TRANSURANIC ELEMENTS AT BIKINI ATOLL 551 



B-24, which were located to the south and east of the area of the highest radionucHde 

 concentrations measured at stations B-2 and B-20. Station B-21 was located at the 

 extreme southern end of the region of high radionuclide concentration, and the fmely 

 divided sediments found there were similar in appearance to those collected at stations 

 B-2 and B-20. Even thougli both stations B-21 and B-20 had similar proportions of fine 

 sediments, only about 23% of the 2 3 9,2 4 op^ measured at station B-20 was found at 

 station B-21. In contrast, stations B-22 and B-24, which were located some distance 

 downstream and to the east of the area of high 2 3 9,24 0p^ concentrations (i.e., stations 

 B-2 and B-20), contained low proportions of finely divided material (less than ~15%) but 

 contained 43% and 39%, respectively, of the total 2 3 9,2 4 Op^^ Pleasured at station B-20. 

 These observations can be explained by two processes: the first is by dilu- 

 tion of the concentration of radioactive particles deposited at station B-21 by 

 material of a lower concentration (resulting from biological activity or erosion of the 

 reef); the second is by physical or chemical fractionation of the radioactivity in, or from, 

 debris particles that are transported in suspension. (Physical fractionation could arise 

 from differences in the concentrations and rates of dissolution of different-size particles.) 

 The plutonium concentrations of relatively larger size particles deposited at station B-21, 

 for instance, may have been lower than those of smaller size particles deposited farther 

 downstream at stations B-22 and B-24. Second, since chemical fractionation of the 

 radionuclides may be a function of the length of time the particles remained in 

 suspension, particles deposited at station B-21 may also have lost a higher proportion of 

 their surface-associated radioactivity than those deposited at stations B-2 and B-20. The 

 relatively high concentrations of the sediments collected at stations B-22 and B-24 would 

 be consistent with the deposition of finely divided material of a high specific activity. 



Distribution in Sediment Cores 



Measurements of the concentration distribution of elements in the sediment column are 

 fundamental to the study of the exchange of materials across the sediment- water 

 interface. In the Bikini lagoon measurements of both the transuranic and fission-product 

 radionuclides with depth were considered to be particularly informative since debris from 

 several detonations have been added to the lagoon at different times. 



Nine sediment cores were collected from various locations in the lagoon. Three types 

 of profiles of the radionuclide concentration with depth were observed. These occurred in 

 (1) crater sediments (stations C-3 and C-12), which had either relatively homogeneous or 

 constant distributions of most radionuclides with depth; (2) northwest quadrant lagoon 

 sediments (stations B-2, B-20, and B-21), which had large proportions of finely pulverized 

 material and which had radionuclide concentrations that changed regularly with depth; 

 and (3) central and eastern lagoon sediments (stations B-15, B-16, B-27, and B-30), which 

 had variable radionuclide concentrations with depth (Marshall, 1975). 



Crater Cores 



The distribution of radionuclides measured in the sediment core collected from the center 

 of Zuni crater (station C-12) showed approximately constant transuranic and fission- 

 product concentrations with depth (Table 3). No appreciable portion of the sediments in 

 the Zuni crater core was fmely pulverized. A unique concentration sequence of the 

 following order was found: 2 39,240p^ ^ ^^s^^ ^ 2 38p^ >^°Co > ' ^'Cs > ^"^^ Am > 

 2^^Bi. 



