588 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



determined from the mean surface concentrations, is 0.14. The average ratio determined 

 in the lagoon water samples during 1972, 1974, and 1976 is identical to the sediment 

 ratio. A steady-state condition is reached where plutonium isotopes are remobiUzed to 

 the aqueous phase in proportion to their concentrations in different regions of the 

 sediments and reef environments. One region of the lagoon sediments with lower or 

 higher isotopic ratios, for example, is not the dominant source term supplying plutonium 

 isotopes to the water column. 



In 1977, several core samples were obtained from the lagoon basin near stations that 

 had been sampled in 1972. The ^^^ Am concentrations in surface-sediment layers sampled 

 in 1972 and 1977 were nearly identical, which showed that there was Httle change in the 

 surface concentrations of transuranics at many lagoon locations during those years. Only 

 small quantities of the transuranics were remobilized or reworked to greater depths in the 

 sediment column during these years. Little resuspended material from other areas of the 

 lagoon having different concentrations of transuranics was transported and deposited to 

 the areas that were resampled. 



The largest inventory of transuranics at Enewetak Atoll is associated with the 

 components of the lagoon sediment. The estimated lagoon sediment inventories given in 

 Table 1 were determined from Figs. 2 and 3 by summing the products of the areas in the 

 lagoon by the average inventory of the transuranics present there. Approximately 250 Ci 

 Q^ 2 39+2 4 0py and 75 Ci of ^"^^ Am are unevenly distributed throughout the 2. 5 -cm-thick 

 surface-sediment layer of the lagoon. The total 2 39+2 40p^j inventory in island soils, 

 sampled to depths of 35 to 150cm, is estimated from available data (U.S. Atomic 

 Energy Commission, 1973; Noshkin et al., 1976) at <25 Ci. Transuranic distributions in 

 surface sediment at Bikini Atoll were constructed, and inventories were estimated from 

 published (Nevissi and Schell, 1975; Noshkin et al., 1975) and unpubhshed (Noshkin 

 et al., 1978a) data. Bikini sediment inventories were estimated from substantially fewer 

 data than were available from Enewetak. Future results from Bikini might change the 

 present estimates of transuranic inventories given in Table 1. Analysis of 25 cores (12 to 

 21 cm deep) from different locations in Bikini and Enewetak lagoons showed that only 

 21 ± 11% of the 2 39+2^0py ^^^ 16 ±6% of the ^^^^ Am (Noshkin et al., 1978a) in the 

 sediment column are associated with components in the top 2.5-cm layer. If the average 

 2 39+24 0p^ inventory in the surface sediment is only 21% of the total inventory to a 

 mean depth of 16 cm for the entire lagoon, then the estimated 2 39+24 op^ inventories in 

 the sediment column to a 16-cm depth at Enewetak and Bikini are 1.2 and 1.5 kCi, 

 respectively. However, in a few deeper cores, which are difficult to obtain from carbonate 

 deposits, 2 3 9+2 40py ^^^ 24 1 ^^ ^^^^ detected at depths below 20 cm. The inventories 

 computed to a depth of 16 cm then can be assumed only to represent lower limits. With 

 the average isotope ratios fron. samples from the Atoll environment (discussed earlier), an 

 estimate of the concentration for each plutonium isotope and ^'^^Am in the Atoll 

 sediments is made (see Table 1). Transuranic isotopes deposited from global fallout of 

 weapons debris are estimated from available data (Krey et al., 1976; Hardy, Krey, and 

 Volchok, 1973) and are also given in Table 1 . 



The inventory at the Atolls of transuranics produced by weapons is only a small 

 fraction of the total quantity deposited to the earth's surface from global fallout debris. 

 Some specific marine environments were contaminated with substantial quantities of 

 transuranics from other source terms. These lagoon sediments, however, are the most 

 contaminated aquatic regions in the world that received transuranic inputs only from 

 nuclear weapons. The estimated ^^^Pu,^'*°Pu, ^'*^Pu, and ^"^^ Am inventories at Bikini 



