Plutonium in the Great Lakes 



M. A. WAHLGREN, J. A. ROBBINS, and D. N. EDGINGTON 



Since 1971 plutonium concentrations have been measured annually in Lake Michigan and 

 Lake Ontario and at less frequent intervals in the other Great Lakes. The concentrations 

 of plutonium in the water column have decreased only slightly during the 7 yr of 

 measurement. Tfie residence times for plutonium in the lakes have been estimated by 

 simple time-concentration models. Tfie apparent sinking rates for plutonium have been 

 found to be essentially constant in all the Great Lakes, which suggest that the basic 

 processes that control the concentrations of dissolved plutonium are similar despite 

 considerable differences in chemical, biological, and physical cliaracteristics of the lakes. 

 Analyses of plutonium in water, suspended solids, material from sediment traps, and 

 sediment cores show that considerable resuspension of previously sedimented material 

 into the hypolimnion occurs throughout a major part of the year. A mechanism is 

 proposed to account for the seasonal cycling of plutonium in the epilimnion of Lake 

 Michigan. Recent studies show that plutonium in Lake Michigan (and in the Irish Sea) 

 exists primarily in the water column as Pu(VI) and on the sediments as Pu(IV). For a 

 better understanding of the long-term geochemical and biological behaviors of plutonium 

 in aquatic environments, further study of the limnological factors that control the 

 chemical forms of plutonium is required. 



Approximately 40% of the population of the United States lives in states bordering the 

 Laurentian Great Lakes (Fig. 1). The economic advantages to the electrical generating 

 industry of using these lakes for once-through cooling have long been recognized in both 

 the United States and Canada. The advent of large multiple-unit nuclear plants has led to 

 the operation of 8 such reactors on Lake Michigan and a total of 16 on the four lower 

 Great Lakes. The rapid growth of the nuclear power industry has generated considerable 

 public concern about possible environmental effects of radioactive discharges, whether 

 routine or accidental, and this concern has been directed primarily toward plutonium. 



Very little, if any, plutonium from nuclear power plants has entered the lakes. The 

 source of plutonium in the Great Lakes is almost entirely stratospheric fallout as a result 

 of nuclear weapons testing. Because of the very low concentration and consequent 

 analytical difficulties of plutonium, neither concentrations nor inventories of ■^■'^'■^'*°Pu 

 or ^^^Pu were measured during the period of maximum fallout. However, excellent 

 records exist for the deposition of ^°Sr from the worldwide fallout monitoring program 

 (Environmental Measurements Laboratory, 1978). Therefore, if a general 239,240p^^ 

 ^°Sr ratio can be assigned, it is possible to obtain a reasonable estimate of the annual 

 deposition of plutonium. Measurements of plutonium and strontium in the atmosphere 

 since 1965 have been summarized by Krey, Schonberg, and Toonkel (1974). who found 



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