228 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



long distances. For example, Carlson and Prospero (1972) have reported the movement of 

 dust from the Sahara desert over the northern equatorial Atlantic Ocean, and Clayton 

 et al. (1972) have reported evidence of transport across the Pacific Ocean to Hawaii. 

 Since most contaminated areas are relatively small in area, one would expect that only a 

 fraction of the dust in the air would originate from the area. Because of preferential 

 deposition of the larger particles from sources some distance away, this "background 

 dust" would contain a higher percentage of the smaller particles that are more readily 

 deposited in the lung than the dust originating from the local, contaminated area. Thus 

 Anspaugh and Phelps (1974) report that measurements at the GMX area with Anderson 

 high-volume cascade impactors for about 1 month indicate that the mass distribution of 

 sizes is about 1.6 iJ.m MM AD with a geometric standard deviation (a^) of about 15, and 

 the plutonium and ^"^^ Am had an activity median aerodynamic diameter of about 3 idm 

 with a Og of about 7. It was also noted that the average activity of the soil was about 

 one-third that found in the soil in close proximity to the sampler. It is noted that even 

 higher activity was upwind. 



From this we conclude that a direct comparison of the size distribution of 

 contaminated particles in the air with those in the soil is probably valid only for very 

 large areas. For the more usual size of contaminated area, the dilution of the total mass in 

 the air, particularly in the smaller particle sizes, could be significant. It is noted, however, 

 that, for resuspension by mechanical disturbance, this dilution may be of lower 

 importance because of the frequently higher concentrations resulting from such 

 disturbances. 



The second question, that of the appropriate concentration of the contaminant in the 

 soil, is more subtle. As was discussed earlier, soil particles that are carried in suspension 

 are the smaller ones because the larger ones will settle rapidly. Tlius, if the concentration 

 of the contaminant in the soil fraction containing the small particles is greatly different 

 from that in the other particle sizes, it would appear that the concentration predicted by 

 the mass-loading approach using the total soil concentration would theoretically be low. 



Tamura (1977) has analyzed the particle sizes and their associated plutonium content 

 in samples from several existing plutonium-contaminated areas and has shown that 

 fractionation of the plutonium content by particle size does exist. Analyses were done 

 using water as the suspending agent, and the effect of this, as compared with the carbon 

 tetrachloride used by Chepil or the liquid Freon used by Gillette, on the aggregate size is 

 unknown. However, in two samples from the NTS, the aggregates less than 20 ;um had 

 plutonium concentration three and five times greater than the total soil mass. (At the 

 NTS the bulk of the activity appears to be in the 20- to 53-iJim size range.) In a bottom 

 sediment from the canal at Mound Laboratory, the concentration in the fraction lower 

 than 20 /im was 1.8 times as high as the total; at the floodplain at ORNL, the soil 

 concentration in the fraction lower than 20 ^m was 1.1 times as higli as the total; and, in a 

 sample from Rocky Flats, the soil concentration in the fraction less than 20 jim was 

 about 3 times as high as the total. It is of interest that these distributions reflect both the 

 method of contamination and the soil type. At the NTS the plutonium was mechanically 

 dispersed by explosive material, and the pa'ticle size distribution reflects the largest 

 amount of the plutonium in the 53- to 125-/jm size range, although the higliest 

 concentration was in the smaller particle sizes. The Mound Laboratory and ORNL 

 samples reflect the distribution expected by adsorption on the smaller particles in the 

 sample, whereas Rocky Flats is intermediate, wliich retlects, perhaps, some adsorption as 



