Sediment fingerprinting with long- and short-lived radionuclide tracers in the Root River watershed, southeastern Minnesota

Abstract:

The field of sediment fingerprinting has evolved rapidly over the past decade and is poised to improve our understanding not only of sediment sources, but also the routing of sediment through watersheds. Such information is essential for understanding and modeling human impacts on erosion and sediment routing at the watershed scale. In this study we use long- (Beryllium-10, ¹⁰Be) and short-lived (Lead-210 and Cesium-137, ²¹⁰Pb and ¹³⁷Cs, respectively) radionuclide tracers associated with suspended sediment to quantify sediment sources and channel-floodplain exchange across a range of watershed scales from 10 km² to 4500 km² in in the Root River, southeastern Minnesota, USA. The uppermost quarter of the Root River watershed was glaciated repeatedly during the late Pleistocene and is characterized by low relief agricultural fields and fine textured soils. The remainder of the watershed lies within the driftless area of the upper Midwestern US, which has not been glaciated in at least the past 500,000 years, and is characterized by karst topography, relatively steep hillslopes and bedrock channels that debouch into a wide, aggrading alluvial valley. The structure of the landscape exerts strong control on sediment generation and transport. Geochemical results indicate a highly variable erosion history, with significant variability of ¹⁰Be concentrations in source areas (agricultural fields, forested hillslopes, and alluvial floodplains and terraces) and inverted ¹⁰Be depth profiles (higher concentrations at depth) in floodplains, suggesting unsteady erosion and significant storage of legacy sediment. Concentrations of ¹⁰Be and ²¹⁰Pb associated with suspended sediment show a systematic disparity in normalized concentrations, indicating that significant storage and re-suspension occurs in both systems as the sediment is routed through the channel-floodplain complex.