Dense Nonaqueous Phase Liquids (DNAPLs)

Treatment Technologies

Permeable Reactive Barriers

This page identifies general resources that contain detailed information on the design and implementation of permeable reactive barriers (PRBs). Information on applications of this technology specific to a chemical class can be found in the class subsections listed to the right. More resources on PRBs for a wide range of contamination problems can be found in the Permeable Reactive Barriers pages of Technology Focus .

A subsurface PRB is an emplacement of reactive materials through which a dissolved contaminant plume must move as it flows, typically under natural gradient. Treated water exits the other side of the PRB. This in situ method for remediating dissolved-phase contaminants in groundwater combines a passive chemical or biological treatment zone with subsurface fluid flow management.

PRBs can be installed as permanent or semi-permanent units. The most commonly used PRB configuration is that of a continuous trench in which the treatment material is backfilled. The trench is perpendicular to and intersects the groundwater plume. Another frequently used configuration is the funnel and gate, in which low-permeability walls (the funnel) direct the groundwater plume toward a permeable treatment zone (the gate). Some gates are in situ reactors that are readily accessible to facilitate the removal and replacement of reactive media. These PRBs use collection trenches, funnels, or complete containment to capture the plume and pass the groundwater, by gravity or hydraulic head, through a vessel containing either a single treatment medium or sequential media (ITRC 2005).

The majority of installed PRBs use zero-valent iron (ZVI) as the reactive medium for converting contaminants to non-toxic or immobile species. ZVI, a mild reductant, has the ability to reductively dehalogenate many halogenated hydrocarbons. Dehalogenation rates will vary for the different halogenated contaminants. The primary determinant of degradation rate is the specific surface area, or the surface area of iron per unit volume of pore water. The reaction pathways by which ZVI reduces halogenated hydrocarbons have been determined for a few major classes of chlorinated hydrocarbons. This information is significant to the optimal design of a PRB, as incomplete dechlorination of a highly chlorinated ethene, for example, could produce an intermediate product (e.g., vinyl chloride) that is more hazardous and more persistent than the parent compound. Even very low concentrations of undesirable byproducts in the reactive barrier effluent must be avoided (Powell 1998).

A variety of other media can be used in PRBs to address a wide range of organic and inorganic contaminants (Powell 1998). For example, organic materials—activated charcoal, cottonseed meal, peat moss, lignite, humite, and compost—have been placed in PRBs to treat groundwater affected by solvent-related compounds, primarily by promoting or enhancing biologically mediated destruction of the target analytes. The treatment zone materials concentrate and either degrade or retain the contaminants and may need to be replaced periodically (ITRC 2005).

In situ redox manipulation (ISRM) is a passive barrier technology based upon the in situ manipulation of natural processes to change the mobility or form of dissolved contaminants in the subsurface. ISRM was developed to remediate groundwater that contains chemically reducible metallic and organic contaminants (i.e., chlorinated solvents). ISRM creates a permeable treatment zone by injection of chemical reagents and/or microbial nutrients into the subsurface downgradient of the contaminant source. The type of reagent is selected according to its ability to alter the oxidation/reduction state of the groundwater, thereby destroying or immobilizing specific contaminants. Because unconfined aquifers are usually oxidizing environments and many of the contaminants in these aquifers are mobile under oxidizing conditions, appropriate manipulation of the redox potential can result in the immobilization of redox-sensitive inorganic contaminants and the destruction of organic contaminants (U.S. DOE 2000).

PRBs can be adapted to sequential treatments to address groundwater plumes that contain a mixture of contaminants. Sequenced reactive barriers have been constructed to treat a variety of mixed contaminant plumes.

ZVI to treat chlorinated hydrocarbons followed by aerobic bioremediation to treat aromatic hydrocarbons.
ZVI to treat carbon tetrachloride and chloroform followed by MNA for dichloromethane.
ZVI to treat chlorinated hydrocarbons followed by nutrient addition or solid carbon sources to promote anaerobic biodegradation of VOCs that cannot be degraded by the iron.
Solid carbon sources to treat nitrate followed by granular iron to treat volatile organics (ITRC 2005).

Typically, PRBs are designed to provide adequate residence time in the treatment zone for the degradation of the parent compound and all toxic intermediate products that are generated. At sites where the groundwater contamination includes a mixture of chlorinated hydrocarbons, the design of the PRB usually is determined by the least reactive constituent (Powell 1998).

The use of a passive PRB requires an unusually comprehensive hydrologic characterization so that the design can be based on a thorough understanding of subsurface heterogeneity rather than on average values for hydraulic parameters. Given the level of investigation required, design costs likely will increase, and the pre-design field work may demonstrate that a passive PRB is not suitable for a particular site (Korte 2001).

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General Resources | Performance Monitoring for Permeable Reactive Barriers

General Resources

An Assessment of Zero Valence Iron Permeable Reactive Barrier Projects in California
J. Muegge, California Department of Toxic Substances Control, Document 1219, 154 pp, 2008

A review of the performance of 10 PRBs installed primarily to address chlorinated contaminants indicates that a ZVI PRB should not be expected to provide near-term improvement of water quality very far below its installation. The same levels observed downgradient of a PRB before its installation can persist for extended periods (often decades) despite the presence of a PRB. The PRBs were installed at Alameda Naval Air Station, BP-Hitco, DuPont Oakley, Fairchild/Applied Materials, Intersil, Moffett Field, Mohawk Laboratory, Sierra Army Depot (2 separate PRBs), and Travis Air Force Base.

Ensuring the Continued Success of a Mulch Biowall at a Trichloroethylene-Contaminated Superfund Site: Lessons Learned
Ghandehari, S. S., S.-H. Cheng, C.J. Hapeman, A. Torrents, B.V. Kjellerup.
Remediation 33(4):323-337(2023)

A review of the entire remediation system was conducted at the Beaver Dam Road Landfill site in Beltsville, Maryland, to determine the reason behind the inefficiency of a biowall designed to promote the bioremediation of TCE and its degradation products. Monitoring data, including the concentration of TCE and its degradation byproducts, and geochemical and physical characteristics were evaluated to understand the conditions and challenges facing decision-makers and possible options to improve biowall efficacy.

Final Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation
A. Gavaskar, N. Gupta, B. Sass, R. Janosy, and J. Hicks.
Strategic Environmental Research and Development Program (SERDP), Arlington, VA. 247 pp, 2000

In Situ Chemical Treatment: Technology Evaluation Report
Yujun Yin and Herbert E. Allen.
Ground-Water Remediation Technologies Center (GWRTAC). TE-99-01, 82 pp, 1999

In situ chemical treatment techniques are useful for treatment of source areas to reduce the mass of contaminants and intercept plumes to remove mobile organics and metals. Chemical injection treatment mechanisms can be oxidative, reductive/precipitative, or desorptive/dissolvable, depending upon the chemical/contaminant interaction. Chemicals can be delivered to the subsurface via well injection techniques, deep soil mixing and hydraulic fracturing, or installation of permeable chemical treatment walls. The main chemical injection in situ treatments discussed are oxidation, flushing, and reduction and immobilization. Treatment wall reactions include immobilization of inorganics and organics via sorption, immobilization of inorganics via precipitation, and degradation of inorganic anions and organics. This report discusses the chemistry and the engineering aspects of available in situ chemical treatment technologies and provides information on costs, lessons learned, and regulatory issues.

Permeable Mulch Biowalls
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, May 2007

A biobarrier is a biologically active flow-through zone in an aquifer that is established downgradient of a source zone or on the leading edge of a contaminant plume. As contaminated groundwater passes through the biobarrier, the contaminants are converted by microorganisms into innocuous byproducts, such as carbon dioxide and water. The microbial population is established with biostimulation and/or bioaugmentation. Biobarriers can be used to create either aerobic or anaerobic conditions. Configurations include biowalls, bioborings, or injection wells to add substrates to groundwater as it passively flows through the biologically active zone. Permeable mulch biowalls are used to remediate chlorinated organic compounds and other contaminants in ground water via the reaction of natural organic substrates, such as mulch, compost, and/or vegetable oil. The organic media create an anaerobic reaction zone that enhances bioremediation within the aquifer. This Web tutorial reviews design and installation considerations for permeable mulch biowalls and highlights case study results at Navy and Air Force sites.

Permeable Reactive Barrier Technologies for Contaminant Remediation
R.M. Powell, et al.
EPA 600-R-98-125, 102 pp, 1998

This document provides sufficient background in the science of PRB technology to allow a basic understanding of the chemical reactions that transform contaminants. It contains sections on PRB-treatable contaminants and the treatment reaction mechanisms; information on feasibility study, site characterization, design, emplacement, and monitoring issues specific to PRBs; and summaries of several field installations.

Permeable Reactive Barriers: Lessons Learned/New Directions
Interstate Technology & Regulatory Council (ITRC) Permeable Reactive Barriers Team.
PRB-4, 202 pp, 2005

While ZVI is the most common medium placed in PRBs to treat a variety of chlorinated organics, metals, and radionuclides, other reactive media—carbon sources (compost), limestone, granular activated carbon, and zeolites—also have been used to address metals and some organic compounds. This document contains compiled information and data on PRBs generated over the last 10 years of technology development and research, as well as information on non-iron-based reactive media that can be used in PRBs. This report also provides an update on a developing technology related to PRBs in which source zone contamination is addressed with iron-based reactive media via backfilling, soil mixing, or injection.

Permeable Reactive Barrier: Technology Update
The Interstate Technology & Regulatory Council (ITRC) PRB Technology Update Team.
PRB-5, 234 pp, 2011

Since inception, the PRB has remained an evolving technology with new and innovative reactive materials introduced to treat different contaminants as well as innovative construction methods. This document gives readers a better understanding of the advantages and limitations of PRBs and helps them navigate the associated regulatory, hydraulic, and engineering challenges.

Zero-Valent Iron Permeable Reactive Barriers: A Review of Performance
N.E. Korte.
ORNL/TM-2000/345, 36 pp, 2001

This review reports on performance issues with PRBs installed at DOE sites. The principal conclusion of the review is that the most significant problems with installed PRBs have been the result of insufficient characterization, which resulted in poor engineering implementation.

Interstate Technology & Regulatory Council (ITRC), 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. PRB-4.

N.E. Korte, 2001. Zero-Valent Iron Permeable Reactive Barriers: A Review of Performance. ORNL/TM-2000/345.

R.M. Powell, et al., 1998. Permeable Reactive Barrier Technologies for Contaminant Remediation. EPA 600-R-98-125.

U.S. DOE, 2000. In Situ Redox Manipulation: Innovative Technology Summary Report DOE/EM-0499.

Performance Monitoring for Permeable Reactive Barriers

Permeable reactive barriers are containment devices that treat dissolved DNAPL chemicals. They do not treat the DNAPL phase directly. Whether PRBs are constructed in a funnel-and-gate configuration or continuous trench/overlapping borings, the object of their design is to pass contaminated groundwater through a reactive zone where the contaminants are transformed into less- or non-toxic chemicals.

Typically for DNAPL chemicals, the barrier has been constructed with some form of solid phase organic carbon (e.g., mulch) or granular zero valent iron (ZVI). The solid phase organic carbon creates the conditions necessary for anaerobic biodegradation. The ZVI primarily destroys the contaminants through an abiotic reaction (β-elimination); however, highly reductive conditions are created by ZVI, and depending upon the barrier's construction, anaerobic biodegradation can also occur (ITRC 2005, Gavaskar et al. 2000). Note that iron walls have the tendency to raise the pH of the water that flows through them, which will affect microbial growth at the downgradient side of the barrier (ITRC 2005).

Performance monitoring is focused on the PRB system itself (including impermeable funnels, if present), rather than the entire site or the compliance boundaries. Effective performance monitoring begins with adequate site characterization to provide a baseline for later comparison, and its objective is to evaluate PRB performance relative to design. Performance monitoring of PRBs includes the evaluation of physical, chemical, and mineralogic parameters over time. It should address verification of emplacement and be able to detect loss of reactivity, decrease in permeability, decrease in contaminant residence time in the reaction zone, and short-circuiting or leakage in the funnel walls. In addition to monitoring the contaminants of concern (COCs) and general water quality, the following are also recommended: contaminant degradation products, precipitates, hydrologic parameters, and geochemical indicator parameters. Understanding the mechanisms controlling contaminant transformation, destruction, or immobilization within the reaction zone is critical to interpretation of performance monitoring data, as these data provide insight into barrier functionality (taken from Powell et al. 1998).

Changes in permeability within a wall can divert groundwater flow laterally around the barrier. Groundwater flow direction also can be affected by changes in off-site activities, such as the addition of a public supply well. Monitoring wells should be placed at the ends of a wall to detect any possible contaminant plume bypassing. There is some evidence that barrier walls have the potential to create a downward gradient at the upgradient side that can result in contaminant plume underflow (ITRC 2005). ITRC (2005) recommends measuring vertical gradients along walls and also placing wells that are screened below the barrier base at the downgradient side of the wall to detect any plume underflow.

Monitoring wells placed upgradient provide information on the contaminant loading and initial geochemistry of the groundwater. Generally, wells also are placed within the reactive zone (ZVI barrier) to provide insight on the reactions that are occurring and under what conditions (e.g., oxidation-reduction potential and pH) as well as providing data for estimation of the barrier's longevity (Gavaskar et al. 2000). The data quality objectives should examine where in the wall the wells should be placed. Studies have shown (ITRC 2005) that most chemical and biological activity in the initial years of the wall's placement occur at its upgradient side.

Downgradient monitoring wells are used to judge the success of the wall in treating the contaminant plume. Wells located close to the wall provide an indication of contaminant destruction, while those located farther away provide insight into what the groundwater's equilibrium geochemistry will look like (e.g., any metals mobilized by the wall should be back to pre-wall concentrations).

Judging the performance of the treatment wall can be complicated if it is constructed in the middle of a plume rather than on the leading edge of the plume. In the latter case, concentrations of COCs should directly reflect the effect the wall is having on the plume; however, in the former case, there will be an initial baseline level of contaminants. While it is anticipated that this baseline will drop off with time, the potential for back diffusion into the treated water can keep COC concentrations in the groundwater downgradient of the wall at levels above the cleanup goals. If the wall is built in the plume, ITRC 2005 suggests placement of monitoring wells in or immediately behind the downgradient side of the PRB to monitor its cleanup effectiveness.

While usually not done as part of performance monitoring, microbial data have been collected for iron PRBs (ITRC 2005 and 2008), and the data might be useful for organic substrates. Microbial data that can be collected include lipid analyses, DNA analyses, and culture analyses.

Reactive-medium core sampling and analysis are specialized techniques that are not required at most PRB field sites; however, core analysis provides important geochemical information for evaluating the longevity of the reactive medium. If problems with field PRB performance relating either to hydraulics or to COC degradation are detected, it might be desirable to investigate the cause by examining the reactive medium directly (Gavaskar et al 2000). Boreholes should be backfilled with fresh reactive material. Recommended analyses and data use are found in Table 1.

Table 1. Recommended Characterization Techniques for Coring Samples

Analysis Method	Description
Total Carbon Analysis Combustion furnace used to quantify total organic and inorganic (carbonate) carbon	Quantitative determination of total carbon. Useful for determining fraction of carbonates in core profile.
Raman Spectroscopy Confocal imaging Raman microprobe	Semiquantitative characterization of amorphous and crystalline phases. Suitable for identifying iron oxides and hydroxides, sulfides, and carbonates.
Fourier Transform Infrared Spectroscopy (FTIR) FTIR coupled with auto-image microscopy	Attenuated total internal reflection (ATR) spectra are collected using a germanium internal reflection element.
Scanning Electron Microscopy Secondary electron images (SEI) Energy-dispersive spectroscopy (EDS)	High-resolution visual and elemental characterization of amorphous and crystalline phases. Useful for identifying morphology and composition of precipitates and corrosion materials.
X-Ray Diffraction (XRD) Powder diffraction	Qualitative determination of crystalline phases. Useful for identifying minerals such as carbonates, magnetite, and goethite.
Microbiological Analysis Heterotrophic plate count PLFA profiling	Identification of microbial populations within the cored material. Useful for determining the presence or absence of iron-oxidizing or sulfate-reducing bacteria.

Source: Gavaskar et al 2000

Table 2 contains an extensive list of parameters that appear in various guidance documents. For the first quarter, ITRC (2005) recommends a monthly monitoring frequency of all parameters selected for the site, and quarterly monitoring thereafter. Both ITRC (2005) and Gavaskar et al. (2000) emphasize that sampling frequency and monitoring parameters are chosen on a site-specific basis.

Performance Monitoring for Permeable Reactive Barriers

Performance Parameter¹	Method	Data Use	Performance Expectation	Recommended Frequency of Analysis²
COCs and potential degradation products	VOCs, EPA SW-846: 8260B (GC/MS), 8021B (GC) SVOCs, SW-846: 8270D GC methods are available for specific classes of SVOCs, e.g., SW-846: 8081B for organochlorine pesticides and 8082A for PCBs.	Determine the effectiveness of the barrier in meeting regulatory goals.	The COCs should be at or below regulatory levels after passing through the barrier³.	Initially site specific based on hydrogeology, type of PRB, and other conditions (ITRC 2005). Quarterly sampling is recommended after the effectiveness of the barrier is ascertained. This can be modified after monitoring shows that the system is operating consistently as designed (ITRC 2005).
Alkalinity	Colorimetric test kits	For a ZVI barrier, indicator parameter for presence of ZVI and the buffering capacity of the aquifer.	For ZVI barrier, alkalinity should drop unless the aquifer is highly buffered (ITRC 2005).	Site specific. Test kit can be used in the field.
pH	Hand-held meter, flow- through cell, down-hole electrode	For mulch or other carbon-based walls: biological processes are pH sensitive, and the ideal range of pH for dechlorinating bacteria is 5-9. Outside this range, biological activity is less likely to occur (ITRC 2008).	For mulch walls: pH levels within a range of 5-9 are desirable. pH <5 indicates that a buffering agent may be required to sustain high rates of anaerobic dechlorination. Desorption toward phase equilibrium is the basis of dissolved CAH rebound, which extends treatment duration (ITRC 2008).	Site specific. This is a field parameter and can be used to monitor water pH changes on a near-continuous basis if desired.
pH	Hand-held meter, flow- through cell, down-hole electrode	For ZVI walls, an indicator parameter for presence of ZVI/COC interaction (Gavaskar et al. 2005, Zhang 2003).	The interaction of COCs with ZVI will tend to raise the pH of the groundwater (Gavaskar et al. 2005a, Zhang 2003).
Temperature	Hand-held meter, flow- through cell, down-hole electrode	For mulch walls, affects the rate of biodegradation.	For mulch walls, biodegradation is slower at lower temperatures (ITRC 2005).	Site specific. This is a field parameter and can be used to monitor water temperature changes on a near-continuous basis if desired.
Temperature	Hand-held meter, flow- through cell, down-hole electrode	Affects the rate of degradation with ZVI.	Degradation is slower at lower temperatures (ITRC 2005)
Dissolved Oxygen (DO)	Flow-through cell, down-hole electrode Note: DO probes might not be effective when measuring conditions within the barrier reactive zone (Gavaskar, et al. 2000).	Indicator parameter for evidence of anaerobic microbial activity.	The presence of DO will inhibit the growth of anaerobic bacteria.	Site specific. This is a field parameter and can be used to monitor water DO changes on a near-continuous basis if desired.
		Measures potential for aerobic microbial activity.	The absence of DO will inhibit the growth of aerobic bacteria.
		Indicator parameter for presence of ZVI (ITRC 2005).	DO is highly reactive with ZVI. DO downgradient or within the reaction zone would show poor distribution of the ZVI (ITRC 2005).
Oxidation Reduction Potential (ORP)	Hand-held meter, flow- through cell, down-hole electrode	Indicator parameter for the potential for microbial growth.	A positive value would favor conditions for aerobic bacterial growth. A negative value would favor anaerobic bacterial growth.	Site specific. This is a field parameter and can be used to monitor water ORP changes on a near-continuous basis if desired.
Oxidation Reduction Potential (ORP)	Hand-held meter, flow- through cell, down-hole electrode	Indicator parameter for presence of active ZVI.	An abundance of ZVI will cause the ORP to fall dramatically. Gavaskar et al. (2005) theorize in their injection studies that a fall to -200mV indicated poor distribution or passivation of ZVI, while a fall to -400mV or more would indicate good distribution and most of the degradation would be abiotic.
Conductivity	Hand-held meter, flow- through cell, down-hole electrode	Indicator parameter for presence of ZVI.	Specific conductance should fall in the vicinity of ZVI (Wilkin and Puls 2003).	Site specific. A field parameter that can be used to monitor water conductivity changes on a near-continuous basis.
Sulfate	Portable Colorimeter Hach Method 8051, Laboratory EPA Method 300.0	Indicator parameter for biological reductive activity.	Sulfate will be reduced to sulfide by sulfate reducing bacteria. A sharp drop in sulfate indicates this activity is occurring (ITRC 2005). Also a drop in sulfate indicates that sulfate reducing conditions are present and this is necessary for the microbial reduction of vinyl chloride (ITRC 1998).	Site specific.
Sulfide	EPA SW-846: 9030B, 9031, and 9034 EPA Method 376.1 and 376.2	Indicator parameter for biological reductive activity. Also useful for geochemical modeling of potential precipitation products that can affect barrier performance (Powell et al. 1998).	Dissolved sulfide may exist within the barrier wall due to biological reduction of sulfate (ITRC 2005).	Site specific.
Methane	GC headspace method (EPA 2002).	Indicator that methanogenic anaerobic biodegradation is occurring. Also indicator that reductive biodegradation of halogenated methanes (when present) is occurring.	Methane can be produced by microbes in the active zone (ITRC 2005)	Site specific.
Ethane/Ethene	GC headspace method (EPA 2002).	Indicator parameters for abiotic and biotic degradation of halogenated ethenes and ethanes.	Ethenes and ethanes are end products of both abiotic and anaerobic degradation of halogenated ethenes and ethanes (ITRC 2005).	Site specific but would generally be done at the same time as the COC.
Total Organic Carbon/ Dissolved Organic Carbon	EPA Method 415.1, SW-846: 9060A	Indicator of natural organic carbon presence during baseline characterization. Some barriers are placed using a biopolymer (guar gum) to aid in keeping the trench open, which raises the organic carbon content and might initially affect barrier performance.	High background concentrations can affect degradation rate of VOCs when using iron (ITRC 2005).	Site specific.
Chloride (for chlorinated COCs)	Flow-through cell, down-hole instrument	Indicator parameter for presence of chlorinated COC remediation.	COC dechlorination should increase the concentration of chloride ions in the groundwater, but the increase may not be distinguishable from background chloride levels (Gavaskar et al. 2005a).	Site specific. This is a field parameter and can be used to monitor chloride changes on a near-continuous basis if desired.
Iron (Total, Dissolved)	SW-846 6010C Hach Colorimetric Meter or other colorimetric test kit	Indicator parameter for ZVI activity.	ZVI is reduced to ferrous ions by water, DO, and many halogenated organic compounds (Gavaskar et al. 2000 and 2005a). An increase in dissolved iron indicates this reaction is occurring.	Site specific. Barring site-specific interferences, the colorimetric meter will allow for field measurements.
Nitrate	Portable Colorimeter Hach Method 8039, Laboratory EPA Method 353.2	Indicator parameter for chemical reductive activity.	Nitrate will be reduced to ammonia/ammonium by ZVI. A sharp drop in nitrate in the target area indicates the iron is active (ITRC 2005).	Site specific. The Hach method can be done in the field..
Aluminum, Barium, Potassium, Sodium, Calcium, Magnesium, Manganese	EPA SW-846: 6010C	Determine the effect of the barrier on groundwater geochemistry.	In ZVI walls, changes in ORP and pH (production of OH^- ions), increases in dissolved iron, and increases in sulfide (reduction of sulfate) both within the barrier and downgradient can affect overall geochemical balances and barrier functioning (Gavaskar et al. 2000).	Site specific.
Dissolved Silica	EPA Method 370.1, SW-846: 6010C	Determine upgradient and downgradient levels of dissolved silica.	High levels of dissolved silica can have a passivating effect on iron barriers (ITRC 2005).	Site specific.
Total Dissolved Solids (TDS)	APHA et al.: 2540C Hand held meter	Determine TDS loading on barrier. TDS can also be used as a check to see if all major ions have been identified (Gavaskar et al. 2000).	For iron barriers, high TDS has been associated with reduced permeability of barrier materials and lower iron activity (ITRC 2005).	Site specific. Can be measured in the field.
Total Suspended Solids (TSS)	APHA et al.: 2540 D Hand-held meter	Determine TSS loading on barrier	For iron barriers, high TSS can affect performance.	Site specific. Can be measured in the field.
Bromide	Flow-through cell, down-hole instrument.	Tracer to measure flow paths through barrier (Gavaskar et al. 2000).	Bromide, a conservative tracer, is not usually found in nature in high concentrations and should move with the groundwater through the barrier.	Optional, site specific (Gavaskar et al. 2000).
Water Levels	Air/water interface probe.	Measure groundwater gradient.	An increasing gradient can indicate reduced permeability in the barrier.	Monthly in the initial monitoring period and quarterly thereafter (ITRC 2005).
Water Levels	Air/water interface probe.	Using multi-depth well clusters, measure vertical head differences along the upgradient and downgradient sides of the barrier.	In some PRBs, the construction could result in a higher vertical gradient with depth, with potential for water to bypass the upper part of the PRB and affect assessment of barrier performance (ITRC 2005).

¹ Parameters are those suggested in Gavaskar et al. 2000 and ITRC 2005.

² ITRC (2005) recommends a site-specific frequency for all parameters when determining the effects from PRB installation and construction, and quarterly for all parameters thereafter (initial and long-term monitoring). The quarterly monitoring might be reduced based on operational stability.

³ Note: Due to pre-existing plume levels, downgradient wells can take time to reflect the values of the water exiting the wall unless the barrier is placed in front of the leading edge of the plume.

References

Capstone Report on the Application, Monitoring, and Performance of Permeable Reactive Barriers for Ground-Water Remediation, Volume 1: Performance Evaluations at Two Sites
2003. R.T. Wilkin and R.W. Puls, U.S. EPA, National Risk Management Research Lab., Ada, OK. EPA 600-R-03-045a, 156 pp.

Capstone Report on the Application, Monitoring, and Performance of Permeable Reactive Barriers for Ground-Water Remediation, Volume 2: Long-Term Monitoring of PRBs — Soil and Ground Water Sampling
2003. C.J. Paul; M.S. McNeil; F.P. Beck, Jr.; P.J. Clark; R.T. Wilkin; R.W. Puls. EPA 600-R-03-045b, 145 pp.

Cost and Performance Report: Nanoscale Zero-Valent Iron Technologies for Source Remediation
A. Gavaskar, L. Tatar, and W. Condit.
Naval Facilities Engineering Service Center, Port Hueneme, CA. CR-05-007-ENV, 54 pp, 2005a

In Situ Bioremediation of Chlorinated Ethene: DNAPL Source Zones
Interstate Technology & Regulatory Council (ITRC), Bioremediation of DNAPLs Team. BioDNAPL-3, 138 pp, June 2008

Long-Term Performance Assessment of a Permeable Reactive Barrier at Former Naval Air Station Moffett Field
A. Gavaskar, W.S. Yoon, J. Sminchak, B. Sass, N. Gupta, J. Hicks, and V. Lal.
Naval Facilities Engineering Service Center, Port Hueneme, CA. CR-05-006-ENV, 37 pp, 2005b

Nanoscale Iron Particles for Environmental Remediation
Zhang, W. Journal of Nanoparticle Research 5: 323-32, 2003.

Permeable Reactive Barriers: Lessons Learned/New Directions
Interstate Technology & Regulatory Council (ITRC) Permeable Reactive Barriers Team.
PRB-4, 202 pp, 2005

Permeable Reactive Barrier Technologies for Contaminant Remediation
R.M. Powell, et al.
EPA 600-R-98-125, 102 pp, 1998

Standard Methods for the Examination of Water and Wastewater, 18th Edition
American Public Health Association (APHA), American Water Works Association, and Water Environment Federation, 1992 Standard Methods commercial Web site

SW-846 On-Line: Test Methods for Evaluating Solid Waste, Physical/Chemical Methods
U.S. EPA, Office of Resource Conservation and Recovery.

Technical and Regulatory Requirements for Enhanced In Situ Bioremediation of Chlorinated Solvents in Groundwater
Interstate Technology and Regulatory Cooperation (ITRC) In Situ Bioremediation Subgroup, ISB-6, 122 pp, 1998

Technical Guidance for the Natural Attenuation Indicators: Methane, Ethane, and Ethene
U.S. EPA Region 1, 18 pp, 2002

Other DNAPLs Treatment Technologies Topics: