PLUTONIUM IN THE GREAT LAKES 66 7 



described in terms of a single parameter, T'r. The preceding discussion suggests that 

 uncertainty in the estimated value for Lake Michigan is around 20% and somewhat higher 

 (~30%) for the other lakes. Thus the variation in the values of T'r from lake to lake by a 

 factor of 6 is real. 



Althougli the variation of values of Tr is large, the apparent settling rate (L/Tr, 

 where Lis the mean lake depth) is essentially independent of the lake. These values are 

 given in Table 4. For all lakes except Lake Superior, the apparent settling rate is 35 ± 2 

 m/yr; for Lake Superior the value is 48 m/yr. However, in view of the uncertainties in 

 the average plutonium concentration for each lake and therefore in calculating Tr, tliis 

 value is not significantly different, and the mean apparent settling rate for the five lakes is 

 37 ± 3 m/yr^ . This result is consistent with the observed loss rate, corrected for 



TABLE 4 Residence Time of Plutonium in the Great Lakes 



degradation, of DDT (Bierman and Swain, 1978) in Lake Michigan and Lake Superior. It 

 is also similar to the apparent settling rate for total phosphorus, 10 m/yr^ (S. C. Chapra, 

 Great Lakes Environmental Research Laboratory, personal communication), and for 

 detrital particles, 36 m/yr^ (D. M. DiToro, Manhattan College, personal communication). 



This rate of 37 m/yr^ is comparable to mean particle settling rates inferred from trap 

 and sedimentation studies. The rate of accumulation of particles in traps placed in 

 southern Lake Michigan shows both a strong seasonal dependence and a marked increase 

 v^th increasing water depth (Wahlgren and Nelson, 1977b). However, for most of the 

 year, the net accumulation rate in near-surface waters is about 0.02 mg cm~'^ day~^. 

 Increases in flux with increasing depth must result either from resuspension or from 

 transient effects associated with earlier particle production in surface waters. The low 

 average downward particle flux from surface waters is comparable to average sedimenta- 

 tion in the southern basin of Lake Michigan (7 mg cm~^ yr"' = 0.02 mg cm~'^ day"') 

 estimated by Edgington and Robbins (1976) on the basis of ^ "^Pb and ' ^ ''Cs profiles in 

 a large number of sediment cores. Since the mean concentration of particles in the water 

 column is about 1 to 2 mg/liter (Wahlgren and Nelson, 1974b; additional unpublished 

 data for the period 1973 to 1977), the mean particle settling rate is about 40 m/yr (0.02 

 mg cm~'^ day"' X 2 mg/liter). That plutonium carrying particles has a net settling rate 

 that is comparable to the mean rate indicates that plutonium is not selectively scavenged 

 by an atypical suite of particles in the water column. This idea is also supported by 

 studies on the distribution of plutonium between particles and water discussed further in 

 the following text. 



