666 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



erosion rate applies to the large watersheds of the Great Lakes, the annual input to Lake 

 Michigan, for example (see Table 1), should be ~0.14 Ci/yr (assuming that there is 270 

 Ci of plutonium deposited on the watershed). The possible effect of such a 0.05%/yr 

 erosion rate on plutonium concentrations in Lake Michigan since 1973 is illustrated in 

 Fig. 4. Equation 3 is evaluated on a monthly time interval starting in 1973, with initial 

 conditions determined by observed concentrations in the lake, and T'r = 2.4 yr. Montlily 

 inputs from the rivers are estimated by the crude assumptions: (l)The total inflow of 

 water to Lake Michigan is proportional to that from the Grand River (U. S. Geological 

 Survey, 1973 — 1976; U. S. Geological Survey, personal communication, for 1977 data); 

 and (2) the concentration of plutonium in river water is constant. Clearly the addition of 

 plutonium from the watershed at a rate of 0.05%/yr (~3 fCi/liter) better reproduces the 

 data. In years like 1976, when there is little new atmospheric deposition, levels do not 

 decrease significantly because of continued inputs from the tributaries. 



Although there may be many ways to account for the minor variations in mean 

 concentration of plutonium in the lakes each year, the overall behavior is adequately 



0.4 



o 

 E 



o 



O- 



o 



0.3 — 



0.2 — 



0.1 — 



0.05 — 



0.025 



0.8 



2 0.6 



o 



Q. 



05 



CM 



0.4 



0.2 — 



Atmospheric input 



Watershed input 

 (a =* 0.05%/vr) 



Plutonium in 



Lake Michigan water 



1973 1974 1975 1976 



YEAR 



1977 



1978 



Fig. 4 Kvaluation of the possible role of watershed erosion in maintaining the recent 



levels of plutonium in Lake Michigan waters. , predicted for unstratified, 



well-mi.xed lake, a = 0.05%. , predicted for unstratified well-mi.xed lake, a = 0. •, 



measured values of mean concentration. 



