318 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



floodplain. Inventory ratios for grass, forb, and tree components of the canyon ecosystem 

 were about 10~^ . The higher IR's of the canyon biota reflect both higher concentrations 

 of plutonium and greater standing crops of biomass. 



Discussion 



There are Hmitations in the use of CR and IR data to describe the distribution of 

 plutonium among components of the ecosystem. There is the difficulty of distinguishing 

 between contributions resulting from several simultaneous transfers to a receptor. 

 Sometimes the relationships between sources and receptors are not clear. Another 

 limitation is that IR's and CR's are static indexes of plutonium distribution in the 

 environment. Both indexes are based on an assumption of equilibrium conditions for the 

 ecosystem; i.e., that biomass and concentrations are constant. The equilibrium assump- 

 tion is not strictly met in terrestrial communities that are dynamic. In spite of these 

 limitations, both CR and IR values can give order-of-magnitude estimates on the 

 availabiUty of plutonium in the environment. 



More than 30 yr after deposition in floodplain sediments, plutonium in the Oak Ridge 

 soil appears to be a monomeric species associated with endemic organic and mineral 

 constituents (Bondietti, Reynolds, and Shanks, 1976). There have been no recent 

 amendments of plutonium to the floodplain ecosystem. In contrast, there are annual 

 additions of plutonium in the waste effluents released to Mortandad Canyon. Tliese 

 effluents contain diverse industrial wastes, including chelating agents; consequently 

 plutonium in the canyon environment can be complexed by chelators. Enlianced mobiUty 

 of plutonium would be expected in the canyon ecosystem if a chelated form of 

 plutonium were a stable and dominant species in the environment. This factor could 

 increase the biochemical assimilation of plutonium by plants and animals; thus, along 

 with surface contamination, it may be partly responsible for the higlier CR's and IR's 

 observed for the canyon ecosystem. 



Plutonium uptake by the root pathway yields a plant : soil concentration ratio of 

 about 10"^ for floodplain species (Table 3). Tliis ratio is about one to two orders of 

 magnitude greater than CR's determined from short-term experiments when plants are 

 grown in soil contaminated with plutonium solutions (Daltlman, Bondietti, and Eyman, 

 1976). Root uptake is the main mechanism of plutonium incorporation by plants in the 

 floodplain ecosystem because the negligible contribution of plutonium to the Oak Ridge 

 atmosphere from resuspension and industrial release would create minimal contamination 

 of vegetative surfaces. As mentioned previously, external contamination of vegetation is 

 considered an important mechanism of uptake at the canyon. 



The fraction of physiologically available plutonium is largely determined by 

 environmental chemistry and reactions of plutonium with soil. The great affinity of 

 plutonium for soil particles results in distribution coefficients of the order of 10^ to 10^ . 

 Sorption of plutonium to colloids is a surface reaction that occurs predominantly with 

 the clay constituents of soil because this component possesses the greatest specific surface 

 area. Enrichment of plutonium in the clay fraction has been observed in several 

 contaminated environments that contain appreciable clay. The differences in percentage 

 clay in canyon and floodplain soil (2% vs. 24%, respectively) may be responsible for 

 diminished sorption of plutonium to canyon soil, and this could account for the increased 

 incorporation of plutonium into biotic components of the canyon ecosystem. Althougli 

 plutonium enrichment in soil clay apparently occurs at both sites, and, althougli 



