Initiated in 2007, the Integrated Field-Research Challenge (IFRC) research will elucidate the mechanisms and rates of removal for important DOE contaminants in plumes that emanate from contaminated source zones, and the effects of source zone remediation measures on those plumes. The Oak Ridge IFRC is one of three IFRCs (others are Old Rifle UMTRA site and the Hanford 300 area) funded by the U.S. Department of Energy's (DOE's) Subsurface Biogeochemical Research (SBR) Program. The Oak Ridge project is entitled Multiscale Investigations on the Rates and Mechanisms of Targeted Immobilization and Natural Attenuation of Metal, Radionuclide and Co-Contaminants in the Subsurface. Together, the three IFRC's seek to support DOE's cleanup mission and long-term stewardship responsibilities by providing new insights into the behavior of contaminants through multidisciplinary field scale research.
The Oak Ridge IFRC consists of multiple directed investigations to improve scientific understanding of contaminant fate and transport and develop practical predictive tools to apply that understanding to most DOE contaminated sites. The Oak Ridge Field Research Center (ORFRC) serves as the field laboratory for studying contaminant attenuation rates and mechanisms. This field laboratory contains the variety of contaminants of interest to DOE (uranium, technetium, nitrate, acidity, and volatile organic carbon species), the diversity and scale of known contaminant ground water pathways, and the promising field-scale manipulation tests of in situ contaminant immobilization already established at this site. Oak Ridge IFRC researchers will integrate the results of the separate studies into a predictive model that informs the selection of remediation alternatives for this highly contaminated ground water plume and for similar plumes at other DOE-managed sites.
Research focuses on measuring the rates and extents of natural attenuation of three main contaminants: nitrate; uranium; and technetium. A combination of geophysical, chemical, microbial, and hydrological analytical tools allow OR IFRC researchers to identify contaminant pathways, characterize active attenuation or transition zones, and understand the dynamics and effects of ground water recharge. Results are critical to establish an accurate predictive capability over the long-term and to determine if active cleanup measures are required.
About the Oak Ridge IFRC project
Why Oak Ridge IFRC research is important
Oak Ridge IFRC research tasks
About the Oak Ridge IFRC project [top]
This project, which began in 2007, is entitled Multiscale Investigations on the Rates and Mechanisms of Targeted Immobilization and Natural Attenuation of Metal, Radionuclide and Co-Contaminants in the Subsurface. Its goal is to provide a mechanistic understanding of the linkages among ground water flow, lithology, biogeochemical processes, and the metabolic activity of microorganisms that limit the fate and transport of metal and radionuclide contaminants in the subsurface of DOE sites. IFC research is motivated by the need to (a) understand the limits of natural processes in attenuating contaminant plumes, (b) delineate uncertainties in how recharge events affect subsurface transport of contaminants, and (c) define the level of source zone mitigation that is required to protect human health and the environment. Because the results of this research will improve our ability to predict the feasibility of natural attenuation of contaminant plumes and the impact of source zone remediation at multiple scales, the research will provide information to aid decision-making about remediation measures that may be required for effective long-term stewardship at DOE sites.
The primary objective of the project is to advance the understanding and predictive capability of coupled hydrological, geochemical, and microbiological processes that control the in situ transport, remediation and natural attenuation of metals, radionuclides, and co-contaminants at multiple scales ranging from the molecular to the watershed. IFRC research focuses on determining the key coupled hydrobiogeochemical factors such as pH, electron donor utilization, and redox conditions along contaminant pathways and within specific transition zones that control the fate and transport of uranium, technetium, and co-contaminant nitrate within spatially distributed source zones and ground water plumes at the ORFRC. Because remedial decisions are made at the watershed scale, investigating and understanding these processes at this scale are necessary for making informed remedial decisions. While natural processes at several uncontaminated watersheds (e.g., DOE Walker Branch Watershed in TN, Coweeta Watershed in NC, and Hubbard Brook Watershed and USGS Mirror Lake site both in NH) are being investigated, there are no comparable watershed-scale contaminated sites where contaminant fate and transport issues are being investigated. The ORFRC is the first watershed-scale research facility for multi-institutional, multidisciplinary investigations of contaminant issues. The specific objectives of ORFRC research are to
Anticipated research products include:
- quantify recharge pathways and other hydraulic drivers for ground water flow and dilution of contaminants along flow pathways and determine how they change temporally and spatially during episodic events, seasonally, and long term;
- determine the rates and mechanisms of coupled hydrological, geochemical, and microbiological processes that control the natural attenuation of contaminants in highly diverse subsurface environments and over scales ranging from molecular to watersheds;
- explore novel strategies for enhancing the subsurface stability of immobilized metals and radionuclides;
- understand the long-term impacts of geochemical and hydrologic heterogeneity on the remobilization of immobilized radionuclides;
- improve our ability to predict the long-term effectiveness of remedial activities and natural attenuation processes that control subsurface contaminant behavior across a variety of scales.
Why Oak Ridge IFRC research is important [top]
- predictive monitoring and modeling tools that can be used at sites throughout the DOE complex to inform and improve the technical basis for decision making, and to assess which sites are amenable to natural attenuation and which would benefit from source zone remedial intervention;
- recommendations and strategies, conveyed via technical reports and stakeholder workshops, that will assist local decision makers in making scientifically informed choices on ground water remediation actions; and
- scientific publications that convey our improved understanding of in situ contaminant attenuation rates and mechanisms and the long-term effectiveness of remedial activities relevant to in situ remediation and stewardship at DOE sites.
Similar to other DOE sites nationwide (Whicker et al. 2004), historical disposal of wastes from the operation of three industrial plant sites (K-25, Y-12, and ORNL) on the Oak Ridge Reservation (ORR) have created extensive areas of subsurface contamination. Inorganic, organic and radioactive wastes were released into thousands of unlined trenches, pits, ponds and streams by intentional disposal and accidental leaks and spills. These wastes have resulted in approximately 1,500 acres of contaminated ground water on the ORR (Fig. 1).
Figure 1. Overview of the Oak Ridge Reservation
Much of the original contamination is now present as secondary sources bound to the sediment-rock matrix material outside of the original disposal sites (DOE 2004). The secondary source areas are extensive and encompass regions on the watershed scale (tens of km). For example, although the S-3 Ponds at Y-12 have been capped, the vast majority of contaminant mass has migrated from the Ponds into the underlying geologic media where it has precipitated or adsorbed onto the solid phase or diffused into the matrix. Leaching of these primary and secondary source zones has created contaminated ground water plumes that tend to be stable and eventually discharge to and contaminate surface water which, in turn, has deleterious ecological impacts and potential adverse consequences for human health. Trend plots (Fig. 2) from several Bear Creek Valley wells show that contaminant concentrations have been relatively stable to slightly decreasing for over 16 years; although concentrations are gradually increasing in some cases.
Figure 2. Ground water contaminant data from Bear Creek Valley over 16 years
shows steady or increasing trends in
contaminant concentrations coming from the S-3 Ponds source.
The ORR is not unique among DOE sites in having a mixture of contaminant plumes and poorly understood hydrobiogeochemical conditions. For sites located in humid regimes, the subsurface transport processes are driven by large annual rainfall inputs (>1400 mm/y) with as much as 50% of the infiltrating precipitation resulting in ground water and surface water recharge (10 and 40%, respectively). The subsurface media are often structured making them conducive to rapid preferential flow coupled with significant matrix storage. Preferential flow paths are highly interconnected and surround a low-permeability, high porosity sediment matrix. When storm flow infiltrates into the media, large hydraulic and geochemical gradients result causing nonequilibrium conditions during solute transport. In these systems, the sediment and bedrock matrix (secondary sources) have been exposed to migrating contaminants for many decades, and thus account for a substantial inventory of the total contamination.
A significant limitation in assessing remediation needs of the secondary contaminant sources is the lack of information on the rates and mechanisms of processes that control contaminant migration. Without this knowledge, it is impossible to assess the contribution of the secondary sources to the total off-site migration of contaminants. Furthermore, the contaminant fluxes emanating from the secondary sources are often so high as to prevent attenuation of the ground water plume. Interventions such as source actions may be a prerequisite for effective and rapid natural attenuation. Two strategies of source control are possible: (1) reduce the soluble contaminant concentration at the source and (2) control the flux from the source to ground water by decreasing recharge. Before determining whether or how to intervene several key questions need to be considered:
- Can source intervention actions significantly and effectively reduce the flux of metals and radionuclides?
- How long is a source action likely to be effective, i.e., will contaminants remobilize if the action is discontinued?
- Will the plume attenuate naturally if the contaminant flux from the secondary source is reduced?
- What types and frequency (spatial and temporal) of measurements are needed to determine if the plume is actually attenuating?
Oak Ridge IFRC research tasks [top]
The Oak Ridge IFRC project consists of four major research tasks:
- systematically exploring geophysical responses to flow and transport properties to characterize contaminant plumes and identify transport pathways
- quantifying the rates and mechanisms of coupled processes that control natural attenuation of U, Tc, and nitrate along contaminant pathways and transition zones using a series of novel tracer tests, isotopes, well sampling, hydraulic testing, and geophysical information
- investigating enhanced contaminant stability strategies for long-term in situ immobilization through a series of laboratory and field experiments that focus on targeted bio-geo-manipulations and the propensity for contaminant remobilization with the primary goal of assessing potential for future contaminant release and remobilization (U and Tc)
- integrating experimental results from the preceding three elements to extrapolate quantitatively the rates and mechanisms of contaminant fate and transport from the plot scale to watershed scale using a multiprocess, multicomponent numerical model
Task A. Geophysical definition of subsurface heterogeneity within pathways [top]
The geophysical objectives of Oak Ridge IFRC research are multifold. This research element will improve the ability to interpret near-surface geophysical signals and relate them to fluid- and solid-phase properties. Multi-scale data will be used to characterize subsurface hydrological and geochemical properties, and to monitor natural and manipulated processes (Fig. 3). Geophysical methods will provide information about hydrological processes associated with temporal and spatial variations (such as ground water recharge during storms, seasonally and over several years) and elucidate biogeochemical variations across transition zones and at the leading front of the plumes. Surface geophysical methods will identify subsurface variations over subregional scales, and local scale tomographic borehole data located along the surface traverses will provide refined estimates of properties and their uncertainties. Results of these surface methods will be checked though the use of the ORFRC Geoprobe, which, through electrical conductivity logging, provides an indication of the depth to bedrock (i.e., depth of penetration), and stratigraphic and ground water quality changes with depth. Integrating the multi-scale geophysical data with more standard characterization data (e.g., geochemistry, mineralogy) will provide information about properties and processes that influence natural attenuation and subsurface manipulations over field-relevant spatial and temporal scales. This endeavor will provide new methods for long-term monitoring of plume attenuation that improve current capabilities to sort out the temporal and spatial well sampling "noise" and assess whether attenuation truly is occurring. Furthermore, geophysical investigations will provide large-scale, spatially integrated data for use in the watershed modeling task (Task D).
Figure 3. Location of research tasks at IFRC
Task B. Quantification of rates and mechanisms of natural attenuation
This task will quantify the effects of flow/recharge, pH, and indigenous electron donors on the metabolic activity of key microbial groups that are likely to affect natural attenuation (Fig. 3). A linked set of investigations (some of which overlap with Task C experiments) will quantify the key hydrological, geochemical, and microbial processes controlling the natural attenuation of U, Tc, and nitrate from the field column scale (cm to m) to the watershed scale (km). Some experiments focus on coupled processes that influence the natural attenuation of U, Tc, and nitrate along the shale and carbonate pathways. Other experiments investigate naturally occurring preferential recharge conditions subregionally along the length of the ground water plume and near the source. Studies will focus on the spatial extent and impact of episodic and seasonal precipitation events to quantify the ground water flux drivers at the site and the effects of recharge on diffusion, advection, dilution, and delivery of oxygen, DOC and higher pH water (which influences coupled geochemical and microbial processes and contaminant migration). These experiments will provide multi-scale data for use in Task D's modeling and data analysis.
Task C. Enhanced contaminant stability strategies for source control [top]
This task is designed to address issues associated with the intensity of the secondary source, enhanced contaminant immobilization and stability, and quantification of the rates and mechanisms of microbial and geochemical controls on contaminant attenuation through targeted remedial strategies and naturally occurring processes. The task consists of a series of laboratory and field experiments that focus on bio-geo-manipulations and assess the propensity for contaminant (U, Tc, and nitrate) immobilization. Experiments include (i) bioreduction and sustainability through controlled geochemical and microbial processes, (ii) biomanipulation and contaminant sequestration using organo-phosphate, and (iii) and manipulations to understand enhanced denitrification of contaminant nitrate and long-term immobilization of U and Tc through subsurface pH adjustment.
|Task C experiments at a glance|
Manipulations in shale/saprolite, near contamination source
Manipulations in carbonate gravel fill
- pH adjustment
- bioreduction strategies
- organo-phosphorus additions
- bioreduction strategies
sustained with oleate
Task D. Multi-scale and multi-process numerical modeling and data analysis [top]
This portion of IFRC research brings together the suite of experimental results into a multi-process, multi-component numerical model. This effort will improve understanding of hydrologic, geochemical, and biological processes and their interactions at multiple scales by intergrating field experiments and laboratory measurements in a usable, site-independent predictive tool. The model will enable the prediction of outcomes of various long-term management strategies for contaminated DOE sites. Using advanced pattern recognition and classification analyses will provide an improved understanding and predictive capability of the natural attenuation mechanisms that control subsurface contaminant behavior across a variety of scales.