640 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



concentrations in samples taken at resuspension sites in the U-Pond region have been 

 significantly higlier tlian fallout levels in other areas (but still less than maximum 

 permissible concentrations). This region has other sources of airborne plutonium besides 

 U-Pond, and it was not possible to determine if the plutonium detected in these samples 

 came from U-Pond, fallout, or some adjacent source. 



However, it appears that U-Pond does not contribute significantly to the plutonium 

 concentrations in the air downwind from the chemical processing areas. This is indicated 

 when plutonium concentrations in the air downwind from these areas are compared with 

 tlie concentrations in the air of distant upwind and downwind perimeter communities. 

 Differences between the plutonium concentrations of the two air masses were not 

 significant. In 1972, for example, the average plutonium concentration in the air of 

 distant communities was 1.8 x 10~^ pCi/m^, whereas the mean air concentration of 

 plutonium downwind from the chemical processing areas was 1.9 X 10~^ pCi/m'^ 

 (Energy Research and Development Administration, 1975). 



The premise that U-Pond does not release appreciable amounts of plutonium particles 

 through wind action is strengthened by an additional consideration. Between 1944 and 

 1955 U-Pond was occasionally flooded to the limits of its basin and subsequently 

 overflowed into an auxiHary basin. Since 1955 the pond has remained within its original 

 shoreline. Plutonium deposited as sediments while these areas were flooded is now 

 exposed to the movements of air. Although this exposed area is larger than the present 

 surface area of U-Pond, the plutonium concentrations of downwind air are not 

 significantly elevated by its presence. 



Summary and Conclusions 



In its 34-yr history, U-Pond has received an estimated 1 Ci of plutonium. Since the same 

 quantity presently resides in the sediments, it appears that U-Pond has retained nearly all 

 the plutonium that has been discharged into it. 



In relative terms, sediments, submerged plants, and gastropods have the highest 

 concentrations of plutonium, ranging from 3.2 X 10° to 6.9 X 10^ pCi/g. Plutonium 

 concentrations of emergent plants and the remaining fauna range from 4.0 X 10~^ to 

 6.1 X 10' pCi/g. Emerging insects had the highest plutonium concentrations of the latter 

 group, ranging from 3.2 x 10' to 6.1 xlO' pCi/g. 



The mean plutonium inventory of the sediment is 1.7 x 10' nCi, ranging from 

 1.3 X 10' to 2.0 X 10' nCi of plutonium (Fig. 5). This essentially represents the total 

 pond inventory since more than 99% of the plutonium in the pond is found in the 

 sediments. The mean plutonium inventory for the biota is 6 x 10^ nCi, ranging from 

 1 X 10^ to 1 X 10*^ nCi (Fig. 5). Among these, biota plant life contains more than 95% of 

 the plutonium. Diatoms and pondweed (Potamogeton) alone account for more than 99% 

 of the plutonium in plants. Emergent insects contain less than 1 X 10~'% of the 

 plutonium in biota and less than 1 X 10"^% of the plutonium in the pond. The inventory 

 of this compartment has particular relevance since it is the only direct biological route of 

 export from the pond. Remaining pond biota contain less than 1 X 10~^% of the total 

 plutonium inventory in the pond and can leave the pond only by the forces of external 

 export vectors. 



If all emergent insects successfully leave the pond, they could export from 3.5 X 10^ 

 to 7x10^ nCi of plutonium. These quantities are more than five orders of magnitude 

 lower than the total pond plutonium inventory (Fig. 5). Estimated quantities of 



