reported mean soluble nitrate content ppm; whereas, a maximum of 15 ppm was is also well established that burrows content than typical desert soils and and more favorable for living systems and Yousef, 1972). in regions of two burrows at 221 to 570 found in surrounding desert soils. It are much higher in oxygen and water temperatures in burrows are more stable than those found in open desert (Bradley The animal burrow, then, is a different environment than that found in the Soluble nutrients, gases, humidity, and temperatures are surrounding desert. conducive for maintenance and growth of organisms. more are which at levels Burrowing activity may significantly influence distribution and resuspension of soil components, including probably Pu and Am. It is important to realize that time spent in burrows by small vertebrates and many invertebrates is significant and, in many cases, is greater than that Nonhibernating rodents spend approximately 85% spent in aboveground activity. of their active lifetime in burrows. For hibernating species (P. longimembris, for example), burrow occupancy increases to approximately 90% of an average year (Bradley, unpublished data). We estimate that lizards may utilize hiber- nals for approximately 30-40% of an average year and inhabit nocturnal retreats throughout their activity season. Food stored in animal burrows provides an ideal substrate for the growth of fungi and bacteria and also a convenient food source for many invertebrates. Au and Beckert (1975) have reported that Aspergillus niger, an ubiquitous soil fungus, can take up several chemical forms of Pu, including plutonium dioxide, from a cultural medium and have reported on microbial numbers and relative abundance of fungi in surface and subsurface soils of NTS. From analyses of stomach samples, Thomas (1975) reported that darkling beetles (Tenebrionidae) consume large quantities of fungi, particularly Aspergillus. In a preliminary effort to estimate tissue burdens in some possible food sources of small vertebrates, darkling beetles were collected from GZ of Area 11-C, washed with alcohol to attempt to remove surface contamination, and gamma scanned by REECo personnel. Plutonium tissue burdens were estimated at 0.5 nCi/g ash. Burrow microenvironments may differ radiochemically from the soil surface and may provide significantly different sources of contamination by inhalation and ingestion. Therefore, Pu and Am concentrations in burrows and hibernals may show higher correlations with tissue concentrations than activity strata developed from surface soil samples. Laboratory error may account for some of the variability in samples, particularly the homogenate problem already dis- cussed. It is obvious, however, that before more conclusive statements can be made, more data on radiochemical and biological inventories of microhabitats, such as burrows, and food sources, of some vertebrates have to be available for evaluation. These data may be critical in order to properly evaluate Pu and Am uptake by inhalation and ingestion. Radioisotopes in these sources of contamination may differ in relative concentrations, physical state, and solubility from those of soil samples analyzed to date. Resident Rodent Populations of Area 11-C One particularly difficult problem in evaluating tissue burdens of small vertebrates occupying different trophic levels and inhabiting defined activity 204