Estimation of Pu Concentration Surface A two-phase estimation procedure, GRID, was used in conjunction with a nearest neighbor data search routine, NEAR (Sampson, 1975a), to estimate the plutonium concentration surface using the 170 data points obtained as described above.* This procedure was applied at each iteration as described in the next section. GRID estimated the 239°249py concentration at each intersection point (grid node) of a grid laid over the study site. The grid mesh spacing was chosen to be 100 feet after trial and error computations of the concentration surface using spacings of 50 and 200 feet.** The final grid had 73 rows (east-west) and 86 columns (north-south) for a total of 6,278 grid nodes. The method used by GRID to estimate the concentration surface is described in detail in Appendix B (from Sampson, 1973). GRID and NEAR are part of a large computer software system called SURFACE II (Sampson, 1975b) developed for the manipulation and display of spatially distributed data. This system is under continuing development by the Kansas Geological Survey. All of the contour and three-dimensional plots presented here were prepared from plots obtained on the Cal-Comp plotter using SURFACE II. A disadvantage of GRID is that it does not yield estimation variances of the estimated grid node values. However, within the next few months, a gridding method known as Kriging is expected to become available on SURFACE II.*** Kriging yields best linear unbiased grid node estimates as well as variances of these estimates if the underlying assumptions of the method are fulfilled. The theory and practice of kriging have too many ramifications for discussion here, but the basic assumption involves second order stationarity of differences between spatial data (the "intrinsic hypothesis"). Introduction to the theory and practice of kriging are given by Huijbregts (1975) and Delfiner and Delhomme (1975). A detailed account of the underlying theory and a worked example are given by Olea (1975). Further insight into the method is given in Davis (1973), Huijbregts and Matheron (1971), Agterberg (1970), Akima (1975), and Olea (1974). *The estimated plutonium contours for Area 13 (Project 57) in Gilbert et al. (1975) were based on 166 observations; those in Gilbert et al. (1976b) on 167 or 168. These earlier efforts exclused the datum 16,400 uCi/m* in Appendix A, the extremely large value near GZ. **Reducing the grid mesh size from 100 to 50 feet increases the number of grid nodes fourfold. The computing expense is similarly increased. In practice, the grid mesh size is determined in part by the desired detail in the concentration surface, the density of data points, and cost factors. **kSince this paper was originally written, kriging has become available on SURFACE II. However, we have instead used the kriging program BLUEPACK written by Dr. Pierre Delfiner (see Delfiner and Gilbert, 1978) since it offers a more general approach to the problem. 325