COMBINING TWO TYPES OF SURVEY DATA FOR ESTIMATING GEOGRAPHICAL DISTRIBUTION OF PLUTONIUM IN AREA 13 P. Delfiner* Centre de Geostatistique in Ecole des Mines de Paris Fontainebleau, France R. O. Gilbert Battelle Memorial Institute, Pacific Northwest Laboratory Richland, Washington ABSTRACT Estimates of average 239°24%Py concentrations in surface (0-5 cm) soil lying within 100-foot by 100-foot cells are obtained for the inner fence region at the Area 13 (Project 57) safety-shot site on the Nevada Test Site. These estimates and approximate 95% confidence intervals are obtained using "kriging" in a two-stage procedure. In Stage 1, initial estimates (a "guess field") of average 239°249py concentrations in 100- x 100-foot cells are obtained by (i) kriging FIDLER 24lam count per minute (cpm) data to obtain average FIDLER readings in 100- x 100-foot cells, then (ii) using these initial estimates in an estimated linear regression equation relating 239’249Pu soil concentrations and 24 1am FIDLER readings (both in logarithmic scale). The results of Stage 1 are given in a figure. In Stage 2, the initial estimates are "corrected" by performing kriging on the differences between observed plutonium concentrations at random locations and those predicted from the linear log Pu-log FIDLER regression. The estimated corrections and the final "cor- rected" arithmetic mean estimates are included in the report. Geometric mean estimates and factors for obtaining confidence intervals are listed. These factors suggest mean plutonium concentrations are estimated within, roughly, factors of 2 with approximate 95% confidence. In general, the kriging estimates seem to be in good agreement with the observed plutonium data at most locations. Exceptions to this are *Work completed under a consulting agreement B~06245-A-L with Battelle Memorial Institute, Pacific Northwest Laboratories, Richland, WA. 361