468 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



where K and n are empirical constants derived from the data. Sehmel and Orgill (1973) 

 found n = 2.1 when they fit Volchok's (1971) data for plutonium resuspension at Rocky 

 Flats to Eq. 6. Shinn and Anspaugli (1975) also found n = 2.1 for dust flux at NTS. We 

 approximately fit Sehmel's (1975) data for calcium molybdate resuspension at Hanford 

 and also arrived at a value of about 2.1 . The only contrary data are Shinn and Anspaugli 's 

 results for a plowed field in Texas, wliich yielded n = 6.4. 



The empirical value of about 2 for n when derived for different tracers, different soils, 

 and different climates (provided that the soil is undisturbed) tends to provide indirect 

 confirmation for the theoretically derived form of Eq. 5. However, both K and Uj in 

 Eq. 5 are functions of particle size, soil moisture content, surface roughness, relative 

 humidity, and the time period over which the wind speed is averaged. Some attempts 

 have been made to theoretically include many of these factors (especially particle size), 

 but the theory does not seem to describe adequately the variations in the data. Thus K 

 and Ux must be treated as empirical constants for the present. Consequently there is no 

 practical benefit in using Eq. 5 in preference to the simpler Eq. 6. However, at least one 

 experimental measurement of resuspension and wind speed must be made to set the value 

 of K in Eq. 6 for the particular area. 



Mass Loading. In the absence of data to implement Eq. 6 for a given area, Anspaugh 

 (1974) suggests that a mass-loading factor (L^) of 100 jug(soil)/m^(air) be used for 

 predictive purposes. If we assume that the radioactivity of one square meter is associated 

 with 50 kg of soil (5-cm depth x 10'* cm^/m^ x 10~^ kg/cm^), a mass-loading factor of 

 100 Mg/m'^ is equivalent to a resuspension factor of 2 X 10"^ m~' . The theoretical basis 

 for the mass-loading approach is described by Anspaugh (1974). Anspaugh et al. (1975) 

 provide comparisons showing that predicted air concentrations based on L^ = 100 jUg/m'^ 

 are in good agreement with measured air concentrations. 



We shall use the suggested mass-loading factor to represent average conditions at NTS, 

 but it must be noted that higlier than average wind velocities (Shinn and Anspaugh, 1975) 

 or mechanical disturbances, such as plowing (Milliam et al., 1976), could cause the 

 mass-loading factor to be temporarily much higher than 100 jug/m^. It should also be 

 noted that some recent work by Sehmel (1977) suggests little if any experimental 

 justification for this approach. 



Vegetation 



As shown in Fig. 1, vegetation can be contaminated externally by deposition of 

 resuspended material or internally by uptake from soil or by both processes simulta- 

 neously. Other mechanisms of external and internal contamination have been identified 

 or postulated, but direct deposition from air and root uptake appear to be the processes 

 most important to consider when attempting to develop a general model. 



In the following paragraphs we discuss the mechanisms involved in contaminating 

 vegetation and present mathematical expressions to simulate the dynamics of the 

 contaminating mechanisms. We also discuss the parameters in these expressions and their 

 variations under the influence of different environmental factors. However, we conclude 

 that there are too few data to develop an adequate dynamic model, and we are forced to 

 use a simple steady-state model with a constant vegetation-to-soil contamination factor in 

 the overall transport model. 



General Hypothesis. Externally deposited material can be removed from plant surfaces 

 by weathering, i.e., the mechanical action of wind and rain, and it can be diluted by plant 



