U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Treatment Technologies


Physical barriers and pump and treat systems are conventional technologies that play a significant role in managing DNAPL source areas by preventing migration of the contaminants. This page identifies general resources that contain detailed information on the design, installation, and monitoring of physical and hydraulic containment systems. On this page, physical barrier and hydraulic systems are described separately and have separate resource lists, but they are not separated in the application pages as they are often used together. Examples of the application of containment technologies to sites affected by DNAPL compounds can be found in the chemical class subsections listed to the right.

Physical Barriers

Containment, both physical and hydraulic, is a remedy commonly applied to contaminant source areas when economic, technical, or site-specific factors make it impractical to address the contaminated areas in any other way. Physical containment removes no mass at all; instead, a physical containment remedy isolates the source area to prevent the migration of contaminants and block any direct route of exposure to the source, thus reducing risk.

Physical containment is accomplished by creating impermeable barriers on all sides of the source zone with standard heavy construction methods and equipment. A typical physical containment remedy consists of a vertical barrier of very low permeability that surrounds the source on all sides and a clay aquitard below the source, topped by a low-permeability cap. Vertical barriers can be constructed using bentonite slurries, slurries combined with polymer sheets, sheet pilings with sealed joints, or pressurized injection methods. Barriers with constructed bottoms can be emplaced by several drilling methods but are not always part of the design (NRC 2004). This section briefly discusses the more commonly used conventional barrier technologies. An innovative approach to barrier technology that provides for both containment and treatment of dissolved-phase groundwater contamination is discussed separately in the section on Permeable Reactive Barriers.

Top barriers, such as cap systems, are used to (1) prevent transfer of contaminants to the atmosphere, (2) prevent generation of contaminated leachate by preventing or minimizing surface water infiltration, (3) prevent contact with treated and untreated hazardous wastes, and (4) contain waste while treatment is being applied. Most landfill top barriers are multilayer systems that contain polymer sheeting and drainage layers. Caps are most effective where most of the underlying waste is above the water table. An interim cap can be installed before final closure of the site to minimize generation of leachate until a better remedy is selected. Caps can be designed to divert water away from waste areas while minimizing erosion and also are used to cover waste masses too large for treatment, such as tailings piles at mining sites (NRC 2004). They are a presumptive remedy for old landfills. A layman's discussion of the technology is available in Community Guide to CappingAdobe PDF Logo. Caps are used by themselves or in conjunction with other waste treatment technologies, such as barrier walls, groundwater pump and treat (P&T), and in situ treatment. By itself, a cap cannot prevent horizontal flow of groundwater through the waste, only vertical entry of water into the waste. Groundwater levels within a contained area can be kept low relative to the adjacent aquifer by operating a small P&T system that withdraws groundwater from inside the system, thus creating an inward groundwater gradient that prevents outward migration of the contaminants. Top barriers help to maintain an inward gradient and lower the cost of P&T (NRC 2004).

Sheet Piles
Sheet pile can be driven into the ground to provide a rigid barrier for contaminant containment. Each sheet is designed to lock into the next sheet, and grout or gaskets are used to seal joints and minimize leaks. If not driven or sealed correctly, the interlocking joints can leak. The use of sheet piles is limited to soils into which the sheets can be driven (e.g., areas with no cobbles or boulders) and to depths of approximately 100 ft. The lifetime of steel sheet pile could be limited by corrosion, although corrosion tends to be significant only under oxidizing conditions, and subsurface contamination frequently creates reducing conditions in the aquifer. Sheet pile is a relatively expensive barrier technology, but it can be installed rapidly without soil excavation and also can be removed when it is no longer needed (NRC 1999).

Slurry Walls
Slurry walls are commonly used subsurface barriers because they are a relatively inexpensive means of reducing groundwater flow in unconsolidated earth material. Contaminated soil, wastes, and groundwater can be physically isolated within surrounding low-permeability barriers by constructing a vertical trench excavated down to and keyed into a deeper confining layer, such as a low-permeability clay or shale, and filling the trench with a slurry. Good keys can be difficult to achieve with rock aquitards. Slurry walls usually consist of a soil, bentonite, or cement mixture. The slurry mix hydraulically shores the trench to prevent collapse during installation. As the excavation continues, additional slurry is added, and the process continues until the depth and length needed to prevent the escape of contaminants from the contained area are completed. Cement and bentonite construction of a wall can adsorb and retard the escape of heavy metals and larger organic molecules but cannot completely stop water movement. Consequently, slurry walls are either interim measures or are typically accompanied by P&T systems. Evidence indicates that soil/bentonite walls may be vulnerable to organic chemicals attack (USACE 2003).

Cement/Bentonite Walls
Cement/bentonite walls are more expensive than soil/bentonite walls and generally are used where there is no room to mix and place soil/bentonite backfill, increased mechanical strength is required, or extreme topography conditions (slopes) make it impractical to grade a site level. Cement/bentonite slurry walls are limited in their use by their higher permeability and their narrow range of chemical compatibilities (i.e., they are more susceptible to attack by sulfates, strong acids or acid bases, and other highly ionic substances) (USACE 2003).

Grouted Barriers
Construction of grouted barriers involves injection of a grout into the subsurface. Pressure grouting and jet grouting are both forms of injection grouting, in which a grout mixture is injected into the pore spaces of the soil or rock. When a vibrating beam is used, the grout is injected through a special H-pile into the space created by the driven pile when the pile is removed. Particulate or chemical grouts can be used for grouted barriers. Particulate grouts include slurries of bentonite, cement, or both and water. Chemical grouts generally contain a chemical base, a catalyst, and water or another solvent. Common chemical grouts include sodium silicate, acrylate, and urethane. Particulate grouts have higher viscosities than chemical grouts and are therefore better suited for larger pore spaces, whereas chemical grouts are better suited for smaller pore spaces. Combinations of particulate and chemical grouts also can be used (U.S. EPA 1998).

In addition to containing dissolved and free-phase contamination migrating from a DNAPL source zone, barrier walls can be used to provide a controlled space for aggressive source zone remediation within the containment area (NRC 1999). Barrier walls also are used to direct or funnel the flow of groundwater to P&T well arrays or in situ treatment areas, such as a biosparging array or permeable reactive barrier (USACE 2003).

Barrier systems can be used to contain any contaminants that are not expected to react with or leach through the components of the containment system. Treatability tests should be performed to evaluate the chemical stability of barrier material in relation to the compounds and conditions to which it will be exposed. For example, wood preserving compounds can affect cement/bentonite barriers. The impermeability of bentonite can decrease significantly with exposure to high concentrations of creosote, water-soluble salts (copper, chromium, arsenic), or fire-retardant salts (borates, phosphates, and ammonia) (U.S. EPA 1992).

A detailed site characterization must identify the areal extent and the depth of source areas to be contained. Knowing the depth and thickness of the underlying aquitard is critical to making the vertical barriers deep enough to key into the aquitard. The aquitard topography must be known so that any depth variations can be taken into account during barrier construction. The internal structure of the source materials or the mass or concentration of contaminants present is not vital to barrier design (NRC 2004), but extremely careful site investigation and modeling is required to ensure that all the contaminant sources lie within the containment structure. The importance and difficulty of this task is illustrated by a hard lesson learned at Hill Air Force Base in Utah, where eight years of intensive site investigation failed to discover a TCE DNAPL pool that lay outside the 1,500 feet long containment wall, rendering it useless in terms of preventing further contamination of the groundwater downgradient (Brusseau et al. 2001). Additionally, the risk of DNAPL mobilization is inherent at all DNAPL-contaminated sites having free-phase pools. Any disturbance of a site to emplace a barrier has the potential to mobilize DNAPLs if a DNAPL pool is penetrated. This mobilization potential underscores the need for accurate site characterization (NRC 1999).

The installation of a subsurface barrier will alter the flow of groundwater, and groundwater modeling is necessary during the containment system design process to identify the changes. Adjacent sites could be affected as water diverts around the barrier, and groundwater mounding can occur upgradient of the barrier. If modeling predicts that mounding will be substantial, then the potential for groundwater to overtop the barrier and flood low areas or basements upgradient would be a significant concern, and a diversion or drainage method might have to be implemented (Brusseau et al 2001).

Barriers and other structural enhancements used for containment generally can be constructed to depths of about 30 meters using equipment such as augers, draglines, clamshells, and special excavators with extended booms. The cost of containment rises as the depth of treated subsurface increases. Costs for containment systems correspond to the types and quantities of construction necessary, including the depth to aquitard, total length of vertical barrier, type of barrier wall construction, type of cap, and the need (if any) to construct a bottom. Monitoring systems are necessary but usually are not complex (NRC 2004).

Advantages of containment:

  • It is a simple and robust technology.
  • Containment typically is inexpensive compared to treatment, especially for large source areas.
  • A well-constructed containment system almost completely eliminates contaminant transport to other areas and thus prevents both direct and indirect exposures.
  • In unconsolidated soils, containment systems substantially reduce mass flux and source migration potential.
  • Containment systems can be combined with in situ treatment and in some cases might allow the use of treatments that would constitute too great a risk with respect to migration of either contaminants or reagents in an uncontrolled setting.

Limitations of containment:

  • Containment does not reduce source zone mass, concentration, or toxicity unless it is used in combination with treatment technologies; generally, only limited treatment will be provided by the P&T systems installed to control groundwater infiltration.
  • Containment systems such as slurry walls are not impermeable and hence provide containment over a finite period.
  • Data are not yet available concerning the long-term integrity of the different types of physical containment systems.
  • Long-term monitoring of the containment system is essential for assuring that contaminants are not migrating.

Adobe PDF LogoBrusseau, M.L., R.G. Arnold, W. Ela, and J. Field. 2001. Overview of Innovative Remediation Approaches for Chlorinated Solvents. Arizona Department of Environmental Quality.

National Research Council (NRC). 1999. Groundwater and Soil Cleanup: Improving Management of Persistent Contaminants. National Academy Press, ISBN: 0-309-06549-6.

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

Adobe PDF LogoPearlman, L. 1999. Subsurface Containment and Monitoring Systems: Barriers and Beyond.

Adobe PDF LogoU.S. Army Corps of Engineers (USACE). 2003. Safety and Health Aspects of HTRW Remediation Technologies. EM 1110-1-4007, Chapter 5: Slurry Walls.

Adobe PDF LogoU.S. EPA. 1992. Contaminants and Remedial Options at Wood Preserving Sites. EPA 600-R-92-182.

Adobe PDF LogoU.S. EPA. 1998. Evaluation of Subsurface Engineered Barriers at Waste Sites. EPA 542-R-98-005.

Physical Barriers: General Resources

Adobe PDF LogoAssessment of Barrier Containment Technologies : A Comprehensive Treatment for Environmental Remediation Applications
S.A. O'Donnell, R.R. Rumer, and J.K. Mitchell, eds. Sponsored by U.S. DOE, U.S. EPA, and Dupont Company. NTIS: PB96-180583, 438 pp, 1995

Contains the edited summary reports from each working session of the first International Containment Technology Workshop, held 29-31 August 1995, in Baltimore, MD. Addresses the gap between what was known and understood about environmental containment technologies and the level of information needed to support consistent decision-making relative to their application in remediation. Discusses design and construction of vertical barrier walls (including sheet piles), barrier floors (indigenous and artificial), caps, geomembrane applications, barrier materials (soil-based and chemical-based), permeable reactive barriers, contaminant transport modeling, performance monitoring, and emplacement verification.

Assessment of the Performance of Engineered Waste Containment Barriers
National Research Council.
National Academies Press, Washington, DC. ISBN-10: 0-309-10809-8, 134 pp, 2007

Focuses on engineered barriers designed to contain municipal solid waste, other nonhazardous solid and liquid waste, hazardous and toxic wastes, and low-level radioactive wastes. Concludes that most engineered waste containment barrier systems that have been designed, constructed, operated, and maintained in accordance with current statutory regulations and requirements have provided environmental protection at or above specified levels (based on as much as 20 years of observations); however, extrapolations of long-term performance have a high level of uncertainty.

Barrier Systems for Environmental Contaminant Containment and Treatment
C.C. Chien, H.I. Inyang, and L.G. Everett.
CRC Press, Boca Raton, FL. ISBN: 9780849340406, 375 pp, 2006

Covers the following topics: damage and system performance prediction, modeling of fluid transport through barriers, materials stability and application, airborne and surface geophysical method verification, and subsurface barrier verification.

Barrier Technologies for Environmental Management: Summary of a Workshop
National Research Council.
National Academies Press, Washington, DC. ISBN-10: 0-309-05685-3, 188 pp, 1997

Notes that barrier technologies (e.g., surface caps and subsurface vertical and horizontal barriers) can provide interim containment while more permanent remedial technologies are being developed as well as longer-term isolation of hazardous contaminants remaining after remediation. Discusses the following themes: (1) employing proper installation techniques and quality control measures, especially during construction, (2) determining effective lifetimes of selected barrier materials and resultant barrier systems, (3) regular inspection, maintenance, and monitoring of containment barriers, (4) data gaps in barrier performance monitoring data, (5) data gathering on both successful and unsuccessful barrier installations, and (6) advantages of using barriers in combination with P&T.

Adobe PDF LogoClay Barrier Layer
U.S. Army Corps of Engineers.
UFGS-02 56 14, 17 pp, 2006

Covers the requirements for construction of a clay barrier layer to isolate contaminated material from the environment.

Adobe PDF LogoContaminant Flux Reduction Barriers for Managing Difficult-to-Treat Source Zones in Unconsolidated Media: Technical Guidance Manual
Newell, C., E.A. Higgins, P.R. Kulkarni, and B.A. Strasters.
ESTCP Project ER-201328, 51 pp, 2017

This containment approach aims to improve groundwater quality at chlorinated VOC sites by (1) physically reducing the mass flux of contaminants leaving the source zone by using permeation grouting, thereby reducing risk and making the downgradient plume more amenable to management by natural attenuation; and (2) increasing the natural depletion rate within the source by diverting competing electron acceptors around it to create an enhanced reductive dechlorination zone. A small-scale demonstration achieved an average 64% reduction in flow through three small barriers, which was lower than the objective of 90% reduction in flow, likely owing to the low permeability of the silty sands in the test area. Applications ≥one acre in area can be significantly less costly than conventional in situ remediation technologies ($996K/ acre and $21/yd3 for a 1-acre site). Based on lessons learned during the small-scale demonstration, the process is moderately complex to implement in the field but with no major problems. Additional information: Final ReportAdobe PDF Logo; Cost & Performance ReportAdobe PDF Logo; Source Barrier ToolAdobe PDF Logo

Adobe PDF LogoDemonstration of a Subsurface Containment System for Installation at DOE Waste Sites
T.J. Crocker and V.M. Carpenter.
U.S. DOE, 173 pp, 2003

Describes the development and demonstration of an innovative subsurface barrier system that provides an in situ containment barrier around existing hazardous waste to protect soil and groundwater from further contamination by combining conventional and specialized construction equipment, high-density polyethylene and bentonite materials, and an innovative construction method.

Engineering and Design: Checklist for Design of Vertical Barrier Walls for Hazardous Waste Sites
U.S. Army Corps of Engineers.
ETL 1110-1-163, 27 pp, 1996

Briefly discusses design aspects for vertical barrier walls with design references, followed by a checklist covering pertinent aspects of design. Focuses primarily on slurry walls, with short descriptions of cement-bentonite slurry walls, vibratory beam walls, sheet pile walls, grout curtains, deep soil mixing, geomembrane walls, Soilsaw(tm) walls, and permeable reactive barriers.

Adobe PDF LogoEngineering and Design: Design of Sheet Pile Walls
U.S. Army Corps of Engineers.
EM 1110-2-2504, 75 pp, 1994

Provides guidance for the safe design and economical construction of sheet pile retaining walls and floodwalls.

Adobe PDF LogoEvaluation of Subsurface Engineered Barriers at Waste Sites
U.S. EPA, Office of Solid Waste and Emergency Response.
EPA 542-R-98-005, 148 pp, 1998

Describes the performance of subsurface engineered barriers at each of 36 sites, including the performance evaluation process, the availability of information upon which to base judgment of barrier performance, and findings and conclusions regarding observed similarities or trends among sites.

Adobe PDF LogoGuidance for Cover Systems as Soil Performance Standard Remedies
Wisconsin Department of Natural Resources, Madison.
PUB-RR-709, 22 pp, Jan 2007

Provides general information (rather than detailed engineering design) with examples on remedy selection, design, construction, and operation and maintenance concepts for cover systems for soil performance standard remedies.

Adobe PDF LogoGuide Specification for Construction: Soil-Bentonite (S-B) Slurry Trench
U.S. Army Corps of Engineers.
UFGS-02 35 27, 27 pp, 2006

Covers the requirements for constructing a soil/bentonite slurry trench at both conventional and hazardous waste project sites, noting that chemical contaminants commonly associated with hazardous waste sites may increase the permeability of S-B backfill, which necessitates undertaking a compatibility testing program (which can take 2 to 6 months to complete) prior to constructing a slurry trench. Suggests performing compatibility testing using two potential backfill materials—soils to be excavated from the trench and an uncontaminated borrow source—if the trench is to be excavated through contaminated material.

Adobe PDF LogoInternational Containment & Remediation Technology Conference and Exhibition, 9-12 February 1997, St. Petersburg, Florida
Sponsored by U.S. DOE, U.S. EPA, and Dupont Company.
CONF-970208-Proceedings, 1,164 pp, 1997

Contains the manuscripts of the papers and posters presented at the 1997 conference.

Adobe PDF LogoLong-Term Verification of Cover Systems Using Perfluorocarbon Tracers
J. Heiser, T. Sullivan and M. Serrato.
Journal of Environmental Engineering, Vol 131 No 6, p 952-960, 2005 (Also as BNL-74670-2005-JA, 31 pp, 2004)

Describes a novel methodology for verifying and monitoring subsurface barriers and cover systems in which gaseous perfluorocarbon tracers (PFTs) are injected on one side of the barrier and searched for on the opposite side of the barrier. Notes that the capability for leak detection in subsurface barriers using PFTs has been proven in multiple demonstrations.

Adobe PDF LogoSealable Joint Steel Sheet Piling for Groundwater Control and Remediation: Case Histories
D. Smyth, R. Jowett, and M. Gamble.
International Containment Technology Conference, St. Petersburg, Florida, 9-12 February 1997, 9 pp, 1997

Describes the Waterloo Barrier(tm) in which steel sheet piling incorporates a cavity at each interlocking joint that is flushed clean and injected with sealant after the piles have been driven into the ground to form a vertical cutoff wall. Presents case histories of Waterloo Barrier(tm) cutoff walls used to prevent off-site migration of contaminated groundwater or soil gases to adjacent property or waterways, including full enclosures to isolate DNAPL source zones or portions of contaminated aquifers for pilot-scale remediation testing.

Slurry Walls
U.S. Army Corps of Engineers.
Engineering and Design: Safety and Health Aspects of HTRW Remediation Technologies. EM 1110-1-4007, Chapter 5: 9 pp, 2003

Briefly discusses the process of excavating contaminated solids and sludges, dewatering, pretreatment, and technology applications, followed by a hazard analysis with controls and control points.

Adobe PDF LogoSubsurface Containment and Monitoring Systems: Barriers and Beyond
L. Pearlman.
U.S. EPA, 66 pp, 1999

Provides a brief survey on the past, present, and future of subsurface barriers—vertical and horizontal—with an emphasis on emerging and innovative vertical barrier technologies in various stages of development.

Abstracts of Journal Articles

Cost-Optimal Contaminant Plume Management with a Combination of Pump-and-Treat and Physical Barrier Systems
P. Bayer, M. Finkel, and G. Teutsch.
Ground Water Monitoring & Remediation, Vol 25 No 2, p 96-106, 2005

Uses a comparative cost analysis to discuss the economic potential of combining hydraulic barriers and P&T systems to manage contaminant plumes, i.e., whether the reduction of the operational costs outweighs the capital costs associated with the construction of physical barriers like slurry walls or sheet piles. Notes that modeling results indicate that physical barriers can yield significant savings in the total costs, particularly if unit costs for on-site treatment are high.

The Compatibility of Slurry Cutoff Wall Materials with Contaminated Groundwater
S. Day.
American Society for Testing and Materials, ASTM STP-1142-EB, 16 pp, 1994

Suggests a suite of indicator tests in which the leachate and the proposed materials are combined and tested in immersion, desiccation, sedimentation, and other modes. Attempts to model a different scenario of the slurry cutoff wall installation and operation with each indicator test. Presents the experience of a specialty contractor from projects where an incompatibility was discovered and alternate materials were used to find a successful solution as verified by subsequent monitoring results. Provides relatively simple lab test methods for use with worst-case scenarios in a step-by-step process that culminates with flexible wall permeability tests.

Evaluation of Two Methods for Constructing Vertical Cutoff Walls at Waste Containment Sites
C.P. Jepsen and M. Place.
Hydraulic Barriers in Soil and Rock. American Society for Testing and Materials, ASTM STP 874, p 45-63, 1985

Compares two methods of constructing vertical cutoff walls when considered in the context of controlling the migration of waste materials, as well as the quality control (QC) of each method, relating its importance to the central issue of cutoff wall continuity. QC considerations include geometry, wall thickness, Darcy's equation, wall composition, and chemical resistance.

Hydraulic Containment

The potential for off-site migration of contaminated groundwater to affect receptors is a critical concern. While stable and attenuating groundwater plumes may not require active remediation and often can be managed with long-term monitoring, migrating plumes, particularly at sites where downgradient receptors are contacting groundwater or vapor intrusion into buildings might be an issue, will require containment or remediation. The use of groundwater extraction wells to prevent migration of aqueous-phase contamination and/or contain the DNAPL source zone hydraulically is a mature technology with a large base of experience.

Conventional pump and treat (P&T) systems serve two main purposes: to contain the contaminant plume by changing the natural hydraulic gradient, which creates a capture zone that draws surrounding groundwater to the extraction wells, and to remove contaminants from the groundwater aquifer. The extracted groundwater typically is treated ex situ in a treatment plant before being discharged to surface water, a sewer system, or reinjected by pump or gravity drain back into the ground.

At most sites where P&T has been used, decreases in contaminant concentrations in extracted water were observed during pumping, but cleanup targets were not met; however, at almost all sites hydraulic containment was achieved, demonstrating that the technology can be effective in simply halting the spread of contaminants from source zones to groundwater (NRC 2004). Where site conditions make source removal technically infeasible, a plume containment strategy may be the only feasible option (AFCEE 2000). Hydraulic controls are particularly useful for deep or large source zones where physical barriers are impractical (Brusseau et al. 2001).

A P&T system can be used in conjunction with a low-permeability barrier. The low permeability barrier slows the flow of contaminated groundwater and prevents the escape of mobile DNAPL. The P&T system reduces the amount of contaminated water impinging on the barrier and maintains a negative groundwater gradient into the containment area, ensuring that potential imperfections in the wall do not allow contamination to escape.

A layman's discussion of the technology is available in Community Guide to Pump and TreatAdobe PDF Logo.

The design for a hydraulic control system is based on consideration of the following factors (Lye et al. 1997):

  • Size of plume or contaminated area;
  • Groundwater flow rate and direction of flow;
  • Proximity of site to existing groundwater extraction wells;
  • Hydrogeologic conditions;
  • Pumping rate needed to control plume;
  • Positioning of extraction wells to achieve containment;
  • Expected height and profile of water table at steady-state pumping;
  • Expected concentration of contaminants in pumped water (modeled over time);
  • Environmental effects of pumping (e.g., impact on levels in nearby surface water); and
  • Effects of re-injection of groundwater, if this option is being considered; and
  • Ability of local wastewater treatment plant to accept the treated water, if this option is being considered.

The effectiveness of P&T can be inhibited by inadequate design and implementation—e.g., too few recovery wells, insufficient pumping rates, deficient well locations or completion intervals, and failure to account for the complex chemistry of contaminants—just as poor system operation (e.g., too much downtime) will restrict its effectiveness. The more complex the hydrogeologic setting, the more challenging the design of an optimal hydraulic containment system. The depth limitations associated with physical barriers do not limit hydraulic containment aside from those associated with well drilling, although costs are likely to increase as well depth increases (NRC 2004).

In systems with high hydraulic conductivities (such as gravel or coarse sand), hydraulic containment can be difficult to achieve because high pumping rates may be required from closely spaced wells. In low-permeability formations (such as clays or silts), effective hydraulic containment also can be difficult to obtain due to the high gradients required to achieve significant capture zone size. For shallow groundwater plumes in low permeability formations, french drains (a trench filled with high-permeability material that drains to sumps with groundwater pumps) can be a very effective way to obtain hydraulic control of a plume. In highly heterogeneous systems, effective hydraulic containment is limited by the lack of hydraulic connectivity resulting from the presence of lower-permeability zones, particularly in fractured systems and karst, for which connectivity can be difficult to determine (NRC 2004).

Evaluation of the effectiveness of groundwater containment systems will require a careful analysis of water levels surrounding the pumping system and of contaminant trends, particularly at wells located at the plume perimeter. In a containment scenario, contaminant concentrations in the plume perimeter monitoring wells should steadily decrease. In theory, the quantity of water pumped from the aquifer should decrease over time as pumping is focused closer and closer to the source area. EPA's 1994 publication, Methods for Monitoring Pump-and-Treat PerformanceAdobe PDF Logo, provides useful guidance in evaluating the effectiveness of both contaminant extraction and hydraulic containment (AFCEE 2000).

Hydraulic containment costs are associated with the operation and maintenance (O&M) of a pumping system and with treatment of extracted water, typically by ex situ processes such as air stripping, thermal oxidation, carbon adsorption, biological reactors, or chemical oxidation or precipitation. As a consequence of the difficulties of identifying and remediating residual DNAPL, continued dissolution and migration of chemicals from a residual source likely will require containment O&M activities for extended timeframes, and can even necessitate perpetual hydraulic containment at some sites (AFCEE 2000, EPA 2001). Continuing implementation of an isolation/containment remedy will include periodic evaluations of emerging technologies for source zone treatment and new regulatory options. Regulatory approval will be required to change primary remedial objectives. To provide more flexibility, technology contingencies can be placed in the decision document that allow changes in technologies, such as turning a P&T system off to implement another technology or converting it from pure P&T to an oxidant or biological stimulant recirculation system.

Hydraulic containment advantages:

  • Techniques and equipment are readily available.
  • May be less costly than construction and operation of alternatives.
  • High degree of design flexibility.
  • Moderate to high operational flexibility, which will allow the system to meet increased or decreased pumping demands as site conditions change.

Hydraulic containment limitations:

  • O&M costs are higher than for artificial barriers (e.g., energy costs for pumping).
  • System failures could lead to migration of dissolved-phase contamination and possibly contact with receptors.
  • Plume volume and characteristics will vary with time, climatic conditions, and changes in the site, resulting in costly and frequent monitoring or the need to re-design the system (USACE 1994).

Adobe PDF LogoAir Force Center for Environmental Excellence (AFCEE). 2000. Remediation of Chlorinated Solvent Contamination on Industrial and Airfield Sites. United States Air Force Environmental Restoration Program.

Adobe PDF LogoBrusseau, M.L., R.G. Arnold, W. Ela, and J. Field. 2001. Overview of Innovative Remediation Approaches for Chlorinated Solvents. Arizona Department of Environmental Quality.

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

Adobe PDF LogoU.S. Army Corps of Engineers (USACE). 1994. Control and Containment Technologies. Chapter 3 in Engineering and Design: Technical Guidelines for Hazardous and Toxic Waste Treatment and Cleanup Activities. EM 1110-1-502.

U.S. EPA. 2001. Groundwater Pump and Treat Systems: Summary of Selected Cost and Performance Information at Superfund-Financed Sites. EPA 542-R-01-021a.

Hydraulic Containment: General Resources

Adobe PDF LogoCost-Effective Design of Pump and Treat Systems
U.S. EPA, Office of Solid Waste and Emergency Response.
OSWER 9283.1-20FS, EPA 542-R-05-008, 38 pp, 2005

Summarizes key aspects to consider for designing cost-effective P&T systems based on professional experience in designing and operating long-term groundwater remedies and on lessons learned from conducting remediation system evaluations of Superfund-financed P&T systems.

Adobe PDF LogoDesign Guidelines for Conventional Pump-and-Treat Systems: Ground Water Issue
R.M. Cohen, J.W. Mercer, R.M. Greenwald, and M.S. Beljin.
EPA 540-S-97-504, 38 pp, 1997

Discusses P&T remediation strategies (including hydraulic containment, restoration, and mixed objectives); site characterization considerations for system design; capture zone analysis for system design; extraction/ injection scheme design; components of a P&T system; selection of treatment technologies; and performance monitoring.

Adobe PDF LogoEffective Contracting Approaches for Operating Pump and Treat Systems
U.S. EPA, Office of Solid Waste and Emergency Response.
OSWER 9283.1-21FS, EPA 542-R-05-009, 22 pp, 2005

Summarizes key aspects to consider for contracting to operate P&T systems based on lessons learned from conducting remediation system evaluations at 20 Superfund-financed P&T systems.

Adobe PDF LogoElements for Effective Management of Operating Pump and Treat Systems
U.S. EPA, Office of Solid Waste and Emergency Response.
OSWER 9355.4-27FS-A, EPA 542-R-02-009, 22 pp, 2002

Summarizes key aspects of effective management for operating P&T systems based on professional experience in designing and operating long-term groundwater remedies and on lessons learned from conducting remediation system evaluations of Superfund-financed P&T systems.

Guidance for Evaluating Technical Impracticability of Ground-Water Restoration
U.S. EPA, Office of Solid Waste and Emergency Response.
OSWER Directive 9234.2-25, EPA 540-R-93-080, 30 pp, 1993

Clarifies how EPA will determine whether groundwater restoration at Superfund and RCRA sites is technically impracticable and if so, what alternative measures must be undertaken to ensure that a final remedy is protective. Describes the types of technical data needed, the criteria for decisions, the types of documentation needed, and alternative remedial strategies for sites with DNAPLs.

Hydraulic Optimization Demonstration for Groundwater Pump-and-Treat Systems
R. Greenwald.
EPA 542-R-99-011A & B, 2 vols., 1999

Presents a spreadsheet-based screening analysis using site-specific values of competing alternatives for quick identification of significant cost savings for an existing or planned P&T system.

Adobe PDF LogoMethods for Monitoring Pump-and-Treat Performance
R.M. Cohen, A.H. Vincent, J.W. Mercer, C.R. Faust, and C.P. Spalding.
EPA 600-R-94-123, 110 pp, 1994

Outlines methods for evaluating the effectiveness and efficiency of P&T remediation systems with extensive discussion of how the process is affected by the presence of NAPLs.

Adobe PDF LogoPump-and-Treat Ground-Water Remediation: A Guide for Decision Makers and Practitioners
U.S. EPA, National Risk Management Research Laboratory.
EPA 625-R-95-005, 90 pp, 1995

Presents the basic concepts of P&T technology and provides decision-makers with a foundation for evaluating the appropriateness of conventional or innovative groundwater remediation approaches.

Adobe PDF LogoRoadmap to Long-Term Monitoring Optimization
U.S. EPA, Office of Superfund Remediation and Technology Innovation.
EPA 542-R-05-003, 48 pp, 2005

Focuses on optimization of established long-term monitoring programs for groundwater and discusses tools and techniques for optimizing the monitoring frequency and spatial (3-D) distribution of wells.

Adobe PDF LogoTechnical Impracticability Waivers: Guidelines for Site Applicability and the Application Process — Draft
U.S. Army Environmental Center, 89 pp, 2002

Presents information on technical impracticability (TI) waivers, including the definition of "technical impracticability," the regulatory implications of a TI Waiver, the TI application process, the review process, and final documentation of TI decisions. Illustrates how the process works with case studies from sites that have obtained TI Waivers, how it may vary with individual sites, and how it may vary within different EPA regions.

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