Areas of Study
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 behavior of organic immiscible liquids such as chlorinated solvents, and their impact on site characterization and remediation. Our research encompasses the pore scale, column scale, intermediate scale, and field scale.
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.
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.
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.
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. We are investigating the transport and fate behavior of a wide variety of contaminants, including organic compounds, heavy metals, colloids and nanoparticles, and pathogenic microorganisms.
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. Diffusive mass-transfer processes may significantly influence contaminant transport, especially in heterogeneous subsurface environments. We investigate the transport behavior of contaminants in heterogeneous media at multiple scales.
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.
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.
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.
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.
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. We are also interested in the development and application of other characterization methods, such as compound-specific stable isotope analysis and mass-flux characterization.
Groundwater is a primary source of potable water for Arizona and the Southwest US. Factors such as continued population growth, changes in land use, and climate variability are significantly impacting the sustainability of groundwater resources in this region. In addition, contamination from a variety of sources is further stressing groundwater sustainability. We are collaborating with policy and social-science experts to characterize the impact of groundwater contamination on water resources sustainability. A current focus involves characterizing contaminant distributions and aquifer vulnerability for watersheds in the US-Mexico border region.
An important part of the research process is ensuring that stakeholders are aware of, and have access to,
innovative research products. Unfortunately, it is all too common that a major divide exists between
the generation of cutting-edge research products and the transfer of those products to stakeholders for
application of technology in the field or for use of information in decision-making. The mission of research
translation is to bridge this divide, and to ensure that all stakeholders have first-hand access to the valuable
resources and research products that academia generates.