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Technology Overview as part of a Web-based Technical and Regulatory Guidance

Permeable Reactive Barrier Systems

1. Introduction
Click Here to view case study table at the end of this document.
In the broadest sense, a permeable reactive barrier (PRB) is a continuous, in situ permeable treatment zone designed to intercept and remediate a contaminant plume. The treatment zone may be created directly using reactive materials such as iron or indirectly using materials designed to stimulate secondary processes, such as by adding carbon substrate and nutrients to enhance microbial activity. In this way, contaminant treatment may occur through physical, chemical, or biological processes. With most PRBs, the reactive material is in direct contact with the surrounding aquifer material. The term “barrier” is intended to convey the idea of a barrier to contaminants but not to groundwater flow. PRBs are designed to be more permeable than the surrounding aquifer materials so that contaminants are treated as groundwater readily flows through without significantly altering groundwater hydrogeology (ITRC 2005).

2. Application
PRBs are often intended as a source management remedy or as an on-site containment remedy. Therefore, PRBs may be designed with different site-specific objectives in mind. For example, a PRB installed near the downgradient site boundary may be designed to protect downgradient properties or receptors such as surface waters or potable wells and reduce contaminants in groundwater to specific numerical objectives. Alternatively, a PRB installed near the source may be designed to reduce mass flux by a given percent, leaving monitored natural attenuation or some other remedy to treat the residual downgradient contamination (ITRC 2005). See Figures 1‑1 and 1-2.

Image-Permeable Reactive Barrier Systems Figure 1-1
Figure 1-1. Dimensions of a permeable reactive barrier. (ITRC 2005)
Image-Permeable Reactive Barrier Systems Figure 1-2
Figure 1-2. Examples of permeable reactive barriers. (ITRC 2005)

3. Advantages
Advantages of using a permeable reactive barrier include the following:

4. Limitations
Using a permeable reactive barrier may have the following limitations:

5. Performance
Hydraulic, geochemical, and microbial assessment of the PRB is all part of the performance assessment of the PRB system. Evaluation of the longevity of a PRB system has been examined using long-term column tests. Early systems predicted decades before the PRBs will lose reactivity (ITRC 2005). Depending on several site-specific conditions, PRBs are now expected to last 10–30 years before reactivity or hydraulic issues will result in the need for maintenance.

6. Costs
The costs of PRB systems are comparable to those of other technologies. While not as cost-effective as groundwater remedies like monitored natural attenuation or bioremediation, PRBs compare favorably to groundwater pump-and-treat systems. Since PRBs provide a mostly passive remediation technology, cost reductions can be found in the operation and maintenance of the system. ITRC (2005) provides site-specific examples of PRB system costs.

7. Regulatory Considerations
Regulatory permits are not specifically required for the operation of a PRB. However, one or more permits may be necessary for the design, construction, monitoring, or closure of a PRB treatment system, to the extent that the activity affects surface water, air, or groundwater quality or involves the management of regulated waste.

Following is a list of key potential regulatory permits that may be required for a PRB:

8. Stakeholders Considerations
Stakeholders have previously expressed the following concerns regarding proposed PRBs:

Special attention should be paid when the PRB is used for radionuclides. Communities are concerned about concentrating radionuclides in underground walls if they are long-lived or are gamma emitters.

9. Lessons Learned
PRBs using zero-valent iron (ZVI) have been operating in the United States since 1994. Considerable information has been collected on the performance of PRBs since that time. Although not all the design and performance issues are perfectly understood, this technology has grown significantly since the early days. ZVI, the most common reactive media for PRBs, is used mainly to treat chlorinated solvents although it has application to other contaminants. See ITRC 2005, Section 2.4 for a more thorough description and list of metals that can be treated with iron-based treatment media.

Other reactive media, such as limestone, compost, zeolites, granular activated carbon, apatite, and others, have also been employed in PRBs in recent years and offer treatment options for controlling pH, metals, and radionuclides (ITRC 2005). This technology has now been applied at more than 200 sites worldwide, including 72 full-scale installations to treat chlorinated solvent compounds. The vast majority of these PRBs are operating as intended. System hydraulics continues to be the main cause of inadequate performance. Ongoing refinements and improvements to construction methods are minimizing adverse impacts due to PRB construction.

10. Case Studies

Table 10-1. Case studies including PRB treatment

Nickel Rim Mine Site, Sudbury, Ontario, Canada (ITRC PRB-4, 2005)
Monticello Mill Tailings Site, Monticello, Utah (ITRC PRB-4, 2005)
Permeable Reactive Wall Treatment of Acid Mine Leachate at the Basin Luttrell Pit, Ten Mile Creek Site, Lewis and Clark County, Montana (R. Semenak and N. Kinghan, Kemron; G. Powell and S. Way, EPA)

11. References
ITRC (Interstate Technology & Regulatory Council). 1999. Regulatory Guidance for Permeable Reactive Barriers Designed to Remediate Inorganics and Radionuclide Contamination. PRB‑3. Washington, D.C.: Interstate Technology & Regulatory Council, Permeable Reactive Barriers Team.

ITRC. 2005. Permeable Reactive Barriers: Lessons Learned/New Directions. PRB-4. Washington, D.C.: Interstate Technology & Regulatory Council, Permeable Reactive Barriers Team.

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