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Title: Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin

Floodplains, heavily used for water supplies, housing, agriculture, mining, and industry, are important repositories of organic carbon, nutrients, and metal contaminants. The accumulation and release of these species is often mediated by redox processes. By understanding the physicochemical, hydrological, and biogeochemical controls on the distribution and variability of sediment redox conditions we can develop conceptual and numerical models of contaminant transport within floodplains. The Upper Colorado River Basin (UCRB) is impacted by former uranium and vanadium ore processing, resulting in contamination by V, Cr, Mn, As, Se, Mo and U. Previous authors have suggested that sediment redox activity occurring within organic carbon-enriched bodies located below the groundwater level may be regionally important to the maintenance and release of contaminant inventories, particularly uranium. To help assess this hypothesis, vertical distributions of Fe and S redox states and sulfide mineralogy were assessed in sediment cores from three floodplain sites spanning a 250 km transect of the central UCRB. Our results support the hypothesis that organic-enriched reduced sediments are important zones of biogeochemical activity within UCRB floodplains. Furthermore, we found that the presence of organic carbon, together with pore saturation, are the key requirements for maintaining reducing conditions, which were dominated by sulfate-reductionmore » products. Sediment texture was found to be of secondary importance and to moderate the response of the system to external forcing, such as oxidant diffusion. Consequently, fine-grain sediments are relatively resistant to oxidation in comparison to coarser-grained sediments. Exposure to oxidants consumes precipitated sulfides, with a disproportionate loss of mackinawite (FeS) as compared to the more stable pyrite. The accompanying loss of redox buffering capacity creates the potential for release of sequestered radionuclides and metals. Because of their redox reactivity and stores of metals, C, and N, organic-enriched sediments are likely to be important to nutrient and contaminant mobility within UCRB floodplain aquifers.« less
Authors:
 [1] ;  [2] ;  [3] ;  [1] ;  [2] ;  [4] ;  [1] ;  [4] ;  [5] ;  [1]
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Synchroton Radiation Lightsource
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Synchroton Radiation Lightsource; Stanford Univ., CA (United States). Dept. of Environmental Earth System Science
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Science Lab.
  4. Stanford Univ., CA (United States). Dept. of Environmental Earth System Science
  5. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth Sciences Division
Publication Date:
Report Number(s):
PNNL-SA-121839
Journal ID: ISSN 0048-9697; PII: S0048969717301195
Grant/Contract Number:
AC02-76SF00515; P41GM103393; AC02-05CH11231; AC0576RL01830
Type:
Accepted Manuscript
Journal Name:
Science of the Total Environment
Additional Journal Information:
Journal Volume: 603-604; Journal ID: ISSN 0048-9697
Publisher:
Elsevier
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23); USDOE Office of Legacy Management (LM)
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; 58 GEOSCIENCES; Floodplain sediments; Redox processes; Iron and sulfur; X-ray absorption spectroscopy; Upper Colorado River Basin
OSTI Identifier:
1348920
Alternate Identifier(s):
OSTI ID: 1349082