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subsequent use of these latter values in the determination of the safe rate of discharge of nuclides into the various marine environments, the following considerations must be taken into account. The ultimate major source of dilution water for the most restrictive environment (harbors, estuaries, and inshore waters) is the next most restrictive environment, the waters of the continental shelf. Likewise, the ability of the waters of the continental shelf to receive wastes depends, in part, upon the rate of exchange of these waters with the waters of the open sea. These dilution waters must have, in each case, significantly lower average concentrations than the ppc values of the neighboring more in- shore environment. The environmental ppc values computed below thus cannot be considered to represent the average permissible concentration throughout the entire volume of each of the subject marine environments. While these ppc values may apply to substantial areas of each environ- ment in an absolute sense, such areas must be only a relatively small fraction of the entire area of the respective marine subdivision. The manner in which this condition is satisfied is presented in a later sec- tion of this report entitled "Basis for Evaluating Safe Discharge Rates". PARTIAL PERMISSIBLE CONCENTRATIONS IN THE ENVIRONMENT Having estimated ppc's in marine food organisms, we can esti- mate ppc's in the environment if we know the factor by which the food organisms concentrate the isotope in question in their bodies from their environment. Concentration factors from seawater to the edible parts of marine invertebrates and marine fishes have been estimated by the authors noted in the footnotes to Table 2. Values are tabulated here for soft (edible) parts of marine invertebrates and for the soft parts and the bones of fish. The latter have been included because some fish (especially canned fish, such as salmon, mackerel, and sardines) are often eaten bones and all. Where the fish bones are eaten, it seems ap- propriate to use 1/10 of the concentration factor for fish bone (since that is roughly the ratio of bone to whole fish) in obtaining a weighted average concentration factor to use in further computations. Based on whichever of the concentration factors (that for inver- tebrates or that for fish) is the higher for each isotope, we have calcu- lated and tabulated the ppc in the environment according to the relation , .. ppc (food) ppc (environment) = *-Â£â*r'. *âr â¢ concentration factor The resulting ppc's in the various subdivisions of the marine environ- ment are tabulated in the last five columns of Table 2. Some investigators have questioned this method of computing ppc values for the marine environment since certain pertinent features of human body chemistry are not evidently included. For the case of strontium 90, an alternate method of computation is based on the so- called "sunshine unit". This computation does not explicitly include the concept of "concentration factors" used in the above evaluation. 21
On the basis of the allowable body burden in man of strontium 90 and of the total amount of body calcium, it has been shown that, for the population as a whole, the ratio of strontium 90 to total body calcium must not exceed 0.1 \ic per kg, i. e. Sr 90 / Ca < 0.1 nc Sr 90 / kg body Ca. Man apparently discriminates against strontium in the uptake of calcium and strontium by a factor of 8:1. Therefore, if man receives his total protein requirements from fish, and the fish contain strontium 90, we must have in the fish: Sr 90 / Ca 1 0.8 \ic Sr 90 / kg Ca. Assuming that fish do not discriminate against strontium in the up- take of calcium and strontium (probably a conservative assumption), the above limiting relation must also hold for the ratio of strontium 90 to calcium in sea water. The calcium concentration in sea water of 34%osalinity is approximately 400 ppm, or 0.40 g of calcium per kg of sea water. Therefore, there must be less than 0.8 |ic of strontium 90 per 2,500 kg of sea water. Hence the maximum permissible value for strontium 90 in the marine environment, assuming that man receives all his protein requirement from fish harvested from that environment, and assuming that this is man's only source of ingestion of radioactive materials, would be 3.2 x 10'7 nc/rnl. Since for the coastal environment we have assigned only one-tenth the maximum permissible dose to the effects of wastes from nuclear-powered ships, the ppc value for coastal water would be 3.2 x 10'8 nc/ml. It is seen that this value differs from the corresponding value given in Table 2 by a factor of only about 6.5. The two methods of com- putation would agree if a concentration factor of about 3, rather than 20, had been used in obtaining the ppc value for the marine environment, for strontium 90, given in Table 2. Thus there is some evidence that slightly conservative values of the concentration factors were employed in computation of the ppc values for the marine environment given in the Table. It is not readily seen how the concept of the "sunshine unit" can be applied to all the radioisotopes with which we are concerned. Also, in view of the many uncertainties involved, conservative estimates must be made at this time. Hence, for our further computations, the approach used to obtain Table 2 is employed in this report. The values thus calculated for ppc's in the environment are prob- ably fairly reliable guides where the "environment" is the water and the food organisms are obtaining their nutrients from the water. This will apply where there is no uptake by organisms of isotopes from bottom sediments. 22