reactor wastes brings about a modification of that environment. The distribution of radioactive elements and their stable isotopes in the organisms from a contaminated marine environment should, therefore, yield useful information with respect to the fate of radioisotopes introduced into the sea. The value of using the specific activity of a radioisotope in a marine organism, as a measure of its distribution in the environment of the organism, factors: ism; is dependent upon several (1) the availability of the radioisotope to the organ- (2) the radioactive half-life of the isotope; (3) the residence time of the element in the environment; and (4) the residence time of the element in the organism (biological halflife). For any given radioisotope existing in a condition in an environment which renders it available to some organism in that environment, its biological half-life in the organism should be no more than a small fraction of the residence time of the element in the environment. of the isotope should be, The radioactive half-life at least, an appreciable fraction of the biological half-life of the element in the organism under study. the radioisotope zinc-65, an induced product of nuclear detonations and a common component of reactor wastes, has the

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