E fd comes at et aeeh : . : Pars al ennai oe 24 In the marine environment the neutron-induced rehabtart S5Fe and 60Co, were predominant together with naturally occy pring 4 K, Tridacna clams remain the best indicator species for ®%o, while »Ere is mst abundant in the liver or viscera of fish, especially large pelagic fish, Change in Radioactivity with Time With the passage of time since the tests which produced most of the fallout on the atolls surveyed, the radionuclides initially deposited in the environment have been concentrated or diluted by many processes, Some of these natural processes are physical-decay rate, weathering of fallout particles, transport by rainwater, wind or wave--and some are biologicalselective uptake or discrimination by the plants and animals which inhabit the contaminated areas, In addition man has redistributed radionuclides by his physical disturbance of the land during construction of houses and planting of food crops (i.e. Bikini, Eneu and Rongelap islands). A summary of the change with time in the amount of ®9co, 90sr, and 137¢s in selected sample types from Bikini and Rongelap atolls is given in Tables 6 and 7. Since samples were not collected in the same locations or for the same purpose during the 20 year span covered in the tables, the data are not strictly comparable or useful in calculating ecological half-life in all cases, The smal] number of samples for some species and normal biological variability also contribute to the fluctuation in the amount of radioactivity measured. However, the general trends indicated by the majority of the data are still distinct. In general, both the number and amounts of radionuclides have decreased in the soil and biota at both Bikini and Rongelap atolls. Several radionuclides 54mn, 57Co, 65Zn, 144Ce, which were present at Bikini Atoll in 1964 (Welander, et al., 1967) were not detectable in 1974, Amounts of the dominant radionuclides at Rongelap in 1958-59 and at Bikini in 1964 (60¢o, 90Sr and 137Cs) were significantly less in 1974-75 although they were still abundant, On Bikini Island the amount of 29Sr or 137c¢s measured in coconut crabs in 1974 is 30 to 60 per cent of the amount measured in 1964, This rate is signi- ficantly greater than the decrease expected from decay alone, hence other processes must also,be acting. One of the possible factors acting to reduce the availability of 90Sr and 137cs to coconut crabs on Bikini Island was the physical mixing of the soil during the rehabilitation of the island. This disturbance and physical decay reduced the amount of radioactivity in the surface soil by almost an order of magnitude between 1964 and 1974. However, the rate of decrease of 20Sr in coconut crabs from Rongelap Atoll (undisturbed between 1957 and 1974) is similar to that seen at Bikini Atoll, hence the soil mixing on Bikini may not have been an important factor in determining the rate of decline of 90sr in the coconut crab on Bikini. SUMMARY The DOS portion of LRE's Pacific Radioecology Program began on 1 July 1974. The purpose of the program is to determine the kinds and amounts of radionuclides in biological and environmental samples from the Central Pacific, especially the Marshall Islands. Five field trips were conducted for this program between April 1974 and August 1975. About 600 samples were collected and about 600 y-spectrum, 70 fron-55, 300 strontium-90, and 200 plutonium analyses were performed. a4 x