AIR TABLE 2. Study Area 3 Mean Bulk Density (g/cm’) of Surface Soils Areas* Study NAEG of (0-5 cm Depth) Nene Samples [| Mean** Bulk Density (g/cm?) Standard Deviation Area 13 169 1.14 0,017 GMX 5 115 1.18 0.013 Double Track 90 1,09 0,032 Clean Siate 1 89 0.918 0,016 Clean Slate 2 94 0.926 0,010 Clean Siate 3 100 0.908 0.011 Area 11-B 50 0.835 0,015 Area 11-C 48 0.866 0,033 Area 11-D 54 1,010 0.019 * ** n). R. O. Gilbert (personal communicatio of these surfaces The relatively low bulk densities re of soils may be re lated to the vesicular structu desert pavement. the soil layers inmediately below As indicated in Figure 1, plutonium contained in surface soils may be resuspended and transported to vegetation via external deposition or to herbivores and man via inhalation and some of it could be carried by wind and redeposited beyond the arbitrary boundary. In the absence of data to the contrary, we have assumed that deposition and resuspension processes in contaminated areas at NTS are in approximate steady state although data presented by Anspaugh and Phelps (1974, pp. 292-294) suggest that resuspension may exceed deposition at least to a small degree. Deposition Velocity The rate at which resuspended plutonium is deposited on soll could be estimated as the product of a deposition velocity (cm/day) and concentration in air (uct/em?) to yield a rate that has dimensions of uCi/cm? day. Deposition velocities are functions of meteorological factors and the aerodynamic propertiles of plutonium-bearing soil particles and soil’ surfaces. Deposition velocities measured under field conditions have been reported by Van der Hoven (1968); Sehmel, Sutter, and Dana (1973); and Healy (1974). Measurements under controlled conditions in a wind tunnel have been reported by Sehmel et af. (1973) and Sehmel (1973, 1975). These data indicate that the deposition velocity increases with increasing air velocity, increases with increasing particle size for sizes greater than about 1 um, increases with decreasing particle size for sizes less than 0.01 um, exhibits a minimum somewhere in the range of 0.0] to 1 um, and is strongly influenced by the type of surface roughness. The wind tunnel data of Sehmel et ai. (1973) for grass surfaces indicate the deposition velocity is approximately proportioned to both air velocity and particle size in the range of 2 to 13 m/sec and 1 to 100 um. These grass data appear to correspond closely to field conditions provided a proper value is assigned to surface roughness. Tamura (1976) has reported that more than 65 percent of the plutonium in soil samples from Area 13 is associated with soil particles in the range of 20 to 53 um. Using the grass data of Sehmel et al. (1973) at 2.2 m/sec, the corresponding range of deposition velocities is from 3 to 20 em/sec. Particles on the order of 20 to 50 pm could play an important role with respect to external contamination of vegetation, but particles this large are of little concern with respect to inhalation. As respirable particles are generally <10 um, the corresponding deposition velocities suggested by the grass data would be <1 cm/sec. Deposition Models Both Healy (1974) and Sehmel (1975) present results of models used to predict deposition velocities. Healy's results indicate deposition velocity is proportioned to air velocity and is strongly dependent on atmospheric stability. Sehmel's results indicate deposition velocity increases as a nonlinear function of air velocity, exhibits a minimum value as a function of particle size, and 628 629