MODEL FOR ESTIMATING Pii TRANSPORT AND DOSE 479 



retained by vegetation than larger particles, the plant interception factor and the effective 

 half-life may increase as the particle size decreases. Variations of the factors of Eq. 16 as 

 functions of particle size and wind velocity may account for the range of vegetation/soil 

 ratios implied by Eq. 11 , but there are still too many unknowns to develop a descriptive, 

 dynamic model for the vegetation compartment. 



For predictive purposes we shall assume that the soil and vegetation compartments 

 are in steady state and that the mean vegetation/soil ratio is 0.1. This ratio is somewhat 

 conservative since it tends to overestimate the plutonium content of vegetation in areas 

 where soil concentrations are greater than 100 pCi/g. 



The steady-state assumption is justified to some extent by the fact that movement of 

 contaminated particles from soil to foliage and back to soil is a more or less continuous 

 process. Since the turnover time is apparently short (between 1 and 2 days), a steady 

 state should be established quickly and characterized by a constant vegetation/soil 

 concentration ratio. The choice of 0.1 is in the range of statistical mean (0.096 ± 0.0004) 

 obtained from actual soil and vegetation samples. 



Cattle 



Transport Pathways. For present purposes the only herbivore assumed to contribute to 

 man's plutonium intake is the cow. Both dairy cattle and beef cattle are considered. The 

 principal plutonium inputs to these herbivores (Fig. 1) are by inhalation and by ingestion 

 of contaminated soil and vegetation. Figure 2 illustrates the assumed pathways of 

 plutonium transport to man via beef cattle and dairy cattle and provides estimates of 

 some of the parameters required to estimate potential concentrations of plutonium in the 

 muscle, liver, and milk of beef and dairy cattle maintained in a contaminated area at NTS. 



Formulation of Cow Model. A general equation for the concentration of plutonium in 

 the muscle, liver, or milk compartment of Fig. 2 can be derived as follows: 



f = ;^ (rb fbi - Ai y.) (17) 



m 



\ 



where i = muscle, liver, and milk 



yj = concentration of plutonium in compartment i at time t (pCi/g) 

 mj = weight of compartment i (g) 

 r^ = rate at which plutonium enters blood (pCi/day) 

 fbi = fraction (dimensionless) transferred from blood to compartment i (Fig. 2: 0.07, 



0.12, and 0.007) 

 Xj = effective elimination rate coefficient (day^' ) for plutonium in compartment i 

 (based on effective half-lives, T, in Fig. 2) 



Estimated values for some of the parameters of Eqs. 17 and 18 are given, as indicated 

 above, in Fig. 2. The transfer fractions from the gastrointestinal tract to blood and from 

 blood to muscle, liver, and milk, the weight of muscle and liver, and the effective half-life 

 in milk are based on experimental results reported by Stanley, Bretthauer, and Sutton 

 (1974). The other values were assumed (Martin and Bloom, 1976) for purposes of 

 estimation. Equation 18 ignores the retention of plutonium in the lungs of cattle and 



