Guidance on in situ flushing technology practices—conceptual design processes, applicability to various contaminants and site conditions, and implementation and performance monitoring—is readily available. This page provides basic information on flushing agents and identifies additional resources for planning and designing flushing remedies, monitoring system performance, and understanding the regulatory context of implementing in situ flushing technology.

Technology Planning and Design

The techniques employed the most in soil flushing are surfactant and cosolvent flooding for fuels and chlorinated solvents. There are many types of surfactants (cationic, anionic, nonionic), and while adjustments can be made in the fluid composition, anionic or nonionic surfactants are generally used. This is because their negative or neutral charge reduces the possibility of their sorption to negatively charged clay particles. They also are generally less toxic than cationic surfactants (EPA 2006).

Surfactants are commonly constructed with hydrophobic and hydrophilic chemical components, meaning that one end of the molecule is attracted to oil (or organic compounds) and the other to water. Surfactants chosen primarily to increase the contaminant (generally a NAPL) solubility are used in a solubilization flood. Surfactants chosen to produce ultra-low interfacial tensions are employed in a mobilization flood (Kueper et al. 1997). Mobilization flooding is an option only when there is a high degree of certainty that the solution can be recovered, such as with a competent bedrock or capillary barrier underlying the treatment zone (EPA 2006).

Surfactant flooding involves the preparation of low-viscosity surfactant solutions that are pumped through the contaminated zone by introduction at injection points and removal from extraction points. Polymer amendments that will increase the viscosity of the surfactant solution might be required for higher viscosity contaminants (e.g., creosote) when temperature augmentation for viscosity reduction is not feasible. The surfactant flooding wellfield generally will be composed of dedicated surfactant injection and extraction points, and water injection points for hydraulic control (NFESC 2003).

Electrolytes and cosolvents are other additives that a typical surfactant solution might contain. In addition to being effective with the target contaminant, the surfactant solution is tailored to be compatible with the site-specific geologic conditions, soil, soil pore water, and groundwater (if applicable). For example, a flushing solution that is compatible with a site's soil chemistry will not adhere to the soil and reduce its effective porosity and permeability. A cosolvent, such as isopropanol, can be used to improve the surfactant solubility in solution and provide the surfactant and contaminant solution with an acceptable viscosity. Chemicals added to the surfactant solution might require treatment along with the contaminant at the recovery end (NAVFAC 2002). Surfactants often can be recycled.

Cosolvents, usually alcohols, are chemicals that dissolve in both water and NAPL. In an alcohol flood, the alcohol may partition into both the NAPL and water phases. Partitioning affects the viscosity, density, solubility, and interfacial tension of the NAPL (Kueper et al. 1997). The physical properties of the co-dissolved NAPL vary with the amount of alcohol available for interaction, and whether the alcohol preferentially dissolves into the NAPL or into the water. Where contact is made between NAPL and cosolvent, complete miscibility is achievable and results in a pumpable solution. Depending upon the density of the NAPL and the proportions of alcohol and water, the solution may be more or less dense than water (EPA 2006). Cosolvents that microbes can use as substrates have the added advantage of promoting bioremediation if they are used at nontoxic levels (Adamson et al. 2011).

Nonvolatile metals are among the classes of chemical compounds treated successfully by in situ flushing (Roote 1997, Lestan et al. 2008, Hashim et al. 2011). The particular reagent necessary is dependent not only on the metal, but its chemical form or species. The pH (buffering capacity) of the soil can affect the amount of flushing solution needed, especially when acids and bases are used. Acidic solutions are applicable for removal of metals and basic organic contaminants; dilute acid solutions also can be used for flushing some inorganic-metal salts, such as carbonates of nickel, zinc, and copper. Metals might also require reducing agents or chelating or complexing agents, although Hashim et al. (2011) cautions that chelate flushing tends to destroy the soil profile because it strips major cations important for plant nutrition and maintenance of soil structure along with the metals. Reductants and oxidants can convert heavy metals to more soluble compounds, and basic solutions might be used to treat phenols and metals bound to the soil organic fraction (Roote 1997, Hashim et al. 2011).

Many site-specific factors can affect the selection, design, and effectiveness of a flushing technology:

• The impact of lateral heterogeneity, such as the presence of discontinuous silt lenses.
• Preferential pathways for water movement caused by either discontinuities in subsurface properties or features such as clastic dikes, which can limit flushing effectiveness if contaminated zones are bypassed.
• Slope to layers or inherent layer anisotropy that could impose lateral flow.
• Surface features that cause ponding of water rather than infiltration. These types of uncertainties are considered in the design of a soil flushing system, including design of a groundwater capture system in terms of the necessary capture zone. Quantification of surface infiltration rates for a site also has significance when evaluating the applicability of soil flushing (Truex et al. 2010).

Before implementing surfactant and/or cosolvent flushing, performance of bench-scale treatability testing is recommended to ensure the selection of the agents best suited to the type of contamination and the site-specific soil and geochemical conditions. Modeling of subsurface conditions is commonly done to identify the best delivery system (NAVFAC 2002). Soil flushing tends to work best at sites that have spaces in the soil that permit the movement of the flushing solution through it, such as sands and gravels. The permeability of the strata can be measured as hydraulic conductivity, with favorable values in the range of 1.0 x10^-4 cm/sec or higher (ITRC 2003), and limiting factors in the range of <1.0 x 10^-5 cm/sec. Heterogeneous soil reduces the efficiency of the flood sweep and may prevent optimum contact between the agent(s) and the target contaminant. Flushing of relatively homogeneous but lower permeability (between 10^-4 and 10^-5 cm/sec) units is possible, but it requires a high induced gradient to move the agent, while greatly increasing the remediation time. Other soil factors that can adversely affect efficiency are high cation exchange capacity, high buffering capacity, high organic soil content, and pH (NAVFAC 2002).

Costs are dependent on whether flushing is used as a long-term source removal technique or as a temporary pathway management solution in conjunction with other technologies (CL:AIRE 2011). The U.S. Army Corps of Engineers explored cost driver information and cost analysis for soil flushing using the 2006 version of the Remedial Action Cost Engineering and Requirements (RACER) software. The RACER analysis identified soil permeability as the primary cost driver. Soils with lower permeability are more recalcitrant to flushing and thus can increase remediation time significantly, which increases costs. Depth to groundwater is the secondary cost driver—a deeper water table increases the cost to complete (FRTR 2007). Other factors to consider include the waste type and the quantity to be treated, as well as design and engineering, which can be a considerable expense. Purchase of surfactants can be a significant component of the total cost, especially if surfactant concentrations of 4 to 8 wt% are used; however, as surfactant concentrations are lowered toward 1 wt% or lower, and as surfactant recovery and reuse are implemented, costs become more economical (NRC 2004).

The Water Science and Technology Board of the National Research Council pointed out in 2004 that additional research is needed to advance good sweep efficiency during flushing, to optimize the implementation of surfactant/cosolvent technologies in karst and fractured bedrock formations, to evaluate the combination of flushing technologies with other source zone and/or plume remedial technologies, and to evaluate the long-term impact of contaminant mass removal on post-flushing water flooding and natural attenuation (NRC 2004). Research continues in these areas.

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Technical Resources

Chelating Agents for Land Decontamination Technologies
Tsang, D.C.W., I.M.C. Lo Rao, and Y. Surampalli (eds.).
American Society of Civil Engineers, Reston, VA. ISBN: 978-0-7844-1218-3, 294 pp, 2012

Chelating agents (or chelants) refer to ligands that can occupy multiple positions in the inner coordination sphere of the central metal ion, leading to the formation of multidentate metal-chelant complexes (or chelates). Chelating agents are able to enhance metal extraction from contaminated soil or sediment and facilitate metal mobility in the subsurface. This book focuses on the engineering applications of chelating agents for soil washing, soil flushing, phytoremediation, and electrokinetic remediation. Table of Contents

Contaminants in the Subsurface: Source Zone Assessment and Remediation
National Research Council, Committee on Source Removal of Contaminants in the Subsurface. National Academies Press, Washington, DC. ISBN: 030909447X, 383 pp, 2004

After discussing the definition of 'source zone' and the characterization thereof, this report reviews the suite of technologies available for source remediation and their ability to reach a variety of cleanup goals, from meeting regulatory standards for ground water to reducing costs. The report proposes elements of a protocol for accomplishing source remediation that should enable project managers to decide whether and how to pursue source remediation at their sites.

Contaminated Land Remediation: Final Report
Contaminated Land: Applications in Real Environments (CL:AIRE). Environment Agency, Defra Research Project SP1001, London, UK. 120 pp, 2011

This report summarizes the current understanding and use of 21 cleanup technologies in the UK, identifies current and likely future factors influencing their selection, and sets out the relative economic, environmental, and social costs and benefits (i.e., the sustainability) of each technique. For each technology (including in situ flushing), the report gives a brief description; information concerning the effectiveness of each method in addressing different contaminants; circumstances (e.g., geology, hydrogeology, contaminant form) that might affect technology implementation; advantages and disadvantages; and barriers to use.

Delivery and Mixing in the Subsurface: Processes and Design Principles for In Situ Remediation
Kitanidis, P.K. and P.L. McCarty (eds.).
Springer, New York. ISBN: 978-1-4614-2238-9. SERDP-ESTCP Environmental Remediation Technology, Vol. 4, 325 pp, 2012

This technology monograph describes the principles of chemical delivery and mixing systems and their design and implementation for effective in situ remediation. In situ technologies discussed include chemical oxidation, surfactant/cosolvent flushing, subsurface reactors, recirculation systems, PRBs, gas delivery via sparging, and intrinsic remediation in natural-gradient systems. Numerous case studies are provided. Table of contents and abstracts.

Evaluating LNAPL Remedial Technologies for Achieving Project Goals
Interstate Technology & Regulatory Council (ITRC) LNAPLs Team. LNAPL-2, 144 pp, 2009

This guidance provides a framework to help stakeholders select the best-suited remedial technology for an LNAPL site and outlines which technologies apply in different site situations. Coverage includes water flooding and flushing with surfactants or cosolvents.

Horizontal Remediation Wells
Appendix A in How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers
EPA 510-B-17-003, 47 pp, 2017

Horizontal directional drilling can be used to install horizontal remediation wells (HRWs) at cleanup sites. The technology uses specialized equipment to produce either a curved surface-to-surface well or a blind well. HRWs are able to access locations beneath surface obstructions and to place long well screens in contact with the contaminated area. The wells can be thousands of feet long, with hundreds of feet of well screen. The potential for HRWs to complement a site remedy is described with reference to air sparging, bioremediation, chemical injection, soil vapor extraction, hot air or steam injection, LNAPL removal, plume containment, injection of treated water, and sampling. A detailed overview is provided of equipment and procedures for drilling a horizontal remediation well. Additional information: The complete UST CAP review manual

INDOT Guidance Document for In-Situ Soil Flushing
Lee, L.S., X. Zhai, and J. Lee, Purdue Univ., West Lafayette, IN.
SPR-2335; FHWA/IN/JTRP-2006/28, 49 pp, 2007

Contains information ranging from the basic chemistry and mechanisms of cosolvent and surfactant flushing to the key factors that need to be considered during the selection, design, and implementation of this technology. Also provides information on several categories of contaminants subject to in situ flushing. This text was written to provide general guidance rather than as a design manual. Additional information: Lab Testing and Field Implementation of Soil Flushing

In-Situ Flushing Technologies: Combined Remedies
Pennell, K.D.
NIEHS/EPA Combined Remedies Workshop, June 27-28, 2006, Tufts University, Medford, MA. 14 slides, 2006

This presentation addresses steps to reduce actual and perceived flushing costs, incomplete mass removal, surfactant compatibility with bioremediation, and minimizing or countering downward mobilization of contaminants, with references to more detailed sources of information.

Innovative Site Remediation Technology, Vol. 3: Soil Washing/Soil Flushing
Anderson, W.C. (ed.). American Academy of Environmental Engineers, Annapolis, MD. 174 pp, 1995

Remediation Hydraulics
Payne, F.C., J.A. Quinnan, and S.T. Potter.
CRC Press, Boca Raton, FL. ISBN: 9780849372490, 432 pp, 2008

This text addresses the need to predict and control fluid movement in the subsurface. It discusses how to conduct realistic assessments of contaminant plume structure and achieve contact between injected reagents and target compounds; offers instruction for the design and application of reagent delivery systems that support in situ chemical and biological treatment strategies for groundwater cleanup; and introduces tracer design and interpretation methods for cost-effective design of large-scale injection systems.

Selecting and Assessing Strategies for Remediating LNAPL in Soils and Aquifers
Johnston, C.D.
CRC for Contamination Assessment and Remediation of the Environment, Adelaide, Australia: CRC CARE Technical Report 18, 194 pp, 2010

This report provides guidance in selecting and matching the performance of remediation technologies to a range of subsurface settings, risk reduction targets, and concerns. Surfactant, cosolvent, and natural flushing are discussed. The emphasis is on LNAPL that has infiltrated the water table. One of the aims is to identify the gaps in the current understanding of remediation process and performance in the various subsurface settings.

Surfactant Selection for Enhanced Contaminant Extraction
Sabatini, D., R. Knox, and J. Harwell.
Groundwater Quality: Remediation and Protection. International Association of Hydrological Sciences, IAHS Publication No 250:361-366(1998)

This paper discusses surfactant-enhanced solubilization versus mobilization for a variety of contaminants. Solubilization increases the density of the contaminant-loaded surfactant solution by only several wt% compared to mobilization, which involves a much more dense, moving front of DNAPL. It is important to identify from the outset whether solubilization or mobilization of DNAPL is desired because not all surfactants can accomplish the low interfacial tension necessary to conduct a mobilization flood. Unit processes for contaminant-surfactant separation and surfactant re-use are summarized and illustrated by examples from field demonstrations.

Surfactant-Enhanced Aquifer Remediation (SEAR) Design Manual
Naval Facilities Engineering Service Center. NFESC Technical Report TR-2206-ENV, 110 pp, 2002

Surfactant-Enhanced Aquifer Remediation (SEAR) Implementation Manual
Londergan, J. and L. Yeh. Naval Facilities Engineering Service Center. NFESC Technical Report TR-2219-ENV54, 54 pp, 2003

Surfactant-Enhanced DNAPL Remediation: Surfactant Selection, Hydraulic Efficiency, and Economic Factors
Sabatini, D.A., R.C. Knox, and J.H. Harwell.
EPA 600-S-96-002, 15 pp, 1996

Surfactants and Cosolvents for NAPL Remediation: a Technology Practices Manual
Lowe, D.F.; C.L. Oubre; C.H. Ward (eds.). Lewis Publications, Boca Raton, FL. ISBN: 0-8493-4117-5. 448 pp, 1999. Additional information: The Table of Contents is available at the publisher's website.

Surfactants and Interfacial Phenomena, 4th Edition
Rosen, M.J. and J.T. Kunjappu.
John Wiley & Sons, ISBN-10: 0-470-54194-6; ISBN-13: 978-0-470-54194-4, 616 pp, 2012

Using a minimum of mathematics, the text describes the properties and applications of surfactants. It explains the mechanisms by which these materials operate in interfacial processes such as foaming, wetting, emulsion formation, and detergency, and shows the correlations between a surfactant's chemical structure and its action. This update features coverage of relevant new literature appearing after the third edition.

Surfactants/Cosolvents: Technology Evaluation Report
Jafvert, C.T.
Ground-Water Remediation Technologies Analysis Center, TE-96-02, 50 pp, 1996

A DNAPL source zone was established within a sheet-pile isolated cell through a controlled release of PCE to evaluate DNAPL remediation by in situ cosolvent flushing. Ethanol was used as the cosolvent, and the main remedial mechanism was enhanced dissolution based on the phase behavior of the water/ethanol/PCE system. Over a 40-day period, 64% of an initial mass of 83 L of PCE was removed by flushing the cell with a solution of 70% ethanol and 30% water; however, tracer results suggest that some PCE was inaccessible to the ethanol solution, which led to the inefficient PCE removal rates observed.

Technical and Regulatory Guidance for Surfactant/Cosolvent Flushing of DNAPL Source Zones
Interstate Technology & Regulatory Council, Dense Nonaqueous Phase Liquids Team, Washington, DC. DNAPLs-3, 151 pp, 2003

Technologies for Dense Nonaqueous Phase Liquid Source Zone Remediation: Technology Evaluation Report
Fountain, J.C. TE-98-02. 1998

Technology Practices Manual for Surfactants and Cosolvents
Kueper, B., et al. Advanced Applied Technology Demonstration Facility (AATDF) Program, Rice University. TR-97-2, 1997.

This manual is intended to assist decision makers with the evaluation and potential application of surfactant/cosolvent flushing for the remediation of subsurface contamination. The report provides a basic understanding of the technologies, their applicability and limitations, and an understanding of the factors to be considered when implementing projects.

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Review Articles

Coupling Surfactants/Cosolvents with Oxidants for Enhanced DNAPL Removal: A Review
Dugan, P.J., R.L. Siegrist, and M.L. Crimi.
Remediation Journal 20(3):27-49(2010)
Additional information: Slide presentation; Abstract

Remediation Technologies for Heavy Metal Contaminated Groundwater
Hashim, M.A., S. Mukhopadhyay, J.N. Sahu, and B. Sengupta.
Journal of Environmental Management 92(10):2355-2388(2011)
Abstract

Soil Flushing : A Review of the Origin of Efficiency Variability
Atteia, O., E. Del Campo Estrada, and H. Bertin.
Reviews in Environmental Science and Biotechnology 12(4):379-389(2013)

Although surfactant alone gives efficiencies of 80-85% in lab experiments, the amounts of product required for field injection may not be economically sustainable. The literature indicates that soil flushing efficiency can vary from 0% to almost 100% in the field, which illuminates the importance of knowledge concerning the site characteristics and contaminant location, amount, and behavior as well as other factors. For initial saturations lower than 1%, soil flushing may be an inefficient technique. This paper provides an overview of recent lab, pilot, and field studies of soil and groundwater remediation, with a focus on chlorinated solvents.

Surfactant-Enhanced Remediation of Contaminated Soil: A Review
Mulligan, C.N., R.N. Yong, and B.F. Gibbs.
Engineering Geology 60(1-4):371-380(2001)

The Use of Chelating Agents in the Remediation of Metal-Contaminated Soils: A Review
Lestan, D., C.-L. Luo, and X.-D. Li.
Environmental Pollution 153(1):3-13(2008)

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Performance Monitoring for In Situ Flushing

The performance of an in situ soil flushing system depends largely upon the amount of contact achieved between the flushing solution and the contaminants, and the effectiveness of recovery. The appropriateness of the flushing solution, the soil adsorption coefficients of the contaminants, and the permeability of the soil are also key factors (EPA 1997).

For inorganic contaminants that might be flushed with plain water, the increase in groundwater concentrations of the target contaminant as flushing begins indicates its transfer from the vadose zone to the saturated zone, where it can be captured by extraction wells. At perchlorate-contaminated Site 285, Edwards Air Force Base, neutron logging was used to monitor wetting front migration down to groundwater, and a potassium bromide tracer released as a slug at the start of soil flushing provided confirmation that the introduced water had reached the aquifer. A perchlorate spike was detected in the groundwater after ~20 weeks of flushing (Battey et al. 2006). With continued flushing of treated water through the vadose and saturated zones, contaminant groundwater concentrations are expected to decline at Site 285. The effectiveness of the treatment for contaminated soil can be documented via confirmation soil boreholes after groundwater contaminant concentrations return to their pre-soil flushing levels (Battey et al. 2007).

The applications most often employed in soil flushing are surfactant and cosolvent flooding for fuels and chlorinated solvents (EPA 2006). The table below provides a list of performance monitoring parameters for surfactant and cosolvent flushing. Most of the table addresses surfactant flushing, which requires a greater amount of monitoring compared with cosolvent flushing alone because of its greater capacity to mobilize NAPL. The type and amount of data needed is site specific and will be affected by the site hydrogeology and size and architecture of the source area.

Discussions of the different sampling and analysis methods appropriate for surfactant/cosolvent flushing can be found in NFESC 2003 and ITRC 2003.

For assessing the performance of a chemical flood, the most direct method would be to measure the amount of DNAPL mass removed, but this approach is complicated by the uncertainty associated with estimating the initial amount of DNAPL mass present at a site. This uncertainty also affects estimating (1) the final average NAPL saturation (i.e., the volume percent of the pore space that still contains NAPL) and (2) the percentage of the initial contaminant mass removed. The performance of a surfactant/cosolvent flushing system generally is measured by the following metrics: the source strength or mass flux of contaminants emanating from the source; the percentage of the injected chemicals recovered (a measure of the efficiency of hydraulic control); and the risk associated with any NAPL remaining after treatment, as well as the risk reduction accomplished with the NAPL removal action (ITRC 2003).

Several indirect techniques can be used to measure the success of a surfactant flood: tracers, such as the partitioning interwell tracer test, where the measurement is performed before and after the flood; contaminant flux measurements, also performed before and after the flood; and geophysical techniques, such as electrical resistivity tomography, performed during the flood. Descriptions of other general techniques that might be useful can be found in Remediation Measurement Tools.

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Performance Monitoring Resources

Controlled Release, Blind Test of DNAPL Remediation by Ethanol Flushing
Brooks, M.C., M.D. Annable, P.S.C. Rao, K. Hatfield, J.W. Jawitz, W.R. Wise, A.L. Wood, and C.G. Enfield.
Journal of Contaminant Hydrology 69:281-297(2004)

A DNAPL source zone was established within a sheet-pile isolated cell through a controlled release of PCE to evaluate DNAPL remediation by in situ cosolvent flushing. Ethanol was used as the cosolvent, and the main remedial mechanism was enhanced dissolution based on the phase behavior of the water/ethanol/PCE system. Over a 40-day period, 64% of an initial mass of 83 L of PCE was removed by flushing the cell with a solution of 70% ethanol and 30% water; however, tracer results suggest that some PCE was inaccessible to the ethanol solution, which led to the inefficient PCE removal rates observed. Longer abstract

Cost and Performance Report for LNAPL Characterization and Remediation: Partition Interwell Tracer Testing (PITT) and Rapid Optical Screening Tool (ROST^TM) Characterization and Evaluation of the Feasibility of Surfactant Enhanced Aquifer Remediation (SEAR) at the Chevron Cincinnati Facility, Hooven, OH
EPA 542-R-05-017, 2005

DNAPL Characterization Methods and Approaches
Kram, M., A.A. Keller, J. Rossabi, and L.G. Everett.
Ground Water Monitoring and Remediation 21(1):67-76(2001) & 22(1):46-61(2002)

Development of Assessment Tools for Evaluation of the Benefits of DNAPL Source Zone Treatment
Abriola, L.M., P. Goovaerts, K.D. Pennell, and F.E. Loeffler.
SERDP Project ER-1293, 173 pp, 2008

This report details the results of work that has enhanced the understanding of significant mechanisms controlling DNAPL source zone behavior and describes lessons learned that can provide improved DNAPL site management strategies. It discusses 4 important concepts: (1) partial source-zone mass removal (mainly through in situ flushing technologies in this study) can result in substantial local concentration and mass flux reductions; (2) potential remediation efficiency is closely linked to source-zone architecture (ganglia-to-pool ratios); (3) biostimulation and bioaugmentation approaches are feasible for treatment of DNAPL source zones; and (4) the uncertainty in mass discharge estimates can be quantified through application of geostatistical methods to field measurements.

Evaluation of In-Situ DNAPL Remediation and Innovative Site Characterization Techniques
Sillan, R.K., M.D. Annable, and P.S.C. Rao.
Florida Center for Solid and Hazardous Waste Management, Gainesville. 69 pp, 1999

This study reports the evaluation of the field-scale performance of in situ cosolvent flushing and innovative tracer techniques for site characterization of the former Sages Dry Cleaner site.

Impacts of DNAPL Source Treatment: Experimental and Modeling Assessment of the Benefits of Partial DNAPL Source Removal
Wood, A.L., M.D. Annable, J.W. Jawitz, R.W. Falta, M.C. Brooks, C.G. Enfield, P.S.C. Rao, and M.N. Goltz.
EPA 600-R-09-096, SERDP Project ER-1295, 172 pp, Sep 2009

For 2002 surfactant flushing at Hill AFB, a transect of passive flux meters (PFMs) used to measure TCE mass flux before and after the surfactant flood indicated a reduction in TCE flux of ~90%. With the integral pumping test method, water was subsequently extracted from the same series of wells, and a contaminant concentration/time series was measured in each pumping well effluent. These two methods were compared to mass flux calculated using water quality data from the transect of fully screened wells (10 monitoring wells on 10-ft centers downgradient from the DNAPL source zone). The three independent measurement techniques provided comparable results. For the Sages Dry Cleaner site 1998 pilot-scale ethanol flush, the performance estimate was supported by soil core results and a post-remedial partitioning tracer test. A second ethanol flood conducted in 2004 to remove additional PCE provided the opportunity to evaluate the impact of source depletion on contaminant mass flux.

Improved Monitoring Methods for Performance Assessment During Remediation of DNAPL Source Zones
Siegrist, R., R. Oesterreich, L. Woods, and M. Crimi.
Strategic Environmental Research and Development Program (SERDP), Project ER-1490, 116 pp, 2010

Investigators evaluated (1) the effects that sampling methods can have on the accuracy of measurements made for chlorinated solvents in samples of porous media collected from intact cores, and (2) the effects that remediation agents can have on the ability to infer CVOC mass levels in the subsurface based on groundwater concentration data. The accuracy of VOC measurements was investigated using an experimental apparatus packed with sandy porous media and contaminated with known levels of VOCs (PCE, TCE, TCA) sampled using different methods under variable, but controlled, conditions. Five sampling methods were examined representing different degrees of porous media disaggregation and duration of atmospheric exposure that can occur during sample acquisition and preservation in the field. CVOCs were studied at dissolved, sorbed, and nonaqueous phases. Five porous media temperatures were examined ranging from 5 to 80 degrees C to represent ambient or thermal remediation conditions, and two water saturation levels were used to mimic vadose zone and groundwater zone conditions. Results show that sampling method attributes can impact the accuracy of VOC measurements in porous media by causing negative bias in VOC concentration data ranging from near 0 to 90% or more. In situ remediation technologies, such as thermal treatment, ISCO, and flushing, have the potential to alter subsurface properties, which can affect the behavior of CVOCs, including DNAPLs.

Site-Specific Verification of Surfactant-Cosolvent Flushing
Vane, L., and S.L. Yeh.
NATO/CCMS Pilot Study. Evaluation of demonstrated and emerging technologies for the treatment of contaminated land and groundwater (Phase III). 2001 Special Session: Performance verification of in situ remediation technologies. EPA 542-R-02-002, p 59-78, 2002

This paper describes the use of soil core analysis and partitioning interwell tracer tests to assess technology performance during a field demonstration of surfactant-based soil flushing of PCE at MCB Camp Lejeune. Although the pre-flush PITT results provided useful information, the post-flush PITT results were unusable because an impurity in the surfactant solution sorbed to the aquifer soils, which caused an unanticipated change in the partitioning behavior of the tracers.

Strategies for Monitoring the Performance of DNAPL Source Zone Remedies
Interstate Technology and Regulatory Council (ITRC) Dense Nonaqueous-Phase Liquids Team. DNAPLs-5, 206 pp., Aug 2004.

Performance monitoring parameters for surfactant/cosolvent flushing include measuring DNAPL volume and dissolved contaminant concentration in extraction wells and monitoring points to assess remedial progress, as well as monitoring fluid chemistry (e.g., temperature, pH, specific conductance, and electrolyte concentrations); injection system pressure; injection and recovery flow rates; and changes in water level and level of free-phase DNAPL.

Tracer Techniques for DNAPL Source Delineation and In-Situ Flushing Techniques for Enhanced Source Removal: Pilot Scale Demonstrations at the Dover National Test Site
Brooks, M.C., M.D. Annable, and S.C. Rao.
AFRL-ML-TY-TR-2003-4512, 293 pp, 2001

A study was performed to evaluate the performance of innovative tracer techniques for DNAPL characterization and in situ cosolvent and surfactant flushing for DNAPL removal at the Dover National Test Site, Dover AFB, DE. The project involved controlled releases of up to 100L of perchloroethene (PCE) into test cells for each remedial technology. After the PCE release, two partitioning tracer tests were conducted: one before and another after the remedial test. The first remedial demonstration involved cosolvent flushing and the second, surfactant flushing. This report focuses on the four partitioning tracer tests and the cosolvent flushing demonstration.

Using Multilevel Samplers to Assess Ethanol Flushing and Enhanced Bioremediation at Former Sages Drycleaners
Brown, Gordon Hitchings, Master's thesis, University of Florida, 98 pp, 2006

A network of multilevel sampling wells (MLS) was installed in the Sages site source area to collect liquid samples before, during, and for 6 years after the 1998 pilot test to determine the initial and post-remedial PCE architecture at the site. Remedial performance was evaluated through comparison of pre- and post-remedial groundwater samples and partitioning tracer tests. The ethanol flushing test was effective at removing significant levels of subsurface PCE and favorably reduced the contaminant flux at most MLS locations. The use of ethanol as the remedial fluid also fostered microbial reductive dechlorination of residual PCE.

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Regulation

Some regulatory concerns are associated with the implementation of surfactant/cosolvent flushing because these technologies involve the introduction of chemical reagents into the subsurface, typically through injection wells, and because the technologies are designed to mobilize contaminants (although the intent is to control their migration and capture them). Use of injection technologies can be restricted or prohibited by regulatory or procedural barriers that vary from state to state (ITRC 2003). Land disposal restrictions and underground injection control regulations, for example, can limit selection of the flushing solution. At a former drycleaner, ethanol was substituted for isopropanol because of regulatory concern about the toxicity and persistence of isopropanol. The applicable federal and state regulatory agencies specify injection permit requirements. When applying for a permit, all chemicals involved, including unreacted compounds and impurities (e.g., benzene used as a drying agent in absolute alcohol), must be listed (NAVFAC 2002).

For CERCLA sites, the remedial alternatives must satisfy Applicable or Relevant and Appropriate Requirements (ARARs). Non-CERCLA sites (RCRA sites, private sites, state Superfund sites, and federal facilities) also can have potentially applicable regulations at the state and federal level (ITRC 2003).

Federal EPA initiatives to reduce regulatory (and legislative) barriers to the use of innovative in situ remediation technologies have been implemented as follows:

• Since 1992, EPA has been granting states the authority to implement the Treatability Exclusion Rule; the Research, Development, and Demonstration Permit Program; and the Subpart X Permit Program. Those authorities are granted to states to simplify the approval process for technologies and to allow flexibility in testing and demonstrating innovative treatment technologies (ITTs).
• To further promote the use of innovative technologies, in 1994 EPA revised its Treatability Study Sample Exclusion Rule (59 FR 8362) to allow treatability studies on up to 10,000 kg of media contaminated with nonacute hazardous waste without the requirement for permitting and manifesting.
• In addition, EPA encouraged streamlining RCRA permits and orders for ITT development and use and encouraged state adoption and streamlining of EPA authorization to administer the treatment study sample exclusion rule.
• In 1993, EPA issued the Superfund Response Action Contractor Indemnification Rule (58 FR 5972), designed to help contractors who use ITTs obtain lower deductibles on liability insurance.
• EPA's 1996 area of contamination (AOC) policy allows soils to be excavated, moved, treated, and redeposited within the AOC without triggering RCRA regulatory requirements (ITRC 2005).
• In December 2000, EPA issued a memorandum, Applicability of RCRA Section 3020 to In-Situ Treatment of Ground Water, which clarifies that reinjection of treated groundwater to promote in situ treatment is allowed under section 3020(b), as long as certain conditions are met. Specifically, the groundwater must be treated prior to reinjection; the treatment must be intended to substantially reduce hazardous constituents in the ground water—either before or after reinjection; the cleanup must be protective of human health and the environment; and the injection must be part of a response action under CERCLA section 104 or 106 or a RCRA corrective action intended to clean up the contamination.
• EPA's Underground Injection Control (UIC) Program is responsible for regulating the construction, operation, permitting, and closure of injection wells that place fluids underground for storage or disposal. The UIC website offers guidance to inform state regulators and owners and operators of injection wells on how to operate injection wells safely to prevent contamination of underground drinking water resources.

Individual state regulations may be more restrictive than EPA requirements; hence, review of individual state regulations is important.

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