TRANSURANIC RADIONUCLIDES IN ENEWETAK LAGOON 593 



The data indicate that all plutonium does not remain associated with the sedimentary 

 material with which it was originally deposited. Small quantities of plutonium are 

 remobilized continuously from the sediments to the lagoon water column by surface- 

 exchange mechanisms. Plutonium is also detected in the interstitial water extracted in situ 

 from sediments (Noslikin et al., 1978b) at concentrations higher than those in the 

 overlying bottom water at Enewetak Atoll. By equilibration, small quantities of 

 2 39+240p^ from the sediments are exchanged and released. Vertical diffusion moves the 

 radionuclides to the sediment water interface where the plutonium mixes with the lagoon 

 water mass. Remobilized plutonium can then be concentrated by members of the marine 

 food chain. Vertical diffusion can also move the exchanged plutonium in the interstitial 

 fluid deeper into the sediment colunin. Exchange of plutonium with exposed carbonate 

 surfaces might account for the concentrations associated with material deeper in the 

 sediment column. 



Transuranic Elements Associated with the Calcareous Algae Halimeda 



Debris from the calcareous algae Halimeda is the second most abundant component of 

 Enewetak lagoon sediments and covers an estimated 26% of the lagoon floor (Emery, 

 Tracy, and Ladd, 1954). Live species have recently been collected by divers and during 

 dredging operations from numerous locations at both Enewetak and Bikini. Because algae 

 were shown previously to concentrate plutonium (Noshkin, 1972), the role of this 

 benthic algae in recycling the transuranic elements at the Atoll should be assessed. 



The mean concentration factor for ^^^ ^'^"Pu associated with algae species from 

 both atolls is 6 X 10"* and ranges from 1 x 10"^ to 32 X 10"^ (Noslikin et al.. 1978a). To 

 within the precision of our measurements, the concentration factors for plutonium at the 

 two atolls and of different Halimeda species from both atolls do not differ (Noshkin 

 etal., 1978a). Concentrations of ^^^ ^^°Pu associated with the hve algae ranged from 

 0.4 to 22 pCi/g (wet weight), and the concentrations in the water where the algae were 

 obtained ranged from 10 to 116 fCi/liter. Surface-sediment concentrations at the stations 

 (Noshkin etal., 1978a; Nevissi and Schell, 1975) were compared to the algae 

 concentrations at these sites. The average ratio of the 239-i-240py concentrations 

 associated with the Halimeda species [pCi/g (dry weight)] to that in the top 2.5-cm 

 sediment layer [pCi/g (dry weight)] was 0.24 ± 0.13, and the ^^^ Am concentration ratio 

 was 0.32 ± 0.24. Concentrations of 239-H240p^ ^^^ 241 ^^^ -^^ ^^^ sediment ranged from 

 9 to 82 pCi/g and from 1.1 to 67 pCi/g, respectively. On an equivalent weight basis, the 

 live benthic algae have lower 239-t-240p^ ^^^ ^"^^Am levels than sediments in the 

 immediate environment. The average plant/sediment concentration ratios of ^^^'''^'^'^Pu 

 and ^ ' Am are not statistically different. Thus there is no discrimination between 

 2 3 9-H2 40p^ and ^^^Am in processes beginning with remobilization of the transuranics 

 from the environment and ending with concentration by the algae. 



Table 4 summarizes data on transuranic concentrations in algae, water, and sediment 

 from Cactus crater at Enewetak. The data show that the ^^^Pu/^^^'^^'*°Pu ratios in 

 plants, water, and sediment are identical. In this crater ecosystem marine algae do not 

 discriminate among the plutonium isotopes in the environment. The plant/sediment 

 concentration ratios of 2 3 9-^2 40p^ ^^^ ^^^ Am are nearly identical, wliich again shows 

 that the processes of environmental release and plant uptake of the two transuranics are 

 similar. 



