TRANSURANIC AND TRACER SIMULANT RESUSPENSION 273 



for ^^^Pu and ^^^Cs of predicted maximum to the experimental maximum airborne 

 concentration show a ratio range from 0.05 for ^^^Pu to 60 for ^^^Cs. Thus there are 

 at least three orders of magnitude uncertainty in using the mass-loading approach in 

 calculating the maximum expected airborne radionuclide concentration. 



This airborne-particle mass-loading approach to calculated airborne radionuclide 

 concentrations does not distinguish the sources of airborne material. For simple wind 

 resuspension, airborne solids included contaminated solids (lower or higher radionuclide 

 concentrations per gram of soUd) blown in from the surrounding area as well as solids 

 resuspended from the prime resuspension study site. In contrast, airborne solids above a 

 mechanically disturbed area are resuspended from tlie study site. In this case possibly 

 such an equality assumption of radionuclide concentration per gram of solids would be 

 appropriate for mechanical disturbances (Milliam et al., 1976) of contaminated soil area. 



Tracer-Particle Resuspension 



Tracer particles placed on selected surfaces were used to measure (Sehmel, 1977b) 

 resuspension rates caused by both mechanical and wind resuspension to determine 

 particle resuspension rates. Mechanical resuspension was measured for vehicular traffic on 

 asphalt and cheat grass areas and for pedestrian traffic on an asphalt area. Wind 

 resuspension was measured as a function of v/ind speed and also as a function of 

 respirable and nonrespirable particle diameters. 



Mechanical Resuspension Rates. Mechanical resuspension includes both vehicular 

 resuspension and pedestrian resuspension. 



Vehicular Resuspension. A /4-ton truck and a car were driven over ZnS tracer 

 particles (8-/jm mass median diameter) placed on one lane of asphalt road. Resuspended 

 tracer was measured to determine resuspension rates (Sehmel, 1973b). Results are shown 

 in Fig. 24 for particle resuspension rates at vehicle speeds of 5, 15, 30. and 50 mph. The 

 resuspension rate is tlie fraction of particles resuspended from the tracer lane each time 

 the vehicle was driven down the road (fraction resuspended per pass). Wlien a car was 

 driven through the tracer lane at speeds up to 30 mph, resuspension rates increased with 

 the square of car speed from about 10""* to lO"'^ fraction resuspended per pass. This 

 means that these resuspension rates were proportional to car-generated turbulence. When 

 tlie car was driven on the lane adjacent to the tracer lane, resuspension rates were lower 

 for each vehicle speed but increased with vehicle speed from about 10~^ to 10~^ 

 fraction resuspended per pass. 



Resuspension was also measured when a ^^-ton truck was driven on the tracer lane. 

 Resuspension rates for truck passage increased from about 10"^ to 10^^ fraction 

 resuspended per pass. Since resuspension rates were higher, truck-generated surface-stress 

 turbulence appears to have been much greater than that for car-generated turbulence. For 

 vehicle speeds above 20 mph, resuspension rates for car and truck passage are comparable. 

 This similarity might be caused by tire surface-stress turbulence rather than by air 

 turbulence. 



Resuspension rates were also a function o{ the time tracer particles were on the 

 asphalt road. As shown in Fig. 25, particle resuspension rates decreased as a function of 

 time. For tliese data the tracer had been on the road for 4 days. Vehicle-generated 

 resuspension rates increased from about 10^^ to about 10~^ fraction resuspended per 

 pass as vehicle speed increased from 5 to 50 mph. For both vehicles resuspension was 



