680 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



TABLE 7 Seston Concentrations, Particle Fluxes, 

 and Apparent Settling Rates in the Upper 10 m of Water 



(ANL-5, 1976) 



*Average of samples taken at beginning and end of sediment-trap 

 collection interval. 



transport of resuspended particles. The fact that regeneration of plutonium proceeds 

 concurrently at offshore stations during the winter indicates that either inshore 

 resuspension is intense and horizontal transport rapid or that dissolution rather than 

 resuspension is more important in recychng plutonium lost to particle phases during the 

 earlier sequence of diatom and calcite production. 



The particle flux profiles (Fig. 13) provide a crude measure of the seasonal variation 

 in apparent particle settling rates. Althougli the lower portions of the profile must result, 

 at least partially, from resuspension, the particle flux measured above the thermocline is 

 hkely to be unaffected by resuspension. Under conditions of a stable thermocline (July 

 and August), the upward particle flux into the epilimnion will be negligible. The ratio of 

 this flux to the estimated concentration of particles in surface waters (Fig. 7) is the 

 apparent particle settling rate" given in Table 7. Apart from extremely high rates during 

 the months of the spring convection mixing period and diatom bloom, the rates during 

 the succeeding months are comparable (22 m/yr; June to September average) to the rate 

 inferred from the concentration— time model. Thus plutonium is apparently scavenged by 

 particles that, on the average, are settling at the same apparent rate as most particles in 

 the water column. Further, it seems likely that resuspension does not have an appreciable 

 effect on this apparent rate of settling. 



Conclusions 



The results of extensive measurements of plutonium in the Great Lakes have shown that 

 for each lake the concentration of plutonium in the water column at the end of the spring 

 convective mixing period can be described by a simple time— concentration model with 

 only one variable for each lake, viz., the residence time of plutonium. This retention time 

 for the loss of plutonium in each lake is controlled by two processes, the outflow of 

 water to the next lower lake (Tr) and transfer to the sediment on settling particles (T'r). 

 With the use of a plutonium source term based on the fallout ^°Sr monitoring values 

 from the Environmental Measurements Laboratory, values of Tr that gave best fit to the 

 experimental data were obtained from the model. The values of Tr were found to be 

 proportional to the mean depth of the lakes, which implies that the appaient settling rate 



