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DNAPL Source Zones
Chlorinated solvents, such as trichloroethene (TCE) and tetrachloroethene (PCE), are found at an estimated 80 percent of all Superfund sites with groundwater contamination and at more than 3,000 Department of Defense sites in the United States. These solvents are released into the subsurface as dense nonaqueous phase liquids (DNAPLs), which can persist for centuries. It is both difficult and costly to remediate DNAPLs. Life-cycle costs to remediate these sites could total several billions of dollars. DoD alone could spend more than $100 million every year for hydraulic containment at these sites, using pump-and-treat technologies. Estimates for the military’s life-cycle costs exceed $2 billion. For a concise overview of current knowledge regarding the management of subsurface chlorinated solvent releases, see ESTCP’s Frequently Asked Questions document.
Over the past two decades, pump-and-treat processes have not fully remediated sites with DNAPL occurrence. Recent tests of innovative source remediation technologies, however, suggest significant mass removal and reductions in mass discharge from sources is possible at certain DNAPL sites. Such innovative techniques include surfactant or alcohol flooding and in situ thermal treatment. These results have led to increasing regulatory and public pressure to remediate sources. Source remediation, however, can be extremely expensive in the short term and it is difficult, if not impossible, to predict with confidence whether it will be effective. Therefore, research and development is essential to better understand whether and how to attempt source remediation.
SERDP and ESTCP convened a workshop, Reducing the Uncertainty of DNAPL Source Zone Remediation, in 2006 to define the research needs in this area. The workshop’s goal was to define a path forward to further reduce the uncertainty regarding DNAPL sites. To that end, the workshop provided a critical review of the progress to date, including a consensus perspective on the implications of the funded research for practical remediation and prioritization of the remaining data gaps. Results of this workshop provided the direction for research and demonstration efforts in SERDP and ESTCP over the next several years.
A greater understanding of the long-term impacts of DNAPL source zone treatment technologies will support the development of performance assessment tools and eventual technical guidance on the selection and use of these technologies in the field. This improved understanding of the benefits and risks of DNAPL source zone treatment will result in more cost-effective remediation strategies at DoD sites in the coming years.
SERDP and ESTCP efforts have encompassed a range of issues related to DNAPL source zone characterization, treatment and monitoring. The major areas of research and demonstrations include the following:
Understanding DNAPL Source Zones and Plume Responses to Source Depletion
Responses to source depletion are not easy to predict or measure. Better predictive models are needed to assist decision-making and evaluate the need for, and impacts of, aggressive remediation. In addition, it is difficult to assess both the source strength - the rate of contaminant release into the groundwater flow - and the downgradient water quality response to source treatment. Yet these factors are key to understanding and assessing the benefits of source treatment.
Understanding Complexities of Remediation in Fractured Rock
DNAPL contamination in fractured geologic media presents many additional complexities. Because of the typically poor characterization of fractured zones, it is difficult to characterize the distribution of the contaminants in the pathways the fractures provide. Several technologies have been applied to remediate fractured geology sites. It has proven extremely difficult, however, to flush contaminants out or to install effective barriers to prevent contaminant migration. Fractured geological settings also can be difficult to characterize and remediate because of the extreme differences in hydraulic conductivities between the fractured zones and the surrounding matrix. Several projects are examining these issues in an effort to improve our ability to treat fractured geologic media.
It is becoming more common for thermal treatment to be tested and implemented at DoD sites. Thermal treatment is considered a rapid, though relatively expensive, in situ method to remove and/or destroy contaminants, such as chlorinated solvents. Several commercial approaches to implementing thermal treatment exist. Still, compared to other techniques, such in situ biological remediation, there has been little fundamental research on thermal treatment that could serve as the basis for guidance on the use of the technology. Significant questions remain regarding the impacts of site conditions on efficacy, the nature and fate of potential byproducts, and the ability to collect representative multimedia samples. Questions also remain regarding potential secondary impacts, such as volatile emissions, contaminant migration into cooler areas, problematic increased contaminant mobility, effects on biological activities and changes in geochemistry. In situ reaction rates and the environmental factors controlling those reaction rates also are not well understood.
In Situ Chemical Oxidation (ISCO)
ISCO involves injection of strong oxidants into the contaminated subsurface, in some cases with other chemicals that function as catalysts. In situ oxidation has only been practiced in commercial settings for the last ten years. As a result, the technology is rapidly evolving and the state of the art has advanced over time. The limitations of the technology are becoming better understood, and engineering approaches to overcoming a number of these limitations have been developed. Though the chemistry involved is relatively simple, the technology is not a simple one to implement. The subsurface environment can be difficult to control, and it can be challenging to achieve adequate distribution of the oxidants within the subsurface. In many cases, in-situ chemical oxidation requires site-specific data that may not be available from typical site characterization investigations. Significant improvements in the ability to distribute oxidants within the subsurface have been made in recent years. In addition, the understanding of the site-specific data needs has improved.
Injection of nanoscale iron represents a promising area for treatment of DNAPL source zones. Issues associated with delivery and reactivity of the nanoscale iron, however, persist. Research efforts focus on examining these critical issues to improve and assess nanoscale iron as a treatment approach for DNAPL.
In Situ Bioremediation
In situ bioremediation, including biostimulation, bioaugmentation, and aspects of monitored natural attenuation (MNA), are well established for treating the dissolved plumes of chlorinated solvents. These techniques also can play a key role in DNAPL treatment. The interest in bioremediation as a remedial approach reflects the potential cost-effectiveness of both passive and active bioremediation approaches. Bioremediation can play a direct role in destroying chlorinated solvents in the source zone and in accelerating the mass removal process. Further, both MNA and enhanced bioremediation may be significant elements of treatment trains for source zone remediation. In certain cases, these methods may be applied following more aggressive treatment, such as thermal or surfactant flushing technologies.
Technology Performance Evaluation and Prediction
Promising source zone cleanup technologies are available. At present, efforts are better spent on understanding the promise of these technologies as opposed to developing newer ones. In many instances, the tools needed to measure performance are inadequate. Improved technology performance evaluation, improved prediction through the development of better diagnostic tools, and guidance on the use of existing tools are critical needs.
Symposium & Workshop
Chlorinated Ethene Source Remediation: Lessons Learned (2012)
Integrated DNAPL Site Strategy (2011)
Frequently Asked Questions Regarding Management of Chlorinated Solvents in Soil and Groundwater (2008)
Reducing the Uncertainty of DNAPL Source Zone Remediation (2006)