TRANSURANIC RADIONUCLIDES IN ENEWETAK LAGOON 595 



et al., 1974; Noshkin et al,, 1976; Noshkin et al., 1978b). Several studies are in progress 

 at the Atoll which require data on concentrations in lagoon water; so the number of 

 samples and the locations sampled are predicated by the requirements of the current 

 program. Contours of ^^^"""^"^^Pu concentrations in the water show complex distribution 

 patterns (Noshkin et al., 1974) in various regions of the lagoon. The spatial patterns of 

 surface and bottom 2 3 9+24 Op^ concentrations in solution and in association with 

 filterable material are very different, as are the ^^^Pu/^^^'^^'*°Pu ratios in the water 

 mass. A detailed discussion of the plutonium levels in the pelagic environment of the 

 lagoon is in preparation (Noshkin et al., 1978b). Instead of relating all results from the 

 analysis of lagoon water samples collected since 1972 with hydrological, seasonal, or 

 spatial factors, I will summarize some of the data that are related to remobihzation and 

 redistribution of plutonium. 



In 1972, 1974, and 1976, a sufficient number of water samples from the lagoon were 

 analyzed for 239+240p^ ^^ permit an estimate of mean concentrations in the lagoon. The 

 mean concentrations are summarized in Table 5. In 1972, the average 239+240p^ 

 concentration in the lagoon was determined for 34 unfiltered surface and bottom 

 samples. A more-detailed water sampling program was conducted in 1974. In 1976, a 

 smaller number of water samples were collected around the perimeter of the lagoon 2 km 

 off the shore of the reef. Water samples collected during 1974 and 1976 were filtered 

 through l-jum filters. In the discussion to follow, the estimated average soluble 

 2 39+240 py concentrations shown in Table 5 refer to material passing through a 1-Mm 

 filter. 



During July 1974, the soluble ^^^"""^"^^Pu in the lagoon water ranged in concentra- 

 tion from 2 to 75 fCi/liter. The percentage of the total activity associated with the 

 filterable material in the water samples during 1974 and 1976 ranged from 2% to 54% 

 and from 12% to 94%, respectively. The concentrations of plutonium radionucUdes in 

 solution above fallout background concentrations in the lagoon water are direct evidence 

 of the remobilization of trans uranics from the soUd phases of the environment. Dissolved 

 plutonium released from the sediments of Cactus crater was traced for considerable 

 distances along the reef by a plutonium radionuclide balance, which involved the change 

 in the 2 3 8py^239+240py j.^^-Q -^ ^j^g water, and dyes to trace the crater water (Noshkin 

 et al., 1978c). The dissolved plutonium moves in solution apparently without interacting 

 with the sediment deposits during transport. The dissolved plutonium passes readily 

 through dialysis membranes (Noshkin et al., 1978c). Equilibration between dissolved 

 plutonium in the crater seawater and low-activity seawater contained in dialysis bags is 

 achieved in 3 days (Noshkin et al., 1978c). These characteristics suggest that the 

 plutonium remobilized to the environmental waters has very solute-like characteristics. It 

 is tempting to suggest, considering the environment, that the remobilized chemical species 

 is some form of carbonate complex. 



The average concentration of total 2 3 9+2 4 Op^^ ^ ^j^^ water was essentially the same 

 in 1972 and 1974, but a marked decrease was noted during 1976. In 1976, the average 

 concentration associated with the filterable material in the lagoon doubled over the mean 

 1974 level, and the mean soluble concentration was reduced to half. Forty percent fewer 

 samples were collected in 1976 than in 1974. During the 1974 program, samples were 

 taken at stations throughout the lagoon, whereas the 1976 samplings were restricted to 

 locations only 2 km from the reef. Similar 239+240py concentrations were found in 

 water samples from the few 1974 locations resampled in 1976, which suggests that any 



