Using Tracers to Derive Sediment Provenance after the Occurrence of a 500-Year Flood in a Midwestern Stream

In June 2008, catastrophic flooding of the Cedar River inundated a large portion of downtown Cedar Rapids, Iowa. As a result, floodwaters deposited a large amount of sediment throughout the city. The sediment origin is of intense scientific and public interest due to the quantity of deposited sediment and the potential presence of persistent organic pollutants (PCBs, chlordanes, and synthetic fragrances) attached to the sediment. The primary objective of this study is to determine the temporal and spatial patterns of deposited sediment. Two potential sediment sources have been identified: the bed of the Cedar River and the channel banks. We hypothesize that sediment from the two sources was distributed heterogeneously throughout the city. Therefore, samples were gathered from the two source areas, as well as from terrestrial areas within and adjacent to the flooded region. We will compare the deposited sediment gathered from the terrestrial areas with the sediment obtained from the source areas using natural biogeochemical tracing techniques. The tracing techniques will include an analysis of radionuclides (e.g., ⁷Be, ²¹⁰Pb, ¹³⁷Cs), stable isotopes ratios (e.g., δ¹⁵N and δ¹³C), and elemental ratios (C/N). This comparison will allow us to determine the provenance of the sediment particles, which will assist us in developing a sediment budget for the segment of the Cedar River that flows through downtown Cedar Rapids. Further, we will identify the key processes affecting sediment delivery and redistribution that occurred during the flood. Sediment dynamics during floods is poorly described in the existing scientific literature. The methods established in this study will provide for engineers and researchers the ability to track the transport of sediments in similar floods.