MIGRATION OF PLUTONIUM FROM FRESHWATER ECOSYSTEMS 63 7 



Althougli we have not determined how much plutonium has percolated out of 

 U-Pond since its formation in 1944, the available evidence points to the retention of 

 virtually all of it in tlie sediments — probably in the upper several centimeters. 

 Furthermore, we find no evidence to indicate that plutonium has migrated from U-Pond 

 into the groundwater below Hanford. Thus we have no reason to beheve that percolation 

 is a significant route of plutonium export from U-Pond. 



Emergent Insects. Insects emerging from U-Pond constitute the only direct route of 

 biological export. However, if the life cycles of these insects are considered, it appears 

 that emergence alone does not account for the export of the entire plutonium inventory 

 contained in this compartment. The cast exoskeletons left in the pond at the final 

 ecdysial stage prior to emergence may contain a substantial portion of the plutonium 

 burden of the insects. It is also possible that some of these insects complete their life 

 cycles without leaving the pond's ecosystem, and their plutonium burdens may ultimately 

 be returned to the pond. We will not attempt to estimate the fraction of plutonium that 

 is left in the pond by these processes, but instead we will assume that the entire inventory 

 of this compartment leaves the pond when these insects emerge. 



With the foregoing considerations taken into account, the mean annual export of 

 plutonium by emerging insects is about 5 X lO^nCi. This quantity is about 9 X 10~^% of 

 the plutonium inventory of the biota and 6 X 10^^*% of the total pond inventory 

 (Tables 3 and 4). 



Waterfowl. The waterfowl that contact the U-Pond ecosystem are mostly mallards and 

 an assortment of other ducks and coots. Some of these waterfowl nest along the shoreline 

 of the pond. Since it is unlikely that these waterfowl contact other locations where they 

 may be exposed to above-background levels of plutonium, it is assumed that most of the 

 plutonium found in their gut and tissues came from U-Pond. 



Examination of crop and gut contents of ducks collected from U-Pond indicates that 

 they feed most heavily on the organically rich floe that covers the pond's sediments and, 

 to a lesser extent, on goldfish and other material. Recall that floe contains most of the 

 plutonium in the pond's ecosystem. 



Concentrations of plutonium in whole bodies (including gut and contents) of four 

 wild ducks (Anas) ranged from 3 X 10"^ to 3 X 10° pCi/g, with a mean of 4 x 10"^ 

 pCi/g. These ducks were in contact with the pond when they were sampled, and most of 

 tlieir plutonium burdens were contained in the gut. Less than 5% of their entire 

 plutonium burdens was contained in the body tissue. 



Knowledge of the relationship between the length of time a duck spends in the 

 U-Pond ecosystem and the amount of plutonium accumulated would be useful. 

 Information about the contact frequency and duration is not available for waterfowl 

 sampled from the pond; thus there is no basis to establish this relationship. However, a 

 short experiment was performed to determine the amount of plutonium accumulated in 

 ducks (Anas) held on tlie pond in large cages for 5 days and fed a continuous diet of 

 organic floe (Emery and Klopfer, 1977). These experimental conditions represent the 

 highest potential for tlie accumulation of plutonium by a duck on a short-term basis. 



Results of this experiment suggest that ducks could accumulate about 6x10° pCi 

 Pu/g (whole duck) in 2 to 5 days of continuous contact with U-Pond (Fig. 4). Under 

 these conditions the accumulation of plutonium in the gut could be around 7x10^ 

 pCi/g, and tlie body tissue may concentrate about 3 X 10° pCi Pu/g. It is evident that in 

 the experimental ducks the gut contained most of the plutonium burden (>95%), which 



