{"id":9470,"date":"2018-07-16T00:40:02","date_gmt":"2018-07-15T16:40:02","guid":{"rendered":"https:\/\/www.envguide.com\/?p=9470"},"modified":"2018-07-25T01:28:28","modified_gmt":"2018-07-24T17:28:28","slug":"vapor-control-and-site-management","status":"publish","type":"post","link":"https:\/\/us.envguide.com\/vapor-control-and-site-management\/","title":{"rendered":"Vapor Control and Site Management"},"content":{"rendered":"
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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.<\/p>\n
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.<\/p>\n
Vapor Control Strategies for Petroleum Hydrocarbons<\/p>\n\t\t\t\t\t<\/div>\n
Types of vapor control strategies for PHCs include the following:<\/p>\n\t\t\t\t\t<\/div>\n\t \n\t\t
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.<\/p>\n
Remediation to Reduce or Eliminate Petroleum Vapors<\/p>\n\t\t\t\t\t<\/div>\n
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.<\/p>\n
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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.<\/strong><\/p>\n 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.<\/p>\n For PVI, three general remedial approaches address volatilization to indoor air:<\/p>\n\t\t\t\t\t<\/div>\n\t \n\t\t Institutional Controls<\/p>\n\t\t\t\t\t<\/div>\n 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: Mitigation Using Building Control Technologies<\/u><\/strong><\/p>\n Building control technologies mitigate potential exposures by reducing or preventing vapors from entering a building\u2014commonly 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.<\/p>\n Vapor Control Designs<\/p>\n\t\t\t\t\t<\/div>\n Factors for Selecting Vapor Control Technologies<\/u><\/strong><\/p>\n A number of factors may affect selection of the technology employed for the mitigation of petroleum vapors, including the following: 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: Operation, Maintenance, and Monitoring<\/p>\n\t\t\t\t\t<\/div>\n 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: Closure for PVI Buildings and ICs<\/p>\n\t\t\t\t\t<\/div>\n 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n","protected":false},"excerpt":{"rendered":"\n
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