. For example, 137 Cs and 90 ~~Sr contribute by far the most significant fraction of the dose over the next several decades; both have radiological half-lives of about 30 years. Because the environmental residence half-lives for these 2 nuclides is not known, the 30 year radiological half-times are used in the dose models. However, if the environmental residence half-time for either or both of the radionuclides could be accurately determined, then it might be learned that the effective half-time might be less than 30 years. For example, if the environmental residence time were 10 years then the effective half-time would be 7.5 y; even if the environmental residence time of 137¢. were 60 years the effective half-life would be 20 y. As a result the estimated doses would be less than those predicted using the 30 year radiological half-time and conclusions regarding resettlement options could possibly be altered. Therefore, a very important part of the research plans in the Marshall Island is to determine the environmental residence time of the most significant radionuclides in the atoll ecosystems. This requires monitoring the soil and vegetation radionuclide concentrations in specific areas and in specific plants over extended periods of time. Use should be made of any historical samples if they can be precisly located and identified and be of value in the determining the residence time (see section £2). The situation is similar in the marine environment. Radionuclides will be lost from the environment by a variety of processes and change accordingly in marine food products. A knowledge of the environmental residence time for each radionuclide is required to evaluate future impacts from the marine food chain. Studies of the residence time o f 137 E6 90¢,. and the transuranics are in progress at Enewetak and Bikini Atolls. to include Rongelap and Utirik Atolls. 35 These studies should be expanded