Much research on Pu behavior in terrestrial environments has been conducted in arid regions contaminated by plutonium from weapons testing. Comparatively little research has been done on the behavior of Pu in environments of the southeastern United States that are characterized by higher precipitation, acid soils, dense vegetation cover on mineral soil, a myriad of ecological niches, and complex trophic and food-chain relationships, all of which may influence the cycling of the trace element, Pu. Limited information is available on the effect of biota and organic matter on the biogeochemical behavior of Pu in ecosystems and the importance of biotransfer (i.e., root assimilation, absorption from GI tract). These biotransfers can ultimately impact on human exposure to Pu via the ingestion pathway. The purpose of this paper is to present comparative information on the contamination of vegetation either on surfaces or by assimilation into tissues, in two different environments of southeastern United States. At Savannah River, South Carolina, vegetation was exposed to both atmospheric and soil sources of Pu while at Oak Ridge, Tennessee, the source of the Pu was soil that had been contaminated 30 years ago during Manhattan Project Operations. PRINCIPAL MODES OF INCORPORATION OF PLUTONIUM BY VEGETATION Atmospheric emission of Pu from nuclear facilities is expected to be a future mode of Pu input to terrestrial environments; although contamination from liquid releases cannot be excluded because liquid effluents have historically deposited Pu in terrestrial environments. For Pu originating from either fallout or nuclear facilities, the main pathways for incorporation into vegetation are illustrated in Fig. 1. The aerial pathway involves direct deposition of particleborne Pu on vegetation or deposition on the sofl surface. Direct deposition leads to levels of contamination that are incipiently high but of short duration. Rapid decontamination occurs with weathering half-times of 15 te 30 days for deposits of fallout particles on foliar surfaces (Dahlman et ai., 1975). Similar rates of removal are projected for particleborne Pu. , Contamination of foliar surfaces also resuits from suspendible particles being transported from the soil surface to aerial portions of vegetation. Suspendibles may be transported to the entire plant canopy as windblown dust or to basal plant parts as mud splashed by rain. Following deposition of atmospheric or liquid forms of Pu in the soil matrix, a fraction becomes available for assimilation by plants via the root pathway. Uptake of Pu by roots and translocation throughout the plant places the element in intimate association with food substances consumed by man over long-term intervals. As a result of Pu uptake by roots and direct assimilation into tissues, the biochemical associations may alter its chemical properties. Just as biogeochemistry affects the uptake of Pu by plants, interactions between Pu and plant biochemicals may influence the long-term metabolism of Pu by humans. Consequently, it is advisable to distinguish between Pu contamination on foliar surfaces, a physical phenomenon, and Pu assimilated via the root pathway, a process involving biophysical and biochemical mechanisms. 304 ste 4 a cages~ iA ADD oe BOMB DEBRIS VEGETATION AEROSOL NUCLEAR FACILITY SUSPENDIBLE LIQUID EFFLUENT PATHWAYS OF PLUTONIUM MOVEMENT TO VEGETATION FIGURE I. 305