668 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



That the apparent sinking rate is essentially invariant from lake to lake is surprising 

 considering the diversity of limnological characteristics of these lakes. Evidently, the 

 processes determining the long-term removal of plutonium from the water column must 

 be similar for each of the Great Lakes, and the rate of removal from the water column to 

 underlying sediments is determined by the net rate at which the particles scavenging 

 plutonium sink. Furthermore, this result suggests that the availability of suitable particles 

 for scavenging plutonium and kinetics of exchange between dissolved and particulate 

 phases are not presently determijiing the long-term removal rate. 



It must be emphasized that the apparent settling rate could be very different from the 

 actual net downward motion of particles. Vertical turbulence and resuspension of bottom 

 sediments can significantly alter the apparent particle settling rate. The role of 

 resuspension will be discussed further in the following text. 



It is clear that, since the apparent settling rate is constant from lake to lake, the 

 processes controlling the removal of plutonium from the water column are similar for 

 each lake, and, therefore, considering the large differences in productivity, it is unlikely 

 that association with autochthonous organic matter is the rate determining step. However, 

 although this generalization is true for the water column as a whole, it will become 

 evident, from the experimental data discussed in the next section, that association with 

 biogenic material can explain, at least in part, the more rapid removal of plutonium from 

 the epilimnion (at least for Lake Michigan) during the period of stratification. 



Seasonal Cycling of Plutonium in Lake Michigan 



The seasonal cycling of plutonium from surface waters was first observed (Wahlgren and 

 Nelson, 1974b) at an offshore station 40 km west of Grand Haven, Mich. Since 1973 

 onward the cycle has been followed in detail in Lake Michigan (Wahlgren and Nelson, 

 1977a; Walilgren, Nelson, and Kucera, 1977, unpublished data). This seasonal cycling of 

 plutonium occurs at other stations in Lake Michigan and probably is common to all the 

 Great Lakes (Bowen, 1975; Alberts, Walilgren, and Nelson, 1977) except Lake Superior. 

 More than 75% of the total (dissolved and suspended) plutonium is lost from the 

 epilimnion at station ANL-5 in Lake Michigan during the sununer months and returned 

 during the fall and winter mixing period each year. The results, summarized in Fig. 5, 

 contrast the strong seasonal cycle in surface waters with mean levels expected t>om the 

 concentration— time model discussed in the preceding text. The data shown are primarily 

 from a single sampling station, ANL-5 (12 km southwest of Grand Haven, Mich.), but the 

 same trends have been observed at other stations farther offshore, including EPA-18, 

 which is in the middle of the southern basin. Farther offshore the onset of stratification 

 occurs later in the spring season, and the initial removal of plutonium from surface waters 

 is delayed correspondingly. A limited number of cross-lake transects during 1976 show 

 that the degree of removal uom surface water by September was comparable across the 

 whole of the southern half o'i the lake. 



The change in concentration of plutonium in the well-mixed epilimnion of depth is 

 given by 



dC C 



dt Li, T'k 



(5) 



provided that there is no upward transfer across the thermocline. (The etTect of outfiow 

 can be ignored.) The results of evaluating Eq. 5 with = as v/ell as with values of 



