DNAPL Source Zone Characterization and Remediation:
An Ongoing Challenge
By Bettina Longino, Geomatrix Consultants, Inc.
On December 7 and 8, 2005, GRA held the 15th symposium of its very popular and successful Series on Groundwater Contaminants: DNAPL Source Zone Characterization and Remediation. The symposium drew over 170 attendees to San Francisco from across California , 14 other U.S. states, and seven countries to discuss the state-of-the-practice in characterizing and remediating DNAPL source zones. Symposium sponsors included ARCAD IS , Daniel B. Stephens & Associates, Geomatrix Consultants, GeoSyntec Consultants, Locus Technologies, Malcolm Pirnie, Thermal Remediation Services, Liquid Boot ® -LBI Technologies, and Precision Sampling.
DNAPLs, or dense nonaqueous phase liquids, such as chlorinated solvents, PCB oils, and creosote, are groundwater contaminants commonly encountered throughout industrial areas of North America as a result of their association with dry cleaning, metal degreasing, manufactured gas production, and wood preservation operations. It is estimated that as many as 25,000 DNAPL contaminant plumes may exist nationwide (NRC, 2005). Because DNAPLs are denser than water, they are able to migrate beneath the water table and will continue to move downward in unconsolidated or fractured media until either pooling above a low permeability zone, or becoming immobilized as residual along the migration pathway. This residual and pooled DNAPL in the subsurface typically is termed the “source zone.” Drinking water MCLs are generally orders of magnitude lower than DNAPL aqueous solubilities; as such, dissolved plumes associated with DNAPL source zones can cause pervasive and persistent contamination of drinking water aquifers.
Professionals involved in decision making for DNAPL sites are all too aware that effectively addressing DNAPL source zones involves not only difficult technical issues, but also policy challenges. Numerous recent academic papers and regulatory documents underscore the ongoing interest in techniques and technologies for characterization, removal, and more effective management of DNAPL source zones (EA, 2003; ITRC, 2002; U.S. EPA, 2003 and 2004; and NRC, 2005). There is also growing interest among groundwater researchers, consultants, and regulators to explore the potential advantages of evaluating the effectiveness of source zone remediation based on reductions in the rate of contaminant release from the source (referred to as contaminant mass discharge, or mass flux ), rather than reductions in concentrations. Since no DNAPL remediation technology has been proven to remove 100% of the contaminant mass from a DNAPL source zone, partial mass removal is (and certainly proved to be at the symposium) a topic of intense debate.
The two-day symposium included talks on DNAPL source zone characteristics and characterization, source controls and remedial technologies, and remediation performance assessment. In addition to 29 oral and poster presentations, there were 18 exhibitors, providing attendees with access to a wide variety of firms demonstrating technologies and expertise in the field of DNAPL source zone characterization and remediation.
The symposium opened in full gear with two superb introductory keynotes. Dr. Linda Abriola , Dean of Engineering and professor of Civil and Environmental Engineering at Tufts University and a leading researcher in the field of DNAPL transport, fate, and remediation, set the stage for sessions to follow by describing the challenges of defining, monitoring, and modeling the behavior of DNAPL source zones. Dr. Abriola also initiated two important dialogues that carried through the two-day symposium: the use of mass flux as a metric for source zone monitoring and regulation, and the potential benefits of partial mass removal. In the second keynote presentation, The DNAPL Source Remediation Challenge: Where are We Today?, Dr. Michael Kavanaugh of Malcolm Pirnie walked participants through six “phases” of evolution of the DNAPL remediation challenge, from “DNAPL denial” in the 1970s and 1980s, to realization of the many technical limitations in remediating source zones and the subsequent introduction of Technical Impracticability waivers, to the implementation of new technologies and metrics for source zone characterization and remediation. Looking to the future, Dr. Kavanaugh encouraged participants to continue efforts to quantify and reduce uncertainties regarding remediation effectiveness, risk, and cost-benefit relationships.
Presentations in the subsequent general sessions highlighted a number of ongoing challenges faced by practitioners attempting to characterize and remediate DNAPL source zones, including:
- time scales for formation and remediation of source zones;
- impact of heterogeneity on source zone architecture and persistence;
- appropriate metrics for field characterization and monitoring; and
- level of uncertainty in source zone conceptual models.
Session platform speaker Dr. Bernard Kueper of Queen's University initiated the time scales discussion with concepts of remediation in fractured rock. He presented compelling research findings on the importance of reverse diffusion, demonstrating the very long time scales required for remediation, even with short exposure to contamination. Dr. Jason Gerhard of the University of Edinburgh continued in this vein with factors affecting the length of time required for a DNAPL release to stop moving. His numerical models demonstrated migration time scales of weeks to centuries in a sandy aquifer, depending on DNAPL type.
Moving from the site scale to the pore scale, Dr. Walt McNab of Lawrence Livermore National Laboratory spoke about the importance of a detailed understanding and fine-scaled quantification of DNAPL mass transfer for accurately simulating flow and transport at the larger scale, and for predicting both plume source area persistence and cleanup times. Dr. Beth Parker of the University of Waterloo echoed the importance of scale of characterization in her information-packed evening keynote: Insights from Field Studies Regarding the Nature of Chlorinated Solvent Source Zones in Sandy Aquifers. She emphasized the importance of heterogeneity – especially permeability variations – in determining source zone architecture, spatial complexity, and plume evolution behavior. Dr. Parker demonstrated that spatial resolution of high concentration mass/flux “bull's-eyes” in source zones and plumes requires detailed characterization at a very small scale (sub-meter in some cases). Because this scale of characterization is infeasible for most sites, source zone mass estimates will almost always carry a high degree of uncertainty. Dr. Parker concluded her talk with four case studies that demonstrated downgradient effects of partial mass reduction and source isolation, including the persistence of groundwater concentrations above MCLs due to reverse diffusion from small zones of mass remaining in low permeability layers.
Day 2 started with two talks focused on mass discharge, or flux, as a metric for field characterization and monitoring. Session platform speaker Murray Einarson of Geomatrix Consultants provided an overview of the practical aspects of measuring contaminant mass discharge in the field, including the use of transects of single and multi-level wells to provide mass discharge “snapshots” within the plume and the use of down-well flux meters capable of integrating local mass discharge over a period of time. Dr. Elizabeth Edwards of the University of Toronto also used mass flux as a metric for changes in a DNAPL source, in this case demonstrating enhanced DNAPL dissolution following bioaugmentation of a PCE source zone. Later in the day, Dick Jackson of INTERA summarized seven years of source zone characterization and remediation at Hill Air Force Base in Utah , again using mass flux as a metric for success. Carmen Lebrón of the Naval Facilities Engineering Service Center (NFESC) furthered the discussion of mass-based versus concentration-based metrics in her presentation of the results of an NFESC survey conducted to compare and evaluate performance of DNAPL source remediation technologies. The survey identified mass removal, mass flux, rebound, and user perception as metrics of success, rather than MCLs or site closure.
Peter Mesard of Exponent underscored the importance of adopting an internally consistent and comprehensive conceptual site model at sites potentially containing a DNAPL source zone, especially when evaluating seemingly anomalous or inconsistent data results. Uncertainty in source zone conceptual models also was addressed in two poster presentations focused on DNAPL delineation methodology. Lucas Goldstein of LFR Levine-Fricke presented results from lab-scale experiments in which X-ray-computed tomography (or “CT scan”) was used to visualize and quantify DNAPL in porous media. Dennis Goldman of TetraTech presented field-scale techniques for source zone characterization, including soil vapor surveys, soil core sampling, cone penetrometer testing, geologic logging, direct push technology, groundwater and soil sampling, groundwater monitoring well sampling, and in situ microbial testing.
The latter part of the symposium focused on remedial technologies and remediation performance assessment. Presenters in these sessions continued to discuss challenges posed by time scales, heterogeneity, appropriate metrics, and level of uncertainty. They also introduced attendees to new tools and technologies for source zone remediation, and presented evidence of successful applications of established dissolved-phase remediation technologies, such as bioremediation and zero valent iron (ZVI), to DNAPL source zones. Platform speaker Dr. Tom Sale of Colorado State University launched the third session with an informative talk on ZVI-Clay, a novel in situ remediation technology that involves the use of conventional soil mixing equipment to deliver an admixture of reactive media (ZVI) and stabilizing agents (clay) to impacted soil. Two field-scale demonstrations have shown significant depletion of chlorinated compounds in the treated zone, as well as reduced flux from the treated zone due to reduced hydraulic conductivity resulting from the soil mixing process. Suzanne O'Hara of GeoSyntec Consultants also discussed the application of ZVI (in this case emulsified ZVI, or EZVI) technology for reducing both aqueous concentrations and DNAPL mass in DNAPL source zones. Dr. Julie Konzuk of GeoSyntec Consultants introduced a new remediation screening tool currently being developed under the Environmental Security Technology Certification Program (ESTCP). The tool is intended to aid users in determining a site-specific remedial approach by providing expected remedial performance data based on site characteristics, preference for remedial approach, and performance metrics.
The symposium included a number of talks and poster presentations describing specific remedial approaches for DNAPL source zones. Dr. Jason Gerhard's poster presentation introduced Project SABRE (Source Area BioREmediation), a consortium of industry, government, and research institutions whose charter is to determine if enhanced anaerobic bioremediation can result in effective and quantifiable treatment of chlorinated solvent DNAPL source areas. Dr. David Major of GeoSyntec Consultants also discussed enhanced in situ bioremediation of source areas, presenting results from a field-scale bioaugmentation demonstration that showed significant mass removal from the TCE DNAPL treatment zone, without the accumulation of dechlorination daughter products (e.g., cis-1,2-DCE and vinyl chloride) often observed following biostimulation. Scott Davis of ARCADIS presented the results of a phased remedial program that incorporated both physical (excavation and soil vapor extraction) and biological (enhanced in situ reductive dechlorination using carbohydrate substrates) methods to remediate a TCE DNAPL source area
Four presenters focused on thermal technologies. James Keegan of Current Environmental Solutions and Angus McGrath of SECOR presented case studies in which electrical resistive heating (ERH) was used to reduce VOC and/or DNAPL concentrations. John LaChance of TerraTherm presented results from a Bay Area site at which in situ thermal destruction ( IS TD) was used to treat soil and groundwater in a chlorinated DNAPL source zone. To conclude the discussion, Gorm Heron of TerraTherm presented a critical review of the practical application of three thermal remediation technologies – ERH, steam-enhanced extraction (SEE), and IS TD – and educated attendees on common mistakes to avoid when designing and implementing thermal remediation at sites with DNAPL source areas.
Dr. Fred Payne of ARCAD IS fittingly concluded the symposium speaker presentations with a summary talk on sources of DNAPL persistence and post-treatment rebound. We were reminded once again of the importance – and challenge – of source zone architecture, sorption, and fine-scale aquifer structure to the post-treatment persistence of DNAPL source zones. Dr. Payne left attendees with two questions to ponder during the subsequent panel discussion: “What must we (or can we) achieve in chlorinated solvent plumes?” and, “Is complete source removal economically sensible?”
The closing panel discussion focused on the value of partial mass removal. The panel was moderated by Avram Frankel of ARCADIS and included Richard Jackson, Dr. Beth Parker, Dr. Fred Payne, Dr. Tom Sale (all symposium presenters), and Paul Hadley of the California DTSC. Although the title of the panel was Partial Mass Removal: Is It Worth It?, the discussion and question period encompassed many of the recurring topics of the symposium: remediation timeframes, performance metrics, evaluation of uncertainty, and the practicality of achieving MCLs. Participants discussed the definition of “success” in the restoration of a DNAPL-impacted resource, with particular emphasis on the regulatory context. This final discussion provided a perfect summation for the two-day symposium, reminding us all of the ongoing challenges facing DNAPL practitioners and of the importance of maintaining an open and constructive dialogue among stakeholders. This symposium was certainly a step forward along that path.
A binder with copies of speakers' slides and a list of references on various aspects of DNAPL behavior, characterization, and remediation was produced for the symposium. To purchase a copy of the binder, please use to the publications order form or call GRA's main offices in Sacramento at (916) 446-3626.
- National Research Council (NRC), 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academy Press. Washington , DC .
- United States Environmental Protection Agency ( U.S. EPA), 2004. DNAPL Remediation: Selected Projects Approaching Regulatory Closure.
- U.S. EPA, 2003. The DNAPL Remediation Challenge: Is There a Case for Source Depletion?
- Environment Agency (EA), 2003. An Illustrated Handbook of DNAPL Transport and Fate in the Subsurface.
- Interstate Technology Research Council, 2002. DNAPL Source Reduction: Facing the Challenge.
Bettina Longino is a Senior Consultant with Geomatrix Consultants, Inc., and is now based in Ontario , Canada . Prior to leaving the Bay Area in 2005, she served as a technical advisor for the San Francisco Bay Branch of GRA.