u 



O ' 



CM 



2 - 





 1950 



PLUTONIUM IN THE GREAT LAKES 665 



I I I I I 



1 rn \ r 



Lake Superior _ 



Lake Michigan — 

 Tr = 2.4 yr 



Lake Huron 

 Tr = 2.3 yr 



1960 1970 



YEAR 



1950 



1960 1970 



YEAR 



Fig. 3 Comparison of concentrations of 2 3 9,240py jj^ ^j^^ ^vater column predicted by 

 the coupled-lakes model with available experimental data points (•). The best estimate 

 of the residence time for deposition in the sediments is given for each lake. 



predict the concentration of plutonium in the early 1970s, give an adequate account of 

 levels in each lake (except Lake Superior, for which there is only one data point) 3 to 7 

 yr later. For Lake Michigan, where plutonium has been measured each year since 1971, 

 the model appears to slightly underestimate concentrations from 1973 onward. 

 Plutonium levels in the water of Lake Erie show comparatively strong fluctuations over 

 the past two decades because of the very short mean residence time resulting from rapid 

 losses to sediments. 



The small differences between observed and calculated mean plutonium concentra- 

 tions in Lake Michigan after 1973 could be due to a combination of small effects because 

 levels were so low in the 1970s. For example, a 20% increase in the Tr gives a better 

 least-squares fit to all the mean concentration data. Thus the lower value of Tr resulting 

 from use of earliest (1971) concentration values alone could be an artifact of the 

 approach or reflect uncertainties in the estimate of the mean concentration in 1971. 

 Alternatively, the sliglitly liigher recent value of Tr could reflect an increasing 

 importance of sediment— water exchange or watershed erosion in the regulation of the 

 very low plutonium concentrations in Lake Michigan. Unfortunately there are insufficient 

 data to discuss the other lakes in these terms. 



Little is presently known about the inputs of plutonium from watersheds of the Great 

 Lakes. The few measurements of the concentration of plutonium in the Grand River, one 

 of the major tributaries to Lake Michigan, suggest total concentrations of 0.5 to 1.0 

 fCi/hter. If this range is representative of average concentrations, tributary rivers would 

 contribute up to ~0.1 Ci/yr at the present time compared with ~0.7 Ci/yr from direct 

 fallout to the surface of Lake Michigan. 



The recent results of Sprugel and Bartelt (1976) suggest that watershed contributions 

 may be more important than previously supposed. They found that 0.05% of the total 

 plutonium stored on a typical midwestern watershed is lost by erosion each year. If this 



