620 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



There is a lack of any definitive infomiation on the other three elements of interest, 

 americium, neptunium, and curium. One study of neptunium in the Columbia River 

 (Davis et al.. 1958) involved an isotope with a half-life of only 2.3 days (vs. 2x10^ days 

 for ^•'^Np); so the radioactive decay limited the quantities of this isotope in the 

 organism. Data from Lake Michigan indicate increased CR's for ^'^^Am over plutonium in 

 the lower trophic levels by factors of 1.5 to 5 (Wahlgren et al., 1976). However, 

 americium CR values for fish are less than or equal to 10 times those for plutonium: the 

 data were not adequate to calculate specific values. Data from the discharge of waste 

 from Windscale (Hetherington et al., 1976) indicate little difference between ^^^^Puand 

 ^"^^ Am concentrations in tlsh within 10 km of the discharge for equal discharge rates of 

 the two isotopes. 



Concentration ratios chosen for use in both freshwater and marine environments are 

 given in Table 7. For americium, neptunium, and curium (■^^' Am, ^■'^Np, ■^'*^Cm, and 

 ^"^^Cm), we have applied a factor of 10 over the plutonium values. These values were 



TABLE 7 Recommended Concentration Ratios 



chosen because these isotopes have a greater availability than plutonium in terrestrial 

 systems (Dahlman, Bondietti, and Eyman, 1976), and it is expected that, when adequate 

 data are available, the same may be true in aquatic systems. Limited data from Lake 

 Michigan and Windscale indicate that the factor foi ^^ ' Am may be less than 10; however, 

 owing to the incomplete nature of the data, we have chosen a more conservative figure as 

 the factor for biota. 



The very high affinity of plutonium for particulate matter in aquatic ecosystems 

 (distribution coefficient, ~10^) suggests tliat it may not be appropriate to use the 

 traditional expression of CR to estimate die concentration of this element in biota. 

 Rather, we feel that the observed concentrations of plutonium in aquatic biota should be 

 related to the primary abiotic source in the system, sediment (both suspended and 

 bottom). To express this relafionship, the temi Trophic Transfer Factor (TTF) has been 

 used by various researchers (Lipke, 1971; Trabalka and Eyman, 1976; Elwood, 

 Hildebrand, and Beauchamp, 1976). The concentration of an element in sediment or food 

 is substituted for the concentration in water, which is normally used in the calculation of 

 a CR. The undedying assumption is that, owing to the higli distribution coefficients 

 (Kd's) observed, element accumulation in tissues of higlier trophic levels will be 

 dominated by gut absorption rather than by direct uptake from water. Trophic transfer 

 of plutonium by aquatic animals is comprised of diree fractions: exterior surface 



