Tracing Find Sediment Transport in Fluvial Systems Using Fallout Radionuclides

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Despite our understanding of the gross paths taken by sediments and sediment-bound pollutants, we have a poor knowledge of the transit time for their passage through the drainage network and to receiving bodies. The goal of this project is to develop an innovative but broadly applicable tool to investigate the erosion mechanisms of fine sediment from the landscape and the transport characteristics of fine suspended particulates in channels. Specifically, we aim to answer questions on the source of the suspended sediment (Is it from sheet erosion, rill erosion, bank erosion, or resuspension of bottom sediments?); the delivery ratios of fine sediment from the watershed (i.e., the proportion of sediment that is eroded from the landscape and delivered to the receiving waters or the proportion that is transported only part way and resides on the landscape or in the streambed); the proportions of suspended sediment that is newly delivered from the soil mantled landscape versus resuspended from the bed; the transit distance of fine particulates; the residence time of particulates; and the flux of fines from the watershed. We will employ a novel technique we have developed that uses fallout radionuclides (7Be, 137Cs, and 210Pb) as tracers of fine sediments suspended by streamflow.The two-year study will be focused in the Yellowstone River watershed in Wyoming and Montana. During the first year we will address the kinetics of the tracing and the characterization of the radionuclide signature of the snowpack, soils, bottom sediment, and suspended sediment. During the second year we will characterize the sources and transport of fine sediment over a distance 900 km during a snowmelt-driven hydrograph. The experimental design includes collection of precipitation, soil cores, floodplain sediment, channel bottom sediment, and suspended sediment samples at various locations in the drainage networks; gama spectroscopy of samples for naturally occurring short-lived (7Be) and lonaer-lived (137Cs 210Pb) fallout radionuclides and other important radionuclide tags (214Bi, 40K) that adsorb to sediment providing tracers of sediment movement over different time scales; tracing the movement of mine tailings that were released from an upland source area in the study area in 1950; analyses of particle sizes and concentrations; and wave kinematic and mass balance modeling.