kriging over more conventional approaches. The necessity of careful planning of field sampling studies in order for kriging (or any statis- tical technique) to give reasonable estimates of inventory and spatial distribution of plutonium and other radionuclides is noted. INTRODUCTION The Nevada Applied Ecology Group (NAEG) has been conducting environmental transuranic studies at safety-shot sites on the Nevada Test Site (NTS) and the Tonopah Test Range (TTR) since 1971. One important objective of these studies is to estimate the total amount (inventory) and spatial distribution of 239°240py and 24lam (henceforth denoted by Pu and Am) in surface soil. Of the 10 safety-shot sites studied by the NAEG, the most intensive soil sampling program took place in Area 13 at the Project 57 site. The first estimates of Pu inventory in surface soil (0-5 cm) were given by Gilbert et az. (1975), with corrections to these estimates being published by Gilbert (1977). These inventory estimates were obtained using stratified random sampling, i.e., by collecting soil samples at random locations within Am activity strata (Figure 1) that had been defined on the basis of FIDLER* surveys taken on 400-foot and 100-foot grids (Figure 2) about GZ. The inventory estimate for a given stratum was obtained by multiplying the average Pu concentration (yCi/m*) for that stratum by the size (m2) of the stratum. Information on the spatial distribution of Am and Pu was available from the FIDLER Am activity strata map for Area 13 (Figure 1) since Pu and Am are correlated at this study site. The spatial distribution of Pu was also studied by estimating isopleth (contour) lines of concentration on the basis of Pu soil concentrations using a computer program "SURFACE IL" (see Figures 16, 20, and 21 in Gilbert et aZ., 1975; also see Figures 8 through 14 in Gilbert et al., 1977, for the Area 5 (GMX) site). These estimated contours were unsatisfactory in several respects. They were found, for example, to be biased in the sense that estimated concentrations near GZ appeared to be too low, and those at distance from GZ were too high. Hence, we began to look for alternative methods. An iterative estimation approach on both untransformed and log-transformed data was investigated by Gilbert (1978). The iterative procedure appeared to reduce the bias in estimates of plutonium concentration contours mentioned above when calculations were done in logarithmic scale with transformation back to arithmetic scale as the last step. This work also pointed out that highly spurious estimates can result in regions of sparse data where Pu concentrations change rapidly within short distances. This iterative approach grew out of suggestions offered by Professor John Tukey at the first ERDA Statistical Symposium (Eberhardt and Gilbert, 1976). *Field Instrument for the Detection of Low Energy Radiation. 363