Determining the sources of fine-grained sediment using the Sediment Source Assessment Tool (Sed_SAT)

Abstract

A sound understanding of sources contributing to instream sediment flux in a watershed is important when developing total maximum daily load (TMDL) management strategies designed to reduce suspended sediment in streams. Sediment fingerprinting and sediment budget approaches are two techniques that, when used jointly, can qualify and quantify the major sources of sediment in a given watershed. The sediment fingerprinting approach uses trace element concentrations from samples in known potential source areas to determine a clear signature of each potential source. A mixing model is then used to determine the relative source contribution to the target suspended sediment samples. The computational steps required to apportion sediment for each target sample are quite involved and time intensive, a problem the Sediment Source Assessment Tool (Sed_SAT) addresses. Sed_SAT is a user-friendly statistical model that guides the user through the necessary steps in order to quantify the relative contributions of sediment sources in a given watershed. The model is written using the statistical software R (R Core Team, 2016b) and utilizes Microsoft Access® as a user interface but requires no prior knowledge of R or Microsoft Access® to successfully run the model successfully. Sed_SAT identifies outliers, corrects for differences in size and organic content in the source samples relative to the target samples, evaluates the conservative behavior of tracers used in fingerprinting by applying a “Bracket Test,” identifies tracers with the highest discriminatory power, and provides robust error analysis through a Monte Carlo simulation following the mixing model. Quantifying sediment source contributions using the sediment fingerprinting approach provides local, State, and Federal land management agencies with important information needed to implement effective strategies to reduce sediment. Sed_SAT is designed to assist these agencies in applying the sediment fingerprinting approach to quantify sediment sources in the sediment TMDL framework. Introduction Identifying the sources of fine-grained instream sediment flux in a watershed is an essential part of developing total maximum daily load (TMDL) management strategies designed to reduce suspended sediment in streams (Gellis, Noe, and others, 2015). Fine-grained silt and clay are of particular concern as they are a source of habitat degradation, affect water supply intakes and reservoirs, and commonly carry pollutants (Larsen and others, 2010). The sediment fingerprinting method uses trace element concentrations from fine-grained sediment samples (<63μm) in potential source areas in a watershed to determine a clear signature or “fingerprint” for each potential source through a series of computational and statistical steps (Gellis and Walling, 2011). Potential target samples include but are not limited to suspended sediment, bed, passive, and floodplain deposits (Miller and others, 2015). Tracer concentrations found in target samples can then be compared to the source fingerprints through a mixing model to determine relative source contributions (Collins and others, 2010; Gellis, Noe, and others, 2015). This approach allows us to answer the important question of whether the fine-grained sediment in target samples is derived from upland areas (such as agricultural fields, forests, or construction sites) or the channel (e.g. streambanks) sources, allowing for more effective, targeted management strategies (Gellis, Noe, and others, 2015).