SEDIMENT FINGERPRINTING FOR IDENTIFICATION OF SOURCES OF FINE-GRAINED CHANNEL DEPOSITS IN SMITH CREEK VA, 2012-2014

Differentiating between upland and in-channel sediment sources is increasingly important to land-management decision making. Sediment fingerprinting offers a tool for analysis of sediment sources and quantifies the relative source percentage of sediment that is delivered to a point of interest in the watershed. Source assessment is accomplished using chemical and physical tracers through a series of statistical operations and concluding with an “unmixing” model. The sediment fingerprinting method does not determine what the temporal aspects of the sourced sediment may have been. Since sediment that is in storage may become remobilized over subsequent events, the source of the stored sediment may be related to previous storm events. For this reason it may be important to apply the sediment fingerprinting approach to see how sources of deposited sediment vary over time in response to different storms.

A study was designed in the Smith Creek watershed (242 km²), Virginia, a tributary to the North Fork Shenandoah River, to determine the sources of fine-grained sediment (< 0.063 mm) in channel deposits. Sediment was collected after storm events at the downstream streamflow station (4^th order), and upstream on 3^rd- and 2^nd- order tributaries. At each site, sediment was collected from two different channel deposits in Smith Creek: 1) channel margin deposits were sampled by coring, and 2) gravel substrate reaches, where fine-grained sediment was sampled by stirring the channel bed and pumping the slurry into a container. The fine-grained portion of the deposited sediment was analyzed for elemental composition (ICP-OES and ICP-MS), stable isotopes (δ¹³C, δ¹⁵N) and radionuclides (⁷Be, ¹³⁷Cs , ²¹⁰Pb). Comparing the deposited sediment to sediment sources sampled in the Smith Creek watershed (agriculture, forest, and streambanks) allows for source apportionment of the deposited sediment for each storm event. This presentation will discuss the results of sediment-source apportionment for storms that occurred between 2012 and 2014.