14 13 divided into a grid consisting of a series of 6-km? regions; at least one sediment sample was obtained from each region to provide vadiological data for areal distributions. eastern reef. All sediments are composed of different quantities of fine and coarse-grained carbonate material, shells, coral fragments, and Halimeda debris. To assess the sediment inventory, no attempt was made to distinguish concentration levels in specific sedimentary components. Figs, 2 through 5 illustrate the main features of the transuranic distributions in the surface layer of the lagoon sediment. Isolated regions of relatively high concentrations of 23942405, are evident in some lesser contaminated areas of the lagoon; other small regions of high surface radioactivity might have escaped detection. The areal distributions are based on available data from the samples that were collected and analyzed, TMumerous nuclear tests. the locations of larger or more Highest plutonium concentrations are associated with the sediments from the northwest quadrant in a north- and Alice and Belle and several km southwest of Mike and Koa craters. A second region of relatively high concentration is in sediments off the The activity in this region is lower than the concentrations in sediments in the northwest, surface concentrations were less than ? pCi/g. All surface sediment samples obtained during and since 1972 contained 2394240, Pu. The inventory in only the top 2.5-cm layer (mC4/ km) of sedinent exceeds the activity deposited to the earth's surface as worldwide fallout in any latitude band in the northern or southern hemisphere (Ha73). Although the surface distribution of 241 Am in the sediments appears similar to that of 2394240, the ratio of 39424 241,2394240, activities (Fig. 5) shows that the radionuclides are not well mixed throughout the surface deposits. The ratio in the sediments ranges from 0,06 to 0.93, Most of the transuranic inventory in the surface sediments can be separated roughly from the lesser contaminated deposits by a line extending from the Southwest is 0.29 + 0.17, and the ratio determined from the mean surface concertrations + 0.06, (lelle 1) is +,36 The average ratio is similar te that found in central Pacific and northeast Atlantic sediments (Li76), which receive only worldwide fallout deposition but have, in contrast, one-half the average south-oriented elliptical area, roughly 2 to 3 km east of the islands of shore of Yvonne Island. range between 2 and 170 pCi/g dry weight, while south of this line the activities from all surface sediment samples, Highest surface concentrations are associated with the sediments near, but not necessarily adjacent to, 42 4 The surface 239 2405, concentrations north of this line The mean ratio, however, determined by averaging PAT Ay A342 40, The transuranics are distributed nonuniformly over the lagoon floor. Passage to the island of Tom (Munjor), which is south of Yvonne on the concentration ratio of surface sédiments at PRikini (Ne75) 2394240 . Cin in the vurfuce sedivcunts, the distribution ratio of 2385 23 shown in Fig. 4, demonstrates the nonunifornuity amon, plutonium isotopes in components of the sediment in the Atoll environment. There are, however, large geographical regions of the lagoon with ¢lialar {sot :pic ratios in the sediment. On the other band, small areas of the day. oa, such as a 600-m strip on the lagoon side of Yvonne Island, contain DOE ARCHIVES