612 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 6 Comparison of the AbUity of Authigenic Silica and Calcite 

 to Remove Plutonium from the EpiUmnion 



*See text. 



mg CaCOa/cm^. From the data in Table 6 and assuming that all the plutonium is carried 

 on calcite, the clearance from the epilimnion could range from 0.6 to 1.2 fCi/cm^. Thus 

 the combined scavenging ability of diatoms and calcite could account for the removal of 

 all the plutonium from the epilimnion. It must be made clear that in these calculations it 

 is assumed that plutonium is carried exclusively by biogenic silica or calcite. No direct 

 measurements have been made as yet. 



The results of analyses for plutonium and silica in samples of phytoplankton from net 

 tows are compared with those for suspended particles filtered from the epilimnion and 

 are summarized in Table 6. Detailed measurements of the concentration of SiOo in the 

 water column have shown that during the phytoplankton bloom its concentration in the 

 epilimnion decreases by about 1 mg/liter. If it is taken that the early epilimnion is about 

 10 m deep, then the total clearance of SiOi is equivalent to about 1 mg/cm^ , and, 

 therefore, since the concentration of plutonium expressed in terms of Si02 content is 58 

 fCi/g Si02 , about 0.06 fCi/cm^ of plutonium will be carried with the sinking diatoms. 

 That is less than 10% of the total removed from the epihmnion during the whole summer. 

 The concentration of plutonium in the ashed material collected on filters is far higher 

 than that in ashed phytoplankton. It is not clear whether this additional plutonium is 

 associated in any way with planktonic detrital silica. However, if plutonium is, in fact, 

 removed in association with this silica, then about 0.2 fCi/cm^, or about 20%, would be 

 lost from the epihmnion. 



The enhanced removal of plutonium from tlie epilimnion is probably accomplished 

 by scavenging particles with a short lifetime. An extremely small fraction (<5%) of the 

 diatoms produced annually is incorporated into permanent sediments (Parker and 

 Edgington, 1976), and most of the siUca tied up in their frustules is redissolved in the 

 water column (Parker, Conway, and Yaguchi, 1977), whereas the remainder is 

 resolubilized within a few weeks after reaching the benthic zone. The concentration of 

 plutonium in surface water starts to increase again with the breakdown of the 



