Iodine-131 8.05 days Barium-140—Lanthanum-140 12.8 days—40 hours Cesium-141 32.5 days Ruthenium-103—Rhodium-103 10 days—57 minutes Zirconium-95—Niobium-95 65 Days—35 days, Zinc-65 245 days Cerium-144 285 days Manganese-54 314 days Ruthenium-106—Rhodium-106 1 year—30 seconds Cesium-137 30 years Potassium-40 1.3 × 10⁹ years (Reprinted from _Radiobiological Laboratory Annual Report_, April, 1, 1964, page 50.) The project has determined that radionuclides are removed from waters in an estuarine environment by several physical, chemical, and biological means. For example, radionuclides are absorbed in river-bed sediments at a rate varying directly with sediment particle size. Mollusks, such as clams, marsh mussels, oysters, and scallops, not only assimilate radionuclides selectively, but do so in sufficient quantity and with sufficient reliability to be useful as indicators of the quantity of the isotopes present. Clams and mussels are indicators for cerium-144 and ruthenium-106, scallops for manganese-54, and oysters for zinc-65 (most of which winds up in the oyster’s edible portions). It was learned that scallops assimilate more radioactivity than any other mollusk. Of the total radioactivity, manganese-54 accounts for 60%: The scallop’s kidney contains 100 times as much manganese-54 as any of the other tissues and 300 times as much as the muscle, the only part of the scallop usually eaten in this country. [Illustration: _On the left are mussels collected near the Columbia River in an environment containing abnormal amounts of zinc-65._] [Illustration: _Mussels suspended in seawater in research to determine how fast they lose their zinc-65 radioactivity._ (_Photograph taken at low tide._)] In a surprising unintended result, it was determined that one acre of oyster beds, comprising 300,000 individual oysters, may filter out the radionuclides from approximately 10,000 cubic meters (18 cubic miles) of water per week! The Radiological Laboratory scientists also have found that plankton are high concentrators of both chromium-51 and zinc-65, and that zinc apparently is an essential nutrient for all marine organisms. Some plants and animals appear to reach a peak of radionuclide accumulation quickly, which then tapers off even though the radiation concentration in the water is unchanged.