Piutonium in the marine environment of the Marshall Islands For further information contact Between 1946 and 1958, the United States conducted 66 nuclear tests in the Pacific atolls of Enewetak and Bikini in the Marshall Islands. Since 1972, LLNL has conducteda variety of studies related to the cycling and transport of longer lived residual radionuclides in the marine environments of these atolls. Our primary concern is to collect sufficient data to develop recommendationsto minimize the transfer of man-made radionuclides to people returning to the atolls. However, we have also attempted to relate our analytical findings to much wider fields of scientific endeavor and to develop models thatare useful, first locally, and secondly in other aquatic en- | vironments receiving inputs from different source terms. One such program deals with the behavior of plutonium radionuclides in the marine environment of Enewetak and Bikini. V. E. Noshkin (422-6621). Significant quantities of plutonium isotopes and other transuranics will be among the longer Lagoon sediment and water with radiological waste materials, some of which are already being disposed of in the ocean. It is, radionuclides deposited in the lagoon water either settled to the bottom or remained as dissolved or particulate species that were eventually discharged to the North Equatorial Pacific by the prevailing exchange of water between ocean and lagoon. Fallout during and after these tests left a very heterogeneous distribution of radionuclides in lagoon sediments. Figure 1 shows the main features of plutonium-239 and plutonium-240 (239+240b.,) activity associated with sediment components in the top 2.9cm of Enewetak lagoon (as measured in 1972}. Isolines distinguish regions with similar con- lived toxic radionuclides associated therefore, essential to understand by what processes and at whatrates plutonium, under the influence of various chemical, physical, and biological phenomena on the sea floor, migrates back to the water coiumn and is accumulated by marine food chains. concentrations After the last nuclear test at Enewetak in 1958, the residual centrations, and mean values are listed within regions. The number of measurements made is given in parentheses. The highest surface concentrations are associated with sediment