AROMATICITY OF DISSOLVED ORGANIC CARBON IN URBANIZED HEADWATER STREAMS, GREENVILLE, SOUTH CAROLINA

Dissolved organic carbon (DOC) in aquatic systems represents a diverse and dynamic class of compounds. These compounds play a dynamic role in aquatic biogeochemistry, for example in binding and transporting transition metals, but its structural complexity makes it difficult to analyze. The object of this study was to better understand how the composition of DOC changes downstream in the first 300 meters of urban headwaters and during storm events. Two hundred and seventy samples were collected from four urban headwater streams in northwestern South Carolina, including storm event samples and “stream continuum” samples. UV spectrum analysis (SUVA₂₅₄) paired with total DOC measured by UV-persulfate digestion was used to estimate the percent aromatic content (aromaticity) of the DOC. Percent aromaticity of DOC has been used to discriminate between terrestrial and aquatic carbon sources. The storm event samples show that DOC consistently peaked (8-10 mg C/ L) near peak stage height, then returned to pre-storm concentrations (2-3 mgC/L) fairly rapidly, while percent aromaticity increased slowly during the storm and continued to do so for hours after the storm or peaked with the storm but remained higher than normal until the end of the data collection period. Samples collected laong the first 300 m of the streams showed a 20-40% decrease in DOC aromaticity with a maximum values of 35-80% aromaticity and minimum values of 20-38% aromaticity. There was no correlation observed between percent aromaticity and total DOC concentration, suggesting that different processes are responsible for each. The percent aromaticity observed at the headwaters of the study streams is twice as great as that observed in other studies conducted in larger rivers and streams. The trends observed during and after storm events can be explained by the high DOC (8-50 mgC/L) and low % aromaticity (18-19%) of impervious surface runoff that is a significant component of the storm peak flow in urbanized watersheds, followed by slower release from the mostly wooded riparian zones, which have buildups of terrestrial organic matter in various stages of decomposition. The rapid changes in DOC composition observed in the first 300 meters of headwater streams and during storms are indicative of the biogeochemical reactivity of headwater streams.