Title: Introduction (1 of 2) Text: A permeable reactive barrier (PRB), in its simplest form, is a trench built across the flow path of a groundwater plume. The trench is filled with a suitable reactive or adsorptive medium that removes the contamin-ation from the groundwater, thus protecting downgradient water resources or receptors. The treatment media is selected for its ability to clean up specific types of contaminants. Question: What is your overall level of experience with the design and use of PRBs? No Experience, Minimal, Moderate, High Visual Direction: Animated graphic of a groundwater plume passing through a permeable reactive barrier. Contaminated groundwater enters and treated water exits.

Title: Text: PRBs can be used to treat a wide variety of contaminants including chlorinated solvents, organics, inorganics, metals, and radionuclides. Most PRBs have been installed to address chlorinated solvents in groundwater, such as trichloroethylene (TCE) and perchloroethylene (PCE). Chlorinated solvents are often present in the form of dense, nonaqueous-phase liquids (DNAPLs) and can produce large dissolved-phase plumes that persist for many years or decades. Visual Direction: Rollover of contaminant plume before permeable reactive barrier.

Title: Text: PRBs are selected for use at many sites as an alternative to conventional remediation methods such as pump-and-treat. A pump-and-treat system requires constant labor and energy input; however, a PRB may be more economical over time due to the passive nature of its action. The use of PRBs is expanding as the range of contaminants that can be treated increases due to new types of reactive media. PRB media generally are classified as reactive, adsorptive, or biodegradation enhancing. Visual Direction: Rollover of media in permeable reactive barrier.

Title: Text: As groundwater flows through the barrier, the contaminants come into contact with the reactive media and are adsorbed or degraded into nontoxic compounds. For example, zero-valent iron will react with chlorinated solvent compounds to form ethene and chloride as the end products. Although the groundwater exiting the barrier has been treated, it is not necessarily potable (drinking) water. Visual Direction: Rollover of treated water exiting the permeable reactive barrier.

Title: Introduction (2 of 2) Text: This training tool is designed to assist Navy Remedial Project Managers (RPMs) in the development and implementation of effective PRB applications. Navy RPMs can use the information in this tool to: Learn about the history and key scientific concepts related to the use of PRBs. Understand the steps needed to ensure the efficient design and effective performance of PRBs. Anticipate longevity issues and their relationship to the economic benefits of PRB use. Benefit from lessons learned at other sites related to the installation and use of PRBs Visual Direction: No graphic.

Title: History of PRBs Text: The concept of a PRB was first developed in the 1990s. Although collection trenches filled with gravel have been used to intercept groundwater flow for many decades, the idea of using a reactive material to interact with the contaminants in the trench itself is relatively new. In the past decade, the PRB technology has grown and progressed in terms of both its use and regulatory acceptance. Roll over the dates in the timeline to read about the history of the PRB technology. Visual Direction: A graphic of the continental United States with a timeline below it including the years: 1993, 1994, 1995, 1996, and 2003.

Title: Text: The first known application of a solid reactive material in the subsurface to intercept and treat groundwater flow was in 1993 at the Canadian Forces Base (CFB) Borden, Ontario, where the University of Waterloo installed a pilot-scale barrier to treat a TCE plume. Visual Direction: Rollover of 1993. Map shows location of Borden, Ontario.

Title: Text: In 1994, the first full-scale PRB was installed as a funnel-and-gate system at a private site in Sunnyvale, California. Visual Direction: Rollover of 1994. U.S. map shows location of Sunnyvale, California.

Title: Text: The U.S. Department of Defense (DoD) was one of the early adopters of PRB technology. One of DoD's first PRB installations was in 1995 at Lowry Air Force Base (AFB), Colorado. This PRB used granular iron as the reactive medium for treating a chlorinated solvent plume. Visual Direction: Rollover of 1995. U.S. map shows location of Lowry Air Force Base, Colorado.

Title: Text: In 1996, the DoD installed a granular iron PRB to treat chlorinated solvents at Naval Air Station (NAS) Moffett Field, California. Visual Direction: Rollover of 1996. U.S. map shows location of Naval Air Station Moffett Field, California.

Title: Text: As of 2003, PRBs have been installed at more than 50 different sites across the United States. Most of the PRBs use iron as the reactive medium and target chlorinated solvent plumes. However, in recent years, there has been growing interest in using other barrier media and applying this technology to a wider range of contaminants. Visual Direction: Rollover of 2003. Map shows locations of PRBs across the United States.

Title: Advantages and Limitations Text: The following are the major advantages/limitations associated with PRBs: Advantages: In situ remediation Passive operation that requires little labor/energy input No required aboveground structures Potentially less expensive than pump-and-treat systems due to low O&M costs No recurring aboveground waste disposal requirements. Limitations: Mass flux of certain native dissolved solids can cause a decrease in PRB longevity When barrier reactivity decreases to the extent that target cleanup levels are not being met, either the barrier may have to be replaced or regenerated, or an alternative remedial action may have to be implemented. As with many in situ technologies, designing a PRB requires a much more detailed understanding of the hydraulic flow characteristics than designing a pump-and-treat system. Installation can be expensive and technically complex for unusually deep plumes. A higher capital investment may initially be required to implement a PRB (compared to a pump-and-treat system). Visual Direction: No graphic.

Title: Configurations Text: The configuration of a PRB refers to the manner in which groundwater is directed through the barrier. One of the most common configurations is a continuous reactive barrier, where the treatment medium extends across the entire width and depth of the contaminant plume. Another common configuration is the funnel-and-gate, where impermeable walls guide groundwater through one or more treatment gates. Visual Direction: Two animated graphics of PRB configurations. First, a continuous reactive barrier with groundwater flowing through it. Second, a funnel-and-gate with groundwater flows being directed towards a PRB in the center.

Title: Barrier Media Text: Zero-valent iron is currently the most common reactive material used in PRBs, but a variety of other adsorptive, reactive, and biodegradation-enhancing materials can be used to target specific contaminants. In general, the permeable barrier concept (creating an in situ reactive zone to intercept a groundwater plume) has broad application under a variety of conditions, and suitable media can be found to address most contaminants. Click here for more information on some media types. Visual Direction: Table of different contaminants and the corresponding media types.

Title: Text: Additional information is provided below on some of the more novel barrier materials: Zeolite, bauxite, and apatite are naturally occurring minerals which can be used to treat contaminants. Zeolite is a glassy mineral containing aluminum silicates of calcium or sodium or potassium. Bauxite is aluminum ore. Apatite is a mineral of calcium fluoride phosphate or calcium chloride phosphate. An electrochemical barrier works through the application of a small current to the zero-valent iron barrier. The applied current increases the pH of the groundwater flowing through the barrier. This reduces the rate of iron oxidation and increases the reductive capacity of the iron, thereby increasing the effectiveness of contaminant treatment and removal. Visual Direction: No graphic.

Title: Iron Barriers (1 of 2) Text: The iron used in PRBs is typically elemental iron (or zero-valent iron) derived from scrap cast iron. A particle size range of -8 to +50 mesh has been used in most applications. In general, finer iron particles provide a larger surface area, and therefore greater reactivity. Larger iron particles typically provide a higher hydraulic conductivity, and therefore more effective groundwater plume capture. In the -8 to +50 mesh-size range, the iron is expected to exhibit optimum reactive and hydraulic properties. Visual Direction: Photo of iron particles and a hammer.

Title: Text: A mesh size of -8 to +50 means that the iron grains are small enough to pass through a number 8 mesh, but too large to pass through a number 50 mesh. The number 8 mesh has openings of about 2,380 microns, and the number 50 mesh has openings of 300 microns. Visual Direction: No graphic.

Title: Iron Barrier Considerations (2 of 2) Text: The use of granular zero-valent iron for in situ groundwater treatment applications has been patented by the University of Waterloo. Other variants of this technology have been patented for groundwater treatment and include the use of colloidal iron (Pacific Northwest National Laboratory), micro-scale iron (ARS Technologies, Inc.), nano-scale iron (Lehigh University), and emulsified zero-valent iron or EZVI (University of Central Florida and National Aeronautic and Space Administration [NASA]). On a unit weight basis, the ultra-fine iron (colloidal or nano-scale iron) is currently more expensive than the granular iron. However, ultra-fine iron is claimed to be more effective in contaminant source areas, such as in DNAPL zones, where solvent concentrations are extremely high. The use of ultra-fine iron in source zones is promising, but remains in the developmental stage. Most current PRB applications have used the coarser granular iron. The iron barrier is typically composed of a mix of sand and iron. At the center of the PRB, or where the most contaminated groundwater will flow through, the barrier is usually 100% iron. Due to cost considerations, the composition of the barrier along the outer edges is likely to be 50% iron or less. Visual Direction: No graphic.

Title: Iron Reactivity (1 of 2) Text: Iron Reaction Mechanisms Zero-valent iron is a strong reducing agent and its properties are well suited to the treatment of many common dissolved contaminants. Under certain groundwater conditions, elemental iron is slowly oxidized to ferrous iron, releasing two electrons in the process. These electrons participate in a variety of reactions leading to the transformation of the target contaminant. The reaction proceeds through two known pathways. In the hydrogenolysis, or sequential degradation pathway shown here, one chlorine atom is removed in each step, so that TCE degrades to cis-1,2 dichloroethylene (DCE), then to vinyl chloride (VC), and finally to ethene and ethane. Visual Direction: Chemical structure diagrams of the hydrogenolysis pathway for converting TCE to cis-DCE to VC to ethene to ethane.

Title: Hydrogenolysis Text: The breaking of a chemical bond in an organic molecule with the simultaneous addition of a hydrogen atom to each of the resulting molecular fragments. Visual Direction: No graphic.

Title: Iron Reactivity (2 of 2) Text: Iron Reaction Mechanisms In the beta-elimination pathway, the formation of partially dechlorinated products (such as DCE and VC) is avoided, and TCE is transformed directly to ethane via the production of some short-lived intermediates, such as chloroacetylene and acetylene. Most experts believe that chlorinated solvents degrade primarily through the beta-elimination pathway when exposed to iron. Very little DCE or VC have been found in laboratory or field PRB studies with iron, which indicates that the dominant mechanism is probably beta-elimination. Visual Direction: Chemical structure diagrams of the beta-elimination pathway converting TCE to chloroacetylene to acetylene to ethane.

Title: Beta-elimination Text: Beta-elimination is an elimination reaction in which a proton, which is beta to a leaving group, is removed by a base. The adjacent atoms (usually carbons) typically develop a pi bond. This pathway for iron degradation was proposed in Roberts, A.L., L.A. Totten, W.A. Arnold, D.R. Burris, and T.J. Campbell. 1996. "Reductive Elimination of Chlorinated Ethylenes by Zero-Valent Metals." Environ. Sci. Technol. Visual Direction: No graphic.

Title: Iron Reactivity Considerations Text: Despite the fact that iron barriers themselves do not generate much DCE or vinyl chloride, many aquifers contain considerable concentrations of these relatively toxic degradation products. This is because most chlorinated solvent contamination is the result of older spills, and the original solvent in these spills has naturally attenuated (through microbial interaction) to DCE and vinyl chloride. Fortunately, iron degrades these partial dechlorination products fairly efficiently, although the design of the barrier should allow for the presence of these products in the aquifer. Visual Direction: No graphic.

Title: Implementation Text: It is necessary to follow several steps in order to ensure the efficient design and effective performance of a PRB. The success of any in situ technology largely depends upon achieving a good understanding of the contamin-ated subsurface and preparing a suitable design that takes into account the variabilities inherent in aquifer systems. This is especially important for PRBs because they rely on natural groundwater flow to bring the contaminants into contact with the reactive medium. Visual Direction: Picture of lab setting in background with flowchart of steps to follow for PRB implementation.

Title: Conceptual Site Model Text: Developing a conceptual site model includes conducting a preliminary assessment of the available data to determine whether a PRB would be an appropriate technology. Factors that can determine whether a PRB is a suitable remedial action at a site include: Depth of contamination Hydrogeology of the aquifer Geochemistry of the groundwater Type and distribution of contaminants Geotechnical considerations that may affect PRB construction Aboveground features and access. Once the conceptual model is developed, a preliminary determination on the suitability of the PRB technology at the specific site is made. Data gaps typically appear, and relate to information about the site characteristics or the suitability of a certain barrier medium. Additional site characterization and treatability testing are required to address these data gaps. Visual Direction: First, a graphic of the distribution of contaminants at a site showing concentration contours. Second, a graphic of the cross section of soil layers beneath a contaminated site.

Title: Site Characterization Text: Although considerable property-wide site investigations may have been completed in the past, PRB application often requires additional characterization in the more localized setting of the prospective PRB location, including determination of the following: Depth of contamination and aquitard; Competency of the aquitard; Stratigraphy and heterogeneity of target aquifer; Horizontal and vertical distribution of contamination; Groundwater flow characteristics; Presence of underground obstacles; Presence of aboveground objects; Groundwater geochemistry. Visual Direction: Photo of onsite drilling for site characterization.

Title: Text: Depth determines the construction method and cost. Visual Direction: No graphic.

Title: Text: Is the aquitard thick enough and impermeable enough so that the PRB can be keyed into it? Visual Direction: No graphic.

Title: Text: Stratigraphy and heterogeneity of the aquifer affect groundwater flow and capture by the PRB. Visual Direction: No graphic.

Title: Text: The distribution of contamination within the aquifer affects the dimensions of the PRB. Visual Direction: No graphic.

Title: Text: Groundwater flow velocity, direction, and other characteristics affect the orientation and dimensions of the PRB. Visual Direction: No graphic.

Title: Text: Subsurface utilities, cobbles, highly consolidated sediments, and other subsurface obstacles may impede construction. Visual Direction: No graphic.

Title: Text: The presence of aboveground objects such as buildings, overhead utility lines, or other structures may impede construction of the PRB or require placement of the PRB in a less desirable location. Visual Direction: No graphic.

Title: Text: Geochemistry could affect the longevity of the PRB or its ability to retain reactivity and hydraulic performance over long periods of time. Visual Direction: No graphic.

Title: Treatability Testing Text: Treatability testing provides information on the efficiency of reactions between the reactive medium and the target contaminants under the specific site conditions. Treatability tests are most often conducted as either batch or column studies. The kinetics of contaminant removal can be determined from these data in terms of a reaction rate or contaminant half-life. In this picture of a typical column study, groundwater from the site is run through a column packed with the candidate reactive medium. Water samples are collected through ports along the length of the column. Each port represents a different residence time of the groundwater in the reactive medium. Visual Direction: Photo of two packed media columns with multiple sampling ports on each. Groundwater enters at the top of the columns, filters through the media, and exits at the bottom.

Title: Batch tests Text: Batch tests can be done to obtain a quick test of the reactivity of the candidate medium, although column tests provide more dynamic and accurate rate information. Often, batch tests are conducted to compare and select from among several candidate media, and these tests are followed by a column test with the most promising medium. Visual Direction: No graphic.

Title: Column Studies Text: In a typical column study, groundwater from the site is run through a column packed with the candidate reactive medium. Water samples are collected through ports along the length of the column. Each port represents a different residence time of the groundwater in the reactive medium. Column tests provide dynamic and accurate rate information, as well as information on how the groundwater residence time affects the barrier material. Column tests are also a good way of determining the behavior of native groundwater constituents upon contact with the reactive medium. Groundwater constituents, such as dissolved oxygen, calcium, magnesium, and alkalinity, are typically affected by iron or other reactive media, and these reactions could have some effect on the long-term performance of the medium. Visual Direction: No graphic.

Title: Design & Modeling (1 of 2) Text: Once all the additional site characterization and treatability testing data are available, the PRB can be designed. The design process involves determining the parameters described below: Location Configuration Orientation Dimensions The figure to the left shows several site-specific measurements that are used to design PRBs. Visual Direction: Diagram showing groundwater flow and different soil zones. Measurements for an iron cell and pea gravel area are included. The soil type, hydraulic conductivity and total porosity of each zone are given.

Title: Location Text: It seems that the ideal location for a PRB would be at the leading edge of a contaminant plume so that all downgradient resources are protected. In reality, a number of practical considerations may govern the location of the barrier. Factors such as property boundaries, aboveground space accessibility, and subsurface utilities play a role in determining the best location for intercepting a plume. Visual Direction: No graphic.

Title: Configuration Text: In its simplest configuration, a PRB is a straight trench filled with reactive medium. Variations of this simple configuration include funnel-and-gate systems (combination of permeable and impermeable sections), serial barriers (combination of two different media in series, separated perhaps by a non-reactive section), and others. Visual Direction: No graphic.

Title: Orientation Text: Ideally, a PRB should be oriented as perpendicular to the groundwater flow as possible. In practice, this may be difficult to implement due to the uncertainty in determining the exact flow direction on the scale of the PRB. At most sites, the regional gradient is generally well known, but across the few feet of thickness that the groundwater will flow through the PRB, determining the hydraulic gradient can be challenging. In addition, flow velocity and direction can change seasonally, sometimes by a considerable amount. An optimum PRB design is based on sufficient site characterization at this local level and should take into account any variability and uncertainty in the flow predictions. Visual Direction: No graphic.

Title: Dimensions Text: Ideally, a PRB should be wide enough to capture the plume, thick enough to provide sufficient residence time for contaminant degradation, deep enough to capture the deepest portion of the plume, and, if possible, keyed into a competent aquitard. (An aquitard is a very low permeability geologic unit that stores groundwater, but delays its flow.) In practice, a number of factors influence the PRB dimensions, including property boundaries, regulatory cleanup targets, variability in the groundwater flow, and strength of the plume. Visual Direction: No graphic.

Title: Design & Modeling (2 of 2) Text: Given the variability and uncertainty involved in determining flow and contaminant conditions at many sites, computerized modeling can help make the design of PRBs more robust. This figure, from NAS Moffett Field, is an example of the use of groundwater flow (MODFLOW) and solute transport (MODPATH) models for determining the optimal location, orientation, and dimensions of a PRB. Each frame in the figure is a horizontal cross-section of a layered two-dimensional model. As seen from the particle tracking maps, plume capture is asymmetrical and varies considerably with depth at this site. This was due to the presence of silty clay deposits. It was critical that the PRB at this site span the more permeable sand channels located at certain depths. Visual Direction: Models of groundwater flow and solute transport at 10 ft bgs and 20 ft bgs.

Title: Construction (1 of 3) Text: Once the PRB has been designed, the installation method should be determined. Depth is the primary factor that determines the construction method and cost. Depths of 25 to 30 ft below ground surface (bgs) are easily accessible to standard backhoe-based excavation methods, and many PRBs have been installed within this depth range. The depths accessible to a backhoe can be extended to about 60 ft bgs by using a backhoe with a modified boom. Beyond these depths, more complex excavation methods may be required. Visual Direction: Table of construction techniques and their maximum depths.

Title: Construction (2 of 3) Text: Trench Box Construction Trenches deeper than about 4 ft often have to be supported to prevent collapse. Sheet piling can be used to support the excavation until the reactive medium has been filled in. The sheet piling is subsequently withdrawn to re-establish flow through the PRB. Continuous Trenching At some relatively shallow sites (about 30 ft bgs or less), a continuous trencher can be used to excavate and fill a trench with reactive medium. The reactive medium is fed into the trench through a hopper that sits on top of a chainsaw-type excavator. Because the trenching and filling occur almost simultaneously, no additional supports are required to keep the trench open. Visual Direction: Photo of a deep trench being supported while excavated and a photo of a continuous trencher excavating and filling a trench.

Title: Construction (3 of 3) Text: Bioslurry Trenching The use of biodegradable slurry (bioslurry) is a recent advance in PRB trenching. A jelly-like guar gum/water mixture is added to the trench to stabilize the excavation walls, thereby eliminating the need for trench boxes or sheet piling. This technique has been implemented at several sites, including Pease AFB, NH. Preliminary tests indicate that the reactivity of the PRB is not affected by this method of construction. Click here for more details on the bioslurry trenching process. Visual Direction: Diagram of bioslurry trenching: mixer adding reactive material to trench, backhoe excavating trench, and biodegradable slurry holding trench open. Also, a photo of bioslurry being pumped into a trench and 2 photos of equipment loading media into trench.

Title: Pease AFB Text: A PRB at Pease AFB that is approximately 60 ft deep is the deepest PRB that has been installed using the conventional or semi-conventional excavation methods described herein. The biodegradable slurry method was used to support the excavation. Visual Direction: No graphic.

Title: Text: Conventional backhoes excavate trenches up to 5.6 feet wide and up to 30 feet bgs. Trench boxes or sheet piling are used to stabilize the excavation prior to back­filling with treatment medium. Modified backhoes used recently at PRB sites have reached depths of 80 feet, although slurry is required to keep the trench open. Crane-operated clamshells have been used to excavate trenches to depths of 120 feet bgs with the help of trench support slurry. This technique has been implemented at PRB installations in Sunnyvale, CA, and former NAS Moffett Field, CA. Visual Direction: Rollover of the backhoe construction technique.

Title: Text: The use of biodegradable slurry (bioslurry) is a recent advance in PRB trenching. The bioslurry mixture is added to the trench to stabilize the excavation walls, thereby eliminating the need for trench boxes or sheet piling. Preservatives and pH adjustments prevent bioslurry breakdown during construction. This technique has been implemented at several sites, including Somersworth Landfill Site, NH. More information on the bioslurry technique can be found on page 19. Visual Direction: Rollover of the biodegradable slurry construction technique.

Title: Text: Caissons are large-diameter load-bearing enclosures that are driven into the ground. Once installed, the native material is excavated and replaced with the treatment medium, and the caisson is removed. Caissons were used to install the Savannah River Site Geosiphon (25 ft bgs), and the Dover AFB treatment gates (45 ft bgs). Visual Direction: Rollover of the caisson excavation construction technique.

Title: Text: The continuous trencher is equipped with a boom apparatus similar to a chainsaw for excavation, a trench box for stabilizing the trench walls, and a hopper to backfill the excavation with reactive medium. It can excavate and immediately backfill trenches 1 to 2 ft wide. This technique has been implemented at several sites, including U.S. Coast Guard site, NC, and Naval Weapons Industrial Reserve Plant, TX. Visual Direction: Rollover of the continuous trenching construction technique.

Title: Text: Two or three special augers equipped with mixing paddles are lined up in series. As the augers penetrate the ground, they mix fine iron and soil together. The iron also can be introduced in biodegradable slurry. Alternatively, iron-filled casings can be driven into the ground with a vibratory hammer, and the iron later mixed with the soil using the mixing paddles. A variation of this method was used at Launch Complex 34, Cape Canaveral Air Station, FL. Visual Direction: Rollover of the deep soil mixing construction technique.

Title: Text: A series of wells are installed along the length of the PRB. A vertical fracture is propagated within each well, and the fractures are filled with granular iron/guar gum slurry. The reactive iron slurry in one fracture coalesces with the adjacent fracture, creating a continuous vertical wall. This technique was implemented at Massachusetts Military Reservation and at the Caldwell Trucking Co. Site, NJ. Visual Direction: Rollover of the hydraulic fracturing construction technique.

Title: Text: Grout or slurry is injected at high pressure into the ground. A triple-rod injection system delivers a high-pressure mixture of granular iron, guar gum, air, and water to the subsurface. Injection starts at the bottom of the PRB wall and continues as the rod is lifted, creating a column or panel of reactive medium. Multiple rows of overlapping columns or panels create the continuous passive treatment wall. This technique has been implemented at Travis AFB, CA. Visual Direction: Rollover of the jetting construction technique.

Title: Text: An H-beam or mandrel with a sacrificial shoe at the bottom is driven into the ground to create a void space. As the beam is raised, a slurry or grout containing the reactive medium is injected into the void space through a special nozzle at the bottom of the beam. By driving beams in an overlapping panel, a continuous treatment wall is created. This barrier can be installed at an angle up to 45 degrees to avoid utilities or structures. The vibrated beam technique was used at an industrial site in Tifton, GA, and a mandrel was used at Hangar K, Cape Canaveral Air Station, FL. Visual Direction: Rollover of the vibrated beam or mandrel construction technique.

Title: Bioslurry Trenching Process Text: PRBs longer than 50 ft should be installed in sections of 40 to 50 ft each to avoid collapse of the trench. End stops can be installed after every 50-ft length has been excavated; the end stop allows filling of the reactive medium to continue in one 50-ft section of the trench while the next 50-ft section is being excavated. A biocide generally is added to the guar gum slurry to avoid early microbe-driven breakdown of the guar gum and collapse of the trench, while the trenching and filling activities are going on. After slurry is added and endstops are installed (if needed), the reactive medium is tremied into the trench. Once the reactive medium has been placed in the trench, an enzyme breaker is circulated through the trench to promote microbial activity, accelerate the breakdown of the guar gum, and re-establish flow through the PRB. Visual Direction: No graphic.

Title: Monitoring Text: A monitoring plan is required to determine how well a PRB is meeting its design objectives. Two main types of monitoring are typically required: compliance monitoring and performance monitoring. A possible monitoring scheme for a PRB is shown to the left. Roll over the suggested monitoring locations to discover what they are used for. Visual Direction: Diagram of monitoring locations around and within a permeable reactive barrier.

Title: Text: Monitoring location A is used for downgradient compliance and hydraulic performance. Visual Direction: Rollover of monitoring location A downgradient from the PRB.

Title: Text: Monitoring location B is used for downgradient compliance, treatment performance, and hydraulic performance. Visual Direction: Rollover of monitoring location B just exiting the PRB.

Title: Text: Monitoring location C is used as a temporary compliance point, and for hydraulic performance. Visual Direction: Rollover of monitoring location C within the PRB.

Title: Text: Monitoring location D is used for hydraulic and treatment performance. Visual Direction: Rollover of monitoring location D just entering the PRB.

Title: Text: Monitoring location E may be used for hydraulic performance assessment. Visual Direction: Rollover of monitoring location E along the upgradient side of the funnel walls.

Title: Text: Monitoring location F may be used for hydraulic performance assessment. Visual Direction: Rollover of monitoring location Fat the outer ends of the wall.

Title: Compliance Monitoring Text: Compliance monitoring is required to ensure that the PRB is meeting its primary goal of protecting downgradient groundwater quality. This typically involves placement of one or more monitoring wells at a downgradient compliance point in the aquifer. However, at many sites, it may take months or even a few years before significant improvements in downgradient contaminant levels are observed. To counter this situation, regulators have been willing to consider a temporary compliance point inside the PRB (in the reactive medium) at the end where the groundwater exits the PRB. Monitoring wells placed at the exit end of the PRB medium indicate the degree of treatment achieved by the PRB and the quality of the groundwater effluent. Visual Direction: No graphic.

Title: Performance Monitoring Text: In addition to determining whether or not the PRB is currently meeting its compliance goals, performance monitoring is generally required. Regulators and site owners may agree on other monitoring locations and parameters (other than the target contaminants), in order to determine whether or not the PRB is meeting its design goals and for how long the designed performance can be sustained. Performance monitoring generally involves: Hydraulic performance Longevity While compliance monitoring is generally required on a quarterly basis, the frequency of performance monitoring should be determined based on site-specific performance objectives and based on the collective judgment of the site managers and regulators. Visual Direction: No graphic.

Title: Hydraulic Performance Text: Field monitoring aided by computer modeling can be used to assess the PRB hydraulic performance. The analysis determines how much water the PRB is capturing, the residence time of groundwater in the reactive medium, and the occurrence of any plume bypass (around, over, or under the PRB). These factors may contribute to a subsequent inability to meet downgradient compliance goals. This figure, based on the results of a groundwater particle tracking model, shows that hydraulic capture zones are not always homogeneous. The heterogeneity of this aquifer is due to the presence of sand channels embedded in clayey soil. The groundwater flows easily through the sandy soil (where the gate is located at the center of the PRB), but moves much more slowly through the clay that occurs mostly around the funnel walls on either side of the treatment gate. Visual Direction: Animated graphic of a groundwater particle tracking model showing flow pattern through the PRB. The particles are not evenly distributed throughout the hydraulic capture zone.

Title: Longevity (1 of 3) Text: Many types of contaminant plumes can last for several years or decades, sometimes much longer than the life of the PRB. Factors that may limit the longevity of a PRB include: Groundwater flow velocity Chemical composition of the groundwater. Depending on the type of reactive medium used, many naturally occurring constituents (such as calcium and carbonate) can react with and potentially passivate the surfaces of the reactive medium, progressively reducing its reactivity. Unfortunately, iron barriers installed in the subsurface have too short a history to fully understand this phenomenon, and how it relates to the barrier's ultimate fate. Visual Direction: Diagram with arrows showing a long contaminant plume lifespan compared to shorter PRB known and expected lifespans.

Title: Text: The PRB reactive media will be consumed more quickly when the groundwater flow velocity is high, which results in greater exposure of the PRB to groundwater constituents that potentially deplete the reactive medium of its properties. Visual Direction: No graphic.

Title: Text: Chemical constituents in the groundwater that could affect the long-term performance of the PRB include high contaminant concentrations and the presence of native groundwater constituents, such as calcium, magnesium, and carbonates. Native groundwater constituents can precipitate on or otherwise affect the reactive surface of the medium in such a way that either the reactivity or the hydraulic properties (porosity and hydraulic conductivity) are potentially affected. Monitoring groundwater parameters such as pH, ORP, cations, anions, and water levels can provide some understanding of an impending loss of PRB performance, although data should be interpreted with caution. In some laboratory studies, the reactivity of iron medium continued to decline while pH, ORP, and pressure drop (water levels) remained relatively constant. Generally, a combination of parameters provides a better indication of changes in performance than any single parameter. If the PRB is thick enough, installing monitoring points along the flow path in the reactive medium itself can provide early warning of loss of performance, as most of the changes first become noticeable near the upgradient end of the medium. Visual Direction: No graphic.

Title: Text: After the permeable barrier has outlived its effectiveness, it can often be left undisturbed in the ground. If the plume persists after the barrier medium has been saturated, passivated, or otherwise lost its functionality, the medium may have to be removed from the ground and replaced with fresh medium. Alternatively, if flow conditions permit, a second barrier can be installed parallel to the first one. Visual Direction: No graphic.

Title: Longevity (2 of 3) Text: Longevity Research The longevity expectations of zero-valent iron PRBs have been studied through geochemical modeling, coring and analysis of iron from field PRBs, as well as accelerated long-term column tests. Analysis of core samples from iron in field PRBs that have been operational for several years can confirm results obtained from geochemical models, but obtaining a quantitative assessment of how inorganic precipitates affect iron's reactivity is more difficult. This figure, obtained using backscatter electron imaging, reveals the elements present in typical precipitates formed on iron from PRBs. Visual Direction: Backscatter electron imaging figures of BE, calcium, magnesium, silicon, and aluminum.

Title: Backscatter electron imaging Text: During backscatter electron imaging, an electron beam interacts with a sample and a proportion of the electrons from the beam are reflected or backscattered. The amount of electrons reflected back indicates the atomic density of the region being probed; this shows compositional variation and allows the identity of the material to be determined. Visual Direction: No graphic.

Title: Longevity (3 of 3) Text: Longevity Research Empirical evidence for the buildup and effect of precipitation has been obtained through accelerated column tests sponsored by SERDP and ESTCP through NFESC. During these tests, groundwater from two sites was run at an accelerated rate through columns packed with iron to simulate long-term PRB performance. Roll over the graphs to learn more about these studies. The half-life of TCE was used as a predictor of the iron's reactive performance. The degree of precipitation and loss of reactivity was found to depend on the level of dissolved solids and the groundwater flow velocity. Visual Direction: Plots from Lowry AFB and Moffett Field of TCE Half-Life (minutes) vs. Number of Pore Volumes.

Title: Half-Life Text: A measure of the time it takes for the iron to degrade the TCE to half its original concentration. An increase in TCE half-life corresponds to a decrease in iron reactivity. Visual Direction: No graphic.

Title: Text: This plot shows the results of an accelerated column test that was conducted with groundwater and iron from former Lowry AFB. Lowry AFB has groundwater with a relatively high level of total dissolved solids, and a fairly slow groundwater velocity. With approximately 1,300 pore volumes of groundwater exposure (which corresponds to about 60 years of operation), the half-life of TCE increased by a factor of about four, indicating that precipitation effects would eventually affect the reactivity of the PRB. Visual Direction: Rollover of the Lowry plot.

Title: Text: An accelerated column test was conducted with iron and groundwater from former NAS Moffett Field, where the groundwater has a relatively low level of dissolved solids, but a moderate to fast groundwater velocity. After 1,300 pore volumes (corresponding to about 30 years of exposure), the half-life of TCE increased by a factor of two. Visual Direction: Rollover of the Moffett plot.

Title: Economics of PRBs Text: This graph shows estimated project costs at a PRB site. The graph shows that a break even point occurs after about 7 years when the present value (PV) of a PRB and an equivalent pump and treat system are compared over a 30-year period of operation. The capital investment required for a PRB is often higher than for a pump-and-treat system, but operation and maintenance (O&M) costs are lower. While the O&M costs of a PRB are generally limited to quarterly monitoring requirements, a pump-and-treat system typically requires considerable O&M labor and energy involvement. Therefore, annual savings start to accrue over the years and in the long term are expected to result in net savings. This analysis is based on several assumptions. Visual Direction: Plot of Cumulative Present Value Cost vs. Years of Operation for a permeable reactive barrier and a pump and treat system. Pump and treat is higher after about 7 years.

Title: Assumptions Text: This economic analysis assumes that the PRB media will be replaced every 10 years. This assumption is extremely conservative, since it may actually be more common for the barrier media to remain effective for 20 to 25 years. Greater cost savings will be realized if the media life expectancy is extended to longer than 10 years. Depending on regulatory requirements, evolution of the plume, and when the PRB performance declines to an unacceptable level, the PRB can either be abandoned or the reactive medium could be regenerated or replaced. Visual Direction: No graphic.

Title: Lessons Learned (1 of 2) Text: A survey of Interstate Technology Regulatory Council (ITRC) member states revealed the following regulatory concerns and the typical deficiencies in PRB applications submitted to state regulatory agencies: Inadequate site characterization at the proposed PRB location Possibility of flow bypass Possibility of reduced permeability of the PRB over time Possibility of groundwater mounding upgradient of the PRB Inadequate reactive medium thickness Inadequate attention to constructability of the PRB Inadequate discussion of the effects of PRB construction materials, such as biocides, that may adversely affect the environment. Visual Direction: No graphic.

Title: Lessons Learned (2 of 2) Text: Contingency plans required by state regulators typically require that the following information be addressed: Ability to operate a pump-and-treat system, in case of PRB failure Extension of the PRB to capture more of the plume, if monitoring shows that the current dimensions are inadequate Blocking the two ends (sides) of the PRB, to avoid flow bypass at the edges Ability to install a second PRB parallel to the first one, if the first one fails. Many of these concerns can be addressed through sufficient site characterization, good design practices (e.g., use of safety factors), preparation of an adequate monitoring plan, and early, thorough discussions with the regulatory agencies involved. Question: How much did this tool increase your understanding of PRB design and performance issues? Minimally, Moderately, Greatly Visual Direction: No graphic.

Title: Summary Text: PRBs are a cost effective remedy at sites where groundwater contamination is expected to persist for many years. They are in situ, passive remediation systems that can be modified in several ways to accommodate site conditions. This tool summarizes the history and key scientific concepts associated with the use of PRBs. It reviews the steps necessary for PRB implementation at a field site. It also provides key information to help Navy RPMs to: Identify sites where the installation of a PRB would be appropriate. Understand available design and construction techniques. Improve monitoring programs that track PRB performance and compliance with regulatory guidelines. Understand how to evaluate the expected lifespan and economic benefits associated with using a PRB. Visual Direction: Photo of the location of a permeable reactive barrier at a site.

Title: References Text: More information on PRBs is available as follows: Advances in Permeable Barrier Technology RITS Spring 2002 Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation Evaluating the Longevity and Hydraulic Performance of Permeable Reactive Barriers at Department of Defense Sites Permeable Reactive Wall Remediation of Chlorinated Hydrocarbons in Groundwater Visual Direction: No graphic.

Title: Contact Information Text: For more information about this project, please contact: NFESC POC (805) 982-2194 PRTH_NFESCT2@navy.mil Visual Direction: No graphic.