It was established that strontium-90 and cesium-137, important in fallout on land, enter the marine cycles only in minute amounts. Practically no fission products are found in fish. Since strontium-90 is not concentrated strongly by marine organisms, the question of what happens to it in the ocean remains unanswered. Studies have suggested, however, that strontium moves in solution and thus indicates the movement of water. If this is true, strontium-90 may be contained in the deep currents and eventually will be brought again to the surface. Some observers believe this process has begun. Rivers, Lakes, and Estuaries The freshwater environment differs from the marine in the greater variety of its minerals, among other things. As sites for radiobiological studies, rivers and lakes present problems of great complexity, but conditions at river mouths or estuaries are even more difficult because of the mixing by tidal action of fresh and salt water. Rivers vary greatly in character and change radically from season to season because of rainfall and other factors. General understanding of their biological workings is difficult to formulate. But rivers are the routes by which minerals and wastes are transported toward the sea, and estuaries are significant because of the many forms of life that flourish there. Studies of radioactivity in rivers and estuaries usually have been made in relation to the fate of effluents from nuclear plants. Among the longest and most intensive studies are those near Hanford, Washington. Observations were started in 1943, when the federal government was preparing to build plutonium-producing reactors to be cooled by waters of the Columbia River. [Illustration: _Fisheries biologists studying hatchery fish reared in water containing radioactivity from the Hanford plutonium reactors._] For more than two decades, observations have been made of the physical dispersion and biological disposition of low-level effluents in the Columbia. Concentration factors have been established for significant radionuclides in phytoplankton, algae, insects, and fish, and typical patterns of dilution and dispersion have been plotted. Similar programs, in an entirely different freshwater system, have been conducted over a similar span of years near the Oak Ridge National Laboratory in Tennessee. One area of interest has been the biological disposition of trace amounts of strontium-90 released to the Tennessee River via tributary streams. Among the few broad estuarial studies yet undertaken is one started in 1961 to plot the dissemination in the lower Columbia River, and in the Pacific Ocean, of radioisotopes transported by the river from the Hanford plant. Radiobiologists are studying biological distribution. Oceanographers are using the trace amounts of effluent radioactivity to verify the patterns of dispersion of river waters in the ocean. Land