TRANSURANIC ELEMENTS AT BIKINI ATOLL 545 



Distribution in Surface Sediments 



Tlie size of the detonation craters and the extent of the impact on the reef ecosystem are 

 shown in Figs. 2 and 3. The Bravo crater, a dish in the reef, is approximately 550 m in 

 diameter and about 40 m deep. The several large craters in the reef are evident from the 

 photographs. Finely divided coral particles, which resulted from the detonations, appear 

 on the reef flat near Bravo crater and on the lagoon terrace extending south toward the 

 islands of Bokdrolulu and Bokaetoktok. The sampling stations for the biogeochemical 

 survey trip* in 1972 are shown in Fig. 4. 



A thin-source survey method for alpha radioactivity was developed to initially scan 

 the surface sediments collected in 1972 (Marshall, 1975). The results of this rapid total 

 alpha analysis are shown in Fig. 5. The highest total alpha radioactivity is shown not to be 

 in the bomb craters but to be distributed widely over the northwestern quadrant of the. 

 lagoon. Thus the principal source of transuranic elements to the water is a large area in 

 the lagoon; the maximum concentrations are near the Namu Island— Bravo crater area. 

 The plutonium and americium concentrations were determined in the surface sediments, 

 and the results of these analyses are shown in Figs. 6 and 7, respectively, and in Table 2 

 (Marshall, 1975; Nevissi and Schell, 1975; Schell and Walters, 1975). The general 

 distribution pattern of plutonium and americium in the isopleths of Figs. 6 and 7 is the 

 same as that shown previously for the results obtained by the total alpha method of 

 analysis of sediments, which indicates that most of the alpha radioactivity is derived from 

 plutonium and americium. 



Many of the sediment samples collected for analysis in the study by Marshall (1975) 

 consisted predominantly of coralline particles, which were much smaller in size than 

 natural Marshall Island Atoll sediments, as described by Emery, Tracey, and Ladd (1954) 

 and Anikouchine (1961). The sediments were probably pulverized by the detonations and 

 were distributed in the lagoon; the finely divided particles contained the highest 

 concentrations of radioactivity. The proportion of the finely divided material (<16^(m) 

 in each sample was estimated visually. 



Surface sediments collected from stations C-1, C-3, C-4 (Bravo crater), B-2, and B-20 

 (lagoon) consisted entirely of fine-grain material. Surface sediments collected from 

 stations C-7, C-8, B-21, and B-30 contained 45 to 95% fine-grain material. Sediments 

 collected from stations B-18 and B-19 contained approximately 20 to 40% pulverized 

 material. All other sediments contained widely varying portions of fine material but 

 generally less than approximately 10 to 15% by volume (Marshall, 1975). 



Two observations were made regarding the distribution of pulverized sediments and 

 the distribution of ^^^'^"^^Pu, for example. Sediments collected at stations C-5, C-10, 

 and C-11 (S-16), which had much lower concentrations of radionuclides than did 

 sediments collected at the nearby stations, C-1, C-2, C-3, C4, C-6, C-8,and C-11 (S-19), 

 respectively, also contained lower proportions of fine-grain material. Although a similar 

 relationship held for most of the sediments collected, there were three obvious 

 exceptions. These exceptions occurred for sediments collected at stations B-21, B-22, and 



*This sampling trip was initiated by the Energy Research and Development Administration. The 

 Puerto Rico Nuclear Center vessel R. V. Palumbo was used for the trip, and the chief scientists were 

 Frank Lowman, Puerto Rico Nuclear Center, Victor Noshkin, Lawrence Livermore Laboratory, and 

 William Schell, University of Washington. 



(Text continues on page 551.) 



