Permeable Reactive Barriers, Permeable Treatment Zones, and Application of Zero-Valent Iron
Guidance
Considerations for the Design of Organic Mulch Permeable Reactive Barriers
Ahmad, Farrukh, T. McGuire, R. Lee, and E. Becvar
Remediation Winter 2007
This paper discusses two technical considerations in the design and performance of mulch PRBs: (1) hydraulic characteristics of the mulch bed and (2) biochemical characteristics of different types of organic amendments used as mulch PRB fill materials. A transport model that can be used to estimate the required PRB thickness to attain cleanup standards also is presented.
Delivery and Mixing in the Subsurface: Processes and Design Principles for In Situ Remediation
Kitanidis, P.K. and P.L. McCarty (eds.).
Springer, New York. ISBN: 978-1-4614-2238-9. SERDP-ESTCP Environmental Remediation Technology, Vol. 4, 325 pp, 2012
This technology monograph describes the principles of chemical delivery and mixing systems and their design and implementation for effective in situ remediation. In situ technologies discussed include chemical oxidation, surfactant/cosolvent flushing, subsurface reactors, recirculation systems, PRBs, gas delivery via sparging, and intrinsic remediation in natural-gradient systems. Numerous case studies are provided. Table of contents and abstracts.
Design Guidance for Application of Permeable Barriers to Remediate Dissolved Chlorinated Solvents
Gavaskar, A., et al., USACE/USAF. DG 1110-345-117, AL/EQ-TR-1997-0014, 192 pp, 1997.
This design guidance addresses treatability testing, design, installation, and monitoring of barrier technologies in variable geologic settings. The document points out the important considerations and various available options applicable to permeable barriers that should be taken into account during design, implementation, and monitoring.
Economic Analysis of the Implementation of Permeable Reactive Barriers for Remediation of Contaminated Ground Water
Powell, R.M., P.D. Powell, R.W. Puls. EPA 600-R-02-034, 42 pp, 2002
This report presents an analysis of the cost of using permeable reactive barriers to remediate contaminated groundwater. When possible, these costs are compared with the cost of pump-and-treat technology for similar situations. PRB cost information was obtained from a variety of sources, including reports, surveys, and interviews. Costs were broken out into four general categories: site characterization, design, construction, and operation and maintenance. Cost comparisons indicate that, depending upon the situation, implementing a PRB can either be more or less expensive than a P&T in terms of capital expenditures, but that routine operation and maintenance costs favor the PRBs.
Evaluating the Longevity and Hydraulic Performance of Permeable Reactive Barriers at Department of Defense Sites. ESTCP Cost and Performance Report
U.S. DoD, Environmental Security Technology Certification Program, Project CU-9907, 69 pp, 2003.
This report describes the evaluation of short- and long-term performance issues associated with permeable reactive barriers (PRBs) installed at several DoD sites. Regulatory interest in this project is driven by the two challenges involved in implementing PRBs--their longevity and hydraulic performance. The longevity evaluation focused primarily on the PRBs at former Naval Air Station (NAS) Moffett Field and former Lowry Air Force Base (AFB) for two reasons: the PRBs there had sufficient history of field operation and the groundwater at these sites had moderate-to-high levels of total dissolved solids, an important factor in precipitation processes that affect longevity. The hydraulic performance evaluation focused primarily on the PRBs at former NAS Moffett Field (funnel and gate), former Lowry AFB (funnel and gate), Seneca Army Depot (continuous reactive barrier), and Dover AFB (funnel with two gates). A present value comparison of the costs of a PRB and an equivalent pump-and-treat system at various sites has shown that it takes approximately 7 to 10 years to obtain a payback on the initial capital investment in a PRB. The longevity evaluation provides some reassurance that, at many sites, the useful life of zero-valent iron PRBs will exceed the projected payback period.
Final Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation
Gavaskar, A., N. Gupta, B. Sass, R. Janosy, and J. Hicks.
Strategic Environmental Research and Development Program (SERDP), Arlington, VA. 247 pp, 2000
This document is an update to the February 1997 version of a design guidance document for the proposed use of site managers, contractors, and regulators. The design guidance covers permeable barrier application to sites with contaminated groundwater. This update includes the lessons learned during the design, construction, and monitoring of a permeable reactive barrier at Dover AFB, DE, as well as progress made at other sites. It also includes the advances in the technology since the completion of the previous version of the report.
Guidance Document: Prediction of Groundwater Quality Improvement Down-Gradient of In Situ Permeable Treatment Barriers and Fully Remediated Source Zones
Johnson, P.C., P. Dahlen, and P.M. Carlson
Environmental Security Technology Certification Program (ESTCP), Project ER-0320, 127 pp, 2008
In situ permeable treatment barriers (more commonly referred to as a permeable reactive barriers, or PRBs) are designed such that contaminated ground water flows through an engineered treatment zone within which contaminants are eliminated or the concentrations are reduced significantly. This project developed a practicable approach to project reasonable order-of-magnitude estimates of ground-water quality improvements with time downgradient of a PRB and used the approach while conducting detailed monitoring and characterization downgradient of a well-understood PRB site, a full-scale biobarrier to address MTBE at the Naval Base Ventura County. Additional information: DGCHANGE Estimation Tool and User's Guide
Performance of Field-Scale Permeable Reactive Barriers: An Overview on Potentials and Possible Implications for In-Situ Groundwater Remediation Applications
Singh, R., S. Chakma, and V. Birke.
Science of The Total Environment [Published 16 September 2022 before print]
This study presents a comprehensive overview of field-scale PRBs applications and their long-term performance after onsite emplacements. In-depth insight into PRBs as a potential passive remedial measure, covering all significant dimensions for eliminating the contaminated plume over a long time in the subsurface, is provided. The overview will allow stakeholders to understand the implications of PRB's field-scale application and undertake the required measures before its onsite application to avoid any potential failure. Longer abstract
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.
Permeable Reactive Barriers: Lessons Learned/New Directions
Interstate Technology and Regulatory Council (ITRC). 202 pp. 2005.
The document compiled the information and data on permeable reactive barriers (PRBs) that have been generated over the last 10 years of technology development and research, and provides information on non iron-based reactive media that can be used in PRBs. This document also provides an update on a developing technology somewhat related to PRBs in which source zone contamination is treated with iron-based reactive media.
Technical Protocol for Enhanced Anaerobic Bioremediation Using Permeable Mulch Biowalls and Bioreactors
AFCEE, 302 pp, 2008
Biowall substrates are typically low-cost materials (mulch, compost). The substrates are mixed with common construction materials (sand, gravel) to prevent compaction and maintain permeability. Amendments can be added to stimulate both biotic and abiotic degradation processes, based on the type of contaminant(s) present and the desired degradation pathway(s) to be stimulated. The technology can be applied in source areas or use groundwater recirculation to capture deeper plumes in an in situ bioreactor configuration.
Treatability Bulletin: Permeable Reactive Barriers
CL:AIRE (Contaminated Land: Applications in Real Environments), 2011
CL:AIRE Treatability Bulletins describe the key factors to be considered in the early stages of designing a remediation project. Treatability studies provide a means of determining, through laboratory- or pilot-scale tests, the practicability and likely effectiveness of remediation, and can be an essential part of a remediation options appraisal.
Performance Monitoring
Monitoring of conditions in and around PRBs generally begins once installation is complete. Typically, monitoring in one form or another is necessary as long as the groundwater contaminants pose a significant concern. The key lines of evidence targeted by most monitoring programs include contaminants of concern (COCs) and their breakdown products (if any), hydraulic flow characteristics through and around the PRB, and groundwater geochemistry (ITRC 2011).
Most PRBs are operated as passive or low-maintenance systems, with performance monitoring typically conducted on a quarterly to annual basis. For biological PRBs, microbial growth and acclimation within the PRB may take 6 to 12 months or more for the system to achieve optimal performance. Frequent sampling at periods of less than a few months may yield unsatisfactory early results and result in an unjustified lack of confidence in the effectiveness of the system (AFCEE 2008 and ITRC 2011).
Table 1 provides a list of parameters that can be used to evaluate the performance of a PRB and the rationale for examining them. A number of these parameters are specific to biotic or abiotic walls and their use is site specific.
| Parameter | Rationale |
|---|---|
| Primary contaminants | Reductions in contaminant concentrations are the primary measure of performance. Many compounds have regulated intermediate products that also are analyzed for. |
| Dissolved oxygen | Determine whether aerobic or anaerobic conditions exist. DO values <0.5 mg/L generally indicate an anaerobic pathway is possible. |
| Oxidation-reduction potential (ORP or Eh) | The ORP of groundwater reflects the relative oxidizing or reducing nature of the aquifer and the treatment zone. Indicator parameter for active ZVI (ITRC 2005). |
| pH | Aerobic and anaerobic processes are pH sensitive. For ZVI walls an indicator parameter for presence of ZVI/CoC interaction (Gavaskar et al. 2005, Zhang 2003). |
| Temperature and conductivity | Primarily used as well-purging parameters. The rates of both biological and chemical reactions (e.g., ZVI) are temperature dependent, and high conductivity may be an indication of high salinity, which may impact chemical precipitation or inhibit biological processes. |
Terminal electron-accepting processes (TEAPs)
|
Primarily for biological PRBs, as indicators of the predominant microbial processes that are occurring. Measures either the production of reduced species (Mn[II] and Fe+2), the reduction of oxidized species (nitrate, sulfate), or the production of CH4. ZVI is reduced to ferrous ions by water, DO, and many halogenated organic compounds (Gavaskar et al. 2000 and 2005). An increase in dissolved iron indicates this reaction is occurring. |
| Major cations (e.g., Al, Ba, Fe, Mn, Ca, Mg, Na, K) | Some metals may be more mobile under highly reducing conditions. May be required for compliance with secondary water-quality standards. May be used for geochemical modeling. Persistence of Fe+2 downgradient may be a concern for ZVI PRBs in settings close to surface water discharge. |
| Major anions (e.g., HS-, Cl-, NO2-, NO3-, SO4-2, PO4-3, CO3-2) | Can be used for geochemical modeling or to evaluate the potential for precipitation of minerals that may inhibit the reactivity of ZVI media. |
| Sulfide | By-product of sulfate reduction. Elevated concentrations of sulfide may inhibit dechlorinating microorganisms and may pose taste and odor problems. If the plume being treated has metals as well as organics the presence of sulfides may aid in precipitating them as metal sulfides. |
| Alkalinity | For biowalls, an indicator of biodegradation and buffering capacity of the aquifer. For ZVI barrier, an indicator of the presence of ZVI and the buffering capacity of the aquifer (ITRC 2005). |
| Carbon dioxide | By-product of both aerobic and anaerobic biodegradation processes. |
| Nitrite (NO2-) | By-product of denitrification of nitrate. |
| Chloride | General water-quality parameter in high CVOC concentration settings may be used to evaluate mass balance. |
| Hardness and total dissolved solids | General water-quality parameter; used as an indication of secondary water quality. |
| Dissolved organic carbon or total organic carbon | Indication of organic substrate available for biological metabolism. For biological PRBs, declining TOC levels in conjunction with elevated levels of contaminants may indicate additional substrate is required to sustain the treatment zone. May indicate the persistence of guar residuals if these methods used or ZVI PRB construction. |
| Total inorganic carbon (TIC) | TIC includes aqueous CO2, carbonic acid, and total carbonate alkalinity. The distribution is a function of pH and an increase in TIC relative to background concentrations indicates zones with increased microbial activity. |
| Biological and chemical oxygen demand | Secondary water-quality parameters that may also be used as an indication of substrate demand. Alternate to TOC/DOC analyses. |
| Volatile fatty acids | Biodegradation breakdown products and fermentation substrates. Indicator of substrate distribution. |
| Phospholipid fatty acids | Indicator of bioactivity, measure of biomass, and characterization of the microbial community. |
| Nitrogen, phosphate, and potassium | Nutrients needed for microbial growth. May be needed as a substrate amendment for biological PRBs. |
| Ethane and ethene | The presence of ethane and ethene are indicative of reductive dechlorination of chlorinated solvents. Note that both are readily degraded by bacteria and their low concentrations or absence does not necessarily mean incomplete dechlorination (Scott et al. 2012). |
| Sodium bromide or sodium iodide | Conservative groundwater tracers for testing flow through, under, or around a PRB. |
| Water levels | Measure groundwater gradient for flow direction determination. This can also be done using flowmeters. When approaching the wall, an increasing gradient can indicate reduced permeability in the barrier. 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). |
Adapted from ITRC 2011
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 are backfilled with fresh reactive material. Recommended analyses and data use are found in Table 2.
| 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
References:
AFCEE. 2008. Technical Protocol for Enhanced Anaerobic Bioremediation Using Permeable Mulch Biowalls and Bioreactors. 302 pp.
Gavaskar, A., N. Gupta, B. Sass, R. Janosy, and J. Hicks. 2000. Final Design Guidance for Application of Permeable Reactive Barriers for Groundwater Remediation. Strategic Environmental Research and Development Program (SERDP), Arlington, VA. 247 pp, 2000.
Gavaskar, A., L. Tatar, and W. Condit. 2005. Cost and Performance Report: Nanoscale Zero-Valent Iron Technologies for Source Remediation. Naval Facilities Engineering Service Center, Port Hueneme, CA. CR-05-007-ENV, 54 pp, 2005.
ITRC (Interstate Technology and Regulatory Council). 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. 202 pp.
ITRC. 2011. Permeable Reactive Barrier: Technology Update. 234 pp.
Scott O. C. et al. 2012. Monitoring Biodegradation of Ethene and Bioremediation of Chlorinated Ethenes at a Contaminated Site Using Compound-Specific Isotope Analysis (CSIA). Environ. Sci. Technol. 46, 1731-1738.
Zhang, W. 2003. Nanoscale Iron Particles for Environmental Remediation Journal of Nanoparticle Research 5: 323-32
