A difficulty with the strata maps obtained using the FIDLER is that the isopleth lines defining the strata are in units of counts per minute (1,000 cpm, 5,000 cpm, etc.) which cannot be related directly to nCi/gm concentrations of 239-2405, or 241an in surface soil. One approach to this problem would be to choose a subset of the surveysample grid points covering a wide range of expected concentrations, and for these locations, to collect a soil sample immediately after the FIDLER survey reading was taken. counted for 241 Am on the Ge(Li) These samples would then be system, and, hopefully, a sufficiently accurate regression relationship between the FIDLER readings and the Ge(Li) scans would result to enable one to associate concentrations of 241an with the FIDLER isopleth lines. This is still one step from the goal of relating FIDLER isopleth lines to 239-240 Pu concentrations in soil, but Figs. Bl through B20 in Gilbert et al. (1975), indicate a 239-240 good linear relationship exists between 241am and Pu in surface soil at the safety-shot sites, especially for higher concentration levels. Hence, it may be possible to obtain at least a rough idea as to plutonium soil concentrations along FIDLER isopleth lines. However, there is considerable doubt at this time that this approach would result in accurate estimates, since data collected to date at random locations indicate the correlation between FIDLER readings and 2392405, concentrations are often low, especially for low concentration areas (see Tables 18, 1975). 19, and 20, and Figs. 33-36 in Gilbert et a?., These data need to be examined more closely to determine the feasibility of the approach outlined above. An advantage of this regression approach is that it would allow for placing limits of error on Am concentrations associated with FIDLER contour lines. The method used thus far to estimate concentration contours of plutonium in soil is to use the 239-2405. concentration data for soil samples collected at random locations within strata (the samples collected to estimate Pu inventory in surface soil) in a computer contouring program called SURFACE II Graphic Systems (Sampson, 1973). This yields computer-drawn contour maps and 3-dimensional representa- tions of the present geographical distribution of plutonium (Figs. 1627, Gilbert et at., 1975) using a nearest-neighbor estimation scheme. Unfortunately, the program does not allow for estimating the precision of its estimated contour lines. Whether or not this is possible using the nearest-neighbor approach remains to be determined. 103