REVIEW OF RESUSPENSION MODELS 225 



contribute to the airstream is limited by the depth to which the saltating particles can 

 cause ejection. For mechanical disturbance the depths over which the forces can be 

 applied varies with the means of disturbance but could reach depths of 1 or 2 ft in 

 plowing. Of course, the probability of resuspension is not the same at all depths, but no 

 data are available to indicate possible variations. The extreme example would be the 

 excavation of a hole, such as a basement, where material could be ejected into the air 

 from considerable depths in the ground. 



Another difference arises from the fact that most mechanical disturbances are a point 

 source; i.e., the disturbance occurs over a fairly Umited area at any one time. There could 

 be multiple disturbances that could result in an approximate area source or the 

 disturbance could move with time, which would result in an average that resembled a line 

 source or an area source. An example of the line source would be traffic moving along a 

 road. The average resuspension rate from the road would be the product of the 

 resuspension per vehicle times the number of vehicles divided by the time over which this 

 number of vehicles passed. An area source would be the average result of a farmer 

 plowing a field and producing a resuspension rate at each point. Here the average 

 resuspension rate would be the instantaneous resuspension rate that occurs at each point 

 divided by the time required to plow the field. Such relationships allow the derivation of 

 calculational methods for finding the average concentration downwind if the resuspension 

 rate from the disturbance can be defined. 



There are also similarities with wind resuspension. One would expect the dust flux to 

 be greater in fields that contain a larger quantity of small aggregates. Many of the factors, 

 such as moisture or vegetative cover that inhibit erosion, would be expected to minimize 

 mechanical disturbance. Thus, when digging in contaminated soil, it is common practice 

 to keep the soil damp to minimize resuspension. However, local areas of low saltation 

 (vegetated strips in the field) will not affect the mechanical-resuspension rate. 



A few measurements of mechanical resuspension can be used to give an order-of- 

 magnitude estimate of the rate of resuspension under different conditions. Sehmel (1972) 

 measured the resuspension caused by an individual walking along a 50-ft length of asphalt 

 10 ft wide that had been previously seeded with zinc sulfide tracer. He reports that 

 1 X 10"^ to 7 X 10"'* (at wind speeds of 3 to 18 mph)of the tracer was resuspended per 

 walk-through. Assuming a walking speed of 3 mph, this would result in resuspension rates 

 of 9 X 10~^ to 9 X 10~^ sec"'. Such values have much uncertainty because of the 

 width of the seeded area, but it is noted that they are about two orders of magnitude 

 greater than the wind-resuspension rates with wind speeds of 2 to 9 mph. 



In a continuation of these same experiments, Sehmel (1973) reports the results of 

 driving vehicles in the adjacent lane and through the tracer material. Both a car and a 

 three-quarter-ton truck were used. His values, reported as fractional resuspension per pass, 

 were converted to a resuspension rate tlirough use of the length of the seeded area and 

 the speed of the vehicle. The resuspension rate varied from 10~^ to 8 x 10~^ sec~\ 

 depending primarily on the speed of the vehicle. Sehmel (1973) had reported that the 

 resuspension was proportional to the square of the speed. The resuspension rate caused 

 by the truck was greater than that caused by the car, presumably because of the greater 

 turbulence from the truck. A rapid weathering of the particles was also noted. In this 

 calculation it was assumed that there was no removal by the winds during the 30 days of 

 the experiment; so the latter rates are the lower Umit of the resuspension rates. However, 

 the resuspension rate 30 days after application was two to three orders of magnitude 



