Radioactive cesium isotope ratios as a tool for determining dispersal and re-dispersal mechanisms downwind from the Nevada Nuclear Security Site

Abstract

Fractionation of the two longer-lived radioactive cesium isotopes (135Cs and 137Cs) produced by above ground nuclear tests have been measured and used to clarify the dispersal mechanisms of cesium deposited in the area between the Nevada Nuclear Security Site and Lake Mead in the southwestern United States. Fractionation of these isotopes is due to the 135-decay chain requiring several days to completely decay to 135Cs, and the 137-decay chain less than one hour decay to 137Cs. Since the Cs precursors are gases, iodine and xenon, the 135Cs plume was deposited farther downwind than the 137Cs plume. Sediment core samples were obtained from the Las Vegas arm of Lake Mead, sub-sampled and analyzed for 135Cs/137Cs ratios by thermal ionization mass spectrometry. The layers proved to have nearly identical highly fractionated isotope ratios. This information is consistent with a model where the cesium was initially deposited onto the land area draining into Lake Mead and the composite from all of the above ground shots subsequently washed onto Lake Mead by high intensity rain and wind storms producing a layering of Cs activity, where each layer is a portion of the composite.