The distribution of plutonium in the different particle size classes and in depth shows an interesting trend. Consider the desert pavement, for example, where the bulk of the acitivity in the first two depths is in the sand sizes and the activity in the second layer is about 2% of that in the surface layer. In the lower two layers, the silt and clay sizes contain the major fraction of In Table 1, the plutonium concentrations in the depth the soil activity. increments of the desert pavement suggest a possible accumulation in the 7.5 The change in percentage contribution by the finer sizes in 10.0 cm layer. the lower depths also suggests downward migration of activity, providing The movement of plutonium downward into further support for this hypothesis. the lower soil horizons at the Nevada Test Site has also been observed by Essington et al. (1975), including several soils in Area 11. The distribution of plutonium with depth in the desert mound sample is more Unlike the desert pavement, which shows an abrupt difficult to explain. decrease in activity below the surface layer, the desert mound shows a gradual Furthermore, unlike the desert pavement, decrease in activity with depth. which shows an abrupt change in particle size distribution above and below the 2.5-cm depth, the desert mound shows a gradual change in the particle size distribution with depth, with the deepest sample approaching a particle size This distribution similar to that in the lower layers of the desert pavement. gradual change in particle size distribution with depth in the desert mound suggests that considerable mixing of the original soil has occurred in the secondary deposit. Considering the closeness (30.7 feet) of the mound to GZ, the possible mechanical disturbance during preparation of the site, and the fact that these data are the result of a single sampling of one mound, further interpretations or speculations about the depth distribution of plutonium in Area 11 is not warranted. Density Gradient Segregation and Plutonium Distribution In order to further characterize the plutonium in the desert pavement and desert mound samples, selected size fractions were subjected to density gradient segregation. The technique separates minerals with different densities by allowing them to accumulate at their respective mineral density positions in the linear density gradient solution. To date, the three silt sizes of the surface layers have been separated in this manner. The results presented in Table 5 include the density range or bands in which the particles concentrated, the weight of the particles, and the percent activity in each of the bands. The activity measurements were obtained with a portable alpha survey meter placed over the dry powder; the activity percentages listed in the table, therefore, should be considered as approximate indications of the distribution and not as definitive values. Results show that as the particles get smaller, their densities become lighter. This observation is consistent with mineral hydration (in these cases, the polyvinylpyrrolidone substitutes for water), which increases the effective diameter and reduces the particle density. There is an indication that the desert mound particles in the 20 - 5 and 5 - 2 wm size classes are lower in density than the corresponding size classes in the desert pavement. In the study of a desert pavement-desert mound system in Area 13 (600 feet from GZ) reported earlier (Tamura, 1975b), the 53 - 20 um size particles of the two 10