Abs: 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.
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.
Vapor Control Strategies for Petroleum Hydrocarbons
Types of vapor control strategies for PHCs include the following:
- environmental remediation
- institutional controls
- mitigation
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.
Remediation to Reduce or Eliminate Petroleum Vapors
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. Remedial technologies, such as soil vapor extraction (SVE), are typically used to reduce contaminant concentrations in site soils and soil gas. Figure 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.
Figure 1. 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.
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:
- A remedial technology is implemented sitewide to address the source of vapors, as well as any vapors migrating into or towards a building.
- A remedial technology is designed and implemented on a smaller scale to address a specific route of entry (such as a preferential pathway) or a specific structure.
- A remedial technology is implemented quickly enough (for instance, source removal through dig and haul) that mitigation is not warranted and no emergency response is required.
Institutional Controls
ICs are non-engineered instruments 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:
- ICs can be difficult to implement and enforce over time.
- An IC may be difficult to identify and not immediately apparent, especially ICs that establish building type and occupancy or prohibit activities on all or part of the property.
- An IC may limit or prevent future development activities.
- Some states or parties do not have adequate statutory authority to implement ICs.
- In most states, ICs can be put in place on either an interim or permanent basis to protect human health while the longer term site-wide remedies are being developed and implemented or to sever the inhalation pathway without any actual remediation occurring.
Mitigation Using Building Control Technologies
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.
Vapor Control Designs
Factors for Selecting Vapor Control Technologies
A number of factors may affect selection of the technology employed for the mitigation of petroleum vapors, including the following:
- new versus existing buildings
- building size
- foundation type and condition
- soil conditions
- high water conditions
- presence of sumps and floor and footing drains
- COCs. Because PHCs (as well as methane that is produced during biodegradation) can approach combustible concentrations, intrinsically safe blowers, wiring, and monitoring systems should always be evaluated for use. Additionally, petroleum vapors may cause degradation of membranes, pipes, or the solvents used to join pipes, so use care in selecting compatible materials—especially for systems that will be used for a long time.
- Location of vapor source. The location of the vapor source plays a key role in the design of a mitigation system. At sites where PVI has been confirmed, the source of the vapors is likely to be near, beneath, and possibly in direct contact with a structure.
- Influence of O2 in the subsurface. Technologies that enhance O2 levels, such as SSV or aerated floors, may help promote biodegradation in underlying soils.
- Common background sources of petroleum compounds. Because of the prevalence of background sources or levels of PHCs in indoor air, demonstrating mitigation performance through indoor air testing may be even more difficult than VI in general.
- Sealing the subsurface without providing venting. For other VI scenarios, often one of the first steps is to seal any subsurface cracks or features. Though this practice is usually appropriate to prevent vapors from entering into a structure (and must be considered), sealing can limit O2 entering into the subsurface, which can affect the rate at which biodegradation occurs.
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:
- use of intrinsically safe equipment because of the potential presence of explosive levels of vapors and methane that may accumulate
- design approach and the level of detail necessary to complete the design
- need for stepped mitigation or future system modification and optimization
- requirements for discharge permits and emission controls
- presence of other chemical compounds
- preferences of the owner or tenant
- limitations on the design and installation of the controls
- constraints on collecting representative samples
- limitations on the placement of necessary system components, such as piping runs and vents
- constraints related to obtaining access and scheduling time for installation and assessment
- presence of lead-based paint or asbestos
- HVAC issues, including altering building pressures
- reliability and life cycle costs of the system to remain effective over time
Operation, Maintenance, and Monitoring
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. 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:
- Monitoring parameters. A mitigation system for PVI may need to monitor for O2 and CO2, as well as CH4 and other potentially explosive gases, especially if the system was not designed to address the presence of explosive conditions.
- Operating period. For many sites, especially those that are undergoing remediation, the mechanisms being used to mitigate vapors will not be maintained or need to be operated over extended periods of time. Therefore, the evaluation of the lifetime average of various parts and components similar to those that ASTM (2005) discusses may require a different approach.
Closure for PVI Buildings and ICs
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. 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.
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. For relatively shallow sources, rebound will likely occur within hours or days, while rebound may take years for sources that are several meters deep. 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.