Because the subsurface cleanup boundaries could not be defined as precisely as the surface boundaries, a conservative approach was taken in determining the boundary. Usually, the cleanup area was extended beyond the last location with observed TRU activity above 160 pCi/g to at least halfway to the adjacent location, Soil volume estimates were based on these boundaries and the maximum depth with TRU activity greater than 160 pCi/g. If the subsurface soil removal area was large enough, it was subdivided into sections with a different maximum depth in each section. The type of sampling used for checking the post-removal activity depended on the size and depth of the excavation, and on whether it was to be backfilled. Soil samples were taken from the sidewalls and sometimes the excavation floor. Portable instruments were sometimes used to roughly characterize the radiological condition of the floor and sidewalls of the excavation. IMP measurements were usually made in a pattern that provided complete coverage of the excavation. In relatively shallow excisions with no backfilling, averages of the TRU activity caleulated from IMP data were used to verify that the cleanup criterion was met. In deeper excisions, soil samples were collected to make sure the contamination did not extend beyond the cleaned area while IMP data provided TRU data to compare with the cleanup criterion. If the excavated area was backfilled, the fill material was measured with the IMP before and after the backfilling. Soil stockpile areas were also measured to confirm that all contaminated soil had been removed. The average TRU activity in the soil removed was estimated by using the arithmetic mean of all the soil profile data taken in the lifted area. This estimate was multiplied by the soil volume removed as reported by JTG to estimate the total TRU activity removed. 5.2.4 Quality Assurance Program The external quality control program was an integral part of the overall quality assurance effort for the EIC Enewetak laboratory. In this program, a large soil sample was collected and thoroughly mixed to form a basis for interlaboratory comparisons. Starting in December 1978, and quarterly thereafter, part of this large sample was dried, ballmilled and prepared for analysis as usual on Enewetak. Then it was split into four aliquots with a minimum of 100g in each. One remained at Enewetak for analysis and the other three were shipped to Nevada for transshipment to independent labs for analysis. Each such set cf samples was designated a "batch." Throughout the cleanup, five batches were examined by at least two laboratories. Batches 1 and 2 consisted of soil from one location on the island Janet and Batches 3 through 5 were from another location on Janet. For the purpose of comparison,all the data from a single location were combined. Two different sets of assumptions could be possible for estimating the population variance for data from a single location. The individual samples all received the same preparation and were aliquots from the same homogenized sample. Therefore, it could be assumed that the only contributor to the variance is the counting error resulting from the approximately Poisson distribution of radioactive decay. The other assumption, which is more realistic, is that the factors such as environment, differences in chemical recovery, and sample inhomogeneity also contribute to the variance. Table 5-1 shows the results from all Batches, along with the two sigma counting error. Lab A is the Enewetak laboratory, Lab Al is the EIC Albuquerque laboratory, and Labs B, C, and D are the independent labs. The values reported for Lab A are actually arithmetic means based on the results of several subaliquots of the initial batch aliquot. The data for the other laboratories are based on a single analysis. Results of the comparison for each radionuclide are discussed below. Americium - 241. The results for all laboratories were within the 99 percent confidence interval on the mean of Batches 1 and 2. All but the Lab B Batch 3 results are within the 99 percent confidence interval on the mean of Batches 3, 4 and 5. Laboratory B showed a distinet tendency to produce low results up until Batches 4 and 5. Overall, interlaboratory agreement is good, especially considering the fairly low activity in the last three Batches. Plutonium - 238. Statistical comparison of this isotope was not very useful because of the lack of data and also because of the very low activity. Based on a general review of the results, the interlaboratory agreement appears to be reasonably good. 144