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Mitigation Measures for Vapor Intrusion

So you have completed a soil vapor survey and indoor air sampling and have determined that volatile organic compounds (VOCs) are present at concentrations in subsurface soil gas and/or indoor air that exceed applicable screening levels and indicate there is a vapor intrusion (VI) risk to existing buildings or future planned buildings. Now you need to determine what options are available to mitigate those risks.

The goal of a VI mitigation system is to successfully mitigate the intrusion of subsurface VOCs to indoor air and prevent human exposure at unacceptable levels. VI mitigation systems are implemented to reduce contaminant entry into existing buildings or planned future buildings until the subsurface contamination is remediated or no longer poses a significant risk to human health. Remediation and mitigation are complimentary components of a VOC response action, addressing both cleanup of subsurface contaminants and protecting current building occupants, respectively. VI mitigation systems and measures are not designed to remove the VOC source mass of the subsurface contamination; this would need to be addressed through separate remedial measures such as soil vapor extraction to remove the source mass. Without mitigating the subsurface contamination, the VI mitigation measures would potentially need to be implemented indefinitely. The VI mitigation measures will prevent human exposure at unacceptable concentrations of contaminates by eliminating the VI pathway and removing low volumes of contaminated vapors.

If the measured or predicted chemical concentration in indoor air, as a result of VI, are estimated to pose a potential risk to human health above 1 x 10-4 (or one in a thousand), both source remediation and VI mitigation may be required. The timing of this response action will depend on whether the VI risk is to an existing building or if future development will proceed prior to remedial goals being achieved.

The air pressure within a building is typically less than the atmospheric pressure surrounding the building. This difference in pressure is caused by thermal differences between indoor air and surrounding soils. Thus, the negative pressure differential typically present in most buildings may cause vapor-phase contaminants in the subsurface to migrate into the building, and it is this pathway that needs to be interrupted. VOCs can enter a building through multiple entry points including cracks or perforations in slabs, openings around sump pumps, elevator shafts, or at utility penetrations through the slab into the structure.

Vapor Intrusion Mitigation Measures

Although many VI mitigation methods are available, the two most commonly accepted mitigation techniques are systems that dilute contamination by ventilation: (1) sub-slab ventilation systems (SSV systems) to stimulate air movement in the shallow soils immediately below a building and (2) sub-slab depressurization systems (SSD systems) that reduce contamination by lowering subsurface pressures.

SSV Systems

An SSV system is typically designed to function by venting sub-slab soil gases or providing a pathway to allow soil gas to migrate to the exterior of the building. SSV systems operate by drawing in outside air to the sub-slab area, which dilutes and reduces VOC concentrations. SSV systems typically consist of a layer of venting material (sand or pea gravel) emplaced below a floor slab to allow soil gas to move laterally under natural diffusion or pressure gradients to a collection piping system for discharge to the atmosphere above the building. SSV systems include a sub-slab liner installed on top of the venting layer. The sub-slab liner aids venting of sub-slab soil gas via the collection pipes rather than upward into the building through entry points discussed above. Usually the collection pipes are connected to a main header that runs up through or along the inner or outer building wall and exhausts above the roofline. Installation of a vertical inlet pipe system within or next to the building allows fresh air to enter into the sub-slab zone, which results in diluting or reducing the VOC concentrations beneath the building.

Because of the extensive foundation work involved in the installation, the SSV systems are generally easier to install in new construction rather than existing buildings. Also, SSV systems may not be appropriate in areas with a high groundwater table or surface drainage problems because the venting system will not function properly if continuously saturated with water.

SSV systems are monitored by measuring VOC concentrations in sub-slab soil gas or by measuring concentrations of indoor air. SSV systems can be designed as passive or active (blower or fan) systems. SSV systems may result in less depressurization and lower air flow rates than SSD systems, and typically do not perform as well as SSD systems; therefore, they are often not appropriate in areas of high concentrations and are the often the preferred technology at sites with lower concentrations of VOCs.

SSD Systems

SSD systems are applicable for slab-on grade building construction. An SSD system is designed to function by continuously creating a lower pressure directly beneath the building floor relative to the pressure within the building. The resulting negative pressure beneath the slab prevents soil gases from flowing into the building, thus reducing entry of VOCs into the building. VOCs captured by this negative pressure field are collected and piped to an ambient air discharge point above the building roofline. The depressurization under the slab is typically accomplished with a motorized blower which draws air form the soil beneath the building and discharges it to the atmosphere through a series of collection and discharge pipes. In most cases a sub-slab liner is an appropriate redundant feature that protects against VI in the event of a blower failure. Additionally, the liner typically increases the efficiency of the system thus allowing a smaller fan to be used.

In existing structures, active SSD systems entail drilling or cutting one or more holes in the existing slab, removing soil from beneath the slab to create an open hole or suction pit, and placing vertical suction pipes into the holes. The suction pipes are manifolded together and routed to the fan and discharges so that the soil gas can be drawn from just beneath the slab. An operating SSD system will induce indoor air to flow down into the subsurface through any cracks or conduits in the existing slab thus eliminating any VI into the building.

The effectiveness of the SSD system can be adequately evaluated by monitoring the blower operation and the reduced pressure beneath the floor. The pressure monitoring requirements for an SSD system are generally easier to implement routinely compared to monitoring VOC concentrations in an SSV system. The magnitude of the VOC concentration reduction in the SSD system is less critical than for the SSV system, because the SSD system is designed to mitigate VI by maintaining a lower pressure beneath the building.

Additional Alternatives to SSV and SSD Systems

Other remedies that can be used in addition to SSV or SSD systems, or even as alternatives depending on the severity of the VI risk, include the following:

  • Sealing Cracks and Openings in the Slab

  • Sealing the Slab with an Epoxy Coating

  • Sub-Slab Liners (Passive Membranes or Vapor Barriers)

  • Submembrane Depressurization (SMD) for Dirt Surface Beneath a Crawl Space

  • Building Pressurization by Adjusting Building Heating, Venting, or Air Conditioning

  • Indoor Air Treatment

  • Podium Style Building with Open Air First Floor

Selection of Vapor Intrusion Mitigation Measures for Your Building

The full range of mitigation approaches should be evaluated to determine which is the most feasible based on the site constraints and severity of the VI risk. Many of the mitigation measures described above can be combined to create a very robust and cost-effective solution. There are a number of cost-effective proven mitigation measures that can be easily implemented for existing buildings to greatly reduce or eliminate the VI risk if needed. During development of VOC impacted properties, SSV and SSD systems are becoming an integral part of many new building designs and can be installed during the building construction for a relatively small incremental upfront cost that can greatly reduce VI risk to building occupants and thus greatly reduce long term liability for the Property Owner.

The Department of Toxic Substances Control (DTSC) has issued the Vapor Intrusion Mitigation Advisory to evaluate and discuss mitigation methods and system designs, implementation and monitoring considerations on property owners and occupants, and document submittal requirements. For assistance in evaluating the feasibility of VI mitigation approaches and determining which approach (or combination of approaches) is best suited for your site, please call me or one of the other Waterstone Environmental project managers to discuss your options.


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