Chemical behavior of long-lived radionuclides in the marine environment

Measurements of long-lived radionuclides in the marine environment have provided a wealth of information regarding the physical, biological, and chemical processes which control the behavior of these and many other pollutants in the oceans. Their value as tracers for the dispersion, transport, and fate of pollutants in the oceans is largely dependent on the chemical properties of each individual radioelement. Differences in these properties, particularly in relation to their interaction with biotic or abiotic particulate matter, result in the separation of parent-daughter radioisotopes in the natural radioelement series or in changes in the ratios of fission and activation products. Such differences have provided the means to provide time scales for a variey of transport processes and to determine sedimentation rates. The properties of these radionuclides in the oceans can, in general, be predicted from the chemical properties of the stable elements. For those elements such as plutonium, for which there are no naturally-occurring stable isotopes, studies of their distribution in the oceans have provided a new important understanding of their chemical behavior. This behavior has not always agreed with what would have been predicted from laboratory studies carried out at far higher concentrations. Differences between observed distributions and laboratory predictions have highlighted the importance of correct experimental conditions in order to avoid confusing experimental artifacts. The interaction of radionuclides with particles in the oceans and marine sediments can be described in terms of simple ion exchange or adsorption equilibria

Plutonium and americium in Lake Michigan sediments

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