Introduction: Uranium is a toxic and problematic redox-active contaminant at U.S. Department of Energy (DOE) legacy nuclear sites, present in more contaminant plumes than any other radionuclide except for tritium. Elevated concentrations of uranium in groundwater pose ongoing threats to human and ecosystem health, and challenges site cleanup and closure.
The ability to predict subsurface fate and transport of redox-active radionuclides, contaminants, and biogeochemical critical elements (“BCEs”, including C, N, S, and Fe) is hindered by a lack of knowledge of their speciation, reactivity, and the biogeochemical pathways by which they are transformed. Organic-rich sediments, common to river basin deposits, as well as marine, lacustrine, and estuarine environments, comprise a particularly important class of subsurface environment; they host a myriad of (coupled) microbial and geochemical redox processes that ultimately control carbon, nitrogen, and metal cycling. By releasing or sequestering soluble species, organic-rich sediments profoundly influence the redox status and quality of groundwaters, floodplains, and other host landforms, and help control global carbon and nitrogen cycles.
Relatively thin lenses of silt‑, clay-, and organic-rich sediments in the subsurface at the contaminated Old Rifle, CO uranium ore processing site strongly accumulate uranium, accounting for up to 95% of the uranium inventory at some locations. These sediments, which contain reduced uranium, iron, and sulfur are referred to as naturally reduced zones (NRZs). NRZs represent biogeochemical hotspots that interact with flowing groundwater along extensive interfaces. NRZ-aquifer boundaries provide lenses through which uranium, BCEs and other redox active species diffuse. Importantly, NRZs are believed to be common depositional features of floodplains, occurring along active river margins as a result of alluvial channel migration and overbank flood events. We predict that NRZs are common in floodplains throughout the upper Colorado River basin (CRB) and surrounding areas and are of regional importance to uranium and BCE fate and transport.
Naturally reduced zones as controls over regional behavior of BCEs, radionuclides, and heavy metals: The large stores of natural organic matter (NOM) in NRZs fuel microbial activity, which oxidizes organic carbon and ultimately transfers electrons to sulfate, Fe(III), nitrate, and U(VI), leading to their reduction. Reduced uranium, U(IV), is much less soluble than its oxidized form, U(VI), and therefore the presence of reducing conditions leads to uranium accumulation in NRZs. Conversely, in the presence of dissolved oxygen, microbial decomposition of organic nitrogen can produce nitrate (and nitrite), potent oxidants for U(IV) and other reduced species. This behavior provides a mechanism for the historical accumulation of uranium within NRZs when surficial mine tailings were present at the site and uranium-bearing water was percolating down into the aquifer. There is concern that uranium and BCEs are being re-released from NRZs under present-day conditions. The presence of NRZs and their attendant biogeochemical redox activity are of intense interest for their likely roles in radionuclide, metal, and BCE fate and transport in the subsurface.
Mission: The mission of the SLAC SFA is to investigate fundamental biogeochemical redox processes that control uranium and BCE behavior, with emphasis on hot spots of microbial and chemical activity in organic-rich fine-grained sediments. This project focuses on alluvial floodplains in the upper Colorado River basin (CRB) and surrounding areas. We will provide knowledge critical to uranium and BCE fate and transport at a range of scales, from atoms and molecules to bacterial cells, sediment pore spaces, floodplains, and river basins (the upper CRB specifically). The results of this research will transcend regional and (sub)surface scales and illuminate BCE transformations of global significance, particularly for carbon and nitrogen.
Seminal scientific challenges: We have identified four major inadequacies in our knowledge that limit the application of modeling strengths of the Subsurface Biogeochemical Research program (of the DOE BER Climate and Environmental Science Division) to understanding radionuclide, metal, and BCE subsurface fate and transport:
- The speciation of U(IV) stored in NRZ sediments remains poorly constrained. This subject is a high research priority because speciation controls subsurface uranium behavior and provides the basis for predictive modeling.
- Microbial degradation of NOM in NRZs is intimately linked to reductive transformations of uranium, iron, sulfate, and nitrate. Our ability to predict and model aquifer redox processes controlling the fate of metals and BCEs is limited by a lack of knowledge about the coupling of microbially mediated NOM decomposition pathways linked to metal fate across scales from bacterial cells to sediment pores. This is an exciting frontier in subsurface system science, for which SBR core strengths will significantly advance our understanding.
- The mechanisms and rates of uranium release from NRZ sediments are not known. Yet, such knowledge is critical for predictive modeling of uranium behavior in floodplains. We posit that uranium mobility is closely linked to nitrogen cycling, particularly the production of nitrate and nitrite, which are powerful oxidants for uranium. This is another exciting frontier involving complex coupled biogeochemical processes that plays to SBR strengths.
- Evidence suggests that NRZs play major biogeochemical roles regionally in moderating uranium and BCE fate and transport. In spite of their likely importance, the wider impacts of NRZs on uranium and BCE behavior in floodplains are poorly known. We posit that biogeochemical models accurately describing subsurface element cycling (inclusive of fate and transport) will need to account for NRZs.
Unique Approach: We are investigating biogeochemical redox processes that control uranium and BCE behavior in alluvial floodplains in the upper CRB and surrounding areas with emphasis on hot spots of microbial and geochemical activity in organic-rich fine-grained sediments.
We will use carefully controlled laboratory studies to interpret data from field studies and materials collected from field sites, while field data and observations will be used to guide laboratory studies. We will examine length scales ranging from molecular (uranium speciation and uranium-ligand interactions) to sediment pores (microbe-NOM-uranium interactions) to millimeters (diffusion-mediated NRZ-aquifer interfaces) to the upper Colorado River basin (plumes). We will use innovative combinations of techniques to accomplish this program, including coupling x-ray and electron microscopy coupled to stable isotope probe (SIP) and catalyzed reporter deposition (CARD) fluorescence in-situ hybridization (FISH) imaging. We also will couple synchrotron-based x-ray spectroscopy to electrochemistry. We will use cultivation and genomic approaches to link microbial metabolism to uranium and BCE behavior. This project will leverage collaborations across two SBR SFA programs and three DOE offices to provide new fundamental understandings of contaminant and BCE behavior region-wide.
Core competencies. The research team has deep expertise in synchrotron-based techniques (Bargar, Dynes, Fendorf, Regier, Webb, Bone), biogeochemistry (Bargar, Fendorf, Francis, Spormann, Williams), low-temperature aqueous geochemistry (Bargar, Fendorf), soil science (Fendorf), molecular microbiology and microbial imaging (Francis, Spormann), SIP techniques (Mayali, Spormann), field biogeochemistry (Bargar, Fendorf, Williams), geophysics (Williams), electrochemistry (Jones, Spormann), spectroscopy (Bargar, Kukkadapu, Persson, Washton) and uranium-contaminated site management (Bush, Metzler).
Expected outcome and relevance: The proposed activities will provide improved understandings of biogeochemical drivers of redox-active radionuclide, metal, and BCE behavior at a range of scales from individual molecules to a basin-wide perspective. Products of this effort will include:
- New knowledge of the speciation, geochemical stability, and biogeochemical reaction kinetics of uranium and BCEs, including carbon (NOM).
- New coupled biogeochemical process models for uranium and BCE fate and transport in complex subsurface environments within the upper CRB and surrounding areas.
- New insights into energy-environment-climate connections and their implications for energy production and use.
- Enabling the application of SBR modeling competencies to radionuclide and metal fate and transport at a regional scale.
- New insights into methods for manipulating subsurface biogeochemical processes and using remediation strategies.
Information obtained through these studies will provide insights valuable to management and cleanup of contaminated DOE legacy sites in this region.
Support: This program is funded by the Subsurface Biogeochemistry program within the U.S. Department of Energy, Office of Biological and Environmental Research, Climate and Environmental Sciences Division. Funding for SSRL is provided by the Department of Energy, Office of Basic Energy Sciences.
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