586 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



Range, 0.10-0.15 



:^'fr^1 



Mean, 0.06 ± 0.02 

 Range, 0.04-0.10 



Range, 

 0.06-0.38 





^>-- 



\ 



I 'I 



Mean, 0.11 ^ 0.02 

 Range, 0.08-0.14 



\ 



/ / 



Fig. 4 Activity ratios of =3 8py^239 +2 4opy j^, jj^^ surface sediments of Enewetak 

 lagoon. 



Although the surface distribution of ^'^^ Am in the sediments appears similar to that 

 of "'^■'^^"Pu, the ratios of ^"^ ^ Am/^^^^^'^^Pu activities (Fig. 5) show that the 

 radionuclides are not well mixed throughout the surface deposits. The ratios in the 

 sediments range from 0.06 to 0.93. The mean ratio, however, determined by averaging 

 ^^^ Am/^^^"''^^^Pu ratios from all surface-sediment samples, is 0.29 ± 0.17, and the ratio 

 determined from the mean surface concentrations (Table 1) is 0.30 ± 0.06. The average 

 ratio is similar to that found in central Pacific and northeast Atlantic sediments 

 (Livingston and Bo wen, 1976), which receive only worldwide fallout deposition but have, 

 in contrast, one-half the average concentration ratio of surface sediments at Bikini 

 (Nevissi and Schell, 1975). 



The ratios of 2 38p^y2 39+24 0py activities in the surface sediments (Fig. 4) demon- 

 strate the nonuniformity among plutonium isotopes in components of the sediment in 

 the Atoll environment. There are, however, large geographical regions of the lagoon with 

 similar isotopic ratios in the sediment. On the other hand, small areas of the lagoon, such 

 as a 600-m strip on the lagoon side of Yvonne Island, contain plutonium with isotopic 

 ratios ranging from 0.05 to 0.38 (U. S. Atomic Energy Commission, 1973). In Cactus 

 crater, at the northern end of Yvonne, the isotopic ratio of 0.55 in the sediments is one 

 of the highest at the Atoll. The average concentration ratio in the lagoon sediments, 



