Dense Nonaqueous Phase Liquids (DNAPLs)

Soil Vapor Extraction and Air Sparging

This page identifies general resources that contain detailed information on the design and implementation of two volatilization technologies that are often used together. A layman's discussion of the technologies is available in Community Guide to Soil Vapor Extraction and Air Sparging. More technical resources on these technologies for a variety of volatile contaminants can be found in the Air Sparging and Soil Vapor Extraction pages of Technology Focus.

In the unsaturated zone, volatile DNAPL compounds can exist in gaseous, aqueous dissolved, sorbed, and free phases. A vapor extraction system basically applies a vacuum through a system of wells or vents completed in zones of contamination above the water table to induce a subsurface gas flow pattern that converges on the extraction vents. The flow of gas results in mass transfer from the aqueous, sorbed, and free DNAPL phases to the sweeping gas phase, thus producing contaminant vapors that are extracted, collected, and treated aboveground. Soil vapor extraction has been available for decades, and it is considered a mature technology. Air sparging systems are designed to inject air below the water table through sparge wells. As the injected gas rises through the saturated zone and contacts dissolved-phase compounds, volatile organics transfer to the gas phase. The contaminant vapors emerge into the unsaturated zone, where the gas is collected by vapor extraction (Enfield 1999). Air sparging is considered a mature technology when it is applied to dissolved contaminants and innovative when it is applied to source zones (NRC 2004).

In the following sections, each technology is discussed separately and supported by its own resource list; however, the two technologies are not treated separately in the case study sections because they are so frequently used together. Information on applications of the technologies specific to a chemical class can be found in the class subsections listed to the right.

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National Research Council (NRC), 2004. Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academies Press, Washington, DC.

Soil Vapor Extraction (SVE)

According to EPA's 11th edition of the Treatment Technologies for Site Cleanup: Annual Status Report (2004), in situ SVE is the most frequently used source-control remediation technology. Although usually implemented in situ, SVE also can be applied to excavated soils. For in situ implementation, a vacuum is applied to induce a controlled subsurface air flow to remove volatile organic compounds (VOCs) and some semivolatile organic compounds (SVOCs) from the vadose zone to the surface for treatment. The configuration of the system usually involves attaching blowers to extraction wells, which generally are constructed with slotted polyvinyl chloride to induce airflow through the soil matrix (USACE 2002). The contaminated air is brought to the surface and passed through a vapor/liquid separator to remove any moisture before the air is treated. Treatment typically is done by adsorption (activated carbon), or for more concentrated waste streams, by thermal oxidation systems (U.S. EPA 2006b). The water generated by the liquid separator also may require treatment. When expected concentrations in the air stream are sufficiently high (1,000 to 5,000 parts per million [ppm] or more) for free product recovery for recycling, a stand-alone condensation treatment system might be considered. The condenser system will need to be changed out when contaminant concentrations drop (USACE 2002).

Concrete, asphalt, geomembrane, or other low-permeability covers often are placed over the soil surface to prevent short-circuiting of air flow and to increase the radius of influence of the extraction wells. Replacement air can be introduced into the subsurface by injecting air via a blower or by allowing air to flow into passive injection wells. While vertical wells are the most widely used SVE design method, horizontal wells or trenches provide better lateral flow and superior formation access when the contamination and/or the water table is shallow. The SVE process is driven by the partitioning of volatile materials from condensed phases (sorbed on soil particles, dissolved in pore water, or nonaqueous liquid) into the soil gas being drawn through the subsurface. The partitioning is controlled by contaminant and soil properties. These properties include contaminant vapor pressure, Henry's law constant, solubility, soil intrinsic permeability, water content (which should be low, but very dry soils also inhibit contaminant mobilization), and organic carbon content (AFCEE 2000).

SVE is best suited in well-drained, high-permeability soil (sand and gravel) with a low organic carbon content. Low permeability soil or heterogeneous soil with high carbon content is more difficult to treat with SVE and often requires a preparatory treatment, such as pneumatic or hydraulic fracturing to create cracks in the subsurface. Fracturing allows for high preferential flow paths, but the bulk of the contaminant load still depends upon low flow or diffusion from the competent soil matrix. Like fracturing, heterogeneous subsurfaces provide differential flow paths that result in efficient removal of contaminants in the permeable layers, with the less permeable layers being subject to slow diffusive forces. Rate-limited diffusion in the less permeable soils extends the time needed for remediation. Experience indicates that it may be more efficient to approach these types of sites with a pulsed pumping strategy, in which the blowers are turned off at predetermined effluent concentrations, and the contaminants are allowed to diffuse into the "clean" permeable layers. After a period of time suitable for the site conditions, the blowers are turned back on to capture the more concentrated soil vapors (AFCEE 2002). Where appropriate, this method can save money on electricity and other costs.

For SVE system design, DiGiulio and Varadhan (2001) advise care in choosing standard radius of influence methods to place extraction wells. These methods generally rely on measuring vacuum differentials with distance from the venting well. Vacuum measurements can indicate the direction of a flow gradient, but as the vacuum measured approaches ambient pressures, they may give a false indication and lead to placing wells too far apart. In addition, vacuum measurements give no information on the effective gas flow through the various subsurface materials. For example, although one-dimensional measurements made on layers of sand and silty clay will yield equivalent vacuums, the effective gas flow goes through the sand, with little going through the silty clay. A more relevant approach to well layout is to achieve a pore velocity that exceeds some minimum rate everywhere within the contaminated zone (USACE 2002).

As the vapor extraction system continues to operate, effluent contaminant concentrations generally become asymptotic (steady-state removal of very low concentrations). Unless the SVE system is addressing a single contaminant species, measurements of the venting effluent should provide the total mass being removed, as well as relative compound concentrations. Speciation data also help in evaluating the system's efficiency. Because the chemicals in a mixture have different chemical and physical properties, they will leave the mixture at different rates; hence, a drop in total concentration does not necessarily mean a drop in available contaminant or system efficiency, but rather exhaustion of certain species.

It is also important to test each extraction well in the system individually to determine if the drop is occurring across all wells (USACE 2002). Testing of the header alone may mask wells that have low flow and high concentrations that are being diluted by other wells in the system. Maintaining asymptotic levels over a period of many months is often interpreted as a sign that the SVE effort has been successful and should be shut down; however, a decrease of concentrations in the extracted vapor does not provide conclusive evidence that the concentrations in the soil have decreased proportionally (USACE 2002).

A decrease in contaminant concentration when cleanup goals have not been reached can be caused by the following factors:

• The system has exhausted the supply of readily available contaminants, and their continued presence at very low concentrations represents a draw upon diffusion rate-limited source areas.
• The water table has risen, and the source areas are no longer available to the SVE system.
• The soil has reached a dryness factor that hinders, rather than promotes, SVE.
• The measured flow represents dilution from fully flushed areas near the extraction well, while understating considerably more contaminated areas farther away, near stagnation points (AFCEE 2000 and DiGiulio and Varadhan 2001).

If no rebound is found after shutting the system down for a site-specific determined time, then confirmation sampling should be done. Confirmation sampling can be accomplished with an extensive soil-gas survey, continuous soil sampling on a statistically determined grid, or professional judgment with sufficient previous characterization information gained by use of direct push tools, such as the membrane interface probe. If a site has contaminated groundwater, it should be addressed along with the vadose zone contamination. Often this can be accomplished using an air sparging system to extract contaminants from groundwater (U.S. EPA 2006a).

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Principle source: U.S. EPA. 2006a. In Situ Treatment Technologies for Contaminated Soil: Engineering Forum Issue Paper. EPA 542-F-06-013.

with the following references:

AFCEE. 2000. Source Reduction Effectiveness at Fuel-Contaminated Sites: Technical Summary Report. 125 pp.

D. DiGiulio and R. Varadhan. 2001. Development of Recommendations and Methods to Support Assessment of Soil Venting Performance and Closure, EPA 600-R-01-070. U.S. EPA Office of Research and Development, 435 pp.

USACE. 2002. Engineering and Design: Soil Vapor Extraction and Bioventing, EM 1110-1-4001, 424 pp.

U.S. EPA. 2006b. Off-Gas Treatment for Soil Vapor Extraction Systems: State of the Practice, EPA 542-R-05-028. Office of Solid Waste and Emergency Response, 129 pp.

Soil Vapor Extraction Resources

Analysis of Selected Enhancements for Soil Vapor Extraction
U.S. EPA, Office of Solid Waste and Emergency Response.
EPA-542-R-97-007, 246 pp, 1997

Provides an engineering analysis of, and status report on, the following selected enhancements for SVE treatment: air sparging, dual-phase extraction, directional drilling, pneumatic and hydraulic fracturing, and thermal technologies. Also offers an evaluation of each technology's applicability to various site conditions, cost and performance information, strengths and limitations, recommendations, and technology vendors.

Commissioning and Demonstration for Soil Vapor Extraction (SVE) Systems
U.S. Army Corps of Engineers.
Unified Facilities Guide Specification, UFGS-02-62-16, 32 pp, 2006

Commissioning is performed after construction has been completed. It is an ordered process for testing and startup of SVE system equipment. During commissioning, pre-commissioning checklists are completed, and functional performance tests are performed to test individual components of the system and subsystems. Demonstration serves as a prove-out period. The purpose of the demonstration is to show that the SVE system, as a whole, is ready to be put into service.

Development of Recommendations and Methods to Support Assessment of Soil Venting Performance and Closure
D.C. DiGiulio and R. Varadhan.
EPA 600-R-01-070, 435 pp, 2001

Proposes a strategy for venting closure using a vadose zone paradigm to link the performance of groundwater remediation dynamically to vadose zone remediation, based on an extensive review of the literature.

Effects of Natural Environmental Changes on Soil-Vapor Extraction Rates
S. Martins and S. Gregory.
Remediation of Chlorinated and Recalcitrant Compounds, 22-25 May 2006, Monterey, California. UCRL-CONF-220094, 10 pp, 2006

Remediation by SVE has been used for chlorinated VOCs for over a decade at Lawrence Livermore National Laboratory. Data on flow-rate observations collected over this time were compared to in situ measurements of several different environmental parameters (soil-gas pressure, soil temperature, soil moisture, rainfall, and barometric pressure) to evaluate how natural changes in environmental conditions affect the rate of SVE. Environmental changes that lead to increased soil moisture are associated with reduced SVE flow rates. The use of higher extraction vacuums combined with dual-phase extraction can help to increase pneumatic conductivity when vadose zone saturation is a problem. Daily changes in barometric pressure and soil-gas temperature were found to change flow rate measurements by as much as 10% over the course of a day.

Engineering and Design: Soil Vapor Extraction and Bioventing
U.S. Army Corps of Engineers.
EM 1110-1-4001, 312 pp, 2002

Provides practical guidance for the design and operation of SVE and bioventing systems to environmental professionals who possess a technical education and some design experience but only the broadest familiarity with SVE or bioventing systems. Notes that PCBs are not amenable to removal by SVE.

Guidance for Design, Installation and Operation of Soil Venting Systems
Wisconsin Dept. of Natural Resources, Madison, WI.
PUB-RR-185, 64 pp, 2002

SVE—sometimes referred to as soil venting, in situ volatilization, in situ vapor extraction, in situ air stripping, enhanced volatilization, in situ soil ventilation, or vacuum extraction—is a multi-disciplinary process that requires a working knowledge of geology and basic engineering to design an optimal system, as well as a basic knowledge of chemistry to develop a quality sampling and monitoring plan. It may be necessary for system designers to deviate from the guidance to accommodate the unique characteristics of a site.

Guidance on Soil Vapor Extraction Optimization
Air Force Center for Environmental Excellence.
NTIS: ADA392205, 90 pp, 2001

Designed to assist environmental managers and engineers responsible for the operation and monitoring of SVE systems and to promote the closure of sites where SVE systems have achieved cleanup goals. Provides practical guidance on performance monitoring and the optimization of SVE systems so that remediation goals can be achieved in a cost-effective manner.

Innovative Site Remediation Technology: Design & Application, Volume 7: Vacuum Extraction and Air Sparging
T.B. Holbrook, et al., American Academy of Environmental Engineers, Annapolis, MD.
EPA 542-B-97-010, 392 pp, 1998

Describes specific details of design, construction, and operation of air sparging (AS) and SVE systems, including potential enhancements to vapor extraction technology.

Off-Gas Treatment Technologies for Soil Vapor Extraction Systems: State of the Practice
U.S. EPA, Office of Superfund Remediation and Technology Innovation.
EPA 542-R-05-028, 129 pp, 2006

Provides state-of-the-practice information on off-gas treatment technologies for SVE systems currently being used to clean up hazardous waste sites, ranging from common practices (carbon adsorption and thermal oxidation), to less frequently used technologies (biofiltration), to emerging alternatives (photocatalytic and non-thermal plasma treatment). Presents the state of the practice for these technologies based on applicability, limitations, performance, engineering considerations, residuals management, cost and economics, and developmental status.

Operation, Maintenance, and Process Monitoring for Soil Vapor Extraction (SVE) Systems
U.S. Army Corps of Engineers.
Unified Facilities Guide Specification UFGS-02 01 50, 30 pp, 2006

Covers requirements to be followed by the operations staff to ensure proper operation of the SVE system equipment (e.g., monitoring vacuum levels and air flow rates, vapor stream monitoring, and performing preventative maintenance) for the initial period of operations (usually a 12-month period following completion of construction, commissioning, and demonstration), or periods of operation beyond the initial period.

Soil Gas Movement in Unsaturated Systems
B.R. Scanlon, J.P. Nicot, and J.W. Massmann.
Soil Physics Companion. CRC Press, ISBN: 0849308372, p 297-341, 2002

Most of this chapter deals with transport of gas and associated chemicals through the unsaturated zone, illustrated with the following case study examples of applications of gas transport theory: estimation of gas conductivity using subsurface attenuation of barometric pressure fluctuations; barometric pressures used to help clean contaminated soils; and optimal vapor velocities for active soil vapor extraction.

Soil Vapor Extraction Implementation Experiences. Engineering Forum Issue Paper
R. Stamnes and J. Blanchard.
EPA 540-F-95-030, 9200.5-223FS, 10 pp, 1997

Identifies issues and summarizes experiences with SVE as a remedy for VOCs in soils based on discussions with over 30 Remedial Project Managers and technical experts.

Soil Vapor Extraction Subsurface Performance Checklist
U.S. Army Corps of Engineers, 8 pp, 1999

This checklist is meant to aid in evaluating the overall performance of an SVE system for removing VOCs from the soil vadose zone.

Air Sparging

Air sparging has been used successfully to clean up plumes of dissolved chlorinated solvents, with volatilization as the primary remediation mechanism. For remediation of DNAPL constituents, air or other gases are injected directly into the groundwater to vaporize and recover the dissolved contaminants. Volatile components of the DNAPLs will vaporize and move upward to the atmosphere or to an SVE system installed in the vadose zone. When air sparging and SVE (AS/SVE) are implemented together, the vapor extraction system creates a negative pressure in the vadose zone through a series of extraction wells that controls the migration of the vapor plume (NRC 1999).

The physical process for mass transport in air sparging is volatilization of the contaminant from a dissolved phase in the groundwater to the air sparged through the zone. The vaporization of VOCs is well understood. Injected air moves laterally, driven by the injection pressure, and upward, due to the buoyancy of air. As the injected air moves through the subsurface and comes in contact with NAPLs, dissolved-phase contamination in water, or contaminated soil, the volatile contaminants partition into the air (NRC 1999). As with SVE, the process is governed by the Henry's Law Constant of a compound for the dissolved phase and by vapor pressure for direct sparging of DNAPL. Only volatile compounds will be readily sparged (Fountain 1998).

Performance will be affected by permeability of the subsurface. Low permeability units may not allow adequate air flow, while vapor flow may become highly localized in high permeability units, resulting in a very small radius of influence for each well. Because the process is more efficient in thick saturated zones, the radius of influence of a well may be small if the saturated thickness is small (Fountain 1998). DNAPL tends to be concentrated above lower permeability units, and sparging of such zones is difficult; pools and lenses on low permeability layers are likely to be resistant to rapid treatment by air sparging (Unger et al. 1995). The effectiveness of sparging technology for remediating contaminated sites is highly dependent on site-specific conditions, and careful site characterization is essential to successful sparging implementation. For example, if saturated thickness is small, the number of wells required for adequate coverage may become cost prohibitive.

Understanding how injected air is distributed within the aquifer and how this affects partitioning of contaminants is critical to the success of this technology. Laboratory and field studies have provided the following findings:

• Air flow is irregular in shape and is sensitive to very subtle changes in soil structure.
• Increased airflow rates generally produce more dense flow field patterns.
• Vertical wells in homogeneous soils generally result in an airflow distribution radius of less than 3 meters.
• Heterogeneous soils may have either a positive or negative effect on air distribution (NRC 2004).

According to Leeson, et al. (2002), contaminant removal during air sparging is initially dominated (over a timeframe of months) by volatilization into the discrete air channels generated by the injection wells. Subsequently, contaminant removal is controlled by mass transfer through the liquid phase surrounding the air channels.

Air sparging expands the remediation capabilities of SVE to the saturated zone. One of the limitations of SVE alone is that it does not address contaminated soils effectively within the capillary fringe and below the groundwater table. Sparging can enhance the remediation capabilities of SVE in the capillary fringe zone to include remediation of chemicals with lower volatilities and/or chemicals that are tightly sorbed (U.S. EPA 1997). Air sparging technology is relatively inexpensive. In addition, when it involves the introduction of air only, rather than other substances, regulatory approval is generally straightforward. It is commercially available, and realistic cost estimates can be obtained for a given site (NRC 1999).

The biosparging process is similar to air sparging, although air sparging removes constituents primarily through volatilization, whereas biosparging promotes biodegradation of constituents rather than volatilization, generally by using lower flow rates than are used in air sparging. In practice, some degree of volatilization and biodegradation occurs when either air sparging or biosparging is used if the contaminants are biodegradable under aerobic conditions. Highly halogenated DNAPL components generally are not readily degraded under aerobic conditions, and significant biodegradation is not expected to accompany the air or oxygen sparging of groundwater affected by halogenated compounds; however, combined injection of oxygen and an organic compound that serves as an electron donor (e.g., methane, phenol, toluene, propane) has shown potential for chlorinated compound degradation via aerobic cometabolism (Fountain 1998).

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

Leeson, et al. 2002. Air Sparging Design Paradigm. Environmental Security Technology Certification Program.

National Research Council (NRC). 1999. Groundwater and Soil Cleanup: Improving Management of Persistent Contaminants. National Academies Press, ISBN: 0-309-06549-6. Committee on Technologies for Cleanup of Subsurface Contaminants in the DOE Weapons Complex.

National Research Council (NRC), 2004. Contaminants in the Subsurface: Source Zone Assessment and Remediation. National Academies Press, Washington, DC.

Unger, A.J.A., E.A. Sudicky, and P.A. Forsyth. 1995. Mechanisms controlling vacuum extraction coupled with air sparging for remediation of heterogeneous formations contaminated by dense nonaqueous phase liquids. Water Resources Research 31(8):1913-1925.

U.S. EPA. 1997. Analysis of Selected Enhancements for Soil Vapor Extraction. EPA 542-R-97-007. OSWER.

Air Sparging Resources

Air Sparging Design Paradigm
A. Leeson, et al.
Environmental Security Technology Certification Program (ESTCP), 150 pp, 2002

Provides details on AS principles; site characterization; pilot testing; system design, installation, and operation; and system monitoring, as well as guidance for both standard and complex designs with decision points to help the user choose the level of sophistication appropriate for the site.

Air Sparging Guidance Document
K. Fields, et al.
Naval Facilities Environmental Service Center. NFESC TR-2193-ENV, 119 pp, 2002

Applies the results from full-scale system use, field studies, and research to provide a complete life-cycle approach to selection, design, installation, operation, optimization, and shutdown of AS systems.

Environmental Quality: In Situ Air Sparging
U.S. Army Corps of Engineers. EM 200-1-19, 178 pp, 2013

Provides guidance for evaluation of the feasibility and applicability of AS for remediation of contaminated groundwater and soil and, as a secondary objective, describes design and operational considerations for AS systems.

Helium Tracer Tests for Assessing Air Recovery and Air Distribution During In Situ Air Sparging
R.L. Johnson, P.C. Johnson, T.L. Johnson, and A. Leeson.
Bioremediation Journal, Vol 5 No 4, p 321-336, 2001 [NTIS: ADA406353]

Helium tracer tests are used as an alternative to soil-gas pressure measurements to assess the effectiveness of SVE systems for capturing contaminant vapors liberated by in situ AS. The tracer approach is simple to conduct and provides more direct and reliable measures than the soil-gas pressure approach. The proposed tracer test can be used both to determine SVE system capture efficiency and to evaluate air distribution during AS pilot tests. The tests also can be conducted on operating, full-scale systems to confirm system performance and are easily repeated, which allows system parameters to be modified and the impact of those modifications to be assessed quickly.

In Situ Air Sparging Subsurface Performance Checklist
U.S. Army Corps of Engineers, 6 pp, 1999

This checklist is meant to assist in evaluating the overall performance of an in situ AS system for removing VOCs from groundwater.

Innovative Site Remediation Technology: Design & Application, Volume 7: Vacuum Extraction and Air Sparging
T.B. Holbrook, et al., American Academy of Environmental Engineers, Annapolis, MD.
EPA 542-B-97-010, 392 pp, 1998

Describes specific details of design, construction, and operation of air sparging (AS) and SVE systems, including potential enhancements to vapor extraction technology.

Remediation of DNAPL Source Zones in Groundwater Using Air Sparging
K.R. Reddy and L. Tekola.
Land Contamination & Reclamation, Vol 12 No 2, p 67-83, 2004

The remedial efficiency of the AS process for volatile DNAPLs can vary considerably depending on a site's subsurface conditions and system variables. Source zones of free-phase (pure liquid) DNAPL may be particularly difficult to remediate because the contaminant tends to form small globules that become trapped within the soil matrix. Experiments conducted in a 2-D physical aquifer simulation apparatus containing a DNAPL-spiked homogeneous sand showed that AS removed DNAPL source zones effectively via increased volatilization, dissolution, and diffusion. Airflow at higher rates enhanced removal substantially over airflow at lower rates because the additional air provided a greater interfacial area for DNAPL mass transfer. AS conducted under groundwater flow conditions also resulted in an increased rate of contaminant removal, due in part to undesirable off-site lateral contaminant migration; however, application of a higher airflow rate in the presence of groundwater flow provided greater air saturation, which reduced the hydraulic conductivity of the sand, thereby hindering groundwater flow and lateral contaminant migration.

Technology Overview Report: Air Sparging
R.A. Miller.
Ground-Water Remediation Technologies Analysis Center, TO-96-04, 18 pp, 1996

Provides a brief overview of AS technology, with an introduction to its general principles, applicability and use, and advantages and limitations.

Performance Monitoring for Soil Vapor Extraction and Air Sparging

In situ soil vapor extraction (SVE) is a remediation technology in which a vacuum is applied to induce a controlled subsurface air flow to remove volatile organic compounds (VOCs) and some semivolatile organic compounds (SVOCs) from the vadose zone to the surface for treatment. The configuration of the system usually involves attaching blowers to extraction wells, which generally are constructed with slotted polyvinyl chloride (PVC) tubing to induce airflow through the soil matrix (U.S. EPA 2006).

Performance monitoring is undertaken during the SVE operating period to gauge cleanup progress and at the end of operations to measure the success in meeting remedial action objectives. Typically, the air pressure in vadose zone wells is periodically measured to ensure that a vacuum is occurring in all parts of the target zone, and groundwater elevation data are collected to verify that upwelling from the applied vacuum is not invading the contaminated zone or the extraction well screens.

Contaminant loading is measured at each extraction well and/or the pressure side of the blower/pump. Depending upon project goals, the contaminant loading can be measured with an instrument that provides a total average concentration (e.g., a hand-held photo-ionization detector) or one that provides contaminant-specific concentrations (field or laboratory GC, GC/MS). When collecting the sample, it is necessary to measure the air flow rate to allow for a subsequent calculation of mass removal. The frequency of sampling is very site specific and depends on the estimated mass of the contaminant(s) of concern (COCs) and the complexity of the site subsurface. If aerobic biodegradation also is expected to play a role in the remediation, then periodic measuring of temperature and total oxygen and carbon dioxide should be done to allow for an estimate of mass being consumed.

If the system is producing water (as can happen in a dual-phase vacuum system) or large amounts of condensate, then the water volume produced along with contaminant concentrations within it should be measured to allow calculation of mass removal rates.

Most SVE systems have three phases of performance:

• the initial pore volume removal, which is characterized by relatively high VOC levels;
• the evaporation phase, which is characterized by removal of NAPL from more permeable soil volumes; and
• the diffusion-limited phase, which is characterized by low recovered concentrations as VOCs slowly diffuse from low permeability soils (AFCEE 2001).

Depending upon the site, periodic testing for rebound concentration can provide insight into soil remediation progress (AFCEE 2001). Equilibrium rebound testing involves shutting the system down for a specified period and comparing the concentrations of the effluent upon restart with those measured at initial startup (AFCEE 2001 and USACE 2002).

Cleanup confirmation monitoring can include systematic placement of soil gas probes that are separate from the SVE unit as well as soil gas testing to determine if there are contaminant concentrations above cleanup levels that were not detected in the extraction wells. USACE 2002 also recommends taking statistically determined, random soil cores in the source zone area to test for COCs or NAPL.

Another technology that might aid in estimating the extent of DNAPL removal is the partitioning interwell tracer test.

References

Engineering and Design: Soil Vapor Extraction and Bioventing, EM 1110-1-4001
USACE. 2002, 424 pp.

Guidance on Soil Vapor Extraction Optimization
Air Force Center for Environmental Excellence.
NTIS: ADA392205, 90 pp, 2001

Innovative Site Remediation Technology: Design & Application, Volume 7: Vacuum Extraction and Air Sparging
T.B. Holbrook, et al., American Academy of Environmental Engineers, Annapolis, MD.
EPA 542-B-97-010, 392 pp, 1998

In Situ Treatment Technologies for Contaminated Soil: Engineering Forum Issue Paper
EPA, EPA 542-F-06-013, 35 pp, 2006.

SW-846 On-Line: Test Methods for Evaluating Solid Waste, Physical/Chemical Methods
U.S. EPA, Office of Resource Conservation and Recovery.