652 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 2 Percent of Plutonium Isotopes 



Associated with Filterable Fraction of Water 



Samples from Rocky Flats Ponds 



*Mean ± standard error. 



Laboratory experiments were performed to study the transfer of plutonium from 

 water to sediment as a function of time. The function appeared to consist of two 

 exponential terms and was described by the equation 



C(t) = Co (0.75 e-o-^2^ + 0.25 e-°-5«^) (1) 



where C(t) is the concentration of plutonium in water at any time t, t is time (in days), 

 and Co is initial concentration of plutonium in water. 



This experimental finding fits remarkably well with actual pond limnological data. 

 The average S. Walnut Creek flow into pond Bi was measured to be 480 m^/day during 

 1971. The water volume of pond Bi was calculated to be 1500 m^. Therefore the mean 

 Ufetime of any parcel of water in pond Bi , if mixing is uniform, can be calculated as 

 follows: 



^ 1500 m^ - ^ , 

 t = 7^7; — rr; — = 3.1 days 

 480 m^ /day 



According to Eq, 1, 75% of the plutonium in water delivered to pond Bi should be 

 exchanged to sediment in an average residence time of 3.1 days, 



Plutonium concentration as a function of sediment depth for the ponds is illustrated 

 in Table 3. These data were plotted and integrated by a planimeter to determine the area 

 (picocuries-centimeter per gram). This value was then divided by the mean sediment depth 

 to give the mean sediment concentration of plutonium. When multiplied by the estimated 

 sediment volume, these data yielded plutonium inventories for the sediment of the ponds. 

 The same sediment cliaracteristics were assumed for each pond. The variation could be 

 due to shunting of water past ponds at unknown times. The calculated value also assumed 

 that no plutonium was being transferred from pond to pond by suspended materials. The 

 agreement between the calculated and measured inventories is shown for the holding 

 ponds in Table 4. 



Seston 



Seston was defined as primarily phytoplankton, some detritus, and some small 

 zooplankton. Planktonic algae constituted by far the majority of aquatic plant material 

 found in the holding-pond chain on S. Walnut Creek (B ponds). 



