Studies of plutonium cycling and sedimentation in Lake Michigan

Estimates of cumulative deposition, together with the results of extensive lakewide sampling in 1972, yielded estimates of mean halftimes for removal from the water column of Lake Michigan of 1.0 +- 0.3 yr for 239240Pu, and 1.4 +- 0.3 yr for 137Cs. However, data from 1971, 1972 and 1973 water samples indicate that the recent half-time is much greater than the 1963-1972 average, presently being on the order of 3 to 4 yrs for both 239Pu and 137Cs. The longer turnover time is consistent with the 137Cs turnover time in the water column deduced from analysis of preserved Lake Michigan alewives. Water filtration experiments have demonstrated that greater than or equal to 75 percent of the plutonium is nonfilterable and is present as colloid or sub-colloidal sized fractions. At an offshore station near Grand Haven, Michigan, the concentration of 137Cs in the epilimnion underwent a slight but significant decline from June to November, whereas 239Pu was reduced to a small fraction of its spring value. In net plankton samples dominated by phytoplankton a much lower mean 137Cs:239Pu ratio is observed than in water samples, indicating a preferential sorption of 239Pu over 137Cs by phytoplankton. The results of these experiments suggest that sorption by phytoplankton (and the subsequent rapid settling from the epilimnion of phytodetritus and/or zooplankton fecal pellets) is responsible for the rapid removal of 239Pu from the epilimnion