Collaborative Development of Conceptual Remediation Portfolios for an Abandoned Uranium Mill Site - 21145

The U.S. Department of Energy (DOE) Office of Legacy Management (LM) is responsible for the long-term stewardship of abandoned uranium mill sites. Although surface remediation was completed for all mill sites in the 1990's, groundwater contamination persists at some of the sites and presents various risks. To efficiently address the most pressing contamination issues, LM generated a site ranking and teamed with DOE's National Laboratories Network (NLN) to collaboratively address some of the highest risk sites, including the Tuba City, Arizona, Disposal Site. Milling occurred at the Tuba City site from 1956 to 1966. Mill tailings were slurried into an unlined impoundment and ponds, resulting in groundwater contamination. Following a period of abandonment, surface remediation was performed in 1988-1990, to safely consolidate contaminated materials under an engineered cover. Groundwater characterization led to the installation of a pump-and-treat system in 2000. Contaminated groundwater was extracted, treated, and returned to the aquifer until 2014, when wellfield and treatment plant challenges led to scaled-back operations. Since 2015, interim groundwater extraction coupled with hydrologic and geochemical evaluations were performed to inform a revised Groundwater Compliance Action Plan. LM engaged the NLN and stakeholders to address risks identified for Tuba City and identify technologies and strategies to mitigate those risks. LM/NLN/stakeholder collaborative meetings were conducted over an 8-week period. Technical subgroups were focused on optimizing characterization strategies and on developing site remediation recommendations. The site characterization and remedy subgroups' efforts were integrated with the stakeholders/end-state vision subgroup. Objectives for groundwater remediation and preferred approaches expressed by stakeholders were the guideposts for the site characterization and remedy subgroups. A refined conceptual site model served as the basis for deliberations in all subgroups. The conceptualization identified zones where contaminants may be present, as primary sources such as mill tailings, mineralized secondary sources in the vadose zone and shallow groundwater, and contaminated groundwater in the plume. The characterization subgroup focused on identifying efficient and effective methods to confirm the quantity and nature of the contamination in each zone to better understand aquifer and contaminant source conditions and to focus development of remedial alternatives on the most significant targets. Remediation technology recommendations were also developed to address the identified zones. The process highlighted the need for a comprehensive strategy to address the most significant contributing sources while providing effective plume control. Recommended technologies fell into categories of source removal (excavation or in situ recovery), in situ immobilization (permeable reactive barrier, geochemical reduction), ex situ immobilization (surface barriers), isolation (groundwater bypass), plume containment (groundwater extraction and treatment), and natural attenuation. High-level implementation paths were plotted for each recommendation, including data collection and contaminant transport projections, pre-feasibility studies, and conceptualizations of how each recommendation could be applied. Go/no-go decision points were built into the implementation plans. As the remediation technology recommendations came into focus, the inherent weaknesses of applying any single technology to sitewide contamination became apparent. For example, a 'pump-and treat' system alone could contain the groundwater plume but would likely require an unacceptably long lifespan of operation due to the gradual release of contaminant mass from solid sources. The need for a comprehensive approach led to the creation of 'remedy portfolios', combining multiple remedial technologies that could be deployed simultaneously, or in time-phased steps. Portfolio concepts were informed by technical evaluation of the source zones and effectiveness of the component technologies and encompassed the end-state vision for the site as expressed by stakeholders. The primary statement of stakeholders' end-state vision is to protect both the quality and quantity of water in the Navajo sandstone aquifer (N-aquifer). As an example, one of the remedy portfolios emphasized containment of the groundwater plume as a short-term component and source removal as the long-term component that will ultimately lead to achievement of the end-state vision. Plume containment would likely involve operation of a pump-and-treat system throughout the duration of source characterization and transport modeling activities and as the source-removal component (excavation or in situ recovery) is being evaluated and implemented (proof of process, design, evaluation against other alternatives, and field implementation). Five portfolios were initially generated and included recommended technologies in time-sequenced or spatial combinations. The viability of each remedy portfolio will be evaluated, as site characterization and contaminant transport projections mature. The process aligns with regulatory guidance for the development of groundwater compliance strategies for abandoned uranium mill sites and with the specific efforts to develop a revised Groundwater Compliance Action Plan for the Tuba City site. Commitment to the process should result in regulatory concurrence and site remediation that successfully achieves technical objectives, meets regulatory requirements, and gains acceptance by tribal partners (authors)