MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 471 



The equation for vegetation concentration (nanocuries per gram) as a function of soil 

 concentration (nanocuries per gram) is 



yv = 0.0620 C,«-'^ (11) 



The corresponding equation tor the vegetation/soil ratio is 



Crv =^= 0.0620 rr°-''' (12) 



Thus, for areas in which soil concentrations are 10, 1.0, 0.1, and 0.01 nCi/g, the predicted 

 vegetation/soil ratios would be 0.036, 0.062, 0.107, and 0.187, respectively. 



Equations 1 1 and 12 demonstrate the dependence of vegetation concentration on soil 

 concentration and the fact that the vegetation/soil ratio tends to increase as soil 

 concentration decreases. The use of either equation for predictive purposes may be 

 limited by the extreme range of soil and vegetation values and/or by various site specific 

 factors that are not considered in the regression analysis. It miglit be better to apply the 

 regression analysis to sampling strata means as shown in Table 3. 



Except for strata 3 and 4 in Area 13, the measured and predicted values given in 

 Table 3 seem to agree quite well, but the equations for Area 13 and GMX-5 (footnote to 

 Table 3) predict higher values (especially at liigher soil concentrations) than would be 

 obtained from Eqs. 1 1 and 12, which are based on samples from all study areas. 



Discussion. Equations 11 and 12 shed some Ught on how vegetation/soil ratios may be 

 expected to vary with respect to soil concentration, but they do not explain why. To 

 approach this and related questions, we refer back to Eqs. 7 and 8, the proposed 

 differential equations tor the external and internal components of plant contamination. 

 For the time being at least, we can dismiss Eq. 8 from further consideration because the 

 greenhouse studies have shown that root uptake cannot account for more than a small 

 fraction of the vegetation/soil ratios observed at NTS. 



Equation 7, for external contamination, has the following solution for a constant air 

 concentration (Ca): . 



yve = ^ h'^' (1 -exp HXw + h + XA)t] } (13) 



Aw + Au + Aa 



where the parameters are defined following Eq. 7, and 



kav = VdFv (14) 



Ca = L,Cs (15) 



where Vj is the deposition velocity on soil (cm/day), Fy is a vegetation interception 

 factor (cm'^/g vegetation), and L^ is a mass-loading factor (g soil/cm^ air). 



If we assume a steady state (large t) between vegetation and soil, the vegetation/soil 

 ratio can be expressed in the parameters of Eqs. 13, 14, and 15 as follows: 



Yve VjFvLa 



