Plutonium in the Laurentian Great Lakes: comparison of surface waters

Studies of fallout 239Pu in Lake Michigan, which is essentially a closed system, have demonstrated that only about 3% of the estimated cumulative deposition is retained in the water column, and that winter mixing results in a homogeneous distribution of radionuclides in the water column by spring. We have assumed that similar mixing behaviour occurs in the other Great Lakes as well, and have obtained surface water and plankton samples during the spring of 1973 from all five Great Lakes. The range of 239Pu concentrations over the five lakes was remarkably narrow in view of the diverse area-to-volume ratios and outflow characteristics of the lakes. The average 239Pu concentrations were ca. 0.6 fCi/l in Lakes Superior, Michigan and Huron, and ca. 0.2 fCi/l in Lakes Erie and Ontario. The most rapid removal of both 239Pu and the chemically dissimilar 137Cs appears to occur in the shallow and eutrophic West Basin of Lake Erie. A consistent relationship between the plutonium concentration in water and that in plankton has been observed in all five Great Lakes, and this suggests that equilibrium exists between aqueous and sorbed plutonium. The concentration of 239Pu in the epilimnion of Lake Michigan is depleted rapidly during early summer but returns to nearly its previous level by the following spring. These results suggest that biological processes are significant factors in determining the turnover time for 239Pu in Great Lakes waters. (orig.)