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. 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. 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. 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. 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).