16 15 plutonium with tsotopic ratios ranping from 0.05 to 0,38 (Ne73). In 2 and 3 by summing the products of the areas in the layoon by the Cactus Crater, at the northern end of Yvonne, the isotopic ratio of 0.55 average inventory of the transuranics present there. in the sediments is one of the highest at the Atoll. approximately 250 Ci of 23942405, and 75 Ci of 24d an unevenly The average There are concentration ratio in the lagoon sediments determined from the mean distributed throughout the 2,5-cm-thick surface sediment layer of the surface concentrations is 0.14, lagoon. The average ratio determined in the The total 23942405, inventory in island soils, lagoon water samples during 1972, 74, and 76 is identical to the depths of 35 to 150 cm, sediment ratio. < 25 Ci. A steady state condition is reached where plutonium sampled to is estimated from available data (Ne73, No76) at Transuranic distributions in surface sediment at Bikini Atoll isotopes are remobilized to the aqueous phase in proportion to their were constructed and inventories were estimated from published concentrations in different regions of the sediments and reef No75) and unpublished data (No78a). environments. estimated from substantially less data than were available from One region of the lagoon sediments with lower or higher isotopic ratios, for example, is not the dominate source term supplying Enewetak. Bikini sediment inventories were Future results from Bikini might change the present estimates of transuranic inventories given in Table 1, plutonium isotopes to the water column. In 1977 several core samples were obtained from the lagoon basin (Ne75, to 21 cm deep) Analysis of 25 cores (12 from different locations in Bikini and Enewetak lagoons near stations sampled in 1972. The 241 an concentrations in surface showed that only 21 + 11% of the 73974 %py and 16 + 6% of the 77am sediment layers sampled in 1972 and 1977 were nearly identical showing (No78a) that there was little change in the surface concentrations of 2,5-cm layer. tranguranics at many lagoon locations during those years. sediment is only 21% of the total inventory to a mean depth ef 1€ em for Only small guantities of the transuranics were remobillized or reworked to greater depths in the sediment column during these years, Little resuspended in the sediment column ts associated with compenents in the top If the average 2394240), inventory in the surface the entire lagoon, then the estimated 23942405, inventory in the sediment column to a l6-cm depth at FPnewetak and Bikini are 1.2 and 1.5 material from other areas of the lagoon and with different kCi, respectively. concentrations of transuranics were transported and deposited to the obtain from carbonate deposits, 235+ Hoy and 24a were detected at areas that were resampled. depths below 20 cm, The largest inventory of transuranics at Fnewetak Atoll is associated with the components of the lagoon sediment. The estimated lagoon sediment inventories given in Table 1 were determined from Figs. Uowever, in a few decper cores that are difficult to 24 The inventories computed to a depth of 16 cm then can be assumed only to represent lower limits. Using the average isotope ratios from samples from the Atel] enviranment (discussed earlier), an estimate of the concentration for each plutonium isotope pon ARCHIVE -_ c Thr