7.3.2 Belle Background Island Belle (Marshallese: Bokombako) with an area of 12.0 hectares is the largest in the six-island ehain that lies west of Irene. The soil on Belle is mostly sandy and, except sparsely-vegetated areas near the east end of the island, is covered with dense vegetation. for two There were no ground zeros on Belle during nuclear testing operations, but there were a few scientific stations on the island. Some of the stations and some metal and concrete debris remained on Belle until the cleanup. There were no known or suspected areas of buried contamination on Belle. As a result of fallout from several nearby nuclear events, Belle ranks 10th among the islands in the Atoll in total H + 1 hour exposure rate with 3,382 R/h. 1972 Survey Results During the 1972 survey, soil samples were taken at 36 locations on Belle, and a few vegetation samples were also taken. At four of the soil sampling locations, 0-35 em profile samples were taken, at one location a 0-55 cm profile sample was taken, and 0-15 em core samples were taken at the other 31 locations. The depth distributions of 137g, 90sp and 2395240py activities all followed a similar pattern, in which activity dropped steeply with depth below 5 cm. The distribution of activity of these isotopes on the island surface appeared to be related to vegetation density. In the sparsely-vegetated areas on the east end of Belle, the average activity was as much as a factor of three lower than in the areas with dense vegetation. However, the actual difference in activity might be less because only a few samples were taken in the less-vegetated sections so they might not be representative. Also, the results of the aerial surveys of 1972 and 1977 (see Section 3.1) did not indicate a difference as large as a factor of three, nor did the IMP measurements during the cleanup. The results of the 1972 sampling for 0-15 em data on 13%, 90sp and 239,240py are summarized in Tables 7-1, 7-2 and 7-3, respectively. Surface Characterization Belle was initially measured by the IMP on a 50 m grid from 13-16 February 1978 at the locations shown in Figure 7-6. There had been some disturbance of the soil when the lanes were cut to allow the grid to be staked. This disturbance had only a minor effect on the IMP measurements, but later subsurface investigations were strongly influenced by the soil mixing. Soil samples to determine the ratio of TRU to 24l1Am were taken at five locations, with two composites at each of three depths for a total of 30 samples (see Section 4.2.1 for sampling procedure). The soil sample results were used to estimate the ratio to be 3.8 + 0.09 (see Tech Note 2.8). Figure 7-6 shows the soil sampling locations. ~ Detector 5N:496 was used for the IMP measurements on Belle, and because it had been operated at an incorrect bias voltage, the calculated 241 Am values were too low. Tech Note 5.0 describes the data and methods used to compute a correction factor of 1.6 for the data. Because the 1.6 factor was applicable to only part of the islands affected by the voltage problem, Belle was later completely remeasured at the original locations on the 50 m grid. The results confirmed that the factor of 1.6 was valid for Belle. The corrected IMP 241 Am data and the estimated ratio of TRU to 2414m were used to estimate TRU values at each location. These values were then used to make kriging estimates of 0.5 ha average TRU activity and of the 0.5 s upper bound on the estimated average where s is the kriging standard deviation (see Section 5.1). No upper bound on a 0.5 ha average exceeded 160 pCi/g in TRU activity based on original data (see Tech Note 23 for discussion of original versus final data). It was concluded that Belle met Condition A without soil removal. 216