4 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



than the above species. ImmobiHzation of those chemical species may occur through 

 cation exchange reactions with particle surfaces or through redox reactions to 

 hydrolyzable forms that become insoluble. 



Transuranic elements entering the environment as stable organocomplexes, as might 

 occur in the vicinity of a spent-fuel reprocessing facility, may be highly soluble initially 

 (Wildung and Garland, 1975). The duration of solubility and mobility will be a function 

 of the stability of the complex to substitution by major competing ions, such as Ca^ "*" and 

 ff*" (Lahav and Hochberg, 1976; Stevenson and Ardakani, 1972; Norvell, 1972), tlie 

 competition of other Hgands forming more stable compounds, and the resistance of the 

 organic ligand to chemical and microbial decomposition (Wildung and Garland, 1975). 

 Disruption of the complex may lead to marked reduction in solubility through hydrolysis 

 and precipitation reactions, as described for acid solutions on dilution. A portion of the 

 ions released may react with other, perhaps more stable, ligands. The mobility of the 

 intact complexes, in turn, will be principally a function of their chemical and 

 microbiological stability and the charge on the complex, which will govern the degree of 

 sorption on particles. 



Initial chemical reactions and tendencies to remain soluble after release to the 

 environment apparently depend on the initial chemical form of transuranic elements. 

 However, the original source characteristics become less important as times goes on and 

 weathering and aging processes proceed. From a consideration of the known distribution 

 of transuranic elements in the environment and expected solubilities in the presence of 

 particle surfaces, it is clear that their behavior will be markedly influenced by their 

 individual chamistries and their chemical interactions in soils and sediments. 



The effect of source and the immediate environment on the distribution of trans- 

 uranic elements can be illustrated by comparison of the concentration of plutonium 

 resulting from global fallout with that from more localized sources in soils, sediments, 

 and waters (Table 2). For nuclear weapons testing, highest concentrations of plutonium 

 in soils occur at the test locations. However, after stratospheric dispersion, concentrations 

 are relatively low [<0.1 (d/min)/g] in surface soils, fresh water, and marine sediments. 

 The lower concentrations of plutonium in marine sediments relative to those in soils 

 reflect the longer residence times in the water column. Where nuclear processing faciUties 

 are known to provide a source of plutonium, soil and sediment concentrations range from 

 fallout levels to several thousand disintegrations per minute per gram in controlled areas. 

 Of major significance from the standpoint of environmental behavior is the fact that 

 concentrations of plutonium in soils and sediments generally exceed tliose in water and 

 other media by many orders of magnitude. 



Terrestrial Ecosystems 



One way of examining the distribution of any element within an ecosystem is through tlie 

 use of an inventory ratio (IR). Two types of data are needed to calculate IR's: the weight 

 (W) of each ecosystem compartment and the concentration (C) of the element within 

 each compartment. The IR differs from the concentration ratio (CR) in that it takes into 

 account the size of the compartments. For our discussion the compartments are soil, 

 vegetation, htter, and animals. 

 The IR is calculated as 



At 



