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Vapor Intrusion Mitigation Advisory

The California Department of Toxic Substances Control (DTSC) and California Environmental Protection Agency (CalEPA) have issued the Vapor Intrusion Mitigation Advisory for projects which are affected by soil gas vapor intrusion.

Vapor intrusion mitigation systems are implemented to reduce the exposure of subsurface contamination intrusion into buildings to prevent the exposure of contaminated vapors within occupied spaces. Vapor mitigation systems are not considered a remediation method, they are considered a preventative measure which redirect contaminated vapors. When feasible, alternative remediation approaches may also have to be taken in addition to the implementation of a Vapor Mitigation System.

Vapor Mitigation System

Evaluation of Vapor Intrusion Mitigation Requirements

Phase I Environmental Site Assessments (Phase I ESA) provide historical information regarding the property and land use. Based on the conclusion of the Phase I, a Phase II Subsurface Investigation (Phase II) and indoor air survey may be recommended.

In cases where soil contamination is concluded to be present within a property, the Phase II may be reviewed by a toxicologist. A risk-based decision process shall be evaluated and determined under the guidelines of the DTSC. Based on the conclusions of the risk-based decision, Vapor Intrusion Mitigation may be required.

Vapor Intrusion Mitigation Design and Construction

Vapor Intrusion Mitigation Systems prevent the build-up of gasses beneath a building. This ensures that the migration of contaminated soil gas into the building is prevented by interrupting the pathway between the vapor source and the occupants of the building. These systems are essential if source removal is not feasible, so cleaning up the subsurface’s contamination and lessening the occupants’ impact through Vapor Intrusion Mitigation Systems will be the most practical way of responding to a vapor intrusion.
There are a variety of design approaches that can be taken to implement Vapor Intrusion Mitigation. The scope of the mitigation design will be determined based on the risk-based evaluation established by the DTSC. Some design approaches that may be implemented are sub-slab venting systems, sub-slab depressurization systems, and vapor barriers.

Vapor Intrusion Mitigation
Sub slab venting

Sub-Slab Venting System

Sub-slab ventilation is implemented to promote clean air flowing in the subsurface. This is done while minimizing the resulting impact on indoor air if the vapors subsequently enter the building due to advective flow or diffusion through any openings in the floor. A sub-slab venting system should take in air from the outside to the area under the floor, which essentially dilutes the concentration of volatile chemicals in that location. This is done by installing a vertical inlet pipe system throughout or next to the building to enable fresh air to enter the sub-slab zone or the gravel blanket.

The sub-slab venting also implements a series of vent pipes to also allow for the redirection of contaminated soil vapors towards the exterior of the building rather than entering the structure. Typical designs include perforated pipe positioned within a gravel blanket strategically located throughout the foundation of a structure. This positioning allows the soil gas to laterally move to the Vapor Vent Risers which are routed through the building and up to the roof for vapor discharge to the atmosphere. The implementation of these vent pipes prevents the accumulation of contaminated soil gas which will naturally enter an enclosed space.

Sub-slab venting systems can either be passive or active. Passive sub-slab venting systems depend on the natural effects of the air and temperature to remove soil gasses from the venting layer. Active sub-slab venting systems use a fan on riser pipes, withdraw and vent soil gasses, or blow breathing air into the venting layer. Upgrading to an active sub-slab venting system should be possible if a passive system is built in the subsurface.
Sub-slab Ventilation systems can be used in both existing and new construction. For existing buildings, Sub-slab Ventilation is most suitable where the sub-slab fill material is highly permeable to allow for appreciable air exchange rates below the slab and where there are minimal constraints to sub-slab ventilation. However, if the area has surface drainage problems or a high groundwater table, a sub-slab ventilation system may not be the most suitable due to the water.

To monitor the adequacy of the dilution or removal of the vapors by these systems, concentrations of the volatile chemicals or of the indoor air is measured. Therefore it is also required to design a sampling port within the horizontal or vertical pipes in the subsurface. This port must be fitted with a screen and a non-restricting rain guard to prevent debris and moisture from entering the system. Results should at least show that the concentration is less than 20 times the acceptable indoor air level.

Sub-Slab Depressurization System

A sub-slab depressurization system may utilize the sub-slab vent pipe and active fans to continuously create lower pressure beneath the structure. This pressure differential will reroute soil vapors to the vent risers or other designated routes to ensure that vapor gasses do not accumulate beneath the foundation of the structure.

Implementing an active sub-slab depressurization system in an existing building involves cutting or drilling one or more holes in the slab and then removing a certain amount of soil in the location to create a suction pit. This pit will make space for the placement of vertical suction pipes, which will connect to the fan to withdraw soil gas from the subsurface. The venting should be routed away from the structure’s windows and at a height significantly higher than the outdoor breathing zone.

Vapor tube

A sub-slab depressurization system may utilize the sub-slab vent pipe and active fans to continuously create lower pressure beneath the structure. This pressure differential will reroute soil vapors to the vent risers or other designated routes to ensure that vapor gasses do not accumulate beneath the foundation of the structure.

Implementing an active sub-slab depressurization system in an existing building involves cutting or drilling one or more holes in the slab and then removing a certain amount of soil in the location to create a suction pit. This pit will make space for the placement of vertical suction pipes, which will connect to the fan to withdraw soil gas from the subsurface. The venting should be routed away from the structure’s windows and at a height significantly higher than the outdoor breathing zone.

The sub-slab depressurization system will be evaluated by monitoring the sub-surface’s blower and pressure. Once the effectiveness is established, routine pressure monitoring will still be monitored. 

Additions or Alternatives to Sub-slab Venting and Depressurization Systems

The Vapor Intrusion Mitigation Advisory recommends that project proponents utilize innovative, more effective, and more sustainable approaches to vapor intrusion mitigation. Sealing cracks and openings in the foundation of the building as these are the main entry points of vapors. This can also be done on the walls, gaps around the utilities, dry utilities, drains in the floor, elevator shafts, and sumps.

Sub-slab Liners installed beneath the structure such as passive membranes or vapor barriers can also be used.  Vapor barriers are typically a spray applied asphalt emulsion that reduces the diffusion rate of methane or other contaminated soil gas. All penetrations that protrude through the foundation of a structure are sealed to prevent any leakage of vapors. The application of the vapor barrier requires a specialty Vapor Mitigation Contractor who is certified for vapor membrane installations. Specialty inspections are required during and after installation to ensure that all deficiencies are properly attended to.

Building pressurization can also be implemented for vapor intrusion mitigation by adjusting the heating, venting, and air conditioning systems of the building or installing new ones, which creates and maintains a positive pressure in the interior of the building compared to the subsurface. This method would cost less if the existing systems already provide positive pressure. If otherwise, increasing the pressure will result in more considerable energy costs. However, building pressurization becomes a practical option if the building is relatively small and tight and only has a few doors and windows. However, DTSC considers this method applicable only to commercial or industrial buildings and not residential structures.

Another approach to be considered is Indoor Air Treatment directs air within the structure to air pollution control equipment to eliminate the contaminants present in the air. Considering that this is a relatively new technology, might encourage the collection of contaminated air within the structure, and depends on uninterrupted treatment system operation, DTSC will only approve of this if no other vapor intrusion mitigation methods can be applied. 

 

Evaluation of Vapor Intrusion Mitigation Approaches

The screening, detailed analysis, and selection of the mitigation technologies to be utilized should be documented in a feasibility study, corrective measures study, remedial action plan, removal action work plan, and other related documents. DTSC prepares these documents and makes sure that the California Environmental Quality Act requirements are met.

Screening of the technologies would initiate once mitigation is deemed to be necessary. Its scope will reflect the specific needs of the project. In some cases, an alternative would not be screened if it is not appropriate for the site conditions or potential land use. The detailed analysis would involve comparing each approach or a combination of methods to a set of evaluation criteria, including Threshold Criteria, Balancing Criteria, Long-term effectiveness and permanence, Reduction of toxicity, mobility or volume through treatment, Short-term effectiveness, feasibility based on technical and administrative feasibility, Cost, State, and local agency acceptance, and Community acceptance. Upon completing the evaluation, the detailed analysis should be presented in a report that the appropriate agencies would then check. Once approved, the selection will be presented in a decision document such as a Record of Decision, Removal Action Workplan, or a Proposed Plan. 

Vapor Barrier

Vapor Barrier

In addition to the sub-slab vent system, it is common to install a Vapor barrier beneath the structure. Vapor barriers are typically a spray applied asphalt emulsion that reduces the diffusion rate of methane or other contaminated soil gas. All penetrations that protrude through the foundation of a structure are sealed to prevent any leakage of vapors. The application of the vapor barrier requires a specialty Vapor Mitigation Contractor who is certified for vapor membrane installations. Specialty inspections are required during and after installation to ensure that all deficiencies are properly attended to.