PLUTONIUM IN THE GREAT LAKES 673 



thermocline but does not attain its maximum value until sometime after early December. 

 In contrast, reactive silica, which also undergoes an annual cycle in the water column, 

 returns to nearly its spring value as early as the end of September. Thus the release of 

 plutonium from dissolving frustules is delayed in relation to the sihcon (and calcite) 

 cycles. The production of calcite later in the year may account for this delay, but the 

 lifetime of calcite particles in the water column is presently unknown for the Great 

 Lakes. Alternatively, this delay could indicate that, if plutonium is released from 

 dissolving frustules or calcite particles, it is transferred to other particles in hypoUmnion 

 or in the vicinity of the sediment— water interface. Alberts, Wahlgren, and Nelson (1977) 

 have shown that almost all the plutonium in floe collected at the sediment-water 

 interface is associated with reducible hydrated oxides, such as ferric and manganese 

 oxides, and not with carbonates or silica. 



Since epilimnetic plutonium losses are mainly due to scavenging by particles that 

 dissolve rapidly, this transient process may not result in appreciable net transfer to 

 sediments during most of a year. In 1975 the mean concentration of total (dissolved and 

 particulate) plutonium at station ANL-5 (from the surface to 60 m) shows a seasonal 

 variation that is consistent with its average residence time in the lake (Fig. 8; the dashed 

 line in the figure corresponds to T'r = 2.4 yr; a = 0.05%/yr). The net loss to the 

 sediments during each month of this year is predicted by the coupled-lakes model. Since 



0.8 



0.6 — 



o 



3 



a. 

 o 



CN 

 O) 

 CO 



0.4 



0.2 



Measured 

 total 



Particulate 



M A M J J A 

 MONTH (1975) 



N 



Fig. 8 Mean concentrations of plutonium in whole water, in solution, and on particles 

 in the upper 60 m of the water column of Lake Michigan (ANL-5; 1975; water depth, 

 67 m). The concentrations expected for a mean residence time (Tr) of 2.4 yr and 

 watershed erosion (a) of 0.05%/yr are indicated by the dashed curve. 



