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On the other hand, this same U. N. Committee (1958) stated that "the possibility cannot be excluded that our present estimates exagger- ate the hazards of chronic exposure to low levels of radiation". There is obviously need for much intensive research on all aspects of the problem. For the present, it is evident that the only proper course to pursue, in regard to exposure of man to radiation, is a cautious one. It should be noted that the maximum permissible concentrations in drinking water tabulated in Handbook 52, and in similar publications, are calculated on the assumption that this is the only source of man's intake of an isotope. If a man is ingesting quantities from several sources, the total intake should not exceed the intake corresponding to the mpc for drinking water times 2,200 (the daily water intake, in mil- liliters, assumed in Handbook 52). Therefore, in estimating the max- imum permissible level in any particular source (such as in food from the sea), account must be taken of all other sources. On the other hand, care must also be taken to appreciate the full significance of the terms "maximum permissible exposure" and "max- imum permissible concentration". These terms, as used here, are based on the effects of continuous exposure to a large human population over the average life span. Thus local, short term conditions in which concentrations of radionuclides exceed the mpc values may be per- missible, provided that the total exposure over a relatively long period of time remains below the required average for that period. PROCESSES DETERMINING THE CONCENTRATION OF NUCLIDES IN THE MARINE ENVIRONMENT AND IN SEA FOOD The physical, chemical, and biological processes affecting the distribution of radio-isotopes in the sea have been considered in some detail by Revelle, et al. (1957), and the processes in coastal waters particularly have been reviewed by Carritt, et al. (1958). It is, how- ever, desirable to consider some of these matters here, with particular reference to the problems of disposal of wastes from nuclear-powered ships. Physical effects: The oceans are stratified in approximately a two layer system, with an upper, mixed layer about 100 meters thick, separated from the deeper waters by a region of rapid increase in den- sity, which acts as a barrier to vertical mixing. In partially enclosed basins (harbors and estuaries) interchange between the waters thereof and the waters of the upper mixed layer of the open sea is often inhib- ited, so that the rate of dilution of pollutant in the water of the basin is a great deal less than the rate of dilution due to eddy diffusion in the open sea. Where the basin has a shallow entrance, there usually will be developed in the basin below the entrance depth a body of water which stagnates seasonally or permanently, and in which there can be a con- siderable accumulation of elements which fall out as particles from the upper layer. The interchange of water from partially enclosed basins with the open sea depends on a number of factors including (1) the to- pography of the basin, (2) the amount of fresh water runoff into the 13
basin, (3) the rate of evaporation from the surface, and (4) the tidal and other currents in the waters of the basin. The flushing time (mean residence time of a water particle) is highly variable among estuaries, from a few days to a year or more. Geochemical effects: Several things can happen to radioactive materials introduced into the sea as liquid wastes. They may remain in solution or be precipitated out, depending on whether or not the solu- bility product of the least soluble compound in seawater has been ex- ceeded. Dissolved substances may also be precipitated by coprecipi- tation with other elements or by sorption on organic or inorganic parti- cles already present in the sea. Both particles and dissolved materials may also be taken up by organisms and enter into biochemical cycles. Krumholz, Goldberg and Boroughs (1957) point out that the ele- ments of Groups I, n, V, VI, and VII usually occur as ionic forms in seawater (these include Cs, Sr, Ba, Zr, Cu, Zn, and I) while the other elements, except the rare gases, occur as solid phases (e. g. Y, Fe, Co, and Ru). These authors also tabulate, from Greendale and Ballou (1954), the physical states of elements following detonation of an atomic bomb, which probably also indicates the physical states resulting from waste products introduced into the sea. The material is reproduced here in Table 1. TABLE 1 PHYSICAL STATES OF ELEMENTS IN SEAWATER (from Greendale and Ballou, 1954) Percentage in given physical state Element Ionic Colloidal Porticulote Cesium 70 7 23 Iodine 90 8 2 Strontium 87 3 10 Antimony 73 15 12 Tellurium 45 43 12 Molybdenum 30 10 60 Ruthenium 05 95 Cerium 24 94 Zirconium 1 3 96 Yttrium 04 96 Niobium 0 0 100 Gravity effects on particulate matter in the sea will tend to re- move from the seawater radio-isotopes incorporated in such particles, which eventually settle to the sea bottom. In the deep sea such sedi- mentation will remove radio-isotopes from the domain of harvested marine organisms. In the near shore and continental shelf waters, 14