Management of petroleum vapors may be required at sites where the results of the site investigation indicate that concentrations of PHCs in indoor air exceed mitigation action levels, or are likely to exceed screening levels in future buildings. PVI can be managed by environmental remediation, institutional controls (ICs), building mitigation, or any combination of these approaches. Traditional remedial technologies applied to the source may be sufficient to mitigate PVI for buildings located close to the source. ICs or building mitigation systems can also provide additional notification or protection from potential exposures while longer-term remedies are being developed and implemented. This chapter explains how to evaluate and select a strategy for mitigating PVI, as well as how to identify metrics for performance and closure strategies.
This chapter focuses on chronic exposures and design of an appropriate response. Some of the technologies discussed may be similar to those for emergency situations where the immediate or short term health and safety of the building occupants is the primary concern. Both short-term and long-term risks should be considered to determine the appropriate response action.
Stakeholders are often concerned about whether proposed approaches adequately address risk. Chapter 7, Community Engagement, provides information and tools for developing an appropriate plan to address these concerns. For further information on site management strategies for vapor intrusion in general, Appendix J provides an updated section for the remedial and vapor control technologies originally identified in Vapor Intrusion Pathway: A Practical Guideline (ITRC 2007).
Types of vapor control strategies for PHCs include the following:
Environmental remediation reduces or eliminates the exposure threat by removing contaminant mass from an environmental medium. Mitigation prevents or minimizes exposure by truncating the exposure pathway before vapors can enter a building. ICs are administrative and legal controls that help minimize the potential for human exposure and often protect the integrity of the site remedy. Typically, mitigation measures can be implemented quickly to lessen exposure, whereas remediation takes longer to achieve risk reduction. For this reason, mitigation may be coupled with remediation of the contaminant source. Mitigation measures may also allow immediate reduction of risk to human health prior to remedy selection. ICs are usually necessary while remediation is ongoing, and mitigation of the pathway is required.
For PVI, the interim response action may be the final response action, or the interim response may include a technology that can be rapidly implemented but that will not fully or permanently address the problem. Interim action may be needed quickly to protect human health and the environment from an imminent threat while a final remedial solution is being developed. Temporary interim measures can also stabilize the site or portion of a site (such as an operable unit) and prevent further migration or site degradation.
These vapor control strategies can also be used preemptively to mitigate vapors. When one or more lines of evidence indicate the potential for VI, preemptive action may be reasonable, especially for CVI sites (where preemptive mitigation may be less expensive than a lengthy VI evaluation). For PVI, however, biodegradation without any mitigation is likely to reduce the time needed to maintain the vapor control system or the IC. Also consider the future operation, maintenance, and monitoring requirements of any preemptive mitigation strategy.
At larger operating petroleum facilities, including refineries and terminals/depots, combinations of engineering and institutional (access) controls may already be in place and may be sufficient to address potential PVI issues. For example, in some buildings positive pressure can be induced in order to meet NFPA electrical requirements.
Effective VI remedial actions reduce exposures to vapors by lowering concentrations in the soil and groundwater to levels that no longer serve as a source of unacceptable vapor concentrations. Levels can be established using the approaches described in Chapter 5, Modeling, or using other state approved methodology or requirements.
Remedial technologies, such as soil vapor extraction (SVE), are typically used to reduce contaminant concentrations in site soils and soil gas. In some cases, these technologies can reverse the flow of vapors from migrating towards or into a structure which may potentially preclude indoor air sampling when the radius of influence can be demonstrated. For these situations, however, an IC may be required. Figure 6-1 shows a small-scale SVE system intended to intercept the vapors migrating to a structure. Remedial technologies such as in situ bioremediation or multiphase extraction can also reduce source contaminant concentrations.
Small-scale SVE system designed to address the source of vapors and protect the on-site building. The housing contains a small SVE blower that services SVE wells and was not designed to address the entire source of vapors.
Source: Vapor Mitigation Sciences, LLC.
In general, remedies that directly address the source of the vapors or remedies that facilitate the removal or redirection of the vapors are likely to have the greatest potential to reduce or eliminate VI. If implemented before the contaminated vapors have a chance to migrate to receptors, these remedies may also preempt mitigation actions or ICs.
For PVI, three general remedial approaches address volatilization to indoor air:
Remediation and site-wide remedies are not the focus of this document; however numerous other resources can help in selecting technologies for source control:
General information on how these remediation technologies can be applied at PVI sites is included in Appendix J, Vapor Intrusion Control.
ICs are non-engineered instruments, such as administrative and legal controls, that help minimize the potential for human exposure to contamination and protect the integrity of the site remedy. ICs are important because they limit land or resource use and guide human behavior at a site. These controls, however, also present significant drawbacks:
At undeveloped sites or at sites where an unrestrictive closure has not been obtained, ICs may also be necessary to address the potential for future exposures to occur. ICs at undeveloped sites can include mechanisms to require the preemptive installation of VI controls, such as vapor barriers or subslab depressurization systems, or additional investigations as part of any new construction or site modifications. This approach avoids some of the difficulties associated with attempting to predict the potential for VI prior to building construction (because soil gas concentrations and distributions might change as a result of construction of the building).
This document does not focus on ICs; however, several guidance documents are available that discuss the implementation of effective and reliable ICs. For more information on ICs, see An Overview of Land Use Control Management Systems (ITRC 2008) and other resources on the USEPA IC website (USEPA 2013n).
Building control technologies mitigate potential exposures by reducing or preventing vapors from entering a building—commonly referred to as mitigation. These building control technologies seal the building entry routes, treat the indoor air, or provide an alternate migration route outside the building envelope for vapors. For PVI, this approach may also consist of implementing smaller scale remediation technologies that are designed to have a small area of influence and may not address the full extent of affected soil and groundwater. Though several remedial technologies are available, SVE is one of the most common methods for small-scale applications.
ITRC’s guidance Vapor Intrusion Pathway: A Practical Guideline (ITRC 2007) describes many of these mitigation technologies. Appendix J, Vapor Intrusion Control, updates and expands the discussion of these mitigation systems. Additional technologies are provided, along with an updated summary listing the advantages and disadvantages of each. Table J-1 provides a summary of the common building control technologies implemented for PVI and the typical range of installed costs.
Building control technologies have been available for decades because of the concerns over exposure to radon. The application of this technology to the VI pathway has led to additional innovative approaches that must be evaluated when determining the appropriate design.
A number of factors may affect selection of the technology employed for the mitigation of petroleum vapors, including the following:
For more information on these factors and other details, see Appendix J.2. Some additional factors are unique to PVI sites, including:
The following design factors and potential limitations should be considered for the installation of vapor controls that are common to the mitigation of all vapors:
More information on these factors related to system design at VI sites is included in Appendix J.
An operation, maintenance, and monitoring (OM&M) plan should be prepared for each mitigation system that has been installed. The OM&M required for a system installed at a PVI site is typically similar to systems installed for other types of VI. Appendix J.5 discusses the various items that relate to the OM&M. Some aspects of OM&M for PVI sites, however, are different from other types of VI sites and should also be considered when developing an OM&M plan:
Remediation of the petroleum sources at most PVI sites will eventually reduce the concentrations of volatile petroleum compounds in soil or groundwater to values that are protective of human health. After appropriate levels are attained, mitigation systems may be shut down. It is typically prudent to collect confirmation samples to verify that the systems are no longer needed. Even without active remediation, most PHC sites are likely to see continued reductions in source concentrations because of naturally occurring biodegradation. This biodegradation will ultimately result in a reduction of PVI as source concentrations are depleted. With acceptable confirmation sampling results, long-term, vapor mitigation systems could be turned off and removed, depending on the preferences of the building owners and obligations of the responsible parties. Likewise, ICs could be updated or removed upon attainment of the remediation goals, as appropriate and as allowed by the local regulatory authority.
Early in the project, regulators and responsible parties for PVI sites should consider how to determine when vapor mitigation is no longer required. Typically, this determination is made by establishing remediation standards for various affected media, such as groundwater and indoor air. The decision to evaluate the shutdown of mitigation systems may be made when remediation goals for groundwater or soil vapor are attained. Tests to evaluate attainment should also be specified in planning documents. Because of the variability of VI and the many factors that affect it, each structure mitigated should be evaluated as part of the shutdown process.
In addition to remediation standards for groundwater, soil gas profiles of O2, CO2, and PHCs can generally be used to demonstrate sufficient biodegradation of PHCs, so that mitigation may no longer be necessary. Mitigation systems and remedial actions that have the potential to influence the flow of vapors must be shut down prior to collecting confirmation samples. Confirmation samples should not be collected immediately after system shutdown because of the potential for rebound of subslab vapor concentrations over time. The amount of time required for rebound, if any, depends on the vapor phase retardation factor of the compound, the effective diffusivity of the soil, and the square of the distance from the source (Johnson et al. 1999). For relatively shallow sources, rebound will likely occur within hours or days, while rebound may take years for sources that are several meters deep (Johnson et al. 1999). Considering that PVI is only likely to occur when sources are relatively shallow, testing after a period of about four weeks may be reasonable at most PVI sites. Another approach is to monitor subslab PID and O2 levels over time and to collect confirmation samples when these values reach an asymptote or are unchanging. Because of the potential for temporal variability and uncertainty regarding rebound time, at least one additional confirmation sample (for instance, during the following heating season in cooler clients) may be warranted.
Some building owners may choose to continue operation of their mitigation systems to provide radon control. The building owner would be responsible for mitigation system O&M if the system continued operation for radon control.
Appendix J provides additional detailed information that should be considered as part of the shutdown process.