MODEL FOR ESTIMATING Pu TRANSPORT AND DOSE 481 



about 1.6 kcal/g (dry weiglit). On the basis of these considerations, the vegetation 

 ingestion rate for a cow grazing desert vegetation can be estimated as follows: 



^ 163.5 W"-^^ kcal/day . 



'"^ 0.36 X 4.5 kcal/g ^^ 



where V,n^ is the vegetation ingestion rate (g/day) for maintenance of a mature cow 

 grazing desert vegetation. For cows that are gaining weight, producing milk, or pregnant, 

 the energy requirement, and thus the vegetation ingestion rate, would be higlier than 

 estimated by Eq. 20. 



Grazing cattle also ingest soil. In earlier papers (Martin, Bloom, and Yorde, 1974; 

 Martin and Bloom, 1976) we assumed that the soil ingestion rate might be as high as 2000 

 g/day. Data recently reported by Smith (1977) indicate that this value is probably too 

 high. The amounts of sediment (soil) recovered from the reticulum and rumen of three 

 cows that had been grazing in Area 13 before sacrifice were 8.5, 57.3, and 278 g, 

 respectively. As Smith points out, "These data suggest that the total amount of soil 

 ingested is much less than 2 kg per day, and that a reasonable estimate would be between 

 0.25 and 0.5 kg." 



Smith, Barth, and Patzer ( 1976) estimate that a 409-kg cow that grazed for 177 days 

 in the inner compound of Area 13 ingested a total of 100 /jCi of ^^^'^"^^Pu, or 0.565 

 pCi/day. This estimate was based on plutonium concentrations in the rumen contents of 

 fistulated steers allowed to graze in the same compound. Gilbert, Eberhardt, and Smith 

 (1976) made an independent estimate of 0.620 /iCi/day based on plutonium concentra- 

 tions in Eurotia lanata {347o of the cow's diet) and Atriplex canescens {(A% of the cow's 

 diet), as reported for Area 13 by Romney et al. (1975). The average wet weight of 

 vegetation ingested by the fistulated steers was 30 kg/day, and the average dry/wet ratio 

 was about 0.2 (R. 0. Gilbert, personal communication). In other words, the cow's 

 vegetation ingestion rate was estimated to be about 6 kg/day. On the basis of Eq. 20, a 

 409-kg cow would have to ingest about 8 kg/day to meet its energy requirements for 

 maintenance. Neither Smith, Barth, and Patzer (1976) nor Gilbert, Eberhardt. and Smith 

 (1976) include soil ingestion in their estimates of the plutonium ingestion rate for the 

 Area 13 cow. 



On the basis of the methods outlined above, we would estimate this cow's plutonium 

 ingestion rate as follows: 



Iv = 8139 g vegetation/day x 0.1 x 5.5 x lO""* /jCi/g soil 

 = 0.448 juCi/day 



Is = 250 g soil/day x 5.5 x 1 0""^ A/Ci/g soil 

 = 0.138juCi/day 



Iv + Is = 0.585 AJCi/day 



where ly and 1^ are the plutonium ingestion rates through vegetation and soil, 

 respectively. In this calculation, 8139 g/day is the vegetation ingestion rate, which is 

 based on Eq. 20 for a 409-kg cow: 0.1 is the assumed average vegetation/soil ratio; 

 5.5 X 1 "* A^Ci/g is the average soil concentration o^ the Area 13 inner compound 

 (Gilbert, Eberhardt, and Smith, 1976); and 250 g/day is the assumed soil ingestion rate. 



