input parameters, the simulation model provides estimates of I u which are as accurate as those obtained from long-term grazing studies relying on fistulated steers. ¢ INTRODUCTION Studies of large grazing animals in plutonium-contaminated areas of the Nevada Test Site have been somewhat restricted by the fact that most such areas are relatively small and wmable to support more than one or a few animals for any appreciable period of time. Area 13* was chosen for the EPA grazing study because it is larger (~ 402 hectares) than most plutonium-contaminated areas at NTS and because the vegetation available to grazing animals is better than average. One cow was kept in the inner compound of Area 13 for 177 days. On 14 separate occasions, four fistulated steers were pastured for three days in the same enclosure (see the article by D. D. Smith in this publication). Based on plutonium concentrations in the vegetation (semisolid) and fluid portions of the rumen contents of the fistulated steers, Smith et al. (1976) estimated that the cow's total plutonium intake during 177 days of grazing the inner compound was 100 uCi, which amounts to an average plutonium inges- tion rate of about 565 nCi/day. According to Gilbert et al. (1977), the average total weight of material removed from the rumens of the fistulated steers was about 30 kg and the average vegetation ingestion rate was about 6 kg/day (dry weight). Using this estimated ingestion rate, an assumed diet of 64 percent shadscale (Atriplex canescens) and 36 percent winterfat (Zurotia lanata), and the plutonium concentrations for shadscale and winterfat reported by Romney et al. (1975), Gilbert et aZ. (1977) estimated the cow's average plutonium ingestion rate as 620 nCi/day. The hypothetical diet was a composite in which the fraction contributed by each sampling stratum was proportional to the area of the stratum. Neither Smith et al. (1976) nor Gilbert et al. (1977) considered soil ingestion as a separate component of plutonium ingestion. Martin and Bloom (19/7), considering both vegetation and soil ingestion, provided a third estimate of 585 nCi/day based on general theoretical considerations of factors which apply to NTS as a whole but are not site-specific for the inner compound of Area 13. *Maps of Area 13, showing fences and sampling strata, appear elsewhere in this publication. See the article by Delfiner and Gilbert in this report. 484