PLUTONIUM IN THE GREAT LAKES 681 



of the particles carrying plutonium is the same in each lake. Considering the large 

 differences in their limnological properties, such as primary productivity, this result is 

 rather surprising in that it might have been expected that a large proportion of plutonium 

 would be carried with organic detritus. 



The rate of clearance of plutonium from the epilimnion of Lake Michigan between 

 June and September is far faster than that from the whole water column of the lake and 

 is constant from year to year. Enhanced removal of plutonium from the epilimnion 

 results from intensified particle production during the spring and summer months. Most 

 Ukely, plutonium is scavenged by diatoms and calcite particles, which subsequently 

 redissolve. 



Although it is possible to develop an understanding of what appears to be an 

 extremely complex system, such as the behavior of plutonium in the Great Lakes, in 

 terms of a simple model, there are still many unanswered questions. For example, the 

 uptake of plutonium on biogenic siBca or autochthonous calcium carbonate in the 

 epilimnion must be a transient process because it is clear that almost all this material 

 redissolved in the hypoUmnion. Some redissolved plutonium may be taken up by other 

 particulate material. Presently it is impossible to balance the downward flux of plutonium 

 on Si02 and CaCOa, measured near the bottom in sediment traps, with that apparently 

 deposited in the surface sediments each year because of the subsequent horizontal 

 redistribution of older sediment containing plutonium in the lake. 



Since the lakes have very similar general chemical properties, it is possible that the 

 exchange of plutonium between the water column and sediments is controlled by 

 chemical reactions. In fact, it has been suggested that the concentration of plutonium in 

 the water column is largely controlled by chemical equiUbrium between specific species in 

 the water column and the sediments. If this equilibrium is a major factor in controlling 

 the concentration of plutonium in the lake, the value of Tr should have increased 

 significantly during the period of major deposition in the sediments (1963—1970). The 

 data at present demonstrate that little change has occurred in the past 7 yr since the 

 present concentration in the water column can be described by the value of Tr calculated 

 for 1971. 



The situation is complicated by the very recent observations that (1) plutonium 

 in Lake Michigan . (and the Irish Sea) exists primarily in the water column in the VI 

 oxidation state and on the sediments as the IV oxidation state and (2) Lake Michigan 

 water can readily extract sorbed plutonium from high-activity pond sediments. Since the 

 distribution coefficients of Pu(VI) and Pu(IV) v^th sediment are very different, a critical 

 step in the clearance of plutonium from the water column may be the reduction of 

 Pu(VI) to Pu(IV) either in the water or at the sediment surfaces. It is clear, however, that, 

 if there is to be a complete understanding of the long-term behavior of plutonium, 

 especially from other source terms in aquatic environments, more attention must be paid 

 to determining its chemical forms. 



Acknowledgments 



This work was performed under the auspices of the U. S. Department of Energy. 



We gratefully acknowledge the contributions of D. M. Nelson, K. A. Orlandini, and 

 other members of the Ecological Sciences Section to various aspects of the field sampling 

 program that made this chapter possible. 



