-~30— later, contained the lowest levels of gamma radioactivity. levels The low in the plankton from this location indicated that the south- western perimeter of the contaminated water mass at this time was only afew miles away. The low level of radioactivity also suggests that the radioactive water had travelled primarily ina westward rather than a southwestward direction. The radioisotopic composition of the plankton samples before and after the Wahoo detonationis givenin Table 3. collected Np239 was the major contributor in all of the samples, but the relative amounts of Mo99-Tc99m, Tel32_7132 and U237 differed, Mo28- Tc99M was:much higher and Te!32-1132 was much lower in the pre- Wahoo sample. Ru!05-Rn105 was detected in one post-Wahoo sample and Rul06-Rn196 was detected in one pre- Wahoo sample. These differences in radioisotopic content may be used to establish the origin of the radioactive material in the samples during the survey. collected As seen in Table 3, the radioisotopes in samples P-25 and P-26, collected at the western periphery, were similar to the radioisotopes in earlier samples; therefore, it was assumed that the origin of the radioactive material was the same for all samples collected after the Wahoo detonation. | Results of radiochemical separations and gamma spectrum analyses revealed that the percentages of Bal40-1,a140 and U237 were higher in