FLUORESCENCE-BASED TRACKING OF THE SOURCE AND FATE OF FINE ORGANIC SEDIMENT IN WATERSHEDS

Fluorescence has emerged as a robust tool to assess the source and fate of dissolved organic carbon. Fluorescence excitation-emission matrices (EEMs) act as an organic carbon fingerprint sensitive to the provenance of organic matter, composition (e.g., presence of quinones and amino acids) and characteristics (e.g., redox potential) of its local environment. Unlike isotope, trace metal and organic biomarker analyses, fluorescence measurements are cheap and rapid yet highly sensitive. There is unexploited potential for using these methods to track particulate carbon in watersheds to better understand transport processes of fine sediment—the biggest pollutant in US waterways.

We evaluated the possible use for EEMs of organic carbon extracted from particulate samples as a passive tracer in a suburban Chesapeake Bay watershed. The goal was ultimately to learn how variations in the origin and transport of particulate carbon lead to large differences in metabolism observed between restored and unrestored stream reaches. We used parallel factor analysis to statistically decompose EEMs from potential end-member sources and from suspended sediment samples obtained in baseflow and stormflow events into constituent components (i.e., fluorophore groups). The best fit model had four components with similar spectral signals to components from other watersheds. While suspended sediment samples and end members exhibited similar compositions for three components, the last component was present in much higher amounts in the suspended sediment samples. This component’s non-conservative behavior suggests particulate carbon transformation in floods. The conservative behavior of the remaining three implies that fluorescence can be used with other organic carbon quality measurements to make a robust mixing model. Alternatively, complementary experiments show the feasibility of labeling particulate organic sediment with Rhodamine-WT to use as an active particle tracer in watersheds.