TRANSURANIC RADIONUCLIDES IN ENEWETAK LAGOON 589 



exceed the respective isotopic inventories at Enewetak, but ^^*Pu is higher in Enewetak 

 lagoon. Inventories of ^^^ Am at Bikini will increase by 25% from ^"^ ^ Pu decay, but only 

 a 10% increase over present ^^ ^ Am levels is expected at Enewetak from ^'^^ Pu decay. 



Transuranic Elements Associated with Components in the Sediment Column 



In line with the definition of Emery, Tracy, and Ladd (1954) for classifying fines as 

 material less than 0.5 mm in diameter, 23 surface samples and several cores were 

 separated into fine and coarse fractions. The dry weight of the fines ranged from 25 to 

 80% of the total dry weight of the surface volume (Noshkin et al., 1978a). A similar range 

 of fine material was found in Bikini Atoll sediments. At least 93% of the sediment weight 

 in Mike and Koa crater deposits was fine material. In over 98% of the sediment samples 

 from Enewetak lagoon, the ^"^'Am and ^^^ ^"^^Pu concentrations (pCi/g) associated 

 with the fine sediment components were greater than or equal to the concentrations 

 associated with the coarse fraction. The activity of ^^^ Am and 239+240p^ -^^ ^.j^^ ^^^^^ 

 was 0.6 to more than 10 times that in the coarse fraction. These distributions between 

 size fractions are very unlike those encountered for fallout of ^ ''^''"^^^Pu in sediments in 

 Buzzards Bay, Mass., where the 2 3 9+2 4 op^ ^^^ ^^^ preferentially associated with the 

 fine fractions of sedimentary deposits (Bowen, Livingston, and Burke, 1976). This 

 difference is perhaps not unexpected because most of the transuranic inventory deposited 

 to the lagoon environment was probably associated with small particulate carbonates. 

 During the years since nuclear testing, some plutonium has exchanged slowly as a result 

 of chemical reactions with exposed surfaces of the larger sedimentary components. 



The transuranic inventory at lagoon locations, however, is dependent on the local 

 abundance of the fine and coarse materials. Table 2 shows the ^"^^ Am concentrations in 

 the fine and coarse components of two core samples from midlagoon locations at 

 Enewetak. The fraction of the coarse components in the sediment column of core 6 

 decreases with depth and in core 1 increases with depth. The ^"^^Am concentration 

 associated with the fine fraction in the surface 2-cm section is 2 to 5 times the 

 concentration associated with the coarse fraction; but, because the fine material in the 

 surface layer of core. 6 accounts for only 25% of the total dry weight of the sediment 

 volume, 58% of the ^^^Am in the surface 2-cm layer is associated with the coarse 

 fraction. In core 1 , on the other hand, 95% of the total '^^^ Am in the surface 2-cm layer 

 is associated with the fine fraction. Although the ^'*^Am concentrations (pCi/g) 

 associated with the fine material at various depths in the sediment column exceed the 

 concentrations associated with the coarse components in both cores, the inventory of the 

 radionuclide (pCi/cm^) within any depth interval associated with the fine and coarse 

 components can be variable throughout the sediment column. Areal transuranic 

 distributions like those shown in Figs. 2 and 3 but associated with only the fine or only 

 the coarse component of sediment would differ. 



The vertical distributions of the transuranics in the lagoon sediment are very complex. 

 No generalization about the shape of the concentration profile in any region can be made. 

 Table 2, for example, shows a ^'*^ Am peak associated wdth the fine components of core 6 

 at depths of 25 to 30 cm with Httle ^'^^ Am associated with the coarse components at 

 these depths. In core 1, the highest ^'^'Am concentrations are associated with the fine 

 components at depths of between 8 and 10 cm in the sediment column. The ^"^^Am 

 concentrations associated with the coarse component in both cores generally decrease 

 gradually with depth. Transuranic concentrations increase, decrease, or remain constant 



