482 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



The resulting estimate of tiie cow's total plutonium ingestion rate, 0.585 /jCi/day, is 

 comparable to the independent, site-specific estimates of Smith, Barth, and Patzer (1976) 

 (0.565 AtCi/day) and of Gilbert, Eberhardt, and Smith (1976) (0.620 AtCi/day). 

 Considering the probable variability of the measurements and parameters involved, the 

 agreement of results is remarkably good. 



The principal differences between our method and the method of Gilbert et al. are: 

 (1) our estimate of the vegetation ingestion rate, which is based on energy requirements (8 

 kg/day), is higher than their estimate, which is based on the rumen contents of a 

 tlstulated steer (6 kg/day); (2) our estimate of the average concentration of plutonium in 

 vegetation is lower than theirs, 55 pCi/g vs. 103 pCi/g; and (3) we included an input for 

 soil ingestion. 



If we assume that the digestibility of Area 13 vegetation is 49% instead of 36%, a 

 ration of 6 kg/day would be adequate to meet the cow's maintenance energy 

 requirements. The weighted mean vegetation/soil ratio for Area 13 is about 0.15, or 50% 

 higlier than average. If we use this ratio instead of 0.1 and 6 kg/day instead of 8 kg/day, 

 Iv = 0.495 juCi/day and Is = 0.565 - 0.495 = 0.07 jL/Ci/day, or 127 g soil/day. 



Inhalation Rate. Standard Man's respiration rate is 20 m^/day (International Commis- 

 sion on Radiological Protection, 1959), and his digestible energy requirement for 

 maintenance metabolism (no weight gain or loss) is 2600 kcal/day (National Research 

 Council, 1968). We assume that for man and cattle respiration rates are proportional to 

 digestible energy maintenance requirements. Tlie DE requirement (Eq. 19) for a 409-kg 

 cow is 13,185 kcal/day. The cow's estimated respiration rate is therefore 

 13,185x20/2600=101 m^/day. The cow's plutonium inhalation rate (I^) can be 

 estimated by 



20 DE 

 2600 



Ia=^Z7^XLaXCs (21) 



where la = plutonium inhalation rate (pCi/day) 



DE = digestible energy required (Eq. 19) for maintenance metabolism 

 (kcal/day) 

 Respiration rate = 20 m^/day divided by 2600 kcal/day = 7.69 X 10"^ m^/kcal for the 

 Standard Man 

 La = mass4oading factor (lOOjLig soil/m^ air) as recommended by Anspaugh 



(1974) 

 Cs = average plutonium concentration (pCi/g) in the soil of the area grazed 

 by the cow 

 For the 409-kg cow of the Area 13 inner enclosure, where C^ = 550 pCi/g, la = 5.56 

 pCi/day. 



Ingestion vs. Inhalation. It is obvious from the preceding discussion that ly > la, but the 

 accumulation of plutonium in organs or tissues other than the gastrointestinal tract or 

 lungs and the excretion of plutonium in milk depends (Fig. 2 and Eq. 17) on the rate (r^,) 

 at which plutonium reaches the blood. The rate at which ingested plutonium reaches the 

 blood is simply the plutonium ingestion rate (ly) multiplied by the fraction transferred 

 from the gastrointestinal tract to the blood. For the Area 13 cow discussed above, 



= 565,000 pCi/day x (3 x 10~^) 

 = 16.95 pCi/ day 



