Particle Size Distribution Table 2 includes data on the particle size distribution of the desert pavement and desert mound samples. The high gravel content (> 2000 wm) in the surface of the desert pavement is typical of eroded surfaces in the desert. According to Leavitt (1974), the desert pavement soil possesses an Aj horizon which extends down to 10 cm; thus, the lower three depth increments which show similar particle size distribution reflect the more normal Az horizon. Inspection of the data suggests that the finer silt and clay size (< 20 um) particles were eroded from the surface preferentially as evidenced by the low content of these particle sizes in the 0 - 2.5 cm layer. The desert mound, lacking gravel particles, shows a very high content of sands which dominate the texture throughout the samples. With depth, the sand content decreases from 93% at the surface to 77% in the 7.5 - 10.0 cm depth; the silt content increases from 4% to 15%; and the clay content increases from 0.5% to 3%. A comparison of the particle size distribution in the 7.5 - 10 cm sample from the desert mound with the size distribution in lower layers of the desert pavement suggests that the textural composition of the 7.5 - 10.0 cm sample is approaching that of the original residual soil. Plutonium in Particle Sizes Tables 3 and 4 include data on the plutonium content of the different particle sizes. The activity in each particle size fraction is presented in row A. terms of activity per unit mass, the coarse silt (53 - 20 um) contains the highest activity in the desert mound. In Since the activity in the finer particles (less than 20 um) of the surface layer in the desert mound is higher than the corresponding particle sizes in the neighboring desert pavement, the data would suggest that the source of the plutonium associated with the finer sizes in the surface layer of the mound is not the adjacent pavement soil but a source closer to GZ. . Row B presents the plutonium concentrations in disintegrations per minute (dpm) per gram of soil of each particle size class. This contribution takes into account the mass contribution of each size class to the soil particles less than 2 mm. Row C is the percentage contribution of plutonium for each size class to the soil. Thus, in the desert pavement, the 125 - 53 um size class contributes about one-third of the total soil activity as compared to 25% by the 53 - 20 um size class, although the latter has a higher activity per unit mass. The 125 - 53 um fraction contributes 16.28%* of the less than 2 mm soil, whereas the 53 - 20 um fraction contributes 3.54%.* This mass difference compensates for the activity difference noted in Table 3. Similarly, in the desert mound, the 20 - 5 m size class has the highest activity per unit mass (229, 400 dpm/g), but the small weight contribution (0.86%*) lowers this fraction's contribution to about 20%. *Percentage values in Table 2 recalculated excluding gravel (> 2 mm) particles.