662 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Long-Term Behavior of Plutonium in the Great Lakes 



The earliest measurements of plutonium in any of the Great Lakes were made in 1971 

 (Maletskos, 1972). Since that time measurements of plutonium have been made every 

 year in Lake Michigan and Lake Ontario (Bowen and Noshkin, 1972; 1973; Bowen, 1974; 

 1975; 1 976 ;-l 977 ;Wahlgren and Nelson, 1973; 1974a; 1974b; 1975; 1977a; Wahlgren and 

 Marshall, 1976; Wahlgren, Nelson, and Kucera, 1977, unpublished) and occasionally in 

 the other lakes. The annual data for plutonium for the years 1971 to 1977 are 

 summarized in Table 2. Since it was found in 1972 that the fallout radionuclides ^^^Pu, 

 ^ ^^Cs, and ^°Sr in Lake Michigan have a homogeneous distribution throughout the water 

 column following the winter mixing period (Walilgren and Nelson, 1973), the 

 concentrations in early spring or from the hypolimnion during early summer may be 

 taken to represent the mean for the lake. Therefore the numbers in the table are the best 

 estimate of the total concentration of each nuclide remaining in the water column each 

 year. From these data it is possible, knowing the volume of the lake, to calculate the 

 inventory of plutonium in the water column. As indicated in Table 1, the fraction of the 

 total amount in each of the lakes in the water column at the present time is very small. 

 Since very little plutonium can be lost by outflow from the upper Great Lakes (Michigan, 

 Superior, and Huron), it follows that there must be a very efficient transfer of this 

 element to the sediments. In the lower lakes (Erie and Ontario) losses due to outflow and 

 gains from the upper lakes must be considered as well. The mean ^^^Pu concentrations 

 for the three upper lakes (about 0.6 fCi/liter) are significantly different from those for 

 Lake Erie and Lake Ontario (about 0.2 fCi/hter). At first this would appear to be due 

 solely to the greater outflow in the last two lakes. However, the situation is more 

 complicated and interesting. 



To understand the long-term behavior of plutonium in the Great Lakes, one must 

 consider not only the inputs and losses but also the volume and area of each lake and the 

 efficiency of the scavenging of plutonium from the water column. To the extent that 

 each lake is well mixed, the change in concentration of plutonium in the water column is 



^ = ;^(A0 + aW+l-S-O) (1) 



where V = volume of the lake 

 A = area of the lake 



= plutonium flux to the lake surface, femtocuries per square centimeter per year 

 W = amount stored in the watershed, femtocuries 

 a = annual fraction lost to the lake 



Since the lake is assumed to be well mixed, the amount lost by outflow, 0, is given by 

 QC, where Q is the mean annual outflow from the lake (cubic centimeters per year). In 

 addition, 1 is the input (femtocuries per year) from the next higher lake, where 1 = QC, 

 and S is the amount lost to the sediments (femtocuries per year). Introducing subscripts 

 i=l to 5, wliich refer, respectively, to Lakes Superior, Michigan, Huron, Erie, and 

 Ontario, the change in concentration in each lake is given by 



dCj 1 Qi Si 



— •=-(Ai0i + aiWi + Qi_, Ci_i)-^Ci-;^ (2) 



