MIGRATION OF PLUTONIUM FROM FRESHWATER ECOSYSTEMS 631 



The biomasses of terrestrial life contacting the pond annually were estimated by using 

 mean weiglits for designated taxonomic groups and observations of frequency of contact 

 by these groups. Although it was possible to obtain reUable measurements of mean 

 weights for these organisms, the measurements of their contact frequencies were much 

 less precise. It would be desirable to have a more precise understanding of the intensity of 

 these export activities; however, it is possible to suggest a range within which these 

 vectors operate based on the ranges of plutonium concentrations found in these 

 organisms. 



Means and 95% confidence limits of plutonium concentrations in pond compartments 

 were determined on an arithmetic basis. Concentrations, inventories, percentages, and 

 exported quantities of plutonium are expressed in scientific notation rather than in a 

 decimal format to draw attention to the order of magnitude rather than to emphasize the 

 exact quantity as a primary matter of consideration. 



Results and Discussion 



The Inventor}' 



The scope of this study and the resources available for it placed Umitations on the 

 resolution of compartment-size determination. Although it was possible to determine 

 with accuracy the weiglits of sediments down to 10 cm and of emergent macrophytes, the 

 remaining compartment sizes were evaluated with a variety of estimation procedures. 

 Since much of these data are derived in this way, we feel that they represent only a 

 reasonable approximation of compartment sizes. Efforts to examine these data 

 statistically were not redeeming, and it was concluded that statistical confidence intervals 

 and central tendencies are not appropriate expressions for these results. Instead, these 

 results are intended to suggest best approximations without indicating ranges within 

 which the compartment sizes fluctuate. 



The biota in U-Pond contain about 1% of the total mass of the pond, including 

 sediments down to 10 cm (Table 1). Concentrations of plutonium isotopes in the pond's 

 ecosystem compartments are shown in Table 2. Nonfilamentous algae and sediments 

 show the higliest mean plutonium concentrations of 2.8 x 10' and 5.0 x 10^ pCi/g, 

 respectively. Tliis similarly reflects the close association between the two compartments. 

 Submerged macrophytes and gastropods also have mean plutonium concentrations 

 exceeding 1 x 10^ pCi/g (1.6 x 10^ and 2.4 x 10^ pCi Pu/g, respectively). Filamentous 

 algae and emergent insects show mean plutonium concentrations of 8.6 x 10' and 

 4.6 x 10' pCi/g, respectively, whereas the remaining compartments have mean plutonium 

 concentrations ranging from 1 to 2 x 10' pCi/g. 



U-Pond's eutrophic condition is reflected by its higli rate of primary production, 

 which occurs as high as 42kg C ha^' day"'. This rate of productivity can also be 

 expressed as 440 jug C liter" ' hr"' . Verduin (1964) found primary productivity'rates in 

 two Pennsylvania ponds to range from 120 to 760 /ig C liter"' hr"'. Hence U-Pond's 

 primary productivity resembles that in ponds not associated with nuclear facilities. Its 

 rate of carbon assimilation also approaches that of a higlily productive terrestrial 

 community, a cornfield, which has an average assimilation rate of 63 kg C ha"' day"' 

 (Robbins,Weier, and Stocking, 1957). 



Submerged plant Ufe has most of the total plutonium inventory in pond biomass. 

 more than 95% (Tables 3 and 4). Submerged flora are composed mainly of algae. 



