Studies of biota for the WIPP EM have focused on native vegetation because the vegetation is consumed by beef cattle, and consumption of beef from cattle pastured in the vicinity of the WIPP could serve as an exposure pathway to humans for contaminants released from the WIPP. While it would seem more desirable to monitor the presence of contaminants in the beef directly, the livestock industry in the region routinely uses supplemental feeding for range cattle during some portion of most years. This means that the cattle are potentially exposed to contaminants from distant sources via the supplemental feeding, thereby confounding the interpretation of any direct analyses of range cattle tissues. Secondarily, if the occurrence of radioactive contaminants in major food plants is known, the potential uptake of radioactive contaminants by range cattle can be effectively predicted from existing data on livestock diets, consumption rates, and absorption ratios.
During baseline studies, vegetation samples were collected from a total of six species of plants that serve as preferred forage species for cattle during at least some portion of the year. These include fall witchgrass (Leptoloma cognatum), sand paspalum (Paspalum setaceum), spike dropseed (Sporobolus contractus), mesa dropseed (S. flexuosus), honey mesquite (Prosopus glandulosa), and shinnery oak (Quercus havardii). The sampling included collection of discrete samples of each species, because relevant published work indicates that plant species can vary significantly in both uptake of radionuclides into plant tissues and in adherence of radiation-contaminated soil particles to external plant surfaces.
Vegetation is sampled either once or twice annually, depending on rainfall conditions, during the two major periods of new growth for native vegetation (March-May and August-October). Not all of the same species can be sampled at each period because some of the species only produce major new growth during one season, and because the abundance of the species varies among years depending on weather patterns. Six samples of each of three species (contingent on availability) are collected during each sampling period from selected sites on the sampling grid surrounding the WIPP (which encompasses the Near Field aerosol sampling station). The vegetation samples are analyzed for selected radionuclides only. Collecting samples at the same locations used for soils allows for a direct comparison between radionuclide levels measured in soils and vegetation. Vegetation sampling has currently been suspended until analytical results are obtained from baseline studies.
Additional studies are in progress to evaluate the effectiveness of expanding the biota sampling for radionuclides to include arthropods. In many desert ecosystems, total arthropod biomass has been shown to equal or exceed total biomass for any other consumer component, and arthropod communities encompass primary, secondary and tertiary consumers, as well as detritivores and necrovores. As such, arthropods could be used to study the movement of radionuclides through natural ecosystem food chains. Arthropods were collected during 1998 and 1999, and collection may be repeated in future years, pending analytical results.