660 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



Fig. 1 Laurentian Great Lakes, with water sampling sites for comparative studies. O, 

 May lOto June 6, 1973. A, July 1 to 3, 1974. •, June 30 to July 6, 1976. 



that a ^^^•^'*°Pu/^°Sr ratio of 0.017 should give a good estimate of plutonium 

 deposition. The annual inputs- of plutonium to Lake Michigan, which lies almost wholly 

 within the 40°N latitude band, are shown in Fig. 2. A knowledge of this source term is 

 important for the interpretation of residence times in the water column and concentra- 

 tion profiles in the sediments. Measurements of the cumulative deposition of plutonium 

 in soil at Argonne (~2.2 mCi/km^ ; Golchert, Duffy, and Sedlet, 1978) are in agreement 

 with the integral of all the inputs shown in Fig. 2 and the worldwide fallout data 

 summarized by Hardy, Krey, and Volchok (1973). On this basis the total content of 

 plutonium in the Great Lakes basin is 1660 Ci (or 25 kg), of which 540 Ci was deposited 

 directly onto the surface of the lakes. The plutonium content in each lake is summarized 

 in Table 1. Sprugel and Bartelt (1976) have measured the loss of plutonium from a 

 typical midwestern watershed to be about 0.05% of the total accumulated deposition per 

 year. 



The presence in the Great Lakes waters of these low concentrations of long-lived 

 radioactivity from nuclear fallout provides the opportunity to characterize the environ- 

 mental behavior of these isotopes and to study the biogeochemical and geophysical 

 processes that determine the residence times of radioactive and stable trace materials 

 entering the lakes. For plutonium it is of particular importance to determine (l)the 

 potential pathways to man (food chains and drinking water); (2) a radiological baseline 

 data set for the Great Lakes; and (3) the likely distribution of possible future inputs 

 between various compartments of the lake, including the tlnal sinks, if any. 



