TRANSURANIC ELEMENTS AT BIKINI ATOLL 559 



core, rapid accumulation of the sediments containing no unsupported ^'°Pb pre- 

 dominated. Figure 10 shows that both ^°Co and ^""^Bi have concentration profiles that 

 are markedly different from those of other radionuclides in the core above and below 

 about 11 cm. This may indicate that not only the process of deposition but also the 

 source of contaminated debris may have differed for the two depth regions in the 

 sediment column at station B-2. 



Given the dynamic hydrological environment at Bikini, the most significant 

 contamination of the sediment environment a priori would arise from the large surface 

 bursts (such as Bravo, Koon, and Zuni) whose fireballs strongly interacted with the soil or 

 sediment and from similar interactions of deep lagoon or barge bursts (such as the Baker 

 and Tewa tests). At Bikini the initial introduction of highly contaminated debris to the 

 lagoon from detonations o{ this type can be described as fallout deposition of a large 

 mass of chemically altered coralline soils reduced in size and containing the condensed 

 radionuclides. A large mass of crushed coralline material of a relatively low specific 

 activity must also have been ejected by the detonations. The areal distribution of two 

 different types of materials (altered and unaltered coral) would overlap at progressively 

 greater distances away from the detonation craters, and mixed particles would descend at 

 rates depending on their sizes and shapes. In the aqueous environment the particles would 

 be transported a distance that would be determined by their settling velocities, sizes, 

 densities, and the speed of the prevailing lagoon currents. The result of these physical 

 forces with time would be to yield a concentration of fine particles in the surface 

 deposits. The net result of the radionuclide concentrations would be to yield sediment 

 concentrations (picocuries per gram) that were progressively more dilute (by natural 

 sediments) at increasing distances downstream. 



The sedimentation rate measured in the upper 1 1 cm of sediment collected at the 

 station near the Bravo crater (station B-2) showed that the material was deposited at a 

 constant rate between the 1950s and 1972. Although the initial source for the material 

 deposited at these locations was the detonation craters, the present location of the 

 source(s) supplying the material for redistribution at these lagoon stations is not known. 

 The importance of this point should not be underestimated because the location and 

 extent of the source of these fine sediments may determine the continued availability of 

 the radionuclides for redistribution and uptake by biota. 



It is clear, from the large size of the Bravo, Tewa, and Zuni detonation craters, that a 

 huge quantity of pulverized sediment was removed from the reef immediately after the 

 detonations. However, as noted by Welander et al. (1966), lagoon currents were capable of 

 maintaining a large flow of the finely divided sediment out of certain craters at Enewetak 

 long (>1 yr) after the testing stopped. It is quite likely that much of this material at 

 Bikini was deposited outside the detonation craters and was the source for part of the 

 material redistributed in the lagoon. The ^^^Pu/'^'^^''^'*°Pu ratios measured in the craters 

 and at various stations in the northwest quadrant suggest three possibihties for the source 

 of the redistributed material deposited at station B-2: (1) from locations between station 

 B-2 and the Bravo crater; (2) from (1) above and from the area between station B-2 and 

 the northern reef (near station B-19); or (3) from (1) or (2) above and from within the 

 detonation craters. The reason for making these hypotheses is that the 2 3 8py^2 3 9,2 4 0p^ 

 ratios in the top 1 1 cm of redistributed sediments at station B-2 are about 0.0125, 

 whereas the ratios measured in the fine surface sediments collected in Bravo crater are 

 about 0.05, and the ratios at station B-19 are about 0.008. Thus a mixture of sediments 

 from different sources may be deposited at station B-2. 



