expected from theoretical arguments. The relative stability of the arithmetic mean for different aliquot sizes implies it is preferred to the geometric mean and the median when comparing results from studies that have used different aliquot sizes. A linear relationship between logarithms of s and aliquot weight (w) at this sampling location is suggested by the data. This equation was used to obtain expressions for determining aliquot weight and number of aliquots per sample for 2*!am and 2399240py analyses as a function of analysis costs and the desired standard error of the sample mean. An approach used by Cochran (1977) is suggested as a way of obtaining optimum numbers of field samples and number of aliquots per field sample for minimizing either total cost or the variance of the mean computed over all field samples. Additional studies are required to determine the degree to which these results are applicable to other sampling locations on NTS and to other study sites where different sources of contamination and environmental factors are present. The purpose of the variability with distance study was to obtain data useful for estimating the variogram (correlation structure) between 24lam concentrations at various distances. This information is needed in our continuing evaluation of kriging, an optimal procedure (under certain conditions) for estimating spatial distribution of contaminants. This study was conducted along two sampling transects at the Area 13 (Project 57) Site on NTS. Line 1 was near ground zero (GZ) and cut across several activity strata. Line 2 was 3,600 ft north of GZ where activity levels are relatively low (in the pCi/g range). Both 70-g soil aliquots and FIDLER readings at the soil surface and at l-ft height were taken to measure **!Am. These measurements were taken in clusters of four adjacent locations, each cluster separated by 20 ft in Line 1 and 150 ft in Line 2. One hundred adjacent FIDLER readings were also taken near Lines 1 and 2. The data are plotted and the experimental (estimated) variograms computed and displayed for all cases. Some basic concepts of kriging are discussed. lor the relatively high *+!am concentrations along Line 1 (nCi/g range), there appears to be a definite correlation between observations as a function of distance for both soil Ge(Li) analyses on 70-g aliquots and for FIDLER readings. This is evident from the experimental variograms. This does not appear to be the case for the lower levels (pCi/g range) along Line 2. This suggests that kriging may not be advantageous at these low levels for the data collection methods used here (soil and FIDLER). Its use in these low-level areas would seem to depend on using measurement techniques that integrate over a larger region. Additional analyses of the Line 1 and 2 data are given by Delfiner and Gilbert (1978). 406