Superfund Research Program
An Advanced Characterization Study of a Chlorinated Solvent Contaminated Aquifer
The heterogeneous nature of the subsurface presents serious challenges for the design, implementation, and evaluation of groundwater remediation systems. Design and implementation of a successful remediation project requires a thorough understanding of the distribution, transport, and fate of contaminants at the site. In an effort to improve the effectiveness of the groundwater remediation program in place in one section of the Tucson International Airport Area (TIAA) Superfund site, scientists from the University of Arizona are participating in a cooperative effort involving the support and collaboration of the U.S. Air Force, Raytheon Corporation, local consulting firms, and federal and state regulatory agencies. The project consists of several integrated components, including:
- site characterization activities;
- field tracer experiments;
- laboratory experiments conducted with aquifer material collected from the site;
- mathematical modeling; and
- the design and implementation of pilot-scale tests of innovative remediation technologies.
A large, multiple source-plume of trichloroethene and 1,1-dichloroethene exists in the upper portion of the Tucson regional aquifer, the sole source of potable water for the city. Contaminants are believed to have entered the subsurface by seepage from unlined pits and ponds, and concentrations of trichloroethene and 1,1-dichloroethene range from less than 100 m g/L to 10,000 m g/L. In 1987, a large pump-and-treat groundwater remediation system, which involves flushing water through the contaminated formation and treating the extracted fluids, was put in place in one section of the TIAA site. The pump-and-treat system has removed over 8,000 Kg of trichloroethene and significantly reduced the plume size. However, the system has exhibited reduced efficiency over the past several years and concentration rebound has been observed at some of the extraction wells.
Several factors can influence the capacity of a pump-and-treat system to completely remediate a contaminated site. The presence of regions of high concentrations or large masses of contaminants, known as "source zones", is a common cause of ineffectiveness of pump-and-treat systems. Pump-and-treat systems can handle dilute portions of the plume, but successful remediation of heterogeneous subsurface systems requires complete characterization and the implementation of technologies that can control or remove source zone contamination.
University of Arizona scientists conducted an "advanced-characterization" at one section of the TIAA site to:
- characterize the transport and fate behavior of contaminants at the site;
- identify the factors controlling contaminant removal of the pump-and-treat system; and
- provide information useful for improving the remediation program.
During the field experiments, the University of Arizona researchers observed strong elution tailing and rebound for trichloroethene and dichloroethene. This indicates the presence of a significant mass of contaminant whose transfer to the advecting water is rate limited. The contaminant could be sorbed to aquifer material, or could be associated with low permeability zones or an immiscible-liquid phase. Based on the results of laboratory and field-scale tracer experiments, and several laboratory experiments designed to examine sorption/desorption behavior, the scientists believe that all three factors influence contaminant transport and removal at the site. Mathematical modeling at both the source zone and regional scales indicates that dissolution of immiscible liquid contributes most significantly to the observed elution tailing. The University of Arizona team estimates that the contaminant mass associated with the presence of immiscible-liquid represents nearly two thirds of the total initial contaminant mass.
The University of Arizona scientists used these results to design and implement pilot-scale tests of innovative technologies focused on remediation of the source zones. There are relatively few options currently available for remediating deep saturated zones contaminated by immiscible-liquid phases of chlorinated solvents. They evaluated two candidate technologies: cyclodextrin-enhanced vertical flushing and in-situ chemical oxidation. Both of these technologies are designed to enhance the removal of contaminant mass from the source zones - the first by use of a solubilization agent (cyclodextrin) and the second by promoting chemically-mediated transformation using potassium permanganate solution. Pilot tests of both technologies have been successful.
These tests provide a direct demonstration of the influence of subsurface heterogeneity on water flow as well as the potential impact of subsurface heterogeneity on the ability of a pump-and-treat system to remediate the TIAA site. Such information is critical for the design of a remediation program that is capable of complete, successful cleanup. The success of this project illustrates the benefits associated with endeavors based on partnerships between industry, government, and academia. Such partnerships can be synergistic, resulting in optimal use of intellectual and capital resources. The knowledge gained, as well as the methods used, in this study should be of use for other sites, particularly those contaminated by chlorinated solvents.
For More Information Contact:
To learn more about this research, please refer to the following sources:
- Brusseau ML, Rohrer J, Decker T, Nelson NT, Linderfelt WR. 1999. Contaminant Transport and Fate in a Source Zone of a Chlorinated-Solvent Contaminated Superfund Site: Overview and Initial Results of an Advanced Site Characterization Project. In: Innovative Subsurface Remediation: Field Testing of Physical, Chemical, and Characterization Technologies. American Chemical Society, Washington, DC. pp.chapter19.
- Zhang Z, Brusseau ML. 1999. Nonideal transport of reactive solutes in heterogeneous porous media: 5. Simulating regional-scale behavior of a trichloroethene plume during pump-and-treat remediation. Water Resour Res 35(10):2921-2935.
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