(Reprinted from Nature, Vol. 203, No. 4945, pp. 571-573, August 8, 1964) ANALYSES OF RADIONUCLIDES OF BIOLOGICAL INTEREST IN PACIFIC WATERS By D. CHAKRAVARTi, G. B. LEWIS, R. F. PALUMBO and A. H. SEYMOUR Laboratory of Radiation Biology, University of Washington, Seattle NVESTIGATIONS of the distribution of radionuclides in marine environments have been made by the Laboratory of Radiation Biology, University of Washington, since 1946. These investigations have included radicchemical analyses of marine samples from the western and central Pacific, the north-eastern Pacific and the Arctic Ocean (Chukehi Sea). The analyses diseusscd here are those of samples collected recently in the Pacific Ocean. One group of samples was collected in the vicinity of Christmas Island during the 1962 tests of nuclear devices by the United States; another group was taken froin near the mouth of the Columbia River where the effluent, carrying radioactive wastes from the Hanford reactors, mixes with the Pacific Ocean; and one sample was taken 250 miles off the Oregon coast. Radionuelides from both natural and artificial sources occur in these waters. The most abundant and easily detected naturally occurring radionuclide is potassium-40. Other natural radionuclides to be expected in sea-water are radium, uranium, thorium and their decay products. Most of the artificial radionuclides generally present in the oceans are added as fall-out from nuclear detonations. The fall-out nuclides of biological interest include the fission products: strontium-89, 90, zirconium-95-niobium95, rutLhenium-103, 106, caesium-137 and certum-141, 144; and the induced radionuclides: manganese-54, tron-55, 59, cobalt-57, 58, 60 and zine-65. The induced radionuclides phosphorus-32, chromimm-51] and zine-65, carried to the sea by the Coluinbia River, are produced by neutron activation of clements in tho water used to cool the Hanford reactors. Radionuchdes in sea-water may be concentrated many thousand-fold by plankton. However, plankton constitutes only a mimuto fraction of the total water mass, hence the amount of radioactivity in the total water mass is greator than in the total plankton. The concentration of radioactivity by plankton provides a convenient method for detormining the presence, but not the amounts. of radionuclides in sea-water. A quantitative mincasurement can be mado only by water