PLUTONIUM DISTRIBUTION IN A DESERT PAVEMENT-—DESERT MOUND SOIL SYSTEM IN AREA 11 Tsuneo Tamura Oak Ridge National Laboratory Environmental Sciences Division Oak Ridge, Tennessee ABSTRACT This interim report presents data on plutonium distribution in a desert pave- ment desert-mound soil system in Area 11 of the Nevada Test Site. Total plutonium soil concentrations with depth, plutonium concentrations in the different particle size classes, and preliminary information on plutonium distribution as a function of mineral density fractions are included. Analyses of plutonium in the desert-pavement sample showed that 95% of the activity is in the surface layer (0 - 2.5 cm). Evidence of plutonium migration downward into the soil profile is suggested by higher contributions to the plutonium concentration in deeper layers by the silt and clay size particles. In the desert-mound sample, 60% of the activity is in the surface layer. Significant amounts of plutonium occur down to the 7.5-cm depth, but there is a possibility that this depth distribution is due to the method of sampling. The soil particle size classes bearing the highest plutonium activity are the coarse silt (53 - 20 um) in the desert pavement and the medium silt (20 - 5 um) in the desert mound. The high activity associated with soil particles less than 20-um particle size in the desert mound sample suggests that the fine-parti- cle activity is from a source close to ground zero. Density gradient segrega- tion of the silt fractions in the surface layer showed that the highest activity is in the heaviest fraction (greater than 2.9 g/cm? density); contributions to the activity in the heaviest fraction decrease with finer particle sizes. INTRODUCTION A common feature in the desert landscape of the Nevada Test Site (NTS) is the occurrence of sandy desert mounds formed under desert shrubbery. The mounds are formed by the filtering action of the vegetation in intercepting saltation and creep particles. In contrast to the desert mound, the adjacent bare area consists of desert pavement soils, frequently witl a gravelly surface texture.