried either at the Hanford Reservation in Washington or at the Savannah River Plant in South Carolina-each of which is already a LLW disposal site."

If they are to be disposed of in the ocean, the submarines would be towed to designated sites and sunk by a system of controlled interior flooding, to rest intact on the sea floor." Since the fuel would be removed before disposal, the remaining radioactivity in the submarine at the time of disposal would be radionuclides that are neutron-activated metal atoms within the structure of the reactor compartment. Those radionuclides could only be released by corrosion of the metal structure. Over a period of about 100 years, the reactor compartment containment barrier would be penetrated by corrosion, and bottom currents would begin to flow through it, transporting corrosion products into the adjacent environment." Naval engineers and radiologists calculate, however, that by then such reduced radionuclide emissions would be harmless to

man. Although the total radioactivity initially contained within the 100 submarines may be 6 x 106 curies, the maximum release in any one year would be 39 curies during the 1,800 years required for decay or release of 99 percent of the radioactivity."

The Navy DEIS does not select potential disposal sites, although study areas avoided regions that: (1) produce large amounts of seafood or which are food sources for commercial fishes; (2) are currently used by humans for any purpose; or (3) have future resources potential-such as oil and gas fields, or ocean mining

Figure V-1 shows two Atlantic study areas over 200 miles east of Cape Hatteras, North Carolina, in depths of 13,000 to 16,000 feet." Figure V-2 shows the Pacific study area, about 160 miles west of Cape Mendocino, California. Water depths here are from 13,500 to 14,800 feet. Study criteria for these areas were developed by the Navy, based primarily upon IAEA standards (noted in Chapter III) with the following additions:"3

1. Sites should avoid such areas as submarine canyons, where high rates of exchange occur between the bottom water mass and surface layers over the adjacent continental shelf-to avoid shortening any potential pathways to man.

2. Bottom current shear stress in the study area should not exceed critical erosional shear stress. This precludes high rates of sediment resuspension, and thus rapid movement of material.

3. Sites should be away from areas of intense mesoscale eddy activity, since eddy diffusivity shortens the pathway to man.

The Navy lists the following adverse effects of ocean disposal:14

1. Submarine disposal restricts the use of the seafloor for other activities. (If all 100 submarines were placed in the same general area, the region would be a circle of 11 statute miles in diameter, or an area of about 100 square miles.)

2. About 3,000 tons of recyclable material-mostly steel-would be lost per submarine; however, the cost to recover the steel is greater than its scrap value.

3. The ocean environment-primarily the sediment area-would absorb about 4,000 tons of corro