Environmental dissolved organic carbon (DOC) modifies microbial respiration and the fate of heavy metals on global scales. Hatcher, Chen, and Waggoner et al. (2014, 2015) indicated that natural DOC can undergo Fenton-like reactions and generate condensed aromatic and alicyclic aliphatic compounds in simulated sunlight and iron. However, the presence of non-pyrogenic “black carbon”, i.e., produced without wildfires or combustion, is not understood outside of laboratory settings.  This research aimed to isolate organic matter across a redox-active, urban, and mine-impacted stream. We are also studying how in-stream carbon compares to the leachates from engineered soils being considered for future remediation. We hypothesized that there could be evidence of condensed aromatic carbon formation due to redox swings, sunlight, and rapid oxidation of iron and organic matter. Colorless groundwater from the mine has DO=0%, ORP = -70 mV, and total dissolved iron = 8.6 ± 0.4 mg/L, suggesting highly reducing conditions. Mine discharge mixes with a sunlit creek and recovers to peak DO=136% in summer, ORP = 139.6 mV, dissolved Fe = 0.09 mg/L with a rust-colored streambed ~400 m downstream. Samples for DOC have also been collected over two field campaigns in the winter and summer of 2022 at the mine discharge outflow, upstream, and downstream. For each trip, over 100L of water has been returned to the lab, filtered, and processed for carbon isolation via a novel combination of tandem resin and anion-exchange columns. This process successfully removed ionic interferants (i.e., S, Ca, Na, Mg, Fe, and Zn &lt; 1 mg/L per ion) and concentrated DOC for further analysis. The carbon within particulate flocs has been extracted via repeated hexane extractions.  Ongoing work applies absorbance, fluorescence spectroscopy, and Fourier-transform ion cyclotron resonance (FT-ICR-MS) mass spectrometry to column-isolated carbon extracts. Preliminary optical results do not show a significant difference in in-situ DOC across the upstream, mine adit, and downstream reaches for the winter campaign. However, carbon quality significantly differed from remedial biochar and engineered soil leachates. This presentation will discuss high-resolution carbon data and why alkaline waters may have quenched Fenton-like reactions in this mine. Overall, the work illustrates a novel suite of analytical techniques for studying aquatic redox reactions and environments where carbon may be added in remediation efforts.

High-resolution carbon analysis for the study of redox reactions in a mine-impacted urban stream (Missouri, USA)

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