670 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 5 Parameters Describing the Removal of Plutonium 

 from the Epilimnion 



*C is the mean plutonium concentration in the epilimnion, May through September. 

 jsC/St is the mean rate of change in plutonium concentration, May through 

 September. 



$Trp is uncorrected for atmospheric inputs (i.e., = in Eq. 5); tJg = Le/Tr£. 



§T'RE=C/6C/6t. 



scavenging by particles produced in the epiHmnion. Limnological factors affecting the 

 cycHng of plutonium in the Great Lakes are summarized in Fig. 6. The major inputs of 

 particles from external sources (allochthonous) occur during the very early spring and late 

 fall months and tend to be rapidly distributed throughout the length of the water 

 column. During the late spring and through the summer and fall, two principal types of 

 particles, diatoms and calcite, are produced in surface waters. The onset of the decrease in 

 plutonium levels occurs just before stratification in June (Fig. 5), which coincides with 

 the end of the major plankton bloom. The reduction is largely completed during the 

 period of in situ calcite formation during August and September (Fig. 5). If it is assumed 

 that the thermocline averages 15 m deep over the whole season, then the total clearance 

 of plutonium is about 1 fCi/cm^. 



The initial decrease in plutonium parallels, in time and extent, that reported for 

 soluble silicon in offshore Lake Michigan waters (Holland and Beeton, 1972). One 

 possible removal mechanism would therefore appear to be the accumulation of plutonium 

 by phytoplankton (primarily diatoms) and the subsequent setthng of phytodetritus and 

 zooplankton fecal pellets from the epilimnion. From a knowledge of the concurrent 

 decrease in the sihca content of the epihmnion and the uptake of plutonium by net 

 plankton, it is possible to estimate the removal of plutonium from the epilimnion due to 

 the production and setthng of diatoms in May and June. 



As shown in Fig. 7, after the spring diatom bloom there is a reduction in the con- 

 centration of particles in the 8- to SO-(j.m size range through June and July followed by 

 a dramatic increase in the concentration of particles due primarily to the appearance of a 

 large number in the 3- to 8-//m size range. Since the particulate matter collected in August 

 and September is predominantly calcium carbonate (up to 75%), it is taken that these 

 particles result from the in situ production of calcite particles, which agrees with 

 observations made elsewhere (Brunskill, 1969). 



Thus the formation of calcite may also be an important mechanism for efficient 

 clearance of plutonium from the epilimnion. From July to September the concentration 

 of calcium in the epilimnion decreases by about 1 to 2 mg/hter. If it is taken that at this 

 time the epihmnion is 20 m deep, then the formation of calcite could clear from 5 to 10 



