594 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 4 Concentrations of Transuranium Radionuclides in 

 Halimeda Algae, Sediment, and Water from Cactus Crater 



Radionuclides 



^'" Am, pCi/g (dry weight) 

 2 3 9 +2 4 py^ pCi/g (dry weight) 



2 3 8pii/2 3 9+240py 



*"* Am (Halimeda /sediment)t 0.28 



2 3 9+24 0py (Halimeda I sediment) t 0.23 



Concentration factor for "'^^""Pu = 7.5 x lO'' [pCi/kg (wet weight) per pCi/kg H^O] 



*Wet weight/dry weight - 2.13. 



t Average concentration in eight water samples from the crater bottom collected between 1974 

 and 1977. 



$ Values are expressed as pCi/g (dry weight) Halimeda/ pCi/g (dry weight) sediment. 



The mean surface -sediment inventory of ^^^'''^'^"Pu at Enewetak is 249 Ci (Table 1), 

 The lagoon is 933 km^ in area, and the average specific gravity of the Halimeda and other 

 sediment components is 1.8g/cm^ (Emery, Tracy, and Ladd, 1954). Activities of 

 2 39 24 op^ associated with the algae are related to the activity in the surface sediment. 

 The mean wet/dry ratio of the Halimeda species is 2.3, and the average wet weight of the 

 plants, without holdfast, is 6.4 ± 3.8 g (Noshkin et al., 1978a). Therefore the average 

 2 39 2 40py concentration associated with tlie Uve Halimeda species at Enewetak is 

 0.62 pCi/g (wet weight). Approximately 4.0 pCi is associated with each plant. If the 

 number of Halimeda plants were known, the mean plutonium inventory associated with 

 the living Halimeda reservoir could be computed. Unfortunately no estimates of Halimeda 

 biomass at Enewetak are available. During the late 1940s, the mean sedimentation rate of 

 Halimeda at Bikini was estimated at 3.8 mm/yr (Emery, Tracy, and Ladd, 1954). If this 

 sedimentation rate is applicable to Enewetak Atoll, approximately 1 Ci of ^^^''"^'^^Pu is 

 deposited annually in the sediments in association with Halimeda detritus. This quantity 

 represents only 0.4% of the surface-sediment inventory and a smaller yet fraction of the 

 total inventory in the sediment column. If, however, the life-span of each plant is 1 yr, 

 for example, a quantity of ^^^^^^^Pu equivalent to half the present sediment inventory, 

 or 125 Ci, could be recycled with the algae in approximately 175 yr. Spies, Marsh, and 

 Colsher (1978) demonstrated that, when live Halimeda from Enewetak were cleaned and 

 treated with IN acetic acid, the acid-soluble fraction, or the carbonate material, 

 contained 58% of the total ^^^'''^'*°Pu, and 42% remained bound to the plant tissue. As 

 the plant decomposes after death, the organic material and associated radioelements are 

 released to the environment, leaving the skeletal carbonate matrix and its associated 

 transuranics in the sedimentary deposits. The transuranics associated with the organic 

 fraction released during decomposition are recycled to the benthic or pelagic environ- 

 ments. Over the long term the algae could play a key role in cycling the transuranics 

 between the sediments and the aqueous environment. 



Plutonium Concentrations in the Lagoon Seawater 



A considerable number of lagoon water samples have been collected for plutonium 

 analysis by this laboratory since 1972 (U. S. Atomic Energy Commission, 1973; Noshkin 



