226 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



lower than the initial rate. These data indicate that there will be rapid depletion of the 

 source for materials deposited on such pavements if any significant traffic occurs. 



Sehmel (1976b) also performed a similar experiment with the zinc sulfide tracer 

 placed on a strip of cheatgrass. The course was the same size as the asphalt course. A 

 ^4 -ton truck was driven through the area at different speeds. The results indicated a 

 relatively high resuspension rate of 3.7 x 10~^ sec"' on the first pass at 2.2 m/sec. For 

 the second pass at 6.7 m/sec, the resuspension rate had decreased to 4 x 10~^ sec"^ , 

 whereas for the tliird pass at 13.4 m/sec, the rate increased to 8 x 10~^ sec"' . With the 

 final pass at 17.9 m/sec, the resuspension rate increased to its highest value of 9.4 x 10"^ 

 sec"'. The high resuspension rate at the low speed was caused by removing the most 

 readily suspendible tracer from the cheatgrass. By the time the higlier speeds were 

 attained, it is likely that tliis more readily suspendible material was removed and the 

 resuspension was from the soil surface. 



Milham et al. (1976) described the results of air sampling during the agricultural 

 preparation of two fields having small concentrations of plutonium accumulated 25 to 30 

 yr earlier as the result of a release from a nearby stack. Samples were taken at several 

 locations during operations in the field. Healy (1977a) converted these to approximate 

 resuspension rates by use of the field sizes and meteorological parameters given in the 

 paper. Tliese results are given in Table 3. 



TABLE 3 Resuspension Rates from Agricultural Operations* 



*From Healy (1977a). 



A somewhat similar experiment was performed by Myers et al. (1976). Here the 

 plutonium was applied to a small field in the form of digested sewage-plant sludge 

 containing a small amount of plutonium. The sludge was allowed to dry for 4 weeks 

 without rain, and the area was rototilled. Tlie rototiller was 2 m wide with the dust cover 

 removed and was pulled beliind a tractor. The sampling results were, again, converted to 

 an approximate resuspension rate by Healy (1977a), and the result is given in Table 3. 



Mass Loading 



The mass-loading concept is an attempt to bypass the details of the soil characteristics 

 and the resuspension process and to relate directly measured soil concentrations of the 

 contaminant to the air concentration by use of the mass of soil particulates in the air. 



