Transport of fallout plutonium to the ocean by the Mississippi River

Abstract

Mississippi River suspended sediment shows a continual decrease of 239.240Pu content over a seven-year time period. An exponential curve best-fit through the data predicts a half-time of decrease equal to 4.3 years. However, unsupported 210Pb, a naturally occurring nuclide at steady state in the environment, maintains constant concentration over the same time period. The decrease in Pu content of river sediment results from several factors: cessation of atmospheric weapons testing; transport of Pu to deeper levels of soil profiles; storage of sediment in floodplains and behind dams; and dilution by erosion of older, prebomb soil material. Most of the fallout Pu in the Mississippi drainage basin will remain on the continent unless there are major changes in erosion and sediment transport patterns in the basin itself.