The Old Rifle site in the upper Colorado River Basin received uranium contamination through mill processing tailings for more than 50 years.  After removal of tailings and surface sediments the aquifer concentration of uranium has failed to naturally attenuate.  A series of exploratory wells identified naturally reduced zones (NRZs) within the subsurface, harboring the bulk of the subsurface solid phase uranium.  These NRZs are discontinuous, thin lenses (1-30 cm) of fine-grained alluvium with elevated concentrations of uranium, sulfur, and organic carbon.  We propose that these organic-rich zones are microbial hot spots for redox transformations within the aquifer and provide a source of uranium to the aquifer maintaining the contamination plume on the site.  Buried plant particulate organic carbon (POC) provides source material driving reducing conditions within the NRZ sediments.  Characterization of sediment organic carbon is critical for constraining the biogeochemical cycling of redox elements in the subsurface, modeling uranium mobility, and understanding the long-term stability of solid-phase uranium within the subsurface.

Here we present characterization of subsurface particulate organic carbon by carbon K-edge near edge X-ray absorption fine structure (C-NEXAFS) spectroscopy and Fourier transform infrared (FT-IR) spectroscopy.  Additionally, extracts of sediment organic carbon were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS).  This combined approach provides unique characterization and insight into the bulk carbon and specific classes of organic carbon within subsurface reduced zones.