486 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



four cows, two areas, and two grazing times are hardly an adequate basis for model 

 validation. The best we can conclude from these comparisons is that they provide no basis 

 for rejecting the model. The discrepancy between the experimental values and values 

 predicted by the model is less than an order of magnitude, and we have little reason to 

 expect better than an order of magnitude accuracy. 



The milk cow. The model milk cow is assumed to weigh 650 kg and to produce milk 

 at a rate of 25 kg/day. Such an animal would require a digestible energy intake of 64,750 

 kcal/day, i.e., 18,500 kcal/day for maintenance plus 25 kg/day x 1850 kcal/kg for milk 

 production (Siegmund, 1967). To meet this high energy requirement and, at the same 

 time, to provide a conservatively higli estimate of plutonium transport to man via milk, 

 we shall assume that the model milk cow consumes 10 kg of desert vegetation per day 

 and 15 kg of alfalfa hay grown in the same contaminated area per day. The remainder of 

 the diet consists of commercial concentrates containing no plutonium. For the model 

 milk cow, the plutonium ingestion rate is estimated as follows: 



Iv = Cs (250 g soil/day + 0.1 x 10,000 g vegetation/day + 0.017 x 15,000 g alfal fa/ day) 

 = 1505Cs(pCi/day) 



where C^ is the soil concentration (pCi/g) and 0.017 is the alfalfa/soil ratio, which is 

 assumed to be one-sixth the desert vegetation/soil ratio due to plowing and mixing of the 

 soil to a depth of 30 cm. The equation for estimating the concentration in milk is 



(1505X3 X lCr^)(0.007) 

 "^m ilk 



Cmilk - 25 X. - ^ ^^^^ 



= 1.37 X 10-^Cs(pCi/kg) 

 where Xmilk = In (2)/0.75 (Fig. 2). 



Man 



In the preceding discussion we have considered the dynamics of the plutonium transport 

 system (Fig. I) and have attempted to establish mathematical relationsliips between 

 compartments. Our present knowledge of the food-chain kinetics of plutonium in 

 contaminated areas at NTS is not adequate for modeling the dynamic aspects of all parts 

 of the transport system. To simplify estimation of the plutonium inhalation and ingestion 

 rates for herbivores (cattle), we assumed a steady-state system and constant intake rates. 

 We now apply the same simplifying assumptions to estimate potential plutonium 

 inhalation and ingestion rates for the hypothetical Standard Man. 



Inhalation Rate. The plutonium inhalation rate (Am ) is defined as the product of the 

 respiration rate (Bm) and the concentration of plutonium in air. The concentration of 

 plutonium in air is, of course, quite variable, but, since it is due to resuspension of 

 contaminated soil, it can be related to the average concentration in surface soil (Cg). 



