For brevity, the in vitro solubility data for field-ingested plutonium-238, -239, and americium-241 are presented for only the duodenal phase in Table 3, since this phase is currently believed to be the most important in regard to plutonium absorption (excerpted from Barth, 1975 and Barth, in press). In the simulated duodenal fluid, plutonium-238 was consistently more soluble than plutonium-239. In the other simulated fluids (data not shown), this wag usually the case. The solubility of americium- 241 in these fluids was found to be similar to that of plutonium-239, In making comparisons of radionuclide concentrations in rumen contents between trials, only very large differences are meaningful since the animals may graze at will throughout the enclosures. Also, types of plants ingested and the resuspension of radionuclides may vary. However, in the avallable data, a trend concerning peak activity of plutonium in the rumen contents appears to be developing. Results to date (Table 3) indicate that the highest concentrations of plutonium in the rumen contents of cattle grazing at Area 13 have accurred during the late summer or fall, followed by reduced concentrations during the winter or early summer. An exception to this occured during the May 29, 1975, trial. Analyses of the vegetal composition of the rumen contents indicated that Eurotia lanata (winterfat}) made up a considerable portion of the vegetation ingested, and was frequently the predominant species. Observations were not made of the field conditions of Furotia lanata during these trials. However, visual observations were made of the field conditions of Furotta TABLE 3. M-241 IN SIMULATED SOLUBILITY OF FIELD INGESTED PLUTONIUM AND AMERICIU BOVINE DUODENAL FLUID - pH 4.5* Concentration In |Date = aspen Contents pli/kg rT eectSOS Nov. 6, 1973 236 8539 - 0.49 0.29 - lanata growing in Area 13 during the summer and fall of 1975 and the winter Feb, 19, 1974 158 3869 - 13.5 0,76 - of Furotta lanata May 17, 1974 14 24 - 95.1 44.5 - 244 443 - 96.6 90.0 - 1742 60994 - of 1975/1976 (data not included in this report). The reproductive phase characterized by the long-hatred fruiting involucres, appeared during October 1975, During January 1976, some fruiting involucres remained. The onset of the reproductive phase will vary with the climatic conditions. This suggests that the involucres could greatly enhance the entrapment of resuspended particles, resulting in higher concentrations of plutonium 238, -239, and americium-241 in the rumen contents. During the May 17, 1974, and the July 10, 1974, trials, a large increase in plutonium solubility, accompanied by a marked reduction in plutonium concentration of the rumen contents, was observed concurrently with a reduction in the intake of Eurotia Zanata and an increase in the intake of Oryzopate hymenoides (Indian rice grass} or Stitanion jubatimn (squirrel tail grass). Barth and Mullen (1974) reported average minimum and maximum solubilities of 1.5% and 8.4% for plutonium-238 dioxide in an artificial rumen and simulated abomasal and intestinal fluids procedure. However, during the May 17, 1974, and July 10, 1974, trials, the solubility of plutonium reached 90% and above. Also, during the January 29, 1975, trial, plutonium-238 -239, and americium-241 solubilities of over 62% were observed during the lower intestine digestion phase (data not shown). This suggests the possibility that during these specific trials most of the ingested plutonium was in a form other than plutonium dioxide and that most of this plutonium represented the low level internally incorporated in plant tissue. July 10,.1974**| Aug. 8, 1974 Nov. 5, 1974 729 28024 0,96 0.14 - §553 1.3 0.24 0.43 0.69 1.1 Jan. 19, 1975 62 2240 356 2.9 Jan. 21, 1975 144 6738 943 + 0.15 0.44 3? 1448 213 3.2 0.64 0.75 Mar. 12, 1975 416 11378 1629 t 0.88 1.4 May 29, 1975 589 20202 2814 0.13 0.10 Jan, 29, 1975** *Excerpted from Barth (1975) and Barth (in press). **Outer enclosure. +Below 238pu detection limit, 426 427 0.22