466 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Air 



As indicated in Fig. 1 , plutonium contained in surface soils can be resuspended and 

 transported to vegetation via external deposition or to herbivores and man via inhalation, 

 and some of it can be carried by wind and redeposited beyond the arbitrary boundary. In 

 the absence of data to the contrary, we have assumed that deposition and resuspension 

 processes in contaminated areas at NTS are in approximate steady state, althougti data 

 presented by Anspaugli and Phelps (1974, pp. 292—294) suggest that resuspension may 

 exceed deposition, at least to a small degree. Several methods have been suggested for 

 analyzing and modeling deposition and resuspension processes. These are discussed in the 

 following paragraphs. Of these, the mass-loading approach requires the least information 

 for implementation and was used in the present model owing to the absence of data to 

 implement the other methods at NTS. 



Deposition Velocity. The rate at which resuspended plutonium is deposited on soil 

 could be estimated as the product of a deposition velocity (centimeters per day) and 

 concentration in air (microcuries per cubic centimeter) to yield a rate that has dimensions 

 of juCi cm"^ day" ^ . Deposition velocities are functions of meteorological factors and the 

 aerodynamic properties of plutonium-bearing soil particles and soil surfaces. 



Deposition velocities measured under field conditions have been reported by Van der 

 Hoven (1968), Sehmel. Sutter, and Dana (1973), and Healy (1974). Measurements under 

 controlled conditions in a wind tunnel have been reported by Sehmel, Sutter, and Dana 

 (1973) and Sehmel (1973: 1975). These data indicate that the deposition velocity 

 increases with increasing air velocity, increases with increasing particle size for sizes 

 greater than about 1 jum, increases with decreasing particle size for sizes less than 0.01 

 /im, exhibits a minimum somewhere in tlie range of 0.01 to 1 jum, and is strongly 

 influenced by tiie type of surface roughness. The wind-tunnel data of Sehmel et al. 

 (1973) for grass surfaces indicate that the deposition velocity is approximately 

 proportioned to both air velocity and particle size in the range of 2 to 12 m/sec and 1 to 

 100 iJim. Tliese grass data appear to correspond closely to field conditions provided that a 

 proper value is assigned to surface roughness. 



Tamura (1976) has reported that more than 65% of the plutonium in soil samples 

 from Area 13 is associated with soil particles in the range of 20 to 53 /jm. Using the grass 

 data of Sehmel, Sutter, and Dana (1973) at 2.2 m/sec, the corresponding range of 

 deposition velocities is from 3 to 20 cm/sec. Particles on the order of 20 to 50/^m could 

 play an important role with respect to external contamination of vegetation, but particles 

 tills large are of little concern with respect to inhalation. Since respirable particles are 

 generally <10 [dm. the corresponding deposition velocities suggested by the grass data 

 would be <1 cm/sec. 



Deposition Models. Both Healy (1974) and Sehmel (1975) present results of models 

 used to predict deposition velocities. Healy's results indicate that deposition velocity is 

 proportional to air velocity and is strongly dependent on atmospheric stability. Sehmel's 

 results indicate that deposition velocity increases as a nonlinear function of air velocity, 

 exhibits a minimum value as a function of particle size, and is not strongly dependent on 

 atmospheric stability. Both sets of results indicate a strong dependence on surface 

 rougliness. To apply either model to field conditions, we must estimate or measure the 

 surface roughness and velocity profile, both of which are variable. 



