478 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 3 Mean Concentrations of ^ ^ ^ '^ ^ ^ Pu in Soil and 



Vegetation and Vegetation/Soil Ratios by Strata in Area 



GMX-5 and Area 13 



*Based on data from Romney et al. (1975). 

 tyv = 0.13 q-''' for Area GMX-5. Yv = 0.07 C°-* 

 equations are based on the strata means in columns 2 and 3 



for Area 13. Both 



where Xv = Xw + Xu + Xa is the effective decay rate coefficient for plutonium-bearing soil 

 particles externally deposited on vegetation. 



As noted in the section on air, the deposition velocity is a function of particle size 

 (Eq. 4). In the soils of Area 13 (Tamura, 1976), most of the plutonium is associated with 

 coarse silt (20 to 53 /im), and the estimated deposition velocity (Vj) for particles of 

 50-jum diameter could be as higli as 20 cm/sec, or 1.73 x 10^ cm/day. 



The plant interception factor (Fy) determined by Miller and Lee (1966) for freshly 

 deposited volcanic dust (50 to lOOjum) was 47.4 cm^/g for dry exposure conditions. 



For predictive purposes Anspaugli (1974) has suggested that a mass-loading factor 

 (La) of 100 iJg/m^ (10"'° g/cm^) be used. This is the amount of dust we would expect 

 to find in the GMX area (Shinn and Anspaugli, 1975) when the wind velocity averages 

 about 1 .4 m/sec (3 mph). 



Substituting these values in Eq. 16, assuming a vegetation/soil ratio of 0.1, and solving 

 for Xv indicates an effective half-life of about 8.5 days. So 



yve_(1.73 X 10^ cm/day) (47.4 cmVg)(10''" g/cm^) 



In (2)/8.5 



0.10 



This exercise proves nothing. It merely demonstrates that Eq. 16 might explain the 

 higli vegetation/soil ratios observed at NTS. Shinn and Anspaugh (1975) have 

 demonstrated that mass loading (L^) increases with wind velocity. The effective half-life 

 may decrease with wind velocity. Sehmel (1975) has shown that deposition velocity (V^) 

 decreases as the particle size decreases for dp > 1 lum. If small particles are more readily 



