Mr. E. M. Bramlitt October 19, 1976 Page 3 our automatic contouring result are an improvement over someone setting down and drawing contours by hand. What is needed are estimates -of variability on the contour lines. One can get a feel for the relative accuracy of some of the contours in certain parts of the island by noting whether any data points are in the vicinity of the contour lines. In general, other things being constant, the more dense the data points, the more confident we can be of the placement of the contour lines. We have talked some about Kriging and how this technique can give estimates of confidence limits on contours if the data are adequate. I understand you have a copy of Dr. Delfiner's report on his attempts to use Kriging to answer the question "Which hectares on Janet exceed an average Pu concentration of 40 pCi/g"? His overall conclusion was that "this question cannot. be answered on the basis of the present data". He indicated that denser sampling was required in order to identify the "structure" (trends around the GZ's or across the island) of the data for spacings tess than 50 meters, This structure must be identified before Kriging can be applied to the above question. Dr. Delfiner suggested that "the best that can be done is to calculate an undifferentiated global mean. This raises the question of whether more samples could be collected around the GZ areas and/or hot spots suggested by the present data before the cleanup crew gets underway next year. These samples might allow the structure to be estimated so that kriging could be applied. Of course, the use of In Situ devices for measuring ¢41Am on the island is another approach for obtaining data for estimating the structure. If the In Situ devices are used, however, it is imperative that the resulting In Situ ©4)Am data be calibrated with Pu concentrations in soil by taking a large number of soil samples close to each Other and in the area "read" by the detector. This would need to be repeated at several locations on the island. This should be done before the In Situ device is used to make cleanup decisions. Hope these comments and the enclosed plots are helpful. Tom McCall at ERDA and Bruce Church at NVO, also. Best regards, PBmiela) < Cheer ae. 0. Gilbert Senior Research Scientist Statistics Section Systems Department ROG:m11 Copies with enclosures to T. F. McCraw, ERDA Headquarters, Washington, D.C. B. W. Church, ERDA, NVO,Las Vegas. I'm sending copies to