1g not strongly dependent on atmospheric stability. Both sets of results indicate a strong dependence on surface roughness. In order to apply either model to field conditions, it is necessary to estimate or measure the surface roughness and velocity profile. For most applications to NTS, the grass data of Sehmel et al. (1973) appear to be the best analog. The trend of these data, in the range of 2 to 13 m/sec air velocity and 1 to 100 um particle diameter, is approximately Resuspension Models There have been many attempts to develop mathematical models to simulate resuspension (Amato, 1971; Mills and Olson, 1973; Killough and McKay, 1976). Most of these are based on models of wind erosion developed by Bagnold (1960) and, as a function of wind speed, take the form C.K W- UL) 3 Coo/U (5) where vg/U = 3x10& where @ US is a threshold wind speed, Vg is the deposition velocity (cm/sec), U is the wind velocity (cm/sec), d, is the particle diameter, (um) - (m/sec) K is a constant (sec/m). Others (Sehmel and Orgill, 1973, and Shinn and Anspaugh, 1975) have used a power-law expression of the form Co For 10 wm particles, Equation (4) yields values similar to Healy's results for neutral atmospheric stability. Resuspension Factor The resuspension of plutonium from soil is often expressed as the ratio of air Many such concentration (vCi/m3) to surface soil concentration (uCi/m2). measurements have been made at NTS (Mork, 1970; Anspaugh and Phelps, 1974) and The measured magniin the vicinity of Rocky Flats, Colorado (Volchok, 1971). tudes of this ratio range generally from 1075 to 10711 m-!, To estimate -1 "acceptable soil concentrations,” Anspaugh (1974) used a value of 10-9 m for NTS. These ratios are, to say the least, extremely variable with respect to time and environmental factors such as wind speed and direction, rainfall, Other and disturbances affecting aerodynamic properties of soil surfaces. factors affecting this ratio are the aerodynamic properties of plutonium-bearing There is particles and their susceptibility to saltation and resuspension. evidence that the ratio tends to decrease with time after fallout-contamination of soil (Anspaugh, Phelps, Kennedy, and Booth, 1973; Anspaugh, 1974; Kathren, Anspaugh et al. (1975) have proposed a model in which the air/soil 1968). ratio decreases as a function of time from a maximum of 10°" to a minimum of 1079 m7}, i.e., _ ane 4 -9 ci/c.. = 10 exp(-A\t) + 10 eKU 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 Equation (6), Shinn and Anspaugh 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 Anspaugh's results for a plowed field ia Texas which yielded n = 6.4. The empirical value of about 2 for n when derived for different tracers, different soils, and different climates (provided the soil is undisturbed) tends to provide indirect confirmation for the theoretically derived form of Equation (5). However, both K and U,, in Equation (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 U, must be treated as empirical constants for the present. Consequently, There is no practical benefit in using Equation (5) in preference for the simpler Equation (6). However, one must still have at least one experimental measurement of resuspension and wind speed in order to set the value of K in Equation (6) for the particular area. Loading where and Cc C is the air concentration (vci/m3), is the soil surface concentration (uCi/m?), aS 1n(2)/0.15. al oe In the absence of data to implemegt Equation (6) for a given area, Anspaugh (1974) suggests that a mass loading factor (L_) of 100 vg(soil)/m?(air) be used for predictive purposes. If one assumes that the radioactivity of one This model is consistent with data collected over the years at NTS. 631 630 (6)