Sub-Structure Vapor Depressurization
There are several mitigation approaches for preventing subsurface vapors from intruding into homes and other buildings. The most common active approaches are subslab depressurization and sub-membrane depressurization. Because air pressure in most homes and buildings is usually lower than the pressure in the surrounding soil, vapors may be drawn into the building. Depressurization lowers the pressure under and around the foundation by creating a negative pressure under the slab. This works by venting gases from soil beneath the slab or membrane to the outside above the building and away from windows and air supply intakes. Alternatively, in some special cases, subslab ventilation is used. A less common approach is drain-tile suction. Subslab depressurization is the most prevalent of these techniques, and most other techniques use similar equipment and principles. Some of these approaches, especially for new construction, have added protective features such as vapor barriers placed below the slab.
Subslab depressurization (SSD). SSD is widely considered the most practical vapor intrusion mitigation strategy for existing and new structures. EPA defines SSD technology as "a system designed to achieve lower sub-slab air pressure relative to indoor air pressure by use of a fan-powered vent drawing air from beneath the slab." Thus, even if there are holes, cracks, or other pathways between the building and the subsurface, vapors flow downward, not upward. Thus, a well-designed depressurization system prevents any toxic vapors from intruding above.
In existing structures, installing an SSD system entails cutting one or more holes in the slab, removing a small quantity of soil from beneath the slab to create a "suction pit," and then placing vertical suction pipes into the holes. These pipes are connected to a manifold containing an exhaust fan, and vapors are in turn vented outdoors. Experience has shown that one or two suction pits are adequate to depressurize typical residential homes. A large building needs more.
Where the potential vapor intrusion is minor, yet of concern, SSD systems can also be passive; that is, they do not have to rely on a fan to create a pressure gradient beneath the slab. In passive systems, the pipes are vented to the outside, relying on air currents and the "stack effect" to draw vapors up from below the house. The stack effect works on the principle that higher pressure gases rise through the path of least resistance to the lower pressure outdoor air. Passive systems are unpredictable, as they rely on changing outdoor air pressure to provide a negative pressure. In warmer months and climates, ambient pressure at the roofline may be greater than the subsurface, and passive systems may provide little help. In most cases, they do not create the same pressure differential between the sub-surface and the indoor air as an active system; they may merely vent harmful vapors intermittingly. EPA reported in 1993 that passive subslab systems are 30 to 90 percent as efficient as active systems.
In new construction, a subslab venting layer can be installed below the slab, and a fan is used to draw soil gas through the gravel underlying the slab prior to discharging it to the atmosphere. For larger buildings, multiple points of pipe installation and horizontal perforated pipe installed beneath the building have been effective reducing vapor intrusion.
Sub-membrane depressurization systems. These systems are similar to sub-slab systems, but they are applied to buildings with crawlspaces, where there is either no slab or a partial slab. A vapor barrier (i.e., membrane) that is impermeable to gases is placed under the floor or directly on the soil, and one or more suction pits are placed beneath the membrane. Like subslab systems, they create a negative pressure under the building so vapors do not get sucked up into the building with lower pressure than the subsurface.
Sub-slab ventilation (SSV). SSV is an alternative design used when the soil permeability of the sub-slab region is so high so that it is not possible or maintain a pressure gradient under the building sufficient to maintain a negative pressure. The hardware used in SSV systems and SSD systems are similar. With this configuration, the fan pulls large quantities of air (largely from the atmosphere) down through the soil thus diluting the contaminant in the sub-slab region, or the fan on the SSD is simply reversed, blowing air beneath the slab to dilute the vapors. If no fans are used, the system is identical to passive depressurization. Some frown upon SSV in that it can exacerbate vapor intrusion if there are preferential pathways in the slab.
Drain-tile suction. Some houses have existing drain tiles or perforated pipe to direct water away from the foundation of the house. These drains (often called "French Drains") are usually placed at the bottom of the foundation. Suction on these tiles or pipes is often effective in creating a negative pressure under a building, so it could have the same effect as an SSD system. This is especially true if the drain tile extends around the entire building.
Limitations and Concerns
* In all cases, any cracks or holes in the slab or membrane should be sealed so that no preferential pathways exist that allow vapors indoors.
* Shifts in earth beneath structures are inevitable, buildings may undergo construction improvement, and preferential pathways (such as electrical conduits) may be added, so routine inspection and corrective sealing is important to maintain system effectiveness.
* For those systems relying on subsurface depressurization, there are a number of factors that can moderate the pressure differential and lead to subsurface under-pressurization. These include thermal differences between indoor air and the surrounding soils; wind and barometric changes; the "stack effects" of chimneys and flues; the operation of exhaust fans/vents; and negative pressures created by the combustion of air in gas and oil furnaces. Thus, SSD systems should be designed to achieve depressurization during winter conditions, when combustion furnaces are in operation and when exterior ventilation limited.
* Sites with impervious soils may need to use a blower unit that can provide the extra pressure needed to create a semi-vacuum. These units are noisy and may not be suitable for residences.
* The location of the suction pits for the SSD is controversial. There are some reports that state that the suction pit location is not critical; others state that they may perform better when pits are located near the perimeter of the home, closer to the major air entry routes (construction joints and utility penetrations); while others indicate that closer to the middle is most efficient.
* In locations that use furnaces or other types of combustion heating, "backdrafting" should be investigated prior to installation of the SSD. Backdrafting is off concern if the negative pressures created by the SSD are stronger than the pressures that would drive the combustion gases up a chimney or stack. In such cases, potentially deadly combustion gases (e.g., carbon monoxide) could be discharged into the building. An HVAC contractor should be able to diagnose this problem.
* "Short-circuiting" problems are of particular concern, where cracks, holes, sumps, or spaces in the building foundation/slab disrupt a negative pressure field. Hollow block wall or cinder block foundation walls may act as migration routes for vapor to enter homes, particularly if the holes in the top row of blocks are open.
* In a small number of cases, off-gases from the SSD may potentially have to be controlled, if they exceed State and Regional discharge requirements.
* The effectiveness of SSD systems must be monitored. Besides indoor air testing, evaluation can include monitoring the blower operation and monitoring the reduced pressure beneath the floor.
* Vapor barriers, while intended to impede any vapors from entering a building, should be not used by themselves as a vapor mitigation strategy. There is a tendency for them to be damaged during construction or from subsequent settling or geological events.
* SSV for existing buildings may not be economical because of the extensive foundation work involved.
* 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.
* It is imperative that if SSV systems are used, any preferential pathways be sealed and checked frequently.
* Mechanical components of the SSD (fans) have a life expectancy of 10-15 years. The operation of the fans should be monitored and maintained on a regular basis. In some instances, incorporating a continuous monitor into the operation of the fan is desirable.
* Achievement of the long-term goal for indoor air relies heavily on the cleanup of soil and/or ground water to reduce or eliminate the contamination source. Source remediation should be part of the long-term remedy for the indoor air pathway, and not overlooked because prophylactic remedies such as SSD are in place.
* Long-term monitoring to demonstrate movement towards achievement of remediation goals for the contaminated environmental media and indoor air must be part of all plans that address vapor intrusion.
SSD and similar systems are used to mitigate indoor vapors arising from subsurface contamination. They are also used to mitigate radon, which is a naturally occurring radioactive gas created by the breakdown of uranium in soil, rock, and water. See http://www.epa.gov/radon/ for more information about radon.
Technology Development Status
SSD and similar systems are well established, well understood technologies that have been used for many years to control radon gas. Retrofitting a small building is relatively inexpensive.
Other Resources and Demonstrations
See description of Vapor Barriers.
See http://nepis.epa.gov/Adobe/PDF/P100AE72.pdf for a full description of VI Technologies.
See http://www.itrcweb.org/Documents/VI-1.pdf for regulatory guidance on vapor intrusion.
See http://www.ttemidev.com/narpm2007Admin/conference/materials/148/03_Curran%20NARPM%20Presentation.pdf for presentation, with pictures of a site in Connecticut where houses were inspected and SSD were installed.
See http://www.dtsc.ca.gov/sitecleanup/upload/VI_Mitigation_Advisory_Apr09.pdf for California's 2009 Vapor Intrusion Mitigation Advisory.
See http://www.epa.gov/osp/presentations/viforum09/Folkes.pdf for new developments in mitigation strategies.
See http://www.brownfieldstsc.org/pdfs/BTSC%20Vapor%20Intrusion%20Considerations%20for%20Redevelopment%20EPA%20542-R-08-001.pdf for U.S. EPA's 2008 Report on Vapor Intrusion Considerations for Redevelopment.
See also http://www.epa.gov/tio/download/citizens/a_citizens_guide_to_vapor_intrusion_mitigation_.pdf and http://www.serdp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-200423.
See also http://radonresources.com/resources/