PLUTONIUM IN THE GREAT LAKES 669 



^'°|l I I I I I I I I I I I M I I I I M M M I M I I I I M I I M I I I I I I I I I I I I I I I I I I I M I I 



0.8 



^ 0.6 



O 



0.4 



0.2 



" I I M I I I I I I I I I I I I I I I I I I 



I I M I 



AMJJASONDJFMAMJJASON DJFMAMJJASO NDJFMAMJJASONDJFMAMJJASOND 

 1973 I 1974 I 1975 | 1976 | 1977 



YEAR 



JFM 



Fig. 5 Comparison of concentrations of 239 ,240 p^ jj^ ^y\q water column predicted by 

 the coupled-lakes model with experimental measurements of surface-water concentra- 

 tions in Lake Michigan. The annual cycling of plutonium is clearly evident in this 



comparison. , surface-water values predicted by the model. , surface-water 



values measured. 



determined from monthly measurements of fallout (Environmental Measurements 

 Laboratory, 1978) are summarized in Table 5. The results of these calculations indicate 

 that, for the years 1973 to 1977, there is a sUghtly greater variation in the calculated 

 residence time in the epilimnion when new inputs are ignored (i.e., = 0). For example, 

 the effect of new fallout on the calculated values was most important in 1977; the 

 residence time for plutonium was reduced from 0.31 to 0.21 yr. When new inputs are 

 included, the residence time is almost constant from year to year. The mean residence 

 time T'rj- = 0.22 yr corresponds to an apparent particle settling rate (coe) of about 90 

 m/yr. The effect of an additional input of plutonium from the underlying waters would 

 be to further decrease the estimate of T^e and increase the calculated value of gJe- 



The losses of plutonium from the epiMmnion are more rapid than expected on the 

 basis of the average residence time of this radionuchde in the lake. To the extent that the 

 epilimnion is isolated from underlying waters and the Tr scales with water depth, the 

 residence time of plutonium in the epilimnion would be T're = Tr X (Le/L), where Le 

 is the mean depth of the epilimnion (~20 m). Thus Tre ~ 2.4 X (20/84) ~ 0.6 yr. Thus 

 the observed value T'r£ is at least 2.5 times lower than that expected from the average 

 residence time of plutonium in the lake. It is therefore clear that the removal of 

 plutonium from the epilimnion is not solely due to its isolation from underlying waters. 



This increased efficiency of the removal of plutonium from surface waters during the 

 period just before and during stratification of the lake is probably due to intensified 



