MIGRATION OF PLUTONIUM FROM FRESHWATER ECOSYSTEMS 629 



of plutonium less than 0.1 jum and soluble plutonium are in very dilute concentrations 

 {< 1 pCi/liter), which are lower than standards that regulate plutonium concentrations in 

 drinking water. It is assumed that tliis small fraction of plutonium remains in the pond 

 water until it enters any one of the ecosystem compartments that could provide a route 

 of export. Dissolved and suspended materials in the pond have a short duration since 

 nearly all the water leaves the pond by percolation after a mean residence time of 40 hr 

 (Emery, Klopfer, and Weimer, 1974). The pond has no surface outflow. 

 The contacting terrestrial system has four compartments: 



1 . Waterfowl. 



2. Birds (other than waterfowl). 



3. Mammals. 



4. Airborne particulates. 



Since these compartments have a transient association with the pond, they also serve as 

 routes of plutonium export. In addition, the transient insect population in the pond, 

 along with the emergent macrophytes, provides means for plutonium to leave the pond. 

 The emergent macrophytes would require assistance from one of the other export routes 

 to release any of their plutonium content to adjacent areas. Tliis is also true for the 

 plutonium that resides in the shoreline sediments. Thus the only export vectors through 

 which plutonium can leave U-Pond are: 



• Percolation. 



• Emergent insects. 



• Waterfowl. 



• Birds. 



• Mammals. 



• Wind (containing airborne particles). 



Methods of sample preparation and plutonium analysis of pond samples, which 

 involve drying, ashing, chemical separation, and electrodeposition, are described by 

 Emery, Klopfer, and Weimer (1974). 



Several techniques were used in the measurements of annual production of biomass in 

 the aquatic system. For all compartments except sediments, the annual production is 

 expressed as the quantity of biomass that is generated in 1 yr. The material quantity of 

 the pond's sediments is expressed as the dry weight of sediments to a depth of 10 cm. 

 The inventory of plutonium in the sediments is then the amount of plutonium in the 

 upper 10 cm. The dry weight of the upper 10-cm layer of sediments is 3.4 x 10^ kg, 

 which extends over an area of 5.6 x lO'^ m^ . 



Nonfilamentous algae are composed mainly of sestonic diatoms (not always a true 

 phytoplankton population) and, to a much lesser extent, Tetraspora, which rests on the 

 bottom in loose globular masses. The annual production of sestonic diatoms is estimated 

 by using the weight of the average fraction of seston that is diatoms. This was done by 

 microscopic examination of concentrated seston samples to determine the mean 

 percentage of the total number of the particles that is diatoms (28%). Seston 

 concentrations were then multiplied by 0.28 to obtain an estimate of the concentration 

 of diatoms (in milligrams per liter). This concentration was proportioned to the volume 

 of the pond to estimate an instantaneous standing crop of diatoms in the pond. Sestonic 

 diatom standing crops were sampled seasonally to establish an annual mean. Since the 

 mean residence time of this water mass is 40 hr, this mean standing-crop value is 



