Permeable Reactive Barriers, Permeable Treatment Zones, and Application of Zero-Valent Iron
Overview
A permeable reactive barrier (PRB) is a subsurface 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 (Figure 1) in which the treatment material is backfilled. The trench is perpendicular to and intersects the groundwater plume.
Figure 1. Elevation View of Trench Style PRB (left); Plan View of Trench Style PRB (right)
Figure 2. In Plan View of Funnel and Gate PRB.
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 2011).
Injection or fracturing techniques also have been used to construct subsurface PRBs. These techniques use permanent or temporary injection wells to place reactive materials such as ZVI, edible oils, or proprietary commercial products (Figure 3) into the subsurface. The injection points are spaced to provide overlapping radii of influence between them—forming a free standing treatment zone (Figure 4). The In Situ Chemical Reduction focus page contains more information on treatment zones.
Figure 3. Microscale ZVI in an Injectable EHC® Slurry.
Figure 4. Injection Well Configuration.
The majority of installed trench style PRBs use zero-valent iron (ZVI) as the reactive medium (Figure 5) for converting contaminants to non-toxic or immobile species. ZVI, a mild reductant, has the ability to reductively dehalogenate many halogenated hydrocarbons as well as remove hexavalent chromium, arsenic, and uranium. In laboratory experiments, Fiore and Zanetti (2009) researched the ability of ZVI to treat acid mine drainage water that contained among other ions Al, Ba, Cu, Cr, Fe, Mn, Pb, and Zn. All of these ions were removed in the ZVI column to levels below the appropriate Italian regulatory levels.
Figure 5. Coarse-Grained ZVI Being Placed
in a Trenched PRB.
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 than the parent compound. Even very low concentrations of undesirable byproducts in the reactive barrier effluent must be avoided (Powell 1998).
However, there are materials other than ZVI that can be used to treat contaminants. For example, petroleum hydrocarbon plumes such as those emanating from a creosote source and are not amenable to ZVI, can be treated using a biosparging or slow release oxygen compound containment wall. The aerobic condition created within the wall allows for biodegradation of the dissolved contaminants as they pass through it.
Mulch and other vegetative materials can be employed in traditional trench style PRBs to treat a number of contaminants. The Air Force biowall protocol (AFCEE 2008) provides guidance for the use of permeable mulch biowalls for remediation of chlorinated solvents, perchlorate, and energetics. A benefit of biological PRB systems over purely abiotic systems is that the treatment processes may extend downgradient of the constructed treatment zone due to migration of soluble organic carbon, enabling the effects of anaerobic degradation beyond the biowall. Another benefit is the ability of a single system to treat multiple contaminants with different chemical characteristics, including both organic compounds (e.g., CVOCs, energetic compounds) and inorganic compounds (e.g., nitrate, sulfate, perchlorate, and metals). However, the longevity of biowalls is anticipated to be shorter than that of ZVI walls, and replenishment of organic substrate may be required (ITRC 2011).
Table 1 provides a list of some common contaminants of concern and reactive materials that have been used to treat them.
| Contaminant | ZVI | Biobarriers | Apatite | Zeolite | Slag | ZVI-carbon combinations | Organophilic clay |
|---|---|---|---|---|---|---|---|
| Chlorinated ethenes, ethanes | F | F | L | F | |||
| Chlorinated methanes, propanes | F | ||||||
| Chlorinated pesticides | P | ||||||
| Freons | L | ||||||
| Nitrobenzene | P | ||||||
| Benzene, toluene, ethylbenzene, and xylenes | F | ||||||
| Polycyclic aromatic hydrocarbons | L | ||||||
| Energetics | P | F | P | ||||
| Perchlorate | F | F | L | L | |||
| Creosote | F | ||||||
| Cationic metals (e.g., Cu, Ni, Zn) | L | F | F | L | F | ||
| Arsenic | F | L | F | F | |||
| Chromium (VI) | F | L | L | F | |||
| Uranium | F | P | F | F | |||
| Strontium-90 | F | F | |||||
| Selenium | L | L | |||||
| Phosphate | F | ||||||
| Nitrate | F | F | F | ||||
| Ammonium | L | ||||||
| Sulfate | F | L | |||||
| Methyl tertiary butyl ether | F |
F=Full Scale Application, L=Laboratory Application, P=Pilot Scale Application
Source: ITRC 2011
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 include sequential treatments (or rely on a monitored natural attenuation [MNA] step) to address groundwater plumes that contain a mixture of contaminants. Sequenced reactive barriers have been constructed to treat various 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 an MNA step for the remaining 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 fieldwork may demonstrate that a passive PRB is not suitable for a particular site (Korte 2001).
Table 2 presents a discussion of site characteristics that are favorable or problematic to Reductive Based PRBs.
| Site Characteristic | Ideal Case for PRBs | Suitability Unclear—Requires Further Evaluation |
|---|---|---|
| Contaminant peak concentrations (chlorinated aliphatic hydrocarbons [CAHs] only) | CAH concentrations <10,000 µg/L, depending on media (e.g., ZVI more robust than some carbon based approaches) | Treat CAH concentrations >10,000 µg/L with caution. Mixed contaminant plumes require further evaluation to determine whether all contaminants can be degraded by one or more selected processes within the same wall or whether multiple walls will be necessary. |
| Evidence of anaerobic dechlorination (CAHs only) | Presence of dechlorination products | Limited evidence of anaerobic dechlorination. No evidence of any degradation of CAHs depending on the specific treatment media applied in the PRB. |
| Lithology | Cohesive silts and sands | Well consolidated or hard bedrock. Loose, flowing sands. |
| Stratigraphy | Optimal: PRB extends to a lower confining layer | Lack of a lower confining layer, but where the PRB may extend to the total depth of contamination. Lack of a lower confining layer and uncertainty about the total depth of contamination requires further evaluation. |
| Hydraulic conductivity (K) | <1.0 ft/day (<3.5 � 10-4 cm/sec) | 1.0-10 ft/day (3.5 � 10-4 to 3.5 � 10-3 cm/sec). |
| Groundwater velocity | <1.0 ft/day (generally but not in all cases) | 1.0-10 ft/day, >10 ft/day. |
| pH | 6.5-7.5 (neutral) | <6.0, >8.0. |
| Dissolved oxygen | <4.0 mg/L | >4.0 mg/L combined with a high rate of groundwater flow (>1.0 ft/day). |
| Sulfate concentration (CAHs) | <1,000 mg/L | >1,000 mg/L with caution, may be suitable for abiotic degradation processes. |
Source: ITRC 2011
References:
AFCEE. 2008. 
Technical Protocol for Enhanced Anaerobic Bioremediation Using Permeable Mulch Biowalls and Bioreactors.
This protocol describes the scientific and technical basis for use of enhanced in situ anaerobic bioremediation using permeable mulch biowalls and in situ bioreactors to promote the appropriate use of the technology. Guidance is provided on technology selection, site screening, design criteria, installation methods, performance monitoring, and data interpretation for the various engineered approaches currently being used.
DOE (Department of Energy). 2000. In Situ Redox Manipulation: Innovative Technology Summary Report DOE/EM-0499.
This report describes a demonstration of using sodium dithionite to create a reductive zone of ferrous ions in an iron rich subsurface that causes reduction of chromium VI to chromium III and its precititation.
Fiore, Silvia and Maria Zanetti. 2009. Preliminary Tests Concerning Zero-Valent Iron Efficiency in Inorganic Pollutants Remediation. American Journal of Environmental Sciences 5 (4), p 556-561, 2009.
ITRC (Interstate Technology and Regulatory Council). 1999. Regulatory Guidance for Permeable Reactive Barriers Designed to Remediate Inorganic and Radionuclide Contamination.
This guidance document was developed to address the regulatory requirements of permeable reactive barriers (PRB) and try to achieve a consensus on requirements. It should prove useful to regulators, stakeholders and technology implementers. The document is divided into sections dealing with site characterization, modeling, permitting, construction, monitoring, waste management, maintenance, closure, health and safety and stakeholder concerns.
ITRC. 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. 202 pp.
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.
ITRC. 2011. Permeable Reactive Barrier: Technology Update. 234 pp.
A comprehensive resource incorporating elements from previous documents and providing updates on additional types of reactive media, treatable contaminants, longevity issues, and new construction/installation approaches and technologies.
Korte, N.E. 2001. Zero-Valent Iron Permeable Reactive Barriers: A Review of Performance. Oak Ridge National Lab., Oak Ridge, TN. Report No: ORNL/TM-2000/345, 36 pp
This report briefly reviews issues regarding the implementation of the zero-valent iron permeable reactive barrier (PRB) technology at sites managed by the U.S. Department of Energy (DOE). The purpose of this report is to suggest reasons for the problems that have been encountered and to recommend whether DOE should invest in additional research and deployments. The principal conclusion of this review is that the most significant problems have been the result of insufficient characterization, which resulted in poor engineering implementation.
Powell, R.M., et al. 1998. Permeable Reactive Barrier Technologies for Contaminant Remediation. EPA 600-R-98-125, 102 pp.
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.
Additional Information
A Review on the Use of Permeable Reactive Barriers as an Effective Technique for Groundwater Remediation
Sakr, M., H. El Agamawi, H. Klammler, and M.M. Mohamed.
Groundwater for Sustainable Development 21:100914(2023)
This study provides a comprehensive review of the permeable reactive barrier (PRB) technique for groundwater remediation for a wide range of contaminants. The fundamentals of installation, including site selection and design are described. Different PRB designs are discussed, including the funnel and gate, continuous trench, and sequential configurations. The article also covers different methods for PRB optimization to achieve maximum removal rates of contaminants.
Community Guide to Permeable Reactive Barriers
EPA 542-F-21-019, 2021
The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? 6) Why use it?
Handbook of Groundwater Remediation Using Permeable Reactive Barriers: Applications to Radionuclides, Trace Metals, and Nutrients
Naftz, David, et al. (eds.). Academic Press, San Diego, CA. ISBN: 0125135637, 550 pp, 2002
This handbook offers numerous case studies to introduce the reader to current applications, innovations, and methods for using PRBs in the removal of inorganic contaminants from groundwater.
Permeable Mulch Biowalls
U.S. Navy, Naval Facilities Engineering Command, Environmental Restoration Technology Transfer, Multimedia Training Tools website, May 2007
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
Powell, R.M., 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 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.
Permeable Reactive Subsurface Barriers for the Interception and Remediation of Chlorinated Hydrocarbon and Chromium (VI) Plumes in Ground Water
EPA 600-F-97-008, 1997
Prepared by EPA's ORD, the document discusses the use of barrier walls employing zero-valent iron as the reactive substrate for treating groundwater contaminated with chlorinated hydrocarbons or chromium.
