674 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



no enhanced transfer to the sediments is expected during the period of diatom and calcite 

 production, there appears to be a dramatic exchange of plutonium between dissolved and 

 particulate phases and temporary loss to the benthic layer (between 60 m and the 

 sediment— water interface). This complementary behavior can be understood in terms of 

 the constancy of the distribution coefficient Kp (Wahlgren and Nelson, 1977c). Since Kp 

 is essentially independent of particle type ('^2 x 10^), the proportion of plutonium tied 

 up with particles should reflect the amount of particulate matter in the water column. 

 From a comparison of the total concentration of particles in surface water each month 

 (Fig. 7) with the amount of plutonium in the water column above the benthic layer 

 (Fig. 8), it can be seen that the proportion of plutonium above the benthic layer reflects 

 the time— dependence of particle concentrations in surface waters. 



The extent to which plutonium is removed from the dissolved phase before fall 

 overturn is illustrated in Fig. 9. The seasonal variation in the profiles of dissolved 

 plutonium from the years 1975 to 1977 shows that the removal from the soluble phase 

 extends progressively deeper into the water column as the season advances. 



With the onset of fall overturn in October, appreciable losses of plutonium from 

 solution extend essentially to the bottom at this station (ANL-5). The effect was most 

 pronounced in 1976. On the basis of a series of samples collected from sediment traps 

 moored at all depths across the southern basin, it appears that this behavior is 

 characteristic of the deeper waters of the lake as well.* During November 1976 there was 

 a significant reduction in the total amount of plutonium in the water column at this 

 station. Provided that horizontal transport is ignored, the subsequent regeneration of 

 plutonium in the water column would require transfers of plutonium from sources below 

 the deepest sampling point, which is 7 m off the bottom. 



The composition of particles reaching this depth illustrates the complementary 

 behavior of silica and calcite in regulating the cycling of plutonium. It can be seen 

 (Fig. 10) that the proportion of silica in the particulate material reflects diatom 

 productivity, with peak values in May and a secondary maximum in November. In 

 contrast, the percentage of calcium carbonate (calcite) remains low through the spring 

 and early summer but rises dramatically in September. There is approximately a 1 -month 

 delay between the onset of the plankton blooms or formation of calcite and the 

 appearance of increased concentrations of SiOj or CaCOa in particulate material 7 m 

 above the bottom. Since it is possible to disfinguish seasonal variations at this depth, it 

 strongly suggests that there is either Utile resuspension of bottom sediment at 7 m above 

 the bottom or that the only material that is resuspended is newly deposited detritus. The 

 sharp drop in proportion of Si02 and CaCOa in November is probably associated with 

 the input of terrigenic material washed into the lake as a result of shoreline erosion from 

 severe early winter storms.f Beyond November both the silica and calcite contents of the 

 seston decline dramatically. The total (dissolved and particulate) plutonium per unit 

 weight of particulate material exhibits a monthly variation that reflects the combined 

 effects of silica and calcite scavenging. The total plutonium concentration is high in the 

 spring, goes through a minimum in June, and then steadily rises to a maximum with the 

 addition of calcite to near-bottom seston. Considerable plutonium, Uke siHca and calcite, 

 is lost from the seston and regenerated in the water column during the period from 



*Earth Resources Technology Satellite satellite photographs show that the "whiting" of the 

 surface waters of Lake Michigan is a lake-wide process. 



fit has been shown that up to 50% of the total annual erosion can occur in November (E. Siebel, 

 University of Michigan, personal communication). 



