Effect of Estimation Technique on Inventory Estimates Before turning to our work on estimating Pu concentration contours, it is important that we mention the considerably different estimates of Pu inventory that can be obtained depending on whether the "average" Pu concentration for a stratum is estimated by the arithmetic mean or some statistic that is less affected by unusually high concentration samples such as the geometric mean or median. Gilbert et al. (1977a) illustrate the problems involved with the Pu data from stratum 6 (near ground zero) in Area 13. If the geometric mean is used in place of the arithmetic mean, the estimate of Pu inventory in surface soil at Area 13 drops from the 46 curies reported by Gilbert (1977b) to roughly 24 curies. This difference occurs because the statistical distribution of the observed Pu concentrations for each stratum is highly skewed (a few concentrations much higher than the bulk of the data). correct? Which estimate is more nearly The answer to this is not known with assurance. Some insight may be gained by examining the most recent estimates of Pu inventory as reported by Delfiner and Gilbert (1978) for strata 3 through 6 of Area 13. These were obtained using kriging techniques making use of a linear relationship between FIDLER and Pu measurements (each in logarithmic scale). The inventory estimate for strata 3 through 6 combined is about 17 curies compared with the 28.6 curies reported by Gilbert (1977b) for these strata. Delfiner and Gilbert note, however, that their estimates are probably biased low due to ignoring changes in mean concentration levels over distances less than 100 feet when transforming inventory results computed in logarithmic scale back to arithmetic scale. Also, the method used to make this transformation is based on the assumption that the Pu data are lognormally distributed, which may not be the case, particularly near ground zero in stratum 6. These authors suggest that taking a large number of FIDLER readings on a fine mesh grid may be a viable option for obtaining more reliable inventory estimates, at least in relatively high activity strata where the linear log FIDLER-log Pu relationship appears to be well established. ESTIMATING SPATIAL DISTRIBUTION OF PLUTONIUM AT SAFETY-SHOT SITES Initial Ffforts The first estimates of the spatial distribution of Pu at safety-shot sites appeared in Gilbert and Eberhardt (1974). They gave threedimensional plots of FIDLER Am count per minute data collected on 100and 400-foot grids over the Area 13 (Project 57) study site. Threedimensional plots of actual Pu concentrations over space were also shown. Additional information was published by Gilbert et aZ. (1975) in the form of FIDLER Am activity strata for nine safety-shot sites, and estimated Pu contours for Area 13 and the GMX site in Area 5. 455