Groundwater Contamination: DOD Uses and Develops a Range of Remediation Technologies to Clean Up Military Sites
Published: 2005-06-30T00:00:00

Fast Facts:

Highlights:
section_title: Why GAO Did This Study
paragraphs: 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 "pump-and-treat" technologies to contain or eliminate hazardous contaminants in groundwater. However, the long cleanup times and high costs of using pump-and-treat 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.
section_title: What GAO Found
paragraphs: 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. 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.

section_title: Letter
paragraphs: 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. 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  entities—the Air Force, Army, Defense Logistics Agency, and Navy—are  responsible for groundwater remediation on active DOD facilities. 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. The Air Force has the greatest number of sites with  contaminated groundwater needing remediation, followed by the Navy,  Army, Corps, and Defense Logistics Agency. 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. Pump- and-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-and- treat technologies that rely on a variety of biological, chemical, or physical  processes to treat the contaminated groundwater underground (in-situ).
As directed by Public Law 108-375, 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.
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.
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. 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  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.
subsections: section_title: Background
paragraphs: 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  as a result of groundwater flowing through the contaminated site. See  figure 1 for an illustration of a site with contaminated groundwater.
subsections: section_title: DOD Facilities Can Have Significant Groundwater Contamination
paragraphs: 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.
section_title: DOD Cleanup Activities Generally Follow the CERCLA Process
paragraphs: 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. 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.
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. 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  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. 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.
section_title: DOD Has Implemented or Field-tested a Wide Range of Technologies to Remediate Sites Contaminated with Groundwater
paragraphs: 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.
subsections: section_title: Fifteen Ex-situ and In-situ Technologies Are Currently Available for Groundwater Cleanup
paragraphs: 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 ex- situ 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.  (See app. II for more information on key characteristics of these ex-situ  technologies.)
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 in- situ 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.,  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 surfactants 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  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 well- understood. 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.
section_title: DOD Has Used the Full Range of Groundwater Remediation Technologies Identified
paragraphs: 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.
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. 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  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.
section_title: Technology Selection Is Also Influenced by Cost and Performance
paragraphs: 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. 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  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.  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.  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.
section_title: DOD Is Proactively Using and Developing New Approaches to Groundwater Remediation
paragraphs: 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.
subsections: section_title: Most New Approaches Employ Novel Materials or Modifications to Existing Technologies
paragraphs: 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  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. 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.
section_title: DOD Supports the Development of New Technologies with the Commercial Sector through Several Programs
paragraphs: 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  agencies in seeking more cost-effective solutions to common problems. 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. 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), N- Nitrosodimethylamine (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  remediation. Using a process similar to that of SERDP, ESTCP solicits  proposals from public and private researchers to field-test laboratory- proven 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 field- test 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 co- located 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.
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.
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  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.
section_title: Agency Comments
paragraphs: 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.
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.
section_title: Objectives, Scope, and Methodology
paragraphs: 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  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.
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 DOD- provided 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.
section_title: Technologies for the Remediation of Contaminated Groundwater
subsections: section_title: Ex-situ Technologies
paragraphs: 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  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  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 cost- effective approach.
section_title: In-situ Technologies
paragraphs: 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  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  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. 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  surfactants 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  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). 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  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 in- situ, 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 in- situ 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, 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  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 in- situ 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  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  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.
section_title: Groundwater Remediation Experts Consulted
section_title: Comments from the Department of Defense
section_title: GAO Contact and Staff Acknowledgments
subsections: section_title: GAO Contact
section_title: Staff Acknowledgments
paragraphs: 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.