Transuranium Radionuclides 



in Components of the Benthic Environment 



of Enewetak Atoll 



V. E. NOSHKIN 



Data on the concentrations and distributions of transuranium radionuclides in the marine 

 environment of Enewetak Atoll are reviewed. The distributions of the transuranics in the 

 lagoon are very heterogeneous. Tlie quantities of transuranics generated during the 

 nuclear-test years at the Atoll and now associated with various sediment components are 

 discussed. Whenever possible, concentrations of "^^^ Am and '^^^'^'^^^Pu are compared. 

 The lagoon is the largest reservoir of transuranics at the Atoll, and radionuclides are 

 remobilized continuously to the hydrosphere from the solid source terms and are cycled 

 with components of the biosphere. Although ^^^ ^^^Pu is associated with filterable 

 material in the water column, the amount that is relocated and redeposited to different 

 areas in the lagoon is small. Barring catastrophic events, little alteration in the present 

 distribution of transuranics in the sediment is anticipated during the next few decades. 

 The Atoll seems to fiave reached a chemical steady state in the partitioning of^^^'^^'^^Pu 

 between soluble and insoluble pliases of the environment. Tlie amount of dissolved 

 radionuclides predicted, with an experimentally determined K(j for '^^^'^^^^Pu, to be in 

 equilibrium with concentrations in the sediment agrees well with recently measured 

 average concentrations in the water at both Enewetak and Bikini atolls. Tlie remobilized 

 2 39+2 40p^ /ws solute-like characteristics. It passes readily and rapidly through dialysis 

 membranes and can be traced as a solute for considerable distances in the water. It is 

 estimated tfiat 50% of the present inventory of ^^^'^^^^Pu in sediment will be 

 remobilized in solution and discliarged to the North Equatorial Pacific over the next 

 250 yr. 



Large inventories of several transuranium radionuclides (U. S. Atomic Energy Commis- 

 sion, 1973) persist in the marine environment of Enewetak Atoll. Forty -three nuclear 

 weapons tests were conducted by the United States at Enewetak between 1948 and 1958. 

 The testing produced close-in fallout debris which was contaminated with transuranics 

 and which entered the aquatic environment of the Atoll. More transuranics were 

 transported westward to Enewetak in airborne debris and water contaminated from 

 nuclear testing at Bikini Atoll. Global fallout deposited a small additional amount of 

 transuranics on the Atoll. Presently, the largest inventory of transuranics introduced from 

 these source terms is associated with components of the benthic environment. 



Because of the high level of deposition, the Atoll is now its own transuranic source 

 term. Plutonium, for example, is not permanently fixed with the carbonates and other 

 material with which it was originally deposited in the lagoon and on the reef during 

 nuclear testing. Small amounts of plutonium are now remobilized, resuspended, 

 assimilated, and transferred continuously within the Atoll environment by physical, 

 chemical, and biological processes. 



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