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An accurate understanding of fluid flow and mass transfer of matter in multi-phase systems requires examination and measurement of pore-scale properties and processes.
Mechanistic investigations of pore-scale phenomena require advanced methods for in-situ observation, characterization, and quantification of fluids and pore networks. Several
methods developed over the past few decades have been used for pore-scale imaging of immiscible fluids in three-dimensional systems of porous media, including photoimaging of
refractive-index matched systems, nuclear magnetic resonance methods, and X-ray microtomography. These methods have great potential for enhancing our mechanistic understanding
of multiphase flow and mass transfer processes. In particular, the high resolution associated with synchrotron microtomography (SMT) has allowed its use for natural porous
media such as soils and sediments.
We are using SMT to examine such phenomena as the movement and distribution of immiscible fluids (water, organic liquid, air), the dynamics of fluid-fluid interfaces, and the
dissolution of organic liquids. We are also using the interfacial partitioning tracer test (IPTT) method as an alternate means by which to characterize fluid-fluid interfacial
area. Interfacial partitioning tracer tests provide indirect measurements of interfacial area based on the retention behavior of tracers that accumulate at the interface. These
tracer tests can be conducted in several ways, including the use of either aqueous-phase or gas-phase modes. We are examining the efficacy of both SMT and IPTT, and evaluating
their associated measurement domains.
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Immiscible liquids are often present in the vadose zone at chlorinated-solvent and fuel contaminated sites. This contaminant mass serves as a long-term source of vapor-phase
contamination. In addition, contaminant flux from the large dissolved-phase groundwater contaminant plumes often present at chlorinated-solvent contaminated sites serves as
another long-term source of vapor-phase contamination. The migration of vapors from these contaminant sources into surface and sub-surface structures (vapor intrusion) has
become of concern with respect to potential impacts on human health. This exposure pathway is a primary risk driver for corrective action plans for many chlorinated-solvent
contaminated sites. Soil vapor extraction or soil venting is a primary method used for remediation of sites for which the vadose zone is contaminated with organic liquids. The
transport and fate behavior of volatile organic contaminants in the vadose zone is the key to risk posed by vadose-zone contamination, and to the efficacy of soil venting
operations. We investigate the many processes that influence vapor-phase transport, such as gas-phase advection in heterogeneous porous media, diffusion, organic-liquid mass
transfer (evaporation, dissolution), sorption/desorption, gas-water mass transfer, and accumulation at the gas-water interface. We are also investigating the impact of
vapor-phase mass flux on contamination and remediation of groundwater.
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The region of the subsurface wherein immiscible-liquid contamination is present is referred to as the source zone. The source
zone is defined in terms of two key aspects, configuration and dynamics. The configuration or “architecture” of the source zone
constitutes the porous-medium heterogeneity (e.g., permeability distribution), total contaminant mass, and contaminant
distribution. Source-zone dynamics comprise the mass-transfer, transport, and transformation processes operative within the
system. Source-zone architecture and dynamics are central to the risk posed by the site in that they dictate the magnitude of the
groundwater contaminant plume generated from the source zone. Source zone architecture and mass-transfer processes also greatly
influence the feasibility and effectiveness of remediation strategies.
We are investigating the impact of porous-medium heterogeneity, organic-liquid configuration, and mass-transfer dynamics on the
magnitudes and rates of mass removal and mass flux. This research is being conducted via experiment- and modeling-based studies at
multiple scales-- pore scale, column scale, intermediate scale, and field scale. In addition, we are examining the temporal
dynamics of the relationship between mass flux reduction and mass removal due to temporal changes in source-zone architecture and
mass-transfer dynamics (e.g., as mediated by source-zone aging or remediation efforts). We are also examining methods for
characterizing source-zone architecture and mass flux.
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Generally, the most critical issue for hazardous waste sites contaminated by organic compounds is whether or not
immiscible-liquid phases are present in the subsurface. Immiscible liquids trapped in the subsurface serve as long-term sources of
contamination, and their presence can greatly impact the costs and time required for site remediation. We are investigating the
mass transfer behavior of organic liquids trapped in porous media. For example we have conducted several studies to examine
organic-liquid dissolution in heterogeneous porous media. We are currently employing microtomography to characterize dissolution
processes at the pore scale. We have developed and are testing mathematical models of differing levels of complexity to simulate
and predict dissolution behavior. An area of specific focus has been long-term dissolution and mass flux phenomena associated with
mass removal for poorly accessible organic liquid.
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Biotransformation processes have a significant impact on the transport and fate of many organic contaminants. We have
investigated the impact of biotransformation on contaminant transport, examining for example the coupled effects of sorption, the
influence of multiple populations, and the impact of remediation efforts. We have implemented the biotracer test method to
characterize microbial activity in the field. Currently, we are investigating the use of integrated approaches, employing
compound-specific stable isotope analysis, molecular-assay methods, and mathematical modeling, for characterizing the natural
attenuation of chlorinated solvents and nitrate.
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We are investigating issues associated with remediation of subsurface environments at hazardous waste sites. One area of focus
has been remediation of source zones. For example, we have conducted several studies to evaluate the use of reagents (e.g.,
surfactants, cosolvents, cyclodextrins) for enhancing the removal of organic-liquid contamination from source zones. A major
effort has been focused on examining the efficacy of cyclodextrin as an alternative to surfactants and alcohols as a
solubilization-enhancement agent for source-zone remediation. In addition, we have examined the use of cyclodextrin for
remediation of mixed wastes (organic-liquid and metals). This research has spanned laboratory batch-reactor experiments to field
studies. We have also conducted batch-reactor, flow-cell, and pilot-scale field studies on the use of in-situ chemical oxidation
(using potassium permanganate and fenton’s reagent) for remediation of chlorinated-solvent contaminated source zones. An area of
current focus is the impact of the mass reduction associated with source-zone remediation efforts on the aqueous-phase mass flux.
Another area of focus is on management of sites that have large groundwater contaminant plumes. Large dissolved-phase
groundwater contaminant plumes often form at chlorinated-solvent contaminated sites because chlorinated solvents typically have
relatively high solubilities (in comparison to maximum contaminant levels), limited retardation, and generally low transformation
potential. In many cases, the plumes are hundreds of meters to several kilometers long. For many sites, a large fraction of the
contaminant mass comprising the plume may reside within laterally extensive lower-permeability units adjacent to the aquifer. The
mass in such domains is poorly accessible to flowing groundwater, and thus is not amenable to removal via hydraulic displacement
methods (e.g., pump and treat). As a result, this mass serves as a long-term contaminant source via diffusive mass transfer back
into the flowing groundwater domain (so-called “back diffusion”). This process is critical to long-term management and ultimate
closure of chlorinated-solvent contaminated sites.
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Sorption/desorption processes often play a significant role in mediating the transport and fate of hydrophobic organic
compounds in natural porous media. We are investigating the sorption/desorption behavior of organic compounds as a function of
molecular and porous-media properties. One phenomenon of interest is the observation that the desorption of many organic
contaminants from soils and sediments is often significantly rate- limited, requiring days, months, or even years to attain
equilibrium. We are investigating the mechanisms responsible for rate-limited sorption/desorption, and the properties and
conditions influencing their manifestation.
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Mathematical modeling has become an indispensable tool for investigating contaminant transport and fate. Two primary uses for
mathematical models are investigation of fundamental processes and site-specific management applications. Regarding the former,
mathematical models are a powerful means by which to integrate process-based information for complex systems, and thus are a key
method for investigating phenomena over a wide range of conditions that are problematic to investigate by other means. We are
involved in the development and application of advanced mathematical models for simulating the transport of reactive contaminants
in heterogeneous porous media.
Regarding the second use, mathematical modeling has become a critical component of risk assessment, characterization, and
remediation-system development efforts for hazardous-waste sites. Unfortunately the use of advanced, multiprocess mathematical
models for such applications is often greatly constrained by insufficient knowledge of subsurface properties and contaminant
distributions. This is particularly the case for immiscible-liquid contaminated sites, for which the location and architecture
of the sources zones is rarely known in detail. Thus, it is often necessary to use simpler models (e.g., lumped-process models)
for simulating transport at the field scale. A critical issue associated with the use of simpler models is how to translate
mechanistic information (e.g., pore-scale and local-scale processes) to the simplified models. This is often referred to as the
“upscaling” issue. We are examining this issue and the use of simplified models for example for simulating mass transfer and
transport for organic-liquid contaminated systems.
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We have been involved in the development, testing, and application of numerous innovative tracer-test technologies. These
include the partitioning tracer test for measuring immiscible-liquid contamination, the partitioning tracer test for measuring
water content, the interfacial partitioning tracer test for measuring fluid-fluid interfacial area, the multiple-solute tracer
test for characterizing diffusive mass-transfer processes, and the biotransformation tracer test.
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The transport and fate behavior of contaminants mediates the risk posed by a site, as well as the viability and effectiveness
of remediation efforts. We are interested in the underlying processes, such as sorption, diffusion, and transformation, that
influence transport and fate of reactive contaminants. We are also interested in the impact of geochemical and physical
heterogeneity of porous media on transport and fate.
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The transport and fate of colloidal matter has become of great interest recently, in relation to the potential human-health
and environmental impacts of pathogenic microorganisms and of nanoparticles. We are investigating the mechanisms and processes
influencing the transport of nanoparticles and biocolloids in natural porous media.
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