a at locations where samples are not collected than did fits obtained on untransformed data. untransformed scale, Fits in either scale, but particularly in the gave questionable estimates in regions of sparse data where Pu concentrations change rapidly within short distances. Plots of sample data on estimated contour maps suggest regions where more data are needed. The two-phase grid estimation procedure used here does not give estimation variances for the grid values. Kriging is mentioned as a method that does provide such estimates. An evaluation of the applicability of Kriging to estimating spatial pattern of radionuclides in the environment is encouraged.* INTRODUCTION Estimating the spatial pattern or geographical distribution of environ- mental contaminants is often of interest in environmental sampling programs. Concentrations of the contaminant are measured at various locations and an estimate is desired of the "true" concentration "surface" for the area from which samples are collected. One approach to this problem is to use the observed data to estimate the surface at regular grid points over the study site. This estimated grid matrix can then be displayed as a three-dimensional concentration surface or as a contour map showing lines of constant concentration. In this paper, we examine whether an iterative fitting procedure for estimating the grid matrix of concentrations would improve estimates of the true concentration surface. The iterative procedure is applied to 239°240by concentrations in surface (0-5 cm) soil samples collected according to a stratified random sampling plan at the Area 13 (Project 57) "safety-shot" site on the Nevada Test Site (NTS). This study site is one of 10 safety-shot sites on the NTS or the adjacent Tonopah Test Range currently being studied by the Nevada Applied Ecology Group (NAEG). These are sites where, during the period 1954-1963, assemblies or devices composed of plutonium and/or uranium were blown *This paper was prepared for presentation at the 1976 Annual Meeting of the American Statistical Association in Boston, Massachusetts, August 23-26, 1976. This analysis of Area 13 (Project 57) 239°240py data was performed prior to the kriging analyses by Delfiner and Gilbert (1978) reported elsewhere in this volume. The plutonium soil concentration data used in this latter paper is identical to that listed in Appendix A of the present paper except as noted in Appendix A and footnote 2 in Delfiner and Gilbert (1978). 320