TRANSURANIC RADIONUCLIDES IN ENEWETAK LAGOON 579 



More than half the U. S. nuclear tests in the Pacific were conducted at Enewetak 

 Atoll. Surface and tower shots left craters and contaminated scrap on land and generated 

 radioactive debris that was redistributed to the adjacent reef and lagoon. Megaton tests 

 that left underwater craters and barge shots in the lagoon contributed significantly to the 

 present transuranic inventory. 



The impact of nuclear testing and the fate of the residual radioactive materials 

 introduced to the aquatic environment at both Enewetak and Bikini atolls are the 

 subjects of reports too numerous to list herein. Not until late 1972, however, when a 

 radiological resurvey of Enewetak Atoll was conducted to gather data for the 

 development of cleanup and rehabilitation procedures for the resettlement of the 

 Enewetak people to their homeland, did extensive measurements of transuranics in the 

 Atoll environment begin. The information was published in a survey report (U. S. Atomic 

 Energy Commission, 1973), which contains data on most long-lived residual radio- 

 nuclides, including plutonium and americium, in components of the marine environment. 

 The survey was followed by other more-extensive investigations, which concentrated on 

 the measurement of transuranics to better assess the impact of these radionuclides on the 

 environment and inhabitants of the Atoll and to increase our understanding of the 

 mobilization, reconcentration, and redistribution processes from sources within the 

 environment. 



This chapter contains a summary of data related to the concentrations of the 

 transuranium elements in components of the benthic and pelagic environment of the 

 Atoll lagoon. Data from the survey report (U. S. Atomic Energy Commission, 1973), 

 more-recent publications, and unpublished results from this laboratory are discussed. 

 Some published and unpublished data from Lawrence Livermore Laboratory (LLL) 

 studies at Bikini Atoll are presented when necessary for comparison with Enewetak data 

 and, in the absence of Enewetak data, for the clarification of characteristics of 

 transuranic radionuclide concentrations at the Atolls. Whenever possible, the Atoll data 

 are compared with those from other marine ecosystems. 



Geography and Atoll Test History 



Enewetak Atoll, with U. S. -assigned and native names and several landmarks, including 

 the locations of craters formed by nuclear tests, is shown in Fig. 1 . The U. S. -assigned 

 island names are used throughout this chapter. 



The Atoll originally consisted of a ring of 42 low islands arranged on a roughly 

 elliptical reef, 40.2 by 32.2 km (Emery, Tracy, and Ladd, 1954), with the elongated axis 

 in the northwesterly direction. Nuclear testing completely destroyed the islands of Gene 

 and Flora, and only a sandbar now remains to distinguish the island of Helen. Only 39 of 

 the original 42 islands of the Atoll remain; these islands make up a total land area of 

 approximately 6.9 km^ , which is situated on the reef which has an area of 84 km^ . The 

 average depth of the lagoon is 47.4 m; the maximum depth is 60 m. The lagoon area is 

 933 km^ . The sedimentary components in Enewetak lagoon were studied extensively 

 during the late 1940s (Emery, Tracy, and Ladd, 1954). The main components in the 

 lagoon sediments included foraminifera, coral, Halimeda remains, shells of moUusks, and 

 tine material. Material finer than 0.5 mm in diameter was too fine to identify and was 

 classified as fine debris. Distributions and average abundance of the sedimentary 

 components were described (Emery, Tracy, and Ladd, 1954). Fine debris made up 57% 

 of the lagoon sediments and was abundant throughout the lagoon to witliin a few 

 hundred feet from the shore. 



