a

GAO

United States Government Accountability Office

Report to Congressional Committees

June 2005 GROUNDWATER 

CONTAMINATION

DOD Uses and 

Develops a Range of 

Remediation 

Technologies to Clean 

Up Military Sites

GAO-05-666
What GAO Found

United States Government Accountability Office

Why GAO Did This Study

Highlights

Accountability Integrity Reliability

www.gao.gov/cgi-bin/getrpt?GAO-05-666.

To view the full product, including the scope 

and methodology, click on the link above. 

For more information, contact Anu K. Mittal at 

(202) 512-3841 or mittala@gao.gov. 

Highlights of GAO-05-666, a report to

congressional committees 

June 2005

GROUNDWATER CONTAMINATION

DOD Uses and Develops a Range of 

Remediation Technologies to Clean Up 

Military Sites 

DOD has implemented or field-tested all of the 15 types of generally 

accepted technologies currently available to remediate contaminated 

groundwater, including several alternatives to pump-and-treat technologies. 

Some of these technologies, such as bioremediation, introduce nutrients or 

other materials into the subsurface to stimulate microorganisms in the soil; 

these microorganisms consume the contaminant or produce byproducts that 

help break down contaminants into nontoxic or less-hazardous materials. 

DOD selects the most suitable technology for a given site on the basis of 

several factors, such as the type of contaminant and location in the 

subsurface, and the relative cost-effectiveness of a technology for a given 

site. DOD has identified a number of contaminants of concern at its 

facilities, each of which varies in its susceptibility to treatment. The table 

below shows the technologies DOD used to remediate contaminated 

groundwater. 

GAO did not identify any alternative groundwater remediation technologies 

being used or developed outside DOD that the department has not 

considered or used. Most of the new approaches developed by commercial 

vendors and available to DOD generally use novel materials applied to 

contaminated sites with existing technologies. DOD actively researches and 

tests new approaches to groundwater remediation largely by developing and 

promoting the acceptance of innovative remediation technologies. For 

example, DOD’s Strategic Environmental Research and Development 

Program supports public and private research on contaminants of concern 

to DOD and innovative methods for their treatment. 

Technologies DOD Components Used for Groundwater Remediation 

Technology Air Force Army 

Army 

Corps of 

Engineers 

Defense 

Logistics 

Agency Navy 

In-situ 

Air sparging X X X X X 

Bioremediation X X X X X 

Enhanced recovery X X X 

Chemical treatments X X X X X 

Monitored natural attenuation X X X X X 

Multiphase extraction X X X X X 

Permeable reactive barriers X X X X X 

Phytoremediation X X X X 

Thermal treatments X X X X 

Ex-situ 

Advanced oxidation processes X X X X 

Air stripping X X X X X 

Bioreactors X X X 

Constructed wetlands X X X X 

Ion exchange X X X X 

Adsorption (mass transfer) X X X X X 

Source: Department of Defense. 

To date, the Department of Defense

(DOD) has identified nearly 6,000 

sites at its facilities that require 

groundwater remediation and has 

invested $20 billion over the past 10

years to clean up these sites. In the 

past, DOD primarily used “pumpand-treat” technologies to contain 

or eliminate hazardous 

contaminants in groundwater. 

However, the long cleanup times 

and high costs of using pump-andtreat technologies often make them 

expensive and ineffective for 

groundwater remediation. 

 

As directed by Public Law 108-375 

and as agreed, GAO (1) described 

current DOD groundwater 

remediation technologies and (2) 

examined whether any new 

technologies are being used or 

developed outside the department 

that may have potential for DOD’s 

use and the extent to which DOD is 

researching and developing new 

approaches to groundwater 

remediation. 

GAO provided the Department of 

Defense with a draft copy of the 

report for its review and comment. 

DOD generally agreed with the 

contents stating that the report is 

an accurate summary of DOD’s use 

and field tests of remedial 

technologies. DOD also provided 

technical clarifications that have 

been incorporated, as appropriate. 
Page i GAO-05-666 Groundwater Contamination

Contents

Letter 1

Results in Brief 4

Background 5

DOD Has Implemented or Field-tested a Wide Range of Technologies 

to Remediate Sites Contaminated with Groundwater 10

DOD Is Proactively Using and Developing New Approaches to 

Groundwater Remediation 19

Agency Comments 24

Appendixes

Appendix I: Objectives, Scope, and Methodology 26

Appendix II: Technologies for the Remediation of Contaminated 

Groundwater 29

Ex-situ Technologies 29

In-situ Technologies 31

Appendix III: Groundwater Remediation Experts Consulted 39

Appendix IV: Comments from the Department of Defense 40

Appendix V: GAO Contact and Staff Acknowledgments 41

Tables Table 1: Technologies DOD Components Used for Groundwater 

Remediation 15

Table 2: Technologies Available for the Treatment of DOD’s 

Contaminants of Concern 17

Figures Figure 1: Example of a Site with Contaminated Groundwater 6

Figure 2: Selected Phases and Milestones in DOD’s Environmental 

Cleanup Process 8

Figure 3: Example of a Conventional Pump-and-Treat System 12
Contents

Page ii GAO-05-666 Groundwater Contamination

Abbreviations

CERCLA Comprehensive Environmental Response, Compensation, and 

Liability Act

DNAPL dense nonaqueous phase liquids

DOD Department of Defense

EPA Environmental Protection Agency

ESTCP Environmental Security Technology Certification Program

ITRC Interstate Technology and Regulatory Council

LNAPL light nonaqueous phase liquids

RCRA Resource Conservation and Recovery Act

SERDP Strategic Environmental Research and Development Program

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Page 1 GAO-05-666 Groundwater Contamination

United States Government Accountability Office

Washington, D.C. 20548

A

June 30, 2005 Letr

The Honorable John Warner

Chairman

The Honorable Carl Levin

Ranking Minority Member 

Committee on Armed Services

United States Senate

The Honorable Duncan L. Hunter

Chairman

The Honorable Ike Skelton

Ranking Minority Member

Committee on Armed Services 

House of Representatives

The Department of Defense (DOD) has identified close to 6,000 sites at its 

active, closing, and formerly used defense facilities where the groundwater 

has been so contaminated by past defense activities and the improper 

disposal of hazardous wastes that cleanup (remediation) of the site is 

required.1

 Groundwater—the water found beneath the earth’s surface that 

fills pores between soil particles, such as sand, clay, and gravel, or that fills 

cracks in bedrock—accounts for about 50 percent of the nation’s 

municipal, domestic, and agricultural water supply. When groundwater 

becomes polluted, it can endanger public health or threaten the 

environment. DOD estimates that cleanup of its contaminated sites will 

cost billions of dollars and may take decades to complete because of the 

extent of the contamination and the complexity of groundwater systems. 

DOD identifies, investigates, and cleans up contaminated groundwater 

through its Defense Environmental Restoration Program. This program 

was established by section 211 of the Superfund Amendments and 

Reauthorization Act of 1986, which amended the Comprehensive 

Environmental Response, Compensation, and Liability Act (CERCLA) of 

1980. In fiscal year 2004, DOD obligated approximately $1.7 billion for 

environmental restoration activities, including groundwater remediation, 

on active, closing, and formerly used defense facilities. Multiple DOD 

1

Remediation of a contaminated site involves efforts to remove, destroy, or isolate 

contaminants found in the groundwater. In some cases, disposal practices at these sites 

predate the enactment of relevant environmental cleanup statutes.
Page 2 GAO-05-666 Groundwater Contamination

entities—the Air Force, Army, Defense Logistics Agency, and Navy—are 

responsible for groundwater remediation on active DOD facilities.2

 In 

addition, the U.S. Army Corps of Engineers (Corps) is responsible for 

groundwater remediation on properties formerly owned, leased, or used by 

the military.3

 The Air Force has the greatest number of sites with 

contaminated groundwater needing remediation, followed by the Navy, 

Army, Corps, and Defense Logistics Agency.4

 DOD must carry out its 

groundwater remediation program in a manner consistent with section 120 

of CERCLA. Section 120 addresses the cleanup of federal facilities and, 

among other things, provides for participation in cleanup decisions by the 

state in which a federal facility is located. Personnel from the installation 

where the contamination is located work with DOD-hired contractors; 

regulators (federal, state, local, or tribal); and other stakeholders to 

evaluate and select appropriate technologies to achieve cleanup goals (e.g., 

treatment or containment of contaminants). DOD may use a single 

technology or a combination of technologies to clean up the groundwater 

at a particular site. 

In the past, DOD primarily used traditional “pump-and-treat” technologies 

to contain or eliminate hazardous contaminants in groundwater. Pumpand-treat technologies extract contaminated groundwater for treatment in 

above-ground (ex-situ) facilities and are often used to prevent the further 

spread of contamination in the groundwater. However, according to DOD, 

the Environmental Protection Agency (EPA), and groundwater remediation 

experts we consulted, pump-and-treat often is expensive because of long 

cleanup times, inefficiencies in removing contaminants from the 

subsurface, and the costs associated with disposing of the contaminant and 

treated water. Recently, DOD has begun to use alternatives to pump-andtreat technologies that rely on a variety of biological, chemical, or physical 

processes to treat the contaminated groundwater underground (in-situ). 

2

The Navy oversees environmental restoration on Marine Corps facilities.

3

The Corps may also participate in groundwater remediation activities on active Army 

installations, some Air Force installations, and properties that are scheduled for closure as 

part of the Base Realignment and Closure Act process.

4

For the purposes of this report, we have defined a “site” as a specific area of contamination 

and a “facility” as a geographically contiguous area under DOD’s ownership or control 

within which a contaminated site or sites are located. A single DOD facility may contain 

multiple sites requiring cleanup.
Page 3 GAO-05-666 Groundwater Contamination

As directed by Public Law 108-375,5 and as agreed with your offices, this 

report (1) describes the groundwater remediation technologies that DOD is 

currently using or field-testing and (2) examines whether any new 

groundwater remediation technologies are being used outside the 

department or are being developed by commercial vendors that may have 

potential for DOD’s use, and the extent to which DOD is researching and 

developing new approaches to groundwater remediation. In addition, this 

report provides limited information on the key characteristics, benefits, 

and limitations of selected groundwater remediation technologies in 

appendix II. 

To determine the range of groundwater remediation technologies DOD is 

currently using or field-testing, we developed a questionnaire that we sent 

to the DOD components responsible for DOD’s groundwater cleanup 

efforts—the Air Force, Army, Corps, Defense Logistics Agency, and Navy. 

In the questionnaire, we listed 15 technologies that are currently available 

for the treatment of contaminated groundwater and asked the DOD 

components to indicate which of the technologies they have used and to 

provide examples of specific groundwater remediation projects.6

 We 

developed this list of technologies by reviewing existing lists developed by 

the National Research Council, EPA, and others, as well as by working with 

a groundwater remediation consulting firm and five nationally recognized 

groundwater remediation experts. To identify DOD components involved 

with groundwater remediation activities, we met with department officials 

responsible for developing policy on groundwater remediation and for 

researching and developing groundwater remediation technologies. We 

reviewed documents, reports, and guidance on groundwater remediation 

from DOD, EPA, and the National Academy of Sciences; and visited an Air 

Force groundwater remediation project and a facility DOD uses to test 

innovative groundwater remediation technologies. In addition, we attended 

a national groundwater remediation conference, and spoke with a number 

of commercial vendors of groundwater remediation technologies about 

their products and efforts to develop innovative approaches to 

groundwater remediation. Information presented in this report is based on 

publicly available documents and information provided by government 

officials, independent consultants, and experts. We did not review 

5

Ronald W. Reagan National Defense Authorization Act for Fiscal Year 2000, Pub. L. No. 108-

375, § 316, 118 Stat. 1811, 1843 (Oct. 28, 2004). 

6

See appendix II for more information on each of the 15 technologies.

GAO Definition of Groundwater 

Remediation Technology

For this report, we define a technology as a 

distinct technical method or approach for 

containing, treating, or removing contaminants 

found in groundwater.

Any modifications or enhancements to a 

technology, such as variations in the material or 

equipment used during treatment, are not 

considered to be a separate technology.
Page 4 GAO-05-666 Groundwater Contamination

nonpublic research and development activities that may be ongoing in 

private laboratories. A more detailed description of our scope and 

methodology is presented in appendix I. We performed our work from 

January 2005 through May 2005, in accordance with generally accepted 

government auditing standards.

Results in Brief DOD has implemented or field-tested all of the 15 types of generally 

accepted technologies currently available to remediate groundwater. These 

various remediation technologies include both in-situ and ex-situ 

treatments, each of which relies on biological, chemical, or physical 

processes to clean up groundwater. Of these 15 types of technologies, the 

Navy reported that it has used all 15 and the Air Force, Army, and Corps 

have used 14 each. The Defense Logistics Agency, which has significantly 

fewer sites to clean up than the other DOD components, reported using 9 of 

the 15 technologies. According to department officials, DOD selects the 

most suitable technology for a given site on the basis of a number of 

factors, such as the type of contaminant and its location in the subsurface, 

and the relative cost-effectiveness of a technology for a given site. DOD has 

identified a number of contaminants of concern at its facilities, each of 

which varies in its behavior and susceptibility to treatment by the various 

technologies. Some of the contaminants, such as chlorinated solvents, can 

potentially be treated using 14 of the 15 technologies, while others, such as 

metals, can only be treated effectively with 7 of the 15 technologies. 

According to analyses conducted by groups such as EPA and the Federal 

Remediation Technologies Roundtable, the cost-effectiveness and 

performance of each technology can vary significantly depending, in part, 

on site-specific conditions. A more detailed description of each of the 

technologies we identified for cleaning up groundwater is presented in 

appendix II.

We did not identify any alternative technologies for groundwater 

remediation being used or developed outside of DOD that it has not 

considered or employed. However, we did identify a number of new 

approaches to groundwater remediation being developed by commercial 

vendors—most of which are also being explored or used by DOD—that are 

based on modifications of or enhancements to existing technologies. Most 

of the new approaches involve the use of novel materials applied to 

contaminated sites using existing technologies. For example, DOD has 

recently used molasses and vegetable oils at several bioremediation 

projects to stimulate microorganisms in the subsurface to biodegrade 

contaminants. Other alternative approaches being developed by 
Page 5 GAO-05-666 Groundwater Contamination

commercial vendors usually involve modifying the design of existing 

technologies. For example, DOD is exploring the use of nanoscale rather 

than granular sized metals to clean up sites contaminated by chlorinated 

solvents. In addition, we found that DOD is actively involved in researching 

and testing new approaches to groundwater remediation, largely through 

its efforts to develop and promote the acceptance of innovative 

technologies. For example, DOD maintains several programs—such as the 

Strategic Environmental Research and Development Program—to support 

the research, development, and testing of innovative cleanup approaches. 

This program, a DOD-funded basic and applied research program, supports 

public and private research on contaminants of concern to DOD and 

innovative methods for their treatment, as well as a variety of other 

activities. DOD also pursues innovative solutions to groundwater 

remediation through its Environmental Security Technology Certification 

Program. This program field-tests and validates promising innovative 

environmental technologies and transfers these technologies to the 

commercial sector. DOD also works with various stakeholders, including 

the regulatory community, to promote understanding and acceptance of 

innovative remediation approaches. For example, DOD participates in the 

Interstate Technology and Regulatory Council, a state-led coalition that 

works with the private sector, regulators, and other stakeholders to 

increase the regulatory acceptance of new environmental technologies. 

Background DOD sites that require cleanup are often contaminated by many different 

types of hazardous materials, have contamination in more than one 

medium (e.g., soil, surface water, or groundwater), and may encompass 

several acres or even square miles. Groundwater stored in subsurface 

formations called aquifers can become contaminated in a number of ways. 

For example, contamination can occur when a liquid hazardous substance 

soaks down through the soil. Often, groundwater contamination is difficult 

to address because of the complexity of groundwater systems. The 

subsurface environment can be composed of numerous layers of diverse 

types of material—such as sand, gravel, clay, and solid rock—and fractured 

layers through which groundwater flows. These variations in the 

subsurface often affect how groundwater flows through a contaminated 

site and can influence how contaminants are spread and accumulate in the 

subsurface. Chemical properties of the contaminant also influence its 

distribution in the subsurface. Typically, contaminated sites consist of a 

source zone where the bulk of the contaminant is concentrated and a 

plume of contamination that develops beyond the source of contamination 
Page 6 GAO-05-666 Groundwater Contamination

as a result of groundwater flowing through the contaminated site. See 

figure 1 for an illustration of a site with contaminated groundwater.

Figure 1: Example of a Site with Contaminated Groundwater 

DOD Facilities Can Have 

Significant Groundwater 

Contamination

According to DOD, the Air Force has identified more than 2,500 sites on its 

active and closing installations with contaminated groundwater; the Navy 

has identified more than 2,000 sites; the Army has identified about 800 

sites; and the Defense Logistics Agency has identified 16 sites. In addition, 

DOD has identified more than 500 contaminated groundwater sites on 

formerly used defense sites for which the Corps is responsible for cleanup. 

Contamination on DOD facilities can pose a threat to military personnel, 

the public, and the sustainability of DOD’s training and testing ranges. DOD 

first initiated its environmental restoration efforts in 1975. Over the last 10 

years, DOD has invested approximately $20 billion for the environmental 

restoration of contaminated sites, including remediation of contaminated 

groundwater on and around active, closing, and formerly used defense 

facilities.7

Bedrock

Groundwater

Soil

Containment

plume

Contaminant

source

Source: Adapted from EPA, Fact Flash #5, Groundwater.

7

Some of DOD’s sites are considered megasites—defined by EPA as sites requiring 

investments of over $50 million to achieve cleanup. 
Page 7 GAO-05-666 Groundwater Contamination

DOD Cleanup Activities 

Generally Follow the 

CERCLA Process

DOD’s policies for administering cleanup programs are outlined in its 

guidance for managing its environmental restoration program and 

generally follow the CERCLA process for identifying, investigating, and 

remediating sites contaminated by hazardous materials.8

 According to 

DOD’s guidance, department officials are required to involve EPA, relevant 

state and local government officials, and the public, among others, at 

specified points in the cleanup process. See figure 2 for more information 

on the phases of DOD’s environmental cleanup process.

8

DOD carries out some groundwater remediation as corrective action under the Resource 

Conservation and Recovery Act of 1976 (RCRA). According to DOD, while RCRA and 

CERCLA contain somewhat different procedural requirements, these differences do not 

substantively affect the outcome of remedial activities.
Page 8 GAO-05-666 Groundwater Contamination

Figure 2: Selected Phases and Milestones in DOD’s Environmental Cleanup Process 

Note: These phases may overlap or occur simultaneously, but cleanup activities at DOD facilities 

generally occur in the order shown.

Investigation Cleanup

Preliminary

assessment

Site inspection

Remedial investigation

Feasibility study

Remedial design

Remedial action construction

Remedial action operation

A remedial investigation characterizes 

site conditions and determines the extent 

and nature of contamination.

Record of Decision

Remedy in place

Response complete

Start

Time span

End

Milestone

A feasibility study examines various 

remedial alternatives and screens 

technologies based on their effectiveness, 

implementability, and cost.

A Record of Decision documents the 

results of the investigative study, the 

selected remediation technologies, and 

planned objectives for the site.

A remedy in place is an important 

milestone in the cleanup process. At 

this point, the selected technologies are 

in place and are operating properly and 

successfully to meet cleanup objectives.

Source: Department of Defense, Defense Environmental Programs, Annual Report to Congress, Fiscal Year 2004.
Page 9 GAO-05-666 Groundwater Contamination

Once DOD identifies potential contamination on one of its facilities, it 

initiates a preliminary assessment to gather data on the contaminated site. 

If DOD finds evidence that the site needs remediation, it consults with EPA 

to determine whether the site qualifies for inclusion on the National 

Priorities List.9

 If EPA places a DOD facility on the National Priorities List, 

CERCLA requires DOD to begin the next phase of cleanup within 6 months. 

During this next phase, called a remedial investigation/feasibility study, 

DOD characterizes the nature and extent of contamination and evaluates 

the technical options available for cleaning up the site. 

DOD also pursues a remedial investigation/feasibility study for sites that do 

not qualify for the National Priorities List but require decontamination. 

Data collected during the remedial investigation influences DOD’s 

development of cleanup goals and evaluation of remediation alternatives. 

During the feasibility study, often conducted concurrently with the 

remedial investigation, DOD identifies applicable regulations and 

determines cleanup standards that will govern its cleanup efforts. CERCLA 

requires that sites covered by the statute be cleaned up to the extent 

necessary to protect both human health and the environment. In addition, 

cleanups must comply with requirements under federal environmental laws 

that are legally “applicable” or “relevant and appropriate” as well as with 

state environmental requirements that are more stringent than the federal 

standards. Furthermore, CERCLA cleanups must at least attain goals and 

criteria established under the Safe Drinking Water Act and the Clean Water 

Act, where such standards are relevant and appropriate under the 

circumstances. 

Once cleanup standards have been established, DOD considers the merits 

of various actions to attain cleanup goals. Cleanup actions fall into two 

broad categories: removal actions and remedial actions. Removal actions 

are usually short term and are designed to stabilize or clean up a hazardous 

site that poses an immediate threat to human health or the environment. 

Remedial actions, which are generally longer term and usually costlier, are 

9

This list represents EPA’s highest priorities for cleanup nationwide, including public and 

private sites considered by EPA to present the most serious threats to human health and the 

environment. To make its determination, EPA uses a hazard-ranking system to evaluate the 

severity of the contamination by examining the nature of the contaminants, the pathways 

through which they can move (such as soil, water, or air), and the likelihood that they may 

come into contact with a receptor—for example, a person living nearby. According to DOD’s 

Defense Environmental Programs, Annual Report to Congress, Fiscal Year 2004, DOD has 

152 facilities that are listed or proposed for listing on the National Priorities List.
Page 10 GAO-05-666 Groundwater Contamination

aimed at implementing a permanent remedy. Such a remedy may, for 

example, include the use of groundwater remediation technologies. Also 

during the feasibility study, DOD identifies and screens various 

groundwater remediation technologies based on their effectiveness, 

feasibility, and cost. At the conclusion of the remedial 

investigation/feasibility study, DOD selects a final plan of action—called a 

remedial action—and develops a Record of Decision that documents the 

cleanup objectives, the technologies to be used during cleanup, and the 

analysis that led to the selection. If EPA and DOD fail to reach mutual 

agreement on the selection of the remedial action, then EPA selects the 

remedy. If the cleanup selected leaves any hazardous substances, 

pollutants, or contaminants at the site, DOD must review the action every 5 

years after the initiation of the cleanup.10 According to DOD policy, this 

may include determining if an alternative technology or approach is more 

appropriate than the one in place. DOD continues remediation efforts at a 

site until the cleanup objectives stated in the Record of Decision are met, a 

milestone referred to as “response complete.” Even if DOD meets the 

cleanup objectives for a site, in some cases the site may require long-term 

management and monitoring to ensure that it does not become 

contaminated from residual sources of pollution.

DOD Has Implemented 

or Field-tested a Wide 

Range of Technologies 

to Remediate Sites 

Contaminated with 

Groundwater

DOD has implemented or field-tested all of the 15 types of generally 

accepted technologies currently available to remediate groundwater. These 

15 technologies include 6 ex-situ and 9 in-situ technologies, each of which 

can be used to treat a variety of contaminants. All of these groundwater 

remediation technologies rely on a variety of biological, chemical, or 

physical processes to treat or extract the contaminant. DOD guidance 

directs department officials to consider cost-effectiveness and 

performance when selecting technologies for cleanup.

1042 U.S.C. § 9621(c). The applicable EPA regulation differs from the statute: It requires the 

five-year reports only if contaminants will remain at the site “above levels that allow for 

unlimited use and unrestricted exposure.” 40 C.F.R. § 300.430(f)(4)(ii).
Page 11 GAO-05-666 Groundwater Contamination

Fifteen Ex-situ and In-situ 

Technologies Are Currently 

Available for Groundwater 

Cleanup

We identified a range of ex-situ and in-situ technologies that DOD can 

employ to clean up a contaminated groundwater site. Ex-situ technologies 

rely on a pump-and-treat system to bring the contaminated water above 

ground so that it can be treated and the contaminants removed. Some exsitu technologies destroy the contaminant, while others remove the 

contaminant from the groundwater, which is subsequently disposed of in 

an approved manner. The decontaminated water can be discharged to 

surface water, used as part of a public drinking water supply, injected back 

into the ground, or discharged to a municipal sewage plant. We identified 6 

categories of ex-situ technologies:

• Advanced oxidation processes often use ultraviolet radiation with 

oxidizing agents—such as ozone or hydrogen peroxide—to destroy 

contaminants in water pumped into an above-ground treatment tank.

• Air stripping separates volatile contaminants from water by exposing 

the water to large volumes of air, thus forcing the contaminants to 

undergo a physical transformation from liquid to vapor (volatilization). 

There is no destruction of the contaminant; therefore, the contaminant 

must be removed and disposed of properly.

• Bioreactors are above-ground biochemical-processing systems designed 

to degrade contaminants in water using various microorganisms, an 

approach similar to that used at a conventional wastewater treatment 

facility. Contaminated groundwater flows into a tank or basin where it 

interacts with microorganisms that degrade the contaminant. 

• Constructed wetlands are artificially built wetland ecosystems that 

contain organic materials, plants, microbial fauna, and algae that filter 

or degrade contaminants from the water that is pumped into the 

wetland. 

• Ion exchange involves passing contaminated water through a bed of 

resin media or membrane that exchanges ions in the contaminants, thus 

neutralizing them into nonhazardous substances. 

• Adsorption (mass transfer) involves circulating contaminated water 

through an above-ground treatment vessel containing a sorbent 

material—such as activated carbon—that removes the contaminant 

from the water. 

Use of Pump-and-Treat Systems

Some groundwater remediation experts believe 

that pump-and-treat systems may be the best 

option in situations such as the following: 

the contaminant is located so deep in the 

subsurface that site characterization and 

potential remediation can be prohibitively 

expensive;

the subsurface is so complex that the 

effectiveness of in-situ approaches is limited 

(e.g., highly fractured systems); 

in-situ approaches are not viable or 

sufficiently proven to remediate a site (e.g., 

contamination by chlorinated solvents in 

fractured bedrock);

the interim cleanup goal is to mitigate risk 

by containing the contaminant plume (e.g., 

to protect a public drinking water supply), 

while an in-situ approach is developed for 

the site; and 

an ex-situ system is needed to augment or 

support an in-situ technology.

See figure 3 for an illustration of a pump-andtreat system.
Page 12 GAO-05-666 Groundwater Contamination

(See app. II for more information on key characteristics of these ex-situ 

technologies.)

Figure 3: Example of a Conventional Pump-and-Treat System 

Similarly, we identified nine in-situ technologies that can be used to 

remediate contaminated groundwater. In contrast to ex-situ technologies, 

in-situ technologies treat contaminants within the subsurface. Some in-situ 

technologies—such as bioremediation and chemical treatment—destroy 

the contaminant within the subsurface by altering the contaminant’s 

chemical structure and converting the toxic chemical to a nontoxic form 

(e.g., benzene to carbon dioxide). Other in-situ technologies—such as 

multiphase extraction and enhanced recovery using surfactant flushing—

facilitate the removal of the contaminant from the subsurface for treatment 

above ground. Still other technologies—such as air sparging—combine insitu treatments with extraction techniques. 

• Air sparging introduces air or other gases into the subsurface to 

remove the contamination from the groundwater through volatilization 

(converting a solid or liquid into a gas or vapor that may be treated at 

the surface), and in some configurations may also introduce oxygen into 

the contaminated area to stimulate in-situ biological breakdown (i.e., 

Bedrock

Groundwater

Soil

Contaminant

plume

Contaminant

source

Land surface

Pumping to

treatment

Drinking water

supply well

Source: Federal Remediation Technologies Roundtable Treatment Technologies Screening Matrix, 2002.
Page 13 GAO-05-666 Groundwater Contamination

bioremediation) or ozone to achieve chemical oxidation of the 

contaminant. 

• Bioremediation relies on microorganisms living in the subsurface to 

biologically degrade groundwater contaminants through a process 

called biodegradation. Bioremediation may be engineered and 

accomplished in two general ways: (1) stimulating native 

microorganisms by adding nutrients, oxygen, or other electron 

acceptors (a process a called biostimulation) or (2) providing 

supplementary pregrown microorganisms to the contaminated site to 

augment naturally occurring microorganisms (a process called 

bioaugmentation).

• Enhanced recovery using surfactant flushing involves the injection of 

active agents known as surfactants11 into contaminated aquifers to flush 

the contaminated groundwater toward a pump, which removes the 

contaminated water and surfactant solution to the surface for treatment 

and disposal of the contaminants. 

• Chemical treatments inject various substances into the groundwater 

that can chemically oxidize or reduce contaminants into less-toxic or 

nonhazardous materials. 

• Monitored natural attenuation involves using wells and monitoring 

equipment in and around a contaminated site to track the natural 

physical, chemical, and biological degradation of the contaminants. 

Although not necessarily considered a treatment technology, this 

approach is often used to monitor contaminant concentrations to 

ensure that human health and the environment are not threatened. 

• Multiphase extraction uses a series of pumps and vacuums to 

simultaneously remove from the subsurface combinations of 

contaminated groundwater, free product (i.e., liquid contaminants 

floating on top of groundwater), and hazardous vapors. This technology 

can be used to remove contaminants from above and below the 

11Surfactants, or surface active agents, are molecules with two structural units: one with an 

affinity for water and one with an aversion to water. This molecular combination is useful 

for dissolving some contaminants and enhancing their mobility by lowering the interfacial 

tension between the contaminant and the water. 
Page 14 GAO-05-666 Groundwater Contamination

groundwater table, thereby exposing more of the subsurface for 

treatment.

• Permeable reactive barriers are vertical walls or trenches built into the 

subsurface that contain a reactive material to intercept and remediate a 

contaminant plume as the groundwater passes through the barrier.

• Phytoremediation relies on the natural hydraulic and metabolic 

processes of selected vegetation to remove, contain, or reduce the 

toxicity of environmental contaminants in the groundwater. 

• Thermal treatments involve either pumping steam into the aquifer or 

heating groundwater to vaporize or destroy groundwater contaminants. 

Vaporized contaminants are often removed for treatment using a 

vacuum extraction system.

(See app. II for more information on key characteristics of these in-situ 

technologies.)

Although most in-situ technologies have the advantage of treating a 

contaminant in place, these technologies may afford less certainty about 

the extent and uniformity of treatment in contaminated areas when 

compared with some ex-situ technologies. For example, enhanced 

recovery using surfactant flushing has not been used extensively and has 

limited data on its remediation effectiveness, whereas air stripping has 

been widely used for several decades to remove certain contaminants, and 

its benefits and limitations as a water treatment technology are wellunderstood. In some cases, a combination of in-situ and ex-situ 

technologies may be used (either concurrently or successively) to clean up 

a site if a single technology cannot effectively remediate an entire site with 

its range of contaminants and subsurface characteristics. According to the 

National Research Council, integration of technologies is most effective 

when the weakness of one technology is mitigated by the strength of 

another technology, thus producing a more efficient and cost-effective 

solution.12

12For more information, see National Research Council, Water Science and Technology 

Board, Contaminants in the Subsurface: Source Zone Assessment and Remediation

(Washington, D.C., 2004).
Page 15 GAO-05-666 Groundwater Contamination

DOD Has Used the Full 

Range of Groundwater 

Remediation Technologies 

Identified 

As shown in table 1, the DOD components involved in groundwater 

remediation activities reported using the full range of technologies that we 

identified as currently available for groundwater remediation. Specifically, 

the Navy reported that it has used all 15 of the currently available 

technologies; the Air Force, Army, and Corps reported using 14 each. The 

Defense Logistics Agency has used 9 of the available technologies for the 

cleanup of the limited number of contaminated groundwater sites for 

which it is responsible. 

Table 1: Technologies DOD Components Used for Groundwater Remediation

Source: Department of Defense responses to GAO data collection instrument.

Notes: This table focuses on technologies used to treat contaminants found in groundwater. It excludes 

technologies used (1) to treat and dispose of the byproducts of groundwater remediation—such as 

emissions of potentially harmful volatile gases; (2) exclusively to treat contaminated soil (such as soil 

washing or excavation), although soil remediation is often conducted in conjunction with groundwater 

remediation; and (3) primarily to physically contain a contaminant—such as soil capping. See 

appendix II for more information on the key characteristics, benefits, and limitations of each of these 

technologies.

a

Includes related remedial approaches and technologies, such as co-metabolic air sparging, oxygen 

and ozone sparging, in-well air stripping, and soil vapor extraction. Soil vapor extraction, although not 

Technology Air Force Army

Army Corps 

of Engineers

Defense 

Logistics 

Agency Navy

In-situ 

Air sparginga X XX X X

Bioremediationb X XX X X

Enhanced recovery/surfactant flushingc X XX

Chemical treatmentsd X XX X X

Monitored natural attenuation X XX X X

Multiphase extractione X XX X X

Permeable reactive barriersf X XX X X

Phytoremediationg X XX X

Thermal treatmentsh X XX X

Ex-situ 

Advanced oxidation processesi X XX X

Air stripping X XX X X

Bioreactors XX X

Constructed wetlands X XX X

Ion exchangej

 X XX X

Adsorption (mass transfer) X XX X X
Page 16 GAO-05-666 Groundwater Contamination

technically a groundwater remediation technology, is often used with air sparging to extract or capture 

emissions that result from treating contaminated groundwater.

b

Includes related bioremedial approaches, such as bioaugmentation, biostimulation, co-metabolic 

treatment, enhanced aerobic biodegradation, enhanced anaerobic biodegradation, and biobarriers.

c

Includes related remedial approaches that use co-solvents to improve the solubility of surfactants in 

the subsurface, and other technologies, such as hydrofracturing and pneumatic fracturing, that attempt 

to increase the permeability of the subsurface.

d

Includes various remedial approaches and technologies that chemically oxidize or reduce 

contaminants in-situ, as well as the in-situ immobilization and stabilization of soluble metals.

e

Includes the related technologies of bioslurping and dual-phase extraction.

f

Includes both biotic and abiotic passive and reactive treatment barriers.

g

Includes the related technologies of phytostabilization, phytoaccumulation, phytoextraction, 

rhizofiltration, phytodegradation, rhizosphere degradation, organic pumps, and phytovolatization.

h

Includes related heating technologies, such as steam flushing, conductive heating, and electrical 

resistance heating.

i

Includes the related technologies of ultraviolet oxidation, ultraviolet photolysis, and photocatalysis.

j

Includes technologies that use ion exchange resins or membranes to remove contaminants from 

groundwater, including dissolved metals and nitrates.

According to department officials, DOD selects the most suitable 

technology to clean up a contaminated site based on a number of factors, 

including the type of contaminant, its location and concentration at 

different levels in the subsurface, and its chemical and physical 

composition.13 These officials identified a number of contaminants of 

concern, such as federally regulated chlorinated solvents (commonly found 

in metal degreasers) and fuels used for military aircraft and vehicles. DOD 

officials also consider some other hazardous materials that are not 

regulated by the federal government—such as the rocket propellant 

perchlorate—to be contaminants of concern because they are regulated by 

some states, such as California, where DOD has active, closing, or formerly 

used defense sites that need groundwater remediation. 

According to the groundwater remediation experts we consulted, some of 

DOD’s contaminants of concern, such as chlorinated solvents, can 

potentially be treated using 14 of the 15 technologies, while others, such as 

metals, can be treated with only 7 of the 15 technologies. For example, 

many chlorinated solvents do not readily dissolve in water; and because 

they are often more dense (heavier) than water, they migrate downward 

and pool at the bottom of aquifers, thereby limiting the number of 

technologies that can treat them. Alternatively, some contaminants 

13A contaminant may exist in aqueous (dissolved in water), nonaqueous, solid (sorbed), or 

gaseous form. 
Page 17 GAO-05-666 Groundwater Contamination

composed of petroleum hydrocarbons (e.g., jet fuel, diesel fuel, and motor 

gasoline) float on top of the water table because they are less dense 

(lighter) than water, and technologies such as air sparging or multiphase 

extraction can often effectively treat or extract them through processes 

such as volatilization or free product recovery. See table 2 for information 

on which of the 15 technologies can potentially treat each of DOD’s 

contaminants of concern. 

Table 2: Technologies Available for the Treatment of DOD’s Contaminants of Concern

Sources: Department of Defense and several groundwater remediation experts.

Notes: This table presents the contaminants of concern to DOD. Depending on their concentrations, 

these contaminants can pose health risks to humans. The ability for any one technology to effectively 

treat a contaminant is greatly influenced by site-specific conditions. Some technologies are generally 

less effective or currently less utilized to treat contaminants.

a

Includes, but is not limited to, perchloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), 

vinyl chloride (VC), and chloroform (CF). 

b

Includes, but is not limited to, trinitrotoluene (TNT); dinitrotoluene (DNT); cyclotrimethylene 

trinitramine, cyclonite, and hexogen (RDX); and octogen and cyclotetramethylene-tetranitramine 

(HMX).

c

Includes gasoline, diesel fuel, jet fuel, and BTEX. BTEX is an acronym for benzene, toluene, 

ethylbenzene, and xylene—a group of volatile organic compounds commonly found in petroleum 

hydrocarbons, such as gasoline.

Technology

Chlorinated 

solventsa Explosivesb Fuelsc Metalsd Oxygenatese Propellantsf

In-situ 

Air sparging X XX

Bioremediation X X X X X X

Enhanced recovery/surfactant flushing X XX

Chemical treatments X X XX X X

Monitored natural attenuation X X XX X X

Multiphase extraction X X X

Permeable reactive barriers X X XX X X

Phytoremediation X XX X X

Thermal treatments X X X

Ex-situ 

Advanced oxidation processes X XX X

Air stripping X X X

Bioreactors X XX X X

Constructed wetlands X X XX X X

Ion exchange X X

Adsorption (mass transfer) X X XX X
Page 18 GAO-05-666 Groundwater Contamination

d

Includes, but is not limited to, arsenic, barium, cadmium, chromium, copper, lead, mercury, selenium, 

silver, and zinc.

e

Includes, but is not limited to, oxygen-bearing chemicals that can be added to fuel to bring additional 

oxygen to the combustion process. These include ethers such as methyl tertiary butyl ether (MTBE) 

and its related compounds. 

f

Includes, but is not limited to, materials such as ammonium perchlorate and potassium perchlorate 

that are used in the manufacturing and testing of solid rocket propellants and other munitions such as 

flares.

Technology Selection Is 

Also Influenced by Cost and 

Performance

According to DOD guidance on groundwater remediation, department 

officials should consider cost-effectiveness and performance of various 

groundwater remediation options when selecting the most suitable cleanup 

technology. A number of factors influence total cleanup costs for a given 

site, such as how long the cleanup is expected to take and the horizontal 

and vertical extent of the contamination. In addition, according to the 

National Research Council, actual cleanup costs associated with each 

technology depend on site-specific hydrogeologic, geochemical, and 

contaminant conditions.14 Thus, a particular technology may be the most 

cost-effective solution for one site and not necessarily for another similarly 

contaminated site. The National Research Council and others have also 

found that performance of most technologies, including time for total 

cleanup, also depends on complexities within the site’s subsurface (i.e., site 

heterogeneities) as well as contaminant characteristics. For example, the 

effectiveness of certain in-situ technologies—such as air sparging—

decrease as site heterogeneity increases because the air will naturally 

follow certain pathways that may bypass the contaminant. Similarly, the 

effectiveness of many in-situ technologies may be limited by the presence 

of some chlorinated solvents that, if heavier than water, can migrate into 

inaccessible zones in the subsurface. Alternatively, in-situ thermal 

treatments that use conductors to heat the soil are not as sensitive to 

heterogeneity in the subsurface and contaminant characteristics because 

thermal conductivity varies little with the properties of subsurface 

materials and certain contaminants are more easily volatilized at elevated 

temperatures. However, equipment and energy costs may make this 

approach more costly than other in-situ technologies. 

While overall conclusions on the cost-effectiveness of each groundwater 

remediation technology are difficult to reach, a few groups have attempted 

14For more information, see National Research Council, Contaminants in the Subsurface: 

Source Zone Assessment and Remediation (Washington, D.C., 2005). 
Page 19 GAO-05-666 Groundwater Contamination

to estimate costs for various technologies. For example, EPA has 

developed a technology cost compendium for several technologies based 

on cost data from various public and private remediation projects.15

Similarly, the Federal Remediation Technologies Roundtable—a federal 

consortium of representatives from DOD, EPA, and other federal 

agencies—has attempted to evaluate the relative overall cost and 

performance of selected remediation technologies in general terms.16

However, according to DOD officials and other experts we consulted, these 

efforts to compare technologies are of only limited utility because of the 

site-specific nature of technology decisions. 

DOD Is Proactively 

Using and Developing 

New Approaches to 

Groundwater 

Remediation 

We did not identify any alternative groundwater remediation technologies 

being used outside the department that DOD has not already either 

employed or tested on some scale (laboratory or pilot). However, we did 

identify a number of new approaches to groundwater remediation being 

developed by commercial vendors, but these approaches are based on 

modifications of or enhancements to existing technologies. Most of these 

new approaches are being used or field-tested by DOD and involve novel 

materials that are applied to contaminated sites using existing 

technologies. In addition, we found that DOD is generally aware of new 

approaches to groundwater remediation, in part through its efforts to 

develop remediation technologies with the commercial sector. DOD also 

works with various stakeholders, including the regulatory community, to 

promote understanding and acceptance of innovative remediation 

approaches. Some DOD officials and groundwater remediation experts 

believe additional resources may be needed in order to develop and 

advance DOD’s process for selecting the most appropriate technology at a 

site. 

15For more information, see EPA, Office of Solid Waste and Emergency Response, 

Remediation Technology Cost Compendium—Year 2000 (Washington, D.C., 2001). 

16For additional information, see the online version of the Federal Remediation 

Technologies Roundtable Treatment Technologies Screening Matrix at 

http://www.frtr.gov/scrntools.htm.
Page 20 GAO-05-666 Groundwater Contamination

Most New Approaches 

Employ Novel Materials or 

Modifications to Existing 

Technologies 

Most of the new remediation approaches commercial vendors have 

developed and made available to DOD use existing technologies to apply 

novel materials to contaminated sites. These materials typically accelerate 

the breakdown of contaminants through biological or chemical processes. 

In particular, multiple commercial vendors have developed proprietary 

compounds used during bioremediation to stimulate microorganisms in the 

subsurface to biodegrade contaminants. Some of these compounds are 

designed to slowly release oxygen or other nutrients into the subsurface in 

an effort to prolong their availability, which microorganisms need to 

biodegrade the contaminants. DOD has also field-tested several novel 

compounds for bioremediation that are derived from food-grade materials 

such as molasses or vegetable oils. These compounds can be injected into 

the contaminated site using pre-existing wells or other existing techniques 

such as direct push injection: 

• The Army used a compound developed by a commercial vendor to 

stimulate the bioremediation of chlorinated solvents at a contaminated 

site at its Rocky Mountain Arsenal. This compound reacted with the 

contaminated groundwater to produce lactic acid, which native 

microorganisms used to produce the hydrogen that ultimately led to the 

biological degradation of the contaminants. In addition, the Air Force 

reported using oxygen-releasing compounds to stimulate aerobic 

biodegradation at several of its cleanup sites, including a site in Florida 

contaminated by spilled fuel. 

• DOD has also field-tested the use of molasses during bioremediation to 

treat chlorinated solvents at Vandenberg and Hanscom Air Force bases. 

In addition, DOD reported using vegetable oils to stimulate 

microorganisms in order to treat groundwater contaminated by 

chlorinated solvents and perchlorate at a variety of locations, including 

naval facilities in Massachusetts, Rhode Island, and South Carolina. 

Commercial vendors have also developed innovative approaches for 

chemically treating contaminants in the subsurface. For example, several 

vendors have developed proprietary approaches for delivering oxidants, 

such as molecular oxygen and ozone with or without hydrogen peroxide, 

into the subsurface to achieve in-situ chemical oxidation of a variety of 

contaminants, including fuels and chlorinated solvents. These oxidants are 

often delivered underground using variations of existing air sparging 

technologies and a variety of injection technologies. In addition to 

achieving in-situ chemical oxidation of target contaminants, the use of 

ozone with or without hydrogen peroxide can enhance the aerobic 
Page 21 GAO-05-666 Groundwater Contamination

biodegradation of contaminants because it increases oxygen levels in the 

subsurface. Commercial vendors have also developed approaches to 

directly injecting other chemicals that are oxidizing agents, such as 

persulfate and permanganate, into the subsurface using existing 

technologies such as injection wells and direct push-probe technologies. 

DOD is exploring with the commercial sector other innovative approaches 

to groundwater remediation that involve modifying the engineering, design, 

or application of existing technologies. For example, DOD is currently 

working with the commercial sector to explore innovative uses of 

nanoscale metallic materials—such as zero-valent iron and palladium 

impregnated iron—to improve the efficacy of in-situ chemical treatments 

of chlorinated solvents commonly found on DOD facilities.17 In the past, 

DOD used metallic materials, such as zero-valent iron in granular form, to 

fill trenches dug into the ground (a form of a permeable reactive barrier) to 

chemically reduce chlorinated solvent plumes. The iron reacts with 

chlorinated solvents, transforming them into benign products, such as 

ethane and ethene. Treating contaminant plumes located deep within the 

subsurface is often difficult, costly, and technically impossible using this 

approach. Because of their size, nanoscale particles can be mixed with 

other materials—such as vegetable oil and water—and injected deep into 

the subsurface using existing technologies to treat contaminant sources or 

plumes. Furthermore, nanoscale particles have high surface areas relative 

to their volume (i.e., more metal is available to contact and react with the 

contaminants), which will lead to increased rates of reaction and more 

effective treatment.

DOD Supports the 

Development of New 

Technologies with the 

Commercial Sector through 

Several Programs

We found that DOD is actively involved in researching and testing new 

approaches to groundwater remediation, largely through its efforts to 

develop and promote the acceptance of innovative groundwater 

remediation technologies. According to the National Research Council, 

research on innovative remediation technologies is sponsored almost 

exclusively by federal agencies such as DOD and, in some circumstances, 

by individual companies and industry groups that have joined with federal 

17Nanoscale refers to miniscule particles that measure less than 100 nanometers in diameter. 

In comparison, an average human hair typically measures 10,000 nanometers in diameter.
Page 22 GAO-05-666 Groundwater Contamination

agencies in seeking more cost-effective solutions to common problems.18 In 

particular, the DOD-funded Strategic Environmental Research and 

Development Program (SERDP) supports public and private research on 

contaminants of concern to DOD and innovative methods for their 

treatment, among other activities. Created in 1990, the program primarily 

focuses on issues of concern to DOD, although it is jointly managed by 

DOD, EPA, and the Department of Energy.19 In fiscal year 2004, SERDP 

spent about $49 million to fund and manage projects in a variety of areas, 

including 27 projects related to groundwater remediation.

In response to technology needs and requirements generated by each of the 

DOD components, SERDP funds research projects in private, public, and 

academic settings on the fundamentals of contaminant behavior, 

environmental toxicity, and the advanced development of cost-effective 

innovative groundwater remediation technologies, among other things. For 

example, SERDP has funded research projects to examine such issues as 

the innovative use of vegetable oils for bioremediation; zero-valent iron 

based bioremediation of explosives; and the behavior of, and treatment 

options for, several emerging groundwater contaminants not yet regulated 

by the federal government, such as 1,4-Dioxane (found in solvents), NNitrosodimethylamine (found in rocket fuel), and trichloropropane (used 

as a degreaser and paint stripper). In addition, SERDP holds workshops 

with the scientific, engineering, academic, regulatory, and DOD-user 

communities to discuss DOD’s issues and identify needs for future 

research, development, and testing of groundwater remediation 

techniques. 

DOD also pursues innovative solutions to groundwater remediation 

through its Environmental Security Technology Certification Program 

(ESTCP). This program, founded in 1995, field-tests and validates 

promising innovative environmental technologies that attempt to address 

DOD’s highest-priority environmental requirements, including groundwater 

18See National Research Council, Water Science and Technology Board, Environmental 

Cleanup at Naval Facilities: Adaptive Site Management (Washington, D.C., 2003).

19SERDP’s goals include supporting basic and applied research and development of 

environmental technologies; providing information and data on environmental research and 

development activities to other governmental and private organizations in an effort to 

promote the transfer of innovative technologies; and identifying technologies developed by 

the private sector that are useful for DOD’s and DOE’s environmental restoration activities.
Page 23 GAO-05-666 Groundwater Contamination

remediation.20 Using a process similar to that of SERDP, ESTCP solicits 

proposals from public and private researchers to field-test laboratoryproven remediation technologies that have broad DOD and market 

application. Once ESTCP accepts a proposal, it identifies a military partner, 

which provides a site on a DOD installation where the researcher can fieldtest the technology and document the technology’s cost, performance, and 

reliability. In fiscal year 2004, ESTCP spent about $35 million to fund and 

manage its program, including 36 projects on groundwater remediation. 

These projects include the demonstration of an enhanced recovery 

technology using innovative surfactants, emulsified zero-valent nanoscale 

iron to treat chlorinated solvents, and an ion exchange technology for the 

removal and destruction of perchlorate. ESTCP and SERDP have colocated offices and, according to DOD officials, the two programs work 

together to pursue the development of innovative groundwater remediation 

technologies from basic research through advanced field-testing and 

validation. ESTCP often funds the demonstration of technologies that were 

developed by private or public researchers with financial support from 

SERDP. 

In addition to funding the development of innovative technologies, DOD 

works with various stakeholders, including the regulatory community, to 

promote the understanding and acceptance of these technologies. For 

example, DOD participates in the Interstate Technology and Regulatory 

Council (ITRC), a state-led coalition that works with the private sector, 

regulators, and other stakeholders to increase the regulatory acceptance of 

new environmental technologies. ITRC develops guidance on innovative 

environmental technologies and sponsors training for regulators and others 

on technical and regulatory issues related to environmental cleanup 

technologies and innovative groundwater remediation approaches. 

According to ITRC, these efforts are designed to help regulators streamline 

their review process and enable wider acceptance of innovative 

environmental technologies across state boundaries. In 2004, ITRC and 

DOD signed a memorandum of understanding on the relationship between 

the two organizations. As a result of the agreement, DOD now provides 

several liaisons to the ITRC’s board of advisers and helps the group develop 

materials and training courses on innovative groundwater remediation 

technologies. According to a DOD official, the department’s partnership 

20According to ESTCP, the program “provides an independent, unbiased evaluation of the 

cost, performance, and market potential of state-of-the-art environmental technologies 

based on field demonstrations conducted under DOD operational conditions.”

National Environmental Technology Test 

Site at Dover Air Force Base

Source: Dover National Environmental Technology Test Site, 

Tim McHale. 

At Dover Air Force Base, DOD has constructed 

three double-walled underground test areas 

(referred to as cells) that enable researchers 

to inject common soil and groundwater 

pollutants into a natural geologic setting as 

test constituents, without allowing the test 

constituents to come into contact with the 

surrounding environment. These test cells, 

known as the Groundwater Remediation 

Field Laboratory, include one large test cell 

and several smaller ones, all sharing the 

same outer containment cell area. The cells 

are constructed of interlocking steel sheet 

pilings with sealed grouted joints that extend 

from the ground's surface to a depth of 40 

feet. This safe testing area is in an area with 

"ideal geology," according to the site program 

manager, because it has a shallow aquifer 

contained by a clay layer, which prevents the 

migration of contaminants. This laboratory is 

the only place in the United States that offers 

such a test setting. A variety of technologies 

have been tested here for cleaning up a 

range of contaminants. For example, tests for 

cleanup of TCE are under way using a 

combination of three technologies: soil vapor 

extraction, bioremediation, and air stripping.
Page 24 GAO-05-666 Groundwater Contamination

with ITRC has led to enhanced cooperation among state regulators, DOD 

personnel, and community stakeholders and increased the deployment of 

innovative technologies at DOD cleanup sites. 

Although DOD is actively involved in the research and development of 

innovative technologies, some groundwater remediation experts and some 

DOD officials with whom we consulted believe that additional resources 

may be needed to develop and advance DOD’s process for selecting the 

most appropriate technology at a site. These individuals believe that a 

better understanding of the nature and extent of contamination at a site is 

critical for selecting appropriate technologies for cleanup. Furthermore, 

these experts and some DOD officials believe that additional resources 

may be appropriate for examining and improving methods and engineering 

approaches for optimizing the performance of the 15 types of groundwater 

remediation technologies that are currently available. Other groundwater 

remediation experts and some DOD officials suggested that more 

resources may be needed to further develop innovative approaches to 

emerging groundwater remediation issues, and to educate DOD personnel 

and regulators on these approaches. 

Agency Comments DOD generally agreed with the content of the report, stating that the report 

is an accurate summary of DOD’s use and field tests of remedial 

technologies; DOD also provided technical clarifications that we have 

incorporated, as appropriate. 

We are sending copies of this report to appropriate congressional 

committees; the Secretary of Defense; the Administrator of EPA; and other 

interested parties. We will also make copies available to others upon 

request. In addition, the report will be available at no charge on GAO’s Web 

site at http://www.gao.gov.
Page 25 GAO-05-666 Groundwater Contamination

If you or your staff have any questions about this report, please contact me 

at (202) 512-3841 or mittala@gao.gov. Contact points for our Offices of 

Congressional Relations and Public Affairs may be found on the last page 

of this report. GAO staff who made major contributions to this report are 

listed in appendix V.

Anu K. Mittal

Director, Natural Resources 

and Environment
Page 26 GAO-05-666 Groundwater Contamination

Appendix I

Objectives, Scope, and Methodology ApApenednxidexiIs

This report (1) describes the groundwater remediation technologies that 

the Department of Defense (DOD) is currently using or field-testing and (2) 

examines whether any new groundwater remediation technologies are 

being used outside the department or are being developed by commercial 

vendors that may have potential for DOD’s use, and the extent to which 

DOD is researching and developing new approaches to groundwater 

remediation. In addition, this report provides limited information on the 

key characteristics, benefits, and limitations of selected groundwater 

remediation technologies. 

To address the first objective, we developed a questionnaire that we sent to 

the DOD components responsible for DOD’s groundwater cleanup 

efforts—the Air Force, Army, U.S. Army Corps of Engineers, Defense 

Logistics Agency, and Navy. In the questionnaire, we listed groundwater 

remediation technologies and asked these DOD components to indicate 

which technologies they have implemented and still currently use. We also 

asked the components to provide examples of specific groundwater 

remediation projects. We developed the list of technologies based on a 

review of reports and existing lists developed by the National Research 

Council, Environmental Protection Agency (EPA), Federal Remediation 

Technology Roundtable, and others, as well as through discussions with a 

groundwater remediation consulting firm and several nationally recognized 

groundwater remediation experts. To better understand DOD’s processes 

for environmental cleanup and technology development, we met with 

officials from the offices of the Deputy Undersecretaries of Defense for 

Installations and Environment and for Science and Technology. We also 

reviewed documents, reports, and guidance on groundwater remediation 

from the Office of the Secretary of Defense and the various DOD 

components involved in groundwater remediation. To obtain information 

on how DOD uses groundwater remediation technologies to treat 

contaminants of concern, we toured several bioremediation projects at 

Dover Air Force Base and spoke with a groundwater remediation program 

manager for the Air Force.

To address our second objective, we contracted with consultants from the 

Washington, D.C., office of Malcolm Pirnie Inc. to gather information from 

commercial vendors on the range of currently available groundwater 

remediation technologies. We also attended a national groundwater 

remediation conference, where we spoke with a number of vendors of 

groundwater remediation technologies about their products, efforts to 

develop innovative approaches to groundwater remediation, and 

remediation work they may have performed for DOD. In addition, we 
Appendix I

Objectives, Scope, and Methodology

Page 27 GAO-05-666 Groundwater Contamination

collected and reviewed reports and studies from these vendors to better 

understand the range of technologies available to DOD. We also consulted 

with four nationally recognized groundwater remediation experts—two 

from academia and two from industry—to provide information on 

innovative remediation technologies currently available or under 

development by the commercial sector. We selected these experts on the 

basis of their independence, knowledge of and experience with 

groundwater remediation technologies, and recommendations from the 

National Academy of Sciences and others. In addition, we consulted with a 

senior groundwater remediation official from EPA’s Groundwater and 

Ecosystem Restoration Division, who is an expert on technologies used for 

groundwater remediation. 

Through these sources, we identified 15 technologies that are currently 

available commercially for the treatment of contaminated groundwater. 

For the purposes of this report, we defined a technology as a distinct 

technical method or approach for treating or removing contaminants found 

in groundwater. We did not consider any modifications or enhancements to 

a technology, such as variations in the material or equipment used during 

treatment, to be a separate technology. To determine whether there were 

any technologies currently being used outside of DOD, we compared the 

list of 15 currently available technologies with information provided to us 

by DOD officials on technologies currently used by DOD for groundwater 

remediation. 

To identify the extent to which DOD supports the research and 

development of new approaches to groundwater remediation, we 

interviewed officials from the Strategic Environmental Research and 

Development Program and the Environmental Security Technology 

Certification Program. We reviewed reports, project portfolios, and other 

documents developed by these two programs. To gain a better 

understanding of DOD’s efforts to field-test innovative approaches to 

groundwater remediation, we visited a DOD National Environmental 

Technology Test Site, located in Delaware, where private and public 

researchers can test innovative groundwater remediation technologies. We 

observed several ongoing research projects and interviewed an official 

responsible for managing the test facility. To gain a better understanding of 

DOD’s relationship with the Interstate Technology and Regulatory Council, 

we reviewed a memorandum of understanding between the two 

organizations and interviewed an official that serves as DOD’s liaison to the 

council. 
Appendix I

Objectives, Scope, and Methodology

Page 28 GAO-05-666 Groundwater Contamination

Information presented in this report is based on publicly available 

documents and information provided by government officials, independent 

consultants, and experts. We did not review nonpublic research and 

development activities that may be under way in private laboratories. We 

reviewed data for accuracy and consistency, and corroborated DODprovided data to the extent possible. We assessed the reliability of the 

DOD-provided data by reviewing related documentation, including DOD’s 

annual reports to Congress on its Defense Environmental Restoration 

Program and information provided by consultants.

We performed our work from January 2005 through May 2005, in 

accordance with generally accepted government auditing standards.
Page 29 GAO-05-666 Groundwater Contamination

Appendix II

Technologies for the Remediation of 

Contaminated Groundwater ApendxiI

Ex-situ Technologies 1. Advanced oxidation processes often use ultraviolet light irradiation 

with oxidizers such as ozone or hydrogen peroxide to produce free 

radicals, which break down and destroy chlorinated solvents, fuels, and 

explosive contaminants as water flows through a treatment reactor 

tank. Depending on the design of the system, the final products of this 

treatment can be carbon dioxide, water, and salts. An advantage of 

advanced oxidation processes is that it destroys the contaminant, 

unlike some other technologies, which only shift the phase of the 

contaminant into something more easily handled and removed. There 

are some limitations to these processes; for instance, maintenance of 

the treatment equipment can be a problem if certain substances—such 

as insoluble oil or grease—are allowed into the system. Also, the 

handling and storage of oxidizers can require special safety 

precautions. The cost of this type of remediation is largely dependent 

on the volume and flow rate of groundwater to be treated, energy 

requirements, and chemicals utilized. Operations and maintenance 

costs are also a factor in the overall cost of this approach. For the 

purposes of this report, advanced oxidation processes also include the 

related technologies of phyotolysis and photocatalysis. 

2. Air stripping involves the mass transfer of volatile contaminants from 

water to air by exposing contaminated water to large volumes of air, so 

that the contaminants, such as chemical solvents, undergo a physical 

transformation from liquid to vapor. In a typical air stripper setup, 

called a packed tower, a spray nozzle at the top of a tower pours 

contaminated water over packing media or perforated trays within the 

tower. At the bottom of the tower, a fan forces air up through the tower 

countercurrent to the water flow, thus stripping the contaminants from 

the water. The contaminants in the air leaving the tower must then be 

removed and disposed of properly. Air strippers can be combined with 

other technologies for treatment of groundwater. Advantages of this 

technology include its potential to effectively remove the majority of 

the volatile organic contaminants of concern. Moreover, this mature 

technology is relatively simple and design practices are standardized 

and well-documented, and, in comparison with other approaches, this 

technology is often less expensive. However, maintenance can be an 

issue with this technology if inorganic or biological material clogs or 

fouls the equipment, and process energy costs can be high. 

3. Bioreactors are biochemical-processing systems designed to degrade 

contaminants in groundwater using microorganisms, through a process 
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similar to that used at a conventional wastewater treatment facility. 

Contaminated groundwater flows into a tank or basin, where it 

interacts with microorganisms that grow and reproduce while 

degrading the contaminant. The excess biomass produced is then 

separated from the treated water and disposed of as a biosolids waste. 

This technology can be used to treat, among other things, chlorinated 

solvents, propellants, and fuels. Potential advantages of bioreactors 

include relatively low operations and maintenance costs and the 

destruction, rather than mass transfer of, the contaminants. Moreover, 

regulators and other stakeholders generally accept bioreactor 

technology as a proven approach for remediation. Nonetheless, there 

are some limitations to the use of bioreactors, including decreases in 

effectiveness if contaminant concentrations in the influent water are 

too high or too low to support microorganism growth and if nuisance 

microorganisms enter the system. Additionally, the sludge produced at 

the end of the process may need further treatment or specialized 

disposal. Bioreactor cost is influenced by the upfront capital needed for 

installation, setup, and start-up, as well as the operations and 

maintenance costs associated with longer-term treatment. 

4. Constructed wetlands use artificial wetland ecosystems (organic 

materials, microbial fauna, and algae) to remove metals, explosives, 

and other contaminants from inflowing water. The contaminated water 

flows into the wetland and is processed by wetland plants and 

microorganisms to break down and remove the contaminants. 

Wetlands, intended to be a long-term remediation approach, can be 

created with readily available equipment and generally can operate 

with low maintenance costs. Furthermore, because this technology 

provides a new ecosystem for plant and animal life, it is generally 

popular with the public. However, this approach is often more suitable 

for groundwater that is ultimately discharged to the surface rather than 

reinjected into the ground. Also, the long-term effectiveness of this 

treatment is not well-known, as aging wetlands may lose their ability to 

process certain contaminants over time. Temperature, climate, and 

water flow rate may negatively impact the processes that break down 

the contaminants. Applicability and costs associated with constructed 

wetlands vary depending on site conditions, such as groundwater flow 

rate, contaminant properties, landscape, topography, soil permeability, 

and climate. 

5. Ion exchange involves passing contaminated water through a bed of 

resin media or membrane (specific to the particular contaminant) that 
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exchanges ions in the contaminants’ molecular structure, thus 

neutralizing them. This approach can be useful for dissolved metals 

(e.g., hexavalent chromium) and can be used to treat propellants such 

as perchlorate. Once the ion exchange resin has been filled to capacity, 

it can be cleaned and reused (following a process called resin 

regeneration). Ion exchange is usually a short- to medium-term 

remediation technology. This technology allows contaminated water to 

be treated at a high flow rate and can completely remove the 

contaminants from the water. However, some substances—such as 

oxidants or suspended solids—in the incoming water may diminish the 

effectiveness of the ion exchange resins. Furthermore, different resin 

types can be needed for different contaminants. Among the factors 

influencing costs are discharge requirements, the volume of water to be 

treated, contaminant concentration (as well as the presence of other 

contaminants), and resin regeneration. For the purposes of this report, 

ion exchange includes technologies that use ion exchange resins or 

reverse osmosis membranes to remove contaminants from 

groundwater, including dissolved metals and nitrates.

6. Adsorption (mass transfer) technologies involve passing 

contaminated water through a sorbent material—such as activated 

carbon—that will capture the contaminants (through either adsorption 

or absorption), thus removing or lessening the level of contaminants in 

the water. The contaminated water is pumped from the aquifer and 

passed through the treatment vessel containing the sorbent material. As 

the contaminated water comes into contact with the sorbent surface, it 

attaches itself to that surface and is removed from the water. Benefits 

of this technology include its ability to treat contaminated water to 

nondetectable levels and its potential for treating low to high 

groundwater flow rates as well as multiple contaminants 

simultaneously. However, some contaminants may not be sorbed well 

or the sorbent unit may require disposal as hazardous waste. 

Furthermore, this approach is impractical if the contaminant levels are 

high due to higher costs resulting from frequent changing of the sorbent 

unit. If the concentrations of contaminants are low or flow rates for 

treatment can be kept low, then adsorption technology may be a costeffective approach.

In-situ Technologies 1. Air sparging introduces air or other gases into a contaminated aquifer 

to reduce concentrations of contaminants such as fuel or chlorinated 

solvents. The injected air creates an underground air stripper that 
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removes contaminants by volatilization (a process similar to 

evaporation that converts a liquid or solid into a gas or vapor). This 

injected air helps to transport the contaminants up into the unsaturated 

zone (the soil above the water table, where pores are partially filled 

with air), where a soil vapor extraction system is usually implemented 

to collect the vapors produced through this process. This technology 

has the added benefit of often stimulating aerobic biodegradation 

(bioremediation) of certain contaminants because of the increased 

amount of oxygen introduced into the subsurface. Typically, air 

sparging equipment is readily available and easily installed with 

minimal disturbance to site operations. However, this technology 

cannot be used if the contaminated site contains contaminants that 

don’t vaporize or are not biodegradable. In some cases, this technology 

may not be suitable for sites with free product (e.g., a pool of fuel 

floating on the water table) because air sparging may cause the free 

product to migrate and spread contamination. Also, this technology is 

less effective in highly stratified or heterogeneous soils since injected 

air tends to travel along paths of least resistance in the subsurface, 

potentially bypassing areas of contamination. This technology can be 

less costly than ex-situ technologies because it does not require the 

removal, treatment, storage, or discharge of groundwater. For the 

purposes of this report, air sparging includes the related remedial 

approaches of co-metabolic sparging, sparging using other gases, and 

in-well air stripping. 

2. Bioremediation relies on microorganisms to biologically degrade 

groundwater contaminants through a process called biodegradation. It 

may be engineered and accomplished in two general ways: (1) 

stimulating native microorganisms by adding nutrients, oxygen, or 

other electron acceptors (a process called biostimulation); or (2) 

providing supplementary pregrown microorganisms to the 

contaminated site to augment naturally occurring microorganisms (a 

process called bioaugmentation). This technology mainly focuses on 

remediating organic chemicals such as fuels and chlorinated solvents. 

One approach, aerobic bioremediation, involves the delivery of oxygen 

(and potentially other nutrients) to the aquifer to help native 

microorganisms reproduce and degrade the contaminant. Another 

approach, anaerobic bioremediation, circulates electron donor 

materials—for example, food-grade carbohydrates such as edible oils, 

molasses, lactic acid, and cheese whey—in the absence of oxygen

throughout the contaminated zone to stimulate microorganisms to 

consume the contaminant. In some cases, pregrown microbes may be 
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injected into the contaminated area to help supplement existing 

microorganisms and enhance the degradation of the contaminant, a 

process known as bioaugmentation. A potential advantage of 

bioremediation is its ability to treat the contaminated groundwater in 

place with naturally occurring microorganisms, rather than bringing 

contaminants to the surface. By using native microorganisms, rather 

than injecting additional ones, cleanup can be more cost-effective at 

some sites. However, heterogeneous subsurfaces can make delivering 

nutrient/oxygen solutions to the contaminated zone difficult by 

trapping or affecting movement of both contaminants and 

groundwater.1

 Also, nutrients to stimulate the microorganisms can be 

consumed rapidly near the injection well, thereby limiting the 

microorganisms’ contact with the contaminants, or stimulating 

biological growth at the injection site. In summary, this technology 

avoids the costs associated with bringing water to the surface for 

treatment; instead, the main costs associated with bioremediation 

include: delivery of the amendments to the subsurface (which varies 

depending on the depth of contamination), the cost of the amendments 

themselves, and monitoring of the treatment. For the purposes of this 

report, bioremediation includes the related bioremedial approaches of 

bioaugmentation, biostimulation, co-metabolic treatment, enhanced 

aerobic biodegradation, enhanced anaerobic biodegradation, and 

biobarriers.

3. Enhanced recovery using surfactant flushing speeds contaminant 

removal in conventional pump-and-treat systems by injecting 

surfactants2

 into contaminated aquifers or soil to flush the contaminant 

toward a pump in the subsurface (some distance away from the 

injection point); this pump removes the contaminated water and 

surfactant solution to the surface for treatment and disposal of 

contaminants. Surfactants are substances that associate with organic 

compounds such as fuels and chlorinated solvents and significantly 

1

Heterogeneities can cause wide variability in hydraulic properties such as hydraulic 

conductivity—a measure of the volume of water that will pass through an area at a given 

time. These changes in hydraulic properties enhance the dispersion of a dissolved 

contaminant spread. Heterogeneities can also create preferential pathways for contaminant 

migration.

2

Surfactants are molecules with two structural units: one with an affinity for water and one 

with an aversion to water. Surfactants are especially useful for dissolving some 

contaminants and enhancing their mobility by lowering the interfacial tension between the 

contaminant and water.
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increase their solubility, which aids cleanup of contaminated aquifers 

with less flushing water and pumping time. This technology is 

applicable to both dense and light nonaqueous phase liquids (DNAPL 

and LNAPL).3

 Benefits of enhanced recovery approaches include the 

rapid removal of contaminants, which may significantly reduce cleanup 

times. However, regulatory issues may require special attention due to 

extra scrutiny for obtaining approvals to inject surfactant solutions; a 

greater degree of site characterization is often required to satisfy both 

technical and regulatory requirements. In addition, subsurface 

heterogeneities and low permeability can interfere with the effective 

delivery and recovery of the surfactant solution. Furthermore, to the 

extent that mobilization of organic liquid contaminants is achieved, this 

approach may be better for LNAPLs than DNAPLs, as LNAPLs tend to 

migrate upward and DNAPLs downward, possibly trapping them in 

previously uncontaminated subsurface areas. In addition to the high 

cost of surfactant solutions, another factor influencing the overall cost 

of this approach may be the treatment of the surfactant solution that is 

pumped out of the aquifer. For the purposes of this report, this 

technology includes related remedial approaches that use co-solvents 

such as ethanol to improve the solubility of surfactants in the 

subsurface.

4. Chemical treatments include remediation technologies that chemically 

oxidize or reduce contaminants when reactive chemicals are injected 

into the groundwater. This approach converts contaminants such as 

fuels and explosives into nonhazardous or less-toxic compounds. 

Depending on the extent of contamination, this process involves 

injecting chemicals into the groundwater and generally takes a few 

days to a few months to observe results in rapid and extensive 

reactions with various contaminants of concern. Additionally, this 

technology can be tailored to the site and does not require rare or 

complex equipment, which may help reduce costs. Generally, there are 

no unusual operations and maintenance costs; however, in-situ 

chemical treatment may require intensive capital investment for large 

contaminant plumes or zones where repeated applications or large 

volumes of reactive chemicals may be required; major costs are 

3

Nonaqueous-phase liquids are liquids that do not mix with, or dissolve in, water. Dense 

nonaqueous-phase liquids (DNAPL) fall to the bottom of a body of water; chlorinated 

solvents are typical examples. Conversely, light nonaqueous-phase liquids (LNAPL) gather 

on top of the water. Gasoline and fuel oil are examples of LNAPLs. 
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associated with injection-well installation (cost influenced by well 

depth), procurement of the reactive chemicals, and monitoring. 

Additionally, site characterization is important for the effective delivery 

of reactive chemicals, as subsurface heterogeneities may result in 

uneven distribution of the reactive chemicals. For the purposes of this 

report, chemical treatment also includes various remedial approaches 

and technologies that chemically oxidize or reduce contaminants insitu, as well as those that result in the in-situ immobilization and 

stabilization of soluble metals. 

5. Monitored natural attenuation is a relatively passive strategy for insitu remediation that relies on the naturally occurring physical, 

chemical, and biological processes that can lessen concentrations of 

certain contaminants in groundwater sufficiently to protect human 

health and the environment. The changes in contaminant 

concentrations are observed through various wells that are placed 

throughout the contaminated groundwater zone to monitor the level of 

contamination over time and its migration from its initial location in the 

subsurface. Some chlorinated solvents and explosives may be resistant 

to natural attenuation; however, it can still be used in cases of 

nonhalogenated chlorinated solvents and some inorganic compounds. 

If appropriate for a given site, natural attenuation can often be less 

costly than other forms of remediation because it requires less 

infrastructure, construction, and maintenance. Furthermore, it is less 

intrusive because fewer surface structures are necessary and it may be 

used in all or selected parts of a contaminated site, alone or in 

conjunction with other types of remediation. However, compared with 

active techniques, natural attenuation often requires longer time frames 

to achieve remediation objectives. 

6. Multiphase extraction uses a series of pumps and vacuums to remove 

free product,4

 contaminated groundwater, and vapors from the 

subsurface, treat them, and then either dispose or reinject the treated 

groundwater. Specifically, one or more vacuum extraction wells are 

installed at the contaminated site to simultaneously pull liquid and gas 

from the groundwater and unsaturated soil directly above it. This type 

of vacuum extraction well removes contaminants from above and 

below the groundwater table, and can expose more of the subsurface 

for treatment, notably in low permeability or heterogeneous 

4

Free products are liquid contaminants floating on top of groundwater.
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formations. The contaminant vapors are collected in the extraction 

wells and taken above ground for treatment. This approach can be used 

to treat organic contaminants—such as chlorinated solvents and 

fuels—and can be combined with other technologies, particularly 

above-ground liquid/vapor treatment, as well as other methods of insitu remediation such as bioremediation, air sparging, or bioventing. 

Potential advantages of this technology include its applicability to 

groundwater cleanup in low permeability and heterogeneous 

formations and its minimal disturbance to site-specific conditions. 

However, the system requires complex monitoring and specialized 

equipment, and it may be difficult or problematic to implement the 

most effective number of pumps. A major contributor to this 

technology’s cost is operations and maintenance, which may run from 6 

months to 5 years, depending on site-specific factors. For the purposes 

of this report, multiphase extraction includes the related technologies 

of bioslurping and dual-phase extraction.

7. Permeable reactive barriers are vertical walls or trenches built into the 

subsurface that contain a reactive material to intercept and remediate a 

contaminant plume as the groundwater passes through the barrier. This 

technology can be used to treat a wide range of contaminants and is 

commonly used to treat chlorinated solvents and heavy metals. 

Reactive barriers usually do not require above-ground structures or 

treatment, allowing the site to be used while it is being treated. 

However, its use is limited by the size of the plume since larger 

contaminant plumes are often more difficult to intercept for treatment. 

Moreover, the barrier may lose effectiveness over time as 

microorganisms or chemicals build up on the barrier, making 

rehabilitation or media replacement necessary. The depth of the 

contaminated groundwater zone and the required barrier may also 

present some technical challenges. Underground utility lines, rocks, or 

other obstacles can increase the difficulty of installing a barrier and 

drive up capital costs. Additionally, because permeable reactive 

barriers do not treat the contaminant source, but simply the plume, 

treatment may be required for extended time periods, thus increasing 

overall cleanup costs. For the purposes of this report, permeable 

reactive barriers include biotic and abiotic, as well as passive and 

active treatment barriers.

8. Phytoremediation is the use of selected vegetation to reduce, remove, 

and contain the toxicity of environmental contaminants, such as metals 

and chlorinated solvents. There are several approaches to 
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phytoremediation that rely on different plant system processes and 

interactions with groundwater and contaminants. One approach to 

phytoremediation is phytostabilization, which uses plants to reduce 

contaminant mobility by binding contaminants into the soil or 

incorporating contaminants into plant roots. Another approach is 

phytoaccumulation, where specific species of plants are used to absorb 

unusually large amounts of metals from the soil; the plants are later 

harvested from the growing area and disposed of in an approved 

manner. A similar process is called rhizofiltration, where contaminated 

water moves into mature root systems and is circulated through their 

water supply. Another process can remove contaminants by 

evaporating or volatilizing the contaminants from the leaf surface once 

it has traveled through the plant’s system. Phytoremediation offers the 

benefit of only minimally disturbing the environment and can be used 

for the treatment of a wide range of contaminants. However, specific 

plant species required for particular contaminants may be unable to 

adapt to site conditions due to weather and climate, and 

phytoremediation may not be an effective approach for deep 

contamination. While maintenance costs, including cultivation, 

harvesting, and disposal of the plants, are substantial for this 

technology, phytoremediation typically has lower costs than alternative 

approaches. For the purposes of this report, phytoremediation includes 

phytostabilization, phytoaccumulation, phytoextraction, rhizofiltration, 

phytodegredation, rhizosphere degredation, organic pumps, and 

phytovolitilization. 

9. Thermal treatments involves either pumping steam into the aquifer or 

heating groundwater in order to vaporize chlorinated solvents or fuels 

from the groundwater. The vaporized contaminant then rises into the 

unsaturated zone and can be removed via vacuum extraction for 

treatment. There are three main approaches for heating the 

groundwater in-situ. The first, radio frequency heating, uses the 

electromagnetic energy found in radio frequencies to rapidly heat the 

soil in a process analogous to microwave cooking. The second, 

electromagnetic heating, uses an alternating current to heat the soil and 

may include hot water or steam flushing to mobilize contaminants. The 

third uses heating elements in wells to heat the soil. Thermal 

treatments may be applied to a wide range of organic contaminants and 

sites with larger volumes of LNAPLs or DNAPLs as well as sites with 

low permeability and heterogeneous formations. However, the 

presence of metal and subsurface heterogeneities in the contaminated 

site may interfere with this process. The heating and vapor collection 
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systems must be designed and operated to contain mobilized 

contaminants, to avoid their spread to clean areas. The major costs 

incurred for thermal treatments are for moving specialized equipment 

to the site, developing infrastructure to provide power, and providing 

energy to run the system. For the purposes of this report, thermal 

treatments include related soil-heating technologies, such as steam 

flushing, conductive heating, and electrical resistance heating.
Page 39 GAO-05-666 Groundwater Contamination

Appendix III

Groundwater Remediation Experts Consulted ApendxiI

Dr. John Fountain

Professor and Head, Department of Marine, Earth and 

Atmospheric Sciences

North Carolina State University

Raleigh, North Carolina

Dr. Robert E. Hinchee

Principal Civil and Environmental Engineer 

Integrated Science and Technology Inc.

Panacea, Florida

Dr. Michael C. Kavanaugh

Vice President 

National Science and Technology Leader

Malcolm Pirnie Inc.

Emeryville, California

Dr. Robert L. Siegrist

Professor and Division Director

Environmental Science and Engineering Division

Colorado School of Mines

Golden, Colorado

Dr. John T. Wilson

Senior Research Microbiologist

Ground Water and Ecosystems Restoration Division, National Risk 

Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Ada, Oklahoma 
Page 40 GAO-05-666 Groundwater Contamination

Appendix IV

Comments from the Department of Defense ApendxiIV
Page 41 GAO-05-666 Groundwater Contamination

Appendix V

GAO Contact and Staff Acknowledgments ApendxiV

GAO Contact Anu K. Mittal, (202) 512-3841

Staff 

Acknowledgments 

In addition to the contact above, Richard Hung, Lynn Musser, Jonathan G. 

Nash, Omari Norman, and Diane B. Raynes made key contributions to this 

report. Jessica A. Evans, Katherine M. Raheb, and Carol Herrnstadt 

Shulman also made important contributions to this report. 

(360539)
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