Radionuclides in the Sea
Before we can follow the atom down into the sea, we must understand something about the potentials, both good and bad, of this incursion of one of our most advanced technologies into one of earth’s least understood environments. This adventurous probing has ramifications for studying both man-produced radioactivity in the sea and the ocean itself as an uncontaminated environment.
| TABLE I | ||
|---|---|---|
| AEC OCEANOGRAPHY PROGRAM | 1968 Expenditures Estimate | |
| Research Activities | ||
| Division of Biology and Medicine | $4,000,000 | |
| Studies of uptake, concentration, distribution and effects of radioisotopes on marine life, of geochemical cycling of elements, and of geophysical diffusion and transport. | ||
| Division of Research | 25,000 | |
| Geological dating of corals and other marine and terrestrial materials. | ||
| Division of Isotopes Development | 190,000 | |
| Radioisotope applications to devices for marine systems, such as current meters, analysis and recovery of sedimentary minerals, and underwater sound transmission. | ||
| Division of Reactor Development and Technology | 197,000 | |
| Studies of factors affecting dissolution and dispersal of accidentally released radionuclides, and site evaluations. | ||
| Division of Space Nuclear Systems | 275,000 | |
| Nuclear power sources for aerospace applications. | ||
| Division of Military Applications | 850,000 | |
| Ocean environmental observation and prediction. | ||
| Total—Research Activities | 5,537,000 | |
| Engineering Activities | ||
| Division of Reactor Development and Technology | 5,900,000 | |
| Radioisotope and reactor power development. | ||
| Division of Naval Reactors | 1,320,000 | |
| Deep submergence research vehicle. | ||
| Total —Engineering Activities | 7,220,000 | |
| Total—ABC Oceanographic Activities | 12,757,000 | |
Radionuclides (radioactive atoms) can find their way into the sea from natural radiation sources or from nuclear energy operations undertaken by the United States and other countries since 1945. Specific man-made sources in the past may have included nuclear weapons tested in the atmosphere and under water, the cooling water and wastes of nuclear reactors, laboratories and nuclear-powered ships, containers of radioactive waste disposed of at sea[5], radioisotope energy devices, and intentional injection of radioisotope tracers for scientific research. In the future, they may also include reentry from space of upper-stage nuclear rockets or satellite-borne nuclear energy sources.
The Nansen bottle, shown being attached to a hydrographic wire, is one of the standard tools of oceanology. When a bottle reaches a desired depth, a sliding weight tips it upside down to collect seawater samples. Thermometers on the sides of the bottles record temperature. The device was designed by the Norwegian oceanographer and explorer, Fridtjof Nansen. (See photo on [page 56].)
In order to evaluate the effects of these materials in the ocean environment, it is necessary to know many things. Just how much radiation is introduced? In what form? Where geographically? How are these radionuclides dispersed or concentrated physically, chemically, biologically, and geologically? What is the net result in each case now, and what will it be many years hence?
These questions are not answered easily. There is, as yet, no satisfactory laboratory substitute for the open ocean. Research for the most part must be conducted at sea, where tests and measurements are difficult at best, and where results therefore are often suspect. Further, if we are to study the effects of man-induced changes in a natural environment, it would have been advantageous to have known the nature of that environment before the changes were introduced—which, by and large, in the case of the ocean we do not. So we must start with a contaminated environment and try to separate what we have put there ourselves from what would have been there anyway. It isn’t an easy task to make the physical and biological observations that will make this distinction.
| Table II CONCENTRATION AND AMOUNTS OF 42 OF THE ELEMENTS IN SEAWATER | |||
|---|---|---|---|
| Element | Concentration (mg/l) | Amount of element in seawater (tons mile³) | Total amount in the oceans (tons) |
| Chlorine | 19,000.0 | 89.5 × 10⁶ | 29.3 × 10¹⁵ |
| Sodium | 10,500.0 | 49.5 x-10⁶ | 16.3 × 10¹⁵ |
| Magnesium | 1,350.0 | 6.4 × 10⁶ | 2.1 × 10¹⁵ |
| Sulphur | 885.0 | 4.2 × 10⁶ | 1.4 × 10¹⁵ |
| Calcium | 400.0 | 1.9 × 10⁶ | 0.6 × 10¹⁵ |
| Potassium | 380.0 | 1.8 × 10⁶ | 0.6 × 10¹⁵ |
| Bromine | 65.0 | 306,000 | 0.1 × 10¹⁵ |
| Carbon | 28.0 | 132,000 | 0.04 × 10¹⁵ |
| Strontium | 8.0 | 38,000 | 12,000 × 10⁹ |
| Boron | 4.6 | 23,000 | 7,100 × 10⁹ |
| Silicon | 3.0 | 14,000 | 4,700 × 10⁹ |
| Lithium | 0.17 | 800 | 260 × 10⁹ |
| Rubidium | 0.12 | 570 | 190 × 10⁹ |
| Phosphorus | 0.07 | 330 | 110 × 10⁹ |
| Iodine | 0.06 | 280 | 93 × 10⁹ |
| Barium | 0.03 | 140 | 47 × 10⁹ |
| Indium | 0.02 | 94 | 31 × 10⁹ |
| Zinc | 0.01 | 47 | 16 × 10⁹ |
| Iron | 0.01 | 47 | 16 × 10⁹ |
| Aluminum | 0.01 | 47 | 16 × 10⁹ |
| Molybdenum | 0.01 | 47 | 16 × 10⁹ |
| Selenium | 0.004 | 19 | 6 × 10⁹ |
| Tin | 0.003 | 14 | 5 × 10⁹ |
| Copper | 0.003 | 14 | 5 × 10⁹ |
| Arsenic | 0.003 | 14 | 5 × 10⁹ |
| Uranium | 0.003 | 14 | 5 × 10⁹ |
| Nickel | 0.002 | 9 | 3 × 10⁹ |
| Vanadium | 0.002 | 9 | 3 × 10⁹ |
| Manganese | 0.002 | 9 | 3 × 10⁹ |
| Antimony | 0.0005 | 2 | 0.8 × 10⁹ |
| Cobalt | 0.0005 | 2 | 0.8 × 10⁹ |
| Caesium | 0.0005 | 2 | 0.8 × 10⁹ |
| Cerium | 0.0004 | 2 | 0.6 × 10⁹ |
| Silver | 0.0003 | 1 | 5 × 10⁸ |
| Cadmium | 0.0001 | 0.5 | 150 × 10⁶ |
| Tungsten | 0.0001 | 0.5 | 150 × 10⁶ |
| Chromium | 0.00005 | 0.2 | 78 × 10⁶ |
| Thorium | 0.00005 | 0.2 | 78 × 10⁶ |
| Lead | 0.00003 | 0.1 | 46 × 10⁶ |
| Mercury | 0.00003 | 0.1 | 46 × 10⁶ |
| Gold | 0.000004 | 0.02 | 6 × 10⁶ |
| Radium | 1 × 10⁻¹⁰ | 5 × 10⁻⁷ | 150 |
Adapted from The Mineral Resources of the Sea, by John L. Mero, American Elsevier Publishing Company, New York, 1964.
Many sea creatures are efficient, selective concentrators of “trace elements”, which occur in seawater only in minute portions. These elements are difficult enough to detect qualitatively and all but impossible to analyze quantitatively. Yet among the elements the sea’s plants and animals concentrate are the very materials with which we are apt to be most concerned: Strontium, cesium, cerium, ruthenium, cobalt, iodine, phosphorus, zinc, manganese, iron, chromium, and others. Radioisotopes[6] of all these elements occur as by-products of human nuclear activities. Many concentrating organisms are microscopic in size and are frequently impossible to raise in captivity. It is apparent that we are faced with a research program of considerable challenge and proportion.
We need to know how each marine species concentrates. Is it from the food it eats, by absorption from the water, or both? Does it concentrate an element by continuous accumulation, or is there a constant turnover of the material in the organism’s system? (In the first case, once the creature became radioactive it would remain so throughout its life or until the radioactivity decayed. In the second case, however, the radioactivity might be a transient condition, assuming the creature could find its way into uncontaminated water and were able to flush itself.) Obviously, both the cycling time of the radioisotope in the organism and its radioactive half-life[7] must be taken into account.
Even if we should manage to identify all the marine concentrators and gain some insight into their metabolic processes, this would be only a first step. For example, one tiny form of planktonic protozoan, acantharia, concentrates up to 15% of its own weight of strontium, including the radioisotope strontium-90. It is eaten by larger zooplankton (animals), such as copepods, which are eaten by little fish, which, in turn, are eaten by bigger fish, etc. Somewhere along this food chain, perhaps, a fish will come along that is favored for human dinner tables. How much strontium-90 has that fish accumulated through swallowing its prey and by absorption from the water? Is the radioactivity in its scales, bones, viscera, and other usually uneaten portions, or in its flesh?
It is probable, though as yet by no means proven, that among the million or so oceanic species of plant and animal life, there are concentrators of virtually all the 60 or more elements found in seawater. To identify and study them is an enormous undertaking, which is often possible only by using radioisotopes as tools.
And what of the immediate and genetic effects of radiation on each species? Studies of reef fish in the nuclear testing area in the Marshall Islands have shown that radioiodine in the water caused thyroid gland damage long after the amount of radioiodine remaining in the water was too low to be detected. Studies of salmon in the Columbia River have shown some physiological variations between those fish whose eggs and young were reared in radioactive waters and those that were not, though these variations have not been determined to be statistically significant or different from variations caused by other contaminants.
Studies are being made of the reproductive efficiency and patterns of sea creatures in a radiation-contaminated environment, compared with those in an uncontaminated environment, to learn such things as the numbers, survival rates, and sex ratios of the offspring, and any genetic abnormalities or mutations. Many more studies are needed. Always, the task is made difficult by insufficient detailed knowledge of the original natural environment, the limitations of laboratory experiments, and the mechanics of trying to follow the reproductive cycles of free-floating or swimming organisms in any statistically meaningful manner through successive generations.
One obviously important kind of research deals with the rate, pattern, and means by which radionuclides are distributed into the sea from a point source, such as the mouth of a river or a nuclear test site. Transport and diffusion of radioactivity can be, and are, influenced by physical, chemical, biological, or geological means, separately or all at once. This has led the AEC to support scientific studies of currents, upwelling, downwelling, convergence, diffusion, mixing rates, air-sea interactions, chemical and geological processes in the sea, and the horizontal and vertical migrations of sea life.
This sound instrument record reveals the layers of planktonic sound scatterers on the continental slope east of New England. Each peak originates from an individual group of organisms.
In much of the ocean there is an acoustic “floor”, known as the deep scattering layer (because of what it does to sound waves), which is believed to consist primarily of zooplankton. Every 24 hours the layer migrates up and down through several hundred feet of water. At night the countless small animals graze in the rich sea-plant pastures near the surface; during daylight, back at the lower level, they undoubtedly are heavily fed upon by larger animals. Over a period of time, the layer accounts for considerable vertical transport of materials. (See figure above.) Other life forms may move materials still farther down, or, in some instances, back up—as when the sperm whale descends to the depths to fight and best a giant squid, and then returns to the surface to eat it.
Constantly drifting downward is a great volume of material—the dead bodies, skeletons, excrement, and other waste from sea life at all depths. As it sinks there is a constant exchange of matter between it and the surrounding water through chemical, physical, and biological processes. Eventually, the molecules of material added to the bottom sediments may be returned to the water mass by bacteriological action or the eating and living habits of sea floor animals.
A school of skipjack tuna photographed from an underwater observation chamber on the research vessel Charles H. Gilbert.
Biological transport works in other ways, too. Most pelagic (free-swimming) fish are great travelers. They account for a tremendous movement of material, namely themselves, from one place to another. Tuna, swordfish, whales, porpoises, and sea birds may travel thousands of miles in a single year. Such migrations may serve, variously, as mechanisms for either dispersal or concentration of elements or nutrients. The anadromous (river-ascending) fishes, such as salmon, herring, sturgeon, and shad, concentrate in freshwater streams in untold numbers to spawn. After hatching, the young seek the ocean and scatter widely until they, too, feel the urge to return to the rivers and lakes whence they came, to spawn and die there as did their ancestors.
Ocean currents may transport concentrations of radionuclides essentially undiluted for thousands of miles. Surface currents move at speeds of up to five knots (nautical miles per hour). Normally current waters do not mix readily with the water mass through which they pass. Because of the slowness of vertical circulation in the ocean, radionuclides deposited on the surface of the ocean may take a thousand years to reach the bottom. But the vertical transport sometimes is much more rapid: When the wind piles too much water against a coastline, the resultant downwelling (sinking) may move radionuclides suddenly into the deeper ocean. Or, conversely, when the wind and the rotation of the earth combine to force the surface water away from the coast, deep water may suddenly rise to replace it, a process known as upwelling.
Mechanisms of nutrient turnover in the sea.
Light energy Dissolved gases Birds and man Rivers and ice Wave action Surface mixed layer 20-100m Suspended matter Elements in true solution Plants phytoplankton Animals Deep water Elements in true solution in deep water Buried in sediment Physical Processes Transport by wind Transport by current Turbulent mixing Sedimentation Transport by animals Volcanic action Diffusion Chemical or Biological Processes Photosynthesis Dissolving Upwelling Decomposition and respiration Sorption by sediment surface Redissolving from sediment Chemical precipitation Combined Processes Sedimentation and decomposition by bacteria Scavenging
Some recent evidence indicates that the passage of a hurricane across the ocean drives surface water out from the storm center in all directions. This, too, produces upwelling. If radionuclides fall on the Arctic ice pack or on the Greenland or Antarctic ice caps, it may be years before they are released to the sea. In more or less stable conditions at sea, radionuclides may remain trapped above the thermocline (a layer of sharp temperature change usually less than 100 meters below the surface) for a considerable period. Then a severe storm may destroy the thermocline and mix the waters to much greater depths. The process of diffusion in the ocean is not well understood, due both to the difficulty of the measurements that have to be made and to the variety of other factors affecting both vertical and horizontal transport of materials. Here again, however, the existence of radionuclides, introduced artificially at a known time and place, is materially aiding these investigations by making a particular water mass detectable and traceable.
Winds of 100 knots (about 115 mph) whip high waves in the Caribbean Sea east of Guadeloupe Island during a hurricane.
In chemical oceanography, the AEC is concerned with the fact that in some instances our society is introducing elements, ions, and compounds that have not been naturally found in the sea, as well as natural materials in greater concentration than is normal. These may combine with other materials in the sea, changing into new forms or substances, or removing them from solution entirely. Any change in the chemical composition of the ocean is quite likely to have biological effects, some of which may prove detrimental to man.
A disturbance of the chemical balance of the sea is thought to be responsible, at least in part, for the periodic, disastrous plankton “blooms” known as “red tides”. Such a sudden, explosive overpopulation of plankton is a natural phenomenon, but one that can be triggered by man-made pollution. When it occurs, plankton multiply so rapidly that the oxygen in the water is depleted and many fish die from suffocation.
Fortunately, nuclear energy operations account for an extremely small portion of the chemical contamination of the sea, when contrasted with the tremendous volume of poisons dumped daily into it in the form of other industrial and municipal waste and agricultural pesticides.