7.4.11 Tilda Background Island Tilda (Marshallese: Bijire) is the middle island of the Sally-Tilda-Ursula complex, interconnected by a landfilled causeway to Sally and a plank-and-pile bridge to Ursula. It has an area of 21 hectares and was moderately to densely vegetated before the cleanup project. There was extensive soil disturbance during the cleanup in the southern part of this island because it was used for a sanitary landfill for the forward camp on Ursula. Several test structures still remain on Tilda but the asphalt runway was removed. No ground zero sites were located on this island and it ranks 18th among the islands in the Atoll with 774 R/h accumulated H + 1 hour exposure rate. There were no knownor suspected burials of radioactive materials on this island, though the landfill causeway to Sally contained a major burial (see Section 6.8). 1972 Survey Soil samples were collected at 32 sites during the 1972 survey and a few vegetation samples were also collected. Of the 32 sites, 28 had 0-15 cm core samples and 4 had 0-35 em profiles. Two of the profiles showed the activities of 1375, 90s and 239524Upy to be declining steadily with increasing depth, and the other two profiles indicated a homogeneousdistribution of low activities for the four isotopes. The results from the core samples indicated a difference in activities related to the amount of vegetation. The more densely vegetated area of Tilda yielded higher average activities of these isotopes than the moderately vegetated area. The 1972 aerial survey also showed this distinction in activity. Tables 7-1, 7-2 and 7-3 summarize the results of the 1972 sampling of Tilda for 0-15 em data on 137 Cs, 90sp and 239,240py, respectively. Characterization Tilda was measured with the IMP on a 50 m grid in March 1978. Soil samples were collected at six locations with two composites at each of three depths for a total of 36 samples. (See Section 4.2.1 for more information on soil sampling.) The results from these soil samples were used to estimate a ratio of TRU to 24lAm of 2.76 + 0.11. (See Tech Note 2.13.) Figure 7-41 shows the locations of the IMP measurementsand the soil sampling. The ratio was used to estimate TRU values from the IMP 241Am data based on original data. (See Tech Note 23 for discussion of original versus final data.) Using these TRU numbers, estimates of the 0.5 s upper bounds on the 0.25 hectare averages were made using the kriging technique, where s is the standard deviation of the kriging error (See Section 5.1). Tilda met Condition C without any soil removal. Tilda was also the site of an experiment to compare soil sample results with IMP measurements on a controlled basis. The details of this experiment are given in Tech Note 8.0. Another experiment conducted by the Joint Task Group on Tilda dealt with different techniques to remove brush andsoil in anticipation of cleanup. Isopleths of surface TRU activity based on final data are shown in Figure 7-42. Table 7-4 gives the island averages for computed TRU, 137Cs and 69Co activities from IMP measurements. Fission Product Sampling Tilda was sampled on a 50 m grid for the Fission Product Data Base Program in support of the dose assessment (see Section 6.11). Samples were collected at 48 sites, and soil from 15 of these was analyzed for 99Sr, Tables 7-1, 7-2 and 7-3 give summary statistics for the 0-15 em depths for the Cs, 90sr and estimated 239,240 py results, respectively, for this sampling. The island average transuranics value reported in the Certification is 7 pCi/gm for surface soil, and the transuranics classification is Residence. 251