SYNTHESIS OF THE RESEARCH LITERATURE 15 



be the most available to biota. Plant uptake studies (Schreckhise and Cline, this volume) 

 and experimental feeding studies (Sullivan, 1979) indicate that this is likely. 



Aquatic Systems 



Freshwater Ecosystems. The behavior of plutonium and americium has been studied in a 

 wide range of freshwater systems (Table 5). Some contaminated areas have been small, 

 such as the U-pond on the Hanford Reservation in Washington (Emery and Klopfer, 

 1976), White Oak Lake in Tennessee (Dahlman, 1976), Rocky Flats ponds (Rees, 

 Cleveland, and Gottschall, 1978), and the ponds and canals at the Mound Laboratory in 

 Ohio (Bartelt et al., 1977). More extensive studies have been carried out in the Hudson 

 River in New York (Simpson, Trier, and Olsen, this volume) and in the Great Lakes 

 system (Wahlgren, Robbins, and Edgington, this volume; Bowen, 1976). The concentra- 

 tion of plutonium in these systems varied by more than four orders of magnitude 

 (Table 6). If concentrations are calculated assuming that the 239,240pjj j^ essentially all 

 ^^^Pu, concentrations in water vary between 3x 10" ^^Af in the Great Lakes and 

 3 X 10~'^M in contaminated systems like U-pond on the Hanford reservation. These 

 concentrations are low relative to concentrations of many other trace elements. For 

 example, the concentration of thorium in Lake Michigan is <4 x 10~'^yif (0.1 fCi/liter) 

 (Wahlgren et al., 1977a). 



Processes controlling the solubility of plutonium in natural waters clearly are more 

 complex than can be explained by a simple solubility product. For example, the 

 concentration measured in Lake Michigan is higher than that predicted for Pu(0H)4 and 

 lower than that for Pu02(0H)2 . These differences have been attributed to the formation 

 of hydroxyl complexes, such as Pu(OH)^ (Bondietti and Sweeton, 1977), carbonate 

 complexes, such as PUO2CO3 or Pu02(C03)2~ (Moskvin and Gel'man, 1958), or 

 PUO2CO3OH- (Rai and Serne, 1977). 



A recent investigation of the reduction of Pu(lV) and (VI) by natural organic 

 compounds showed that up to 15% of Pu(IV) was reduced to Pu(III) at pH 4.0 and up to 

 75% of Pu(VI) was reduced to Pu(IV) by fulvic acid at pH 8. However, the Pu(VI) was 

 more stable in the presence of carbonate (Bondietti, Reynolds, and Shanks, 1976). 



Wahlgren et al. (1977a) studied the behavior of plutonium in tlie water column in the 

 Great Lakes and other smaller freshwater lakes to determine whether differences in 

 chemical characteristics of lake water affect the chemical properties of plutonium 

 (Table 7). The concentration of plutonium in waters of these lakes varied almost 

 100-fold. The highest concentrations of plutonium were observed in the lakes (ELA 241 

 and ELA 661) with low pH, a lake with a very high concentration of sulfate (Little 

 Manitou), and tlie acidic southeastern United States lakes. 



Using techniques to separate Pu(III) + Pu(IV) from Pu(V) + Pu(VI), Nelson and 

 Lovett (1978) showed that in the Irish Sea plutonium was predominantly in the 

 Pu(V) + Pu(VI) states. A similar observation was made in Lake Michigan waters (Wahlgren 

 et al., 1977b). Because Pu(V) was thought to disproportionate at lower pH than does 

 Pu(VI) (Pourbaix, 1966), this fraction was referred to as Pu(VI).* In all other lake waters, 

 Pu(lII) + (IV) apparently predominated. 



*Very recent experiments at Argonne National Laboratory have shown that this assumption is not 

 correct (D. M. Nelson and K. A. Orlandini, Argonne National Laboratory, 1979, personal communica- 

 tion). Techniques have been developed after the method of Inoui and Tochiyama (1977) for 

 separation of Np(V) from Np(VI) to distinguish Pu(V) from Pu(VI) in water samples. Preliminary 

 results indicate that all the plutonium in the higher oxidation state is present as Pu(V). 



