Plutonium and americium in Lake Michigan sediments

The vertical distributions of 239,240Pu, 238Pu, and 137Cs have been measured in sediment cores taken from Lake Michigan. Sections from a limited number of cores have been analyzed for 241Am. In addition, grab samples from ten locations in the southern basin of the lake have been analyzed for phase distribution of 239,240Pu using a sequential extraction technique. The results indicate that the 239,240Pu, 238Pu, and 137Cs from weapons testing, and the 241Am formed in situ are concentrated in the sediments. A comparison of the total deposition of 239,240Pu and 137Cs indicates that 137Cs may be valuable as a monitor for 239,240Pu deposition in the sediments. Values of the 238Pu/239,240Pu ratio are in agreement with values reported in Lake Ontario sediments (and Lake Michigan plankton) and show little variation with depth. 241Am data support the concept of in situ production with little preferential mobility after formation. Studies of sedimentary phase distributions show that 239,240Pu is associated with hydrous oxide phases which are chemically stable under the prevailing conditions in lake sediments. Since Lake Michigan sediments remain aerobic, relatively little 239,240Pu is available for chemical mobilization from the hydrous oxide or organic phases present in the sediments