grazing period in the inner compound area.” These estimates agree fairly well with the 620 + 130 nCi/day and 1.1 x 105 + 2.4 x 10% nCi over 176 days that we estimated would be consumed by a cow grazing the inner fence area. This latter estimate is based on average 239py concentrations for shrubs Eurotta Lanata and Atriplex canescens as reported in Romney et al. (1975, Table 1) for Area 13, assuming the cow's diet consisted entirely of these two shrubs; 36% of the former, 64% of the latter. These proportions are approximately those reported by Smith (1975) as obtained from vegetation analysis of the rumen contents of fistulated steers for grazing date September 5, 1973.6 The latter estimate (620 + 130 nCi/day) does not consider the 2339py that is ingested with soil. Presumably, the estimate of 560 nCi/day obtained by Smith et al. (1976a) is influenced to some extent by plutonium ingested with soil since the estimate is based on concentrations of vegetation and liquid taken from the rumen. Smith's estimate is based on Pu concentrations found in rumen contents ingested over a 24-hour grazing period for months July through November, 1973 (cows 2, 3, and 9 were grazed in May through October, 1973). These concentrations remained relatively stable until August, 1974, when they increased by an order of magnitude (Smith et al. (1976a), Figure 3). Smith noted the increase might be due to a change in vegetation composition in the diet or to the ingestion of greater amounts of soil. Another possibility is that the cattle concentrated their grazing in the GZ area on that 24—hour period. Whatever the cause, this variability illustrates that the ingestion estimates above may not be generally applicable for all field situations. The estimated ingestion rates above may be compared with the estimate of 500 nCi/day obtained using the model of Martin and Bloom (1976, eq. 7.1) (see below) if the amount of Pu ingested with soil is ignored, and using the weighted average soil concentration within the inner compound (0.55 nCi/g dry). An estimate of 770 nCi/day is obtained using their eq. 7.1 if the Pu in ingested soil is included (assumed to be 500 g/day; see footnote 7). None of these estimates for ingested Pu consider the rate at which Pu is inhaled. The only estimate of this rate for beef cattle available to us comes from Martin and Bloom (1976). Their model specifies this rate to be 0.0110 C (pCi/day), where C_ is the Pu concentration (pCi/g) in soil. Using C_ = 550 pCi/g as we did above for estimating the ingestion rate, this equation gives °Using the same data (supplied by D. D. Smith) but substituting the arithmetic mean for the geometric mean, we obtained 700 + 130 nCi/day (mean + standard error) or 1.2 x 10° + 2.3 x 10° nCi over the 176 day period. SThere is considerable variability, however, in the botanical composition in the diet of these fistulated steers over time. Grasses and forbs were often the major items in the diet rather than shrubs. The average intake per day was based only on Eurotia lanata and Atriplex canescens since samples of forbs and grasses were not collected for plutonium analyses. 7Eq. (7.1) assumes cattle ingest 2000 g of soil/day. Smith (1976) found that less than 500 g/day was more accurate and is the figure used in calculations here. 248