Guide to the 2020 California Draft Vapor Intrusion Guidance

By July 20, 2020Uncategorized

In February 2020, the California DTSC, State Water Resources Control Board (SWRCB) and San Francisco Regional Water Board issued the Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion for public review and comment. The aim of this new guidance is to detail how California will address the threat of vapor intrusion into indoor air.

The public review period for the new guidance was postponed to June 1, 2020, due to COVID-19. The review period has now ended, and the final guidance is expected to be released within the next few months.

What is vapor intrusion?

Vapor Intrusion (VI) occurs when chemicals present in the subsurface (soil or groundwater) produce vapors, which travel upward through pore spaces in soil, cracks in foundations and building walls, sewer pipes, or other utilities, and enter into indoor spaces. The presence of certain chemicals in indoor air creates potential health hazards for those who are within the breathing zone – whether it is a retail building, office, or home. Sites that are at particular risk for VI include former gas stations, dry cleaners, and manufacturing facilities.

All environmental health agencies have some form of regulation that governs the investigation and mitigation of VI impacts to properties. For further information on VI, please see the EPA website.


Illustration of potential VI pathways in soil vapor, from EPA website.

What are the key points of the new guidance?

The new California vapor intrusion guidance is expected to recommend the following:

  • All sites in California will use the EPA’s (2015) attenuation factor of 0.03 for sub-slab vapor and soil vapor data. This is a substantial increase (about 15-30 times higher) than the current attenuation factors (DTSC, 2011) of 0.001 for an existing commercial site, and 0.002 for an existing residential site. This attenuation factor is extremely conservative, and will result in the collection of additional data at many sites to evaluate if VI impacts are present.
  • The sewer air pathway into indoor air is an area of greater focus. If a sewer is likely to contain harmful vapors, indoor air sampling will be recommended for any buildings connected to that sewer.
  • The stated goal of the new guidance is to create a “VI database” from an extensive variety of properties in California. Once the database is finished, the State will use that to determine whether the new attenuation factors are valid. The State Board has already begun collecting VI data through Geotracker as of August 2019 – see their guide here.

The overall effect of these changes is that regulatory screening standards for soil vapor will become significantly stricter in California. This will result in the need to collect other types of data, such as indoor air and sub-slab vapor sampling, to evaluate if VI impacts are present.

What can I do to off-set the potential risks associated with these new standards?

There are many options available to reduce or mitigate the potential for VI at a Site. This section lists a few options that may (or may not) be recommended for a given property. The actual path forward will be Site-specific, based on the individual factors present at each Site.

Due Diligence

Before any recommendations can be made, it is critical to do due diligence to evaluate the site for environmental risks, including potential VI impacts. In general, this step should always be taken when considering buying or redeveloping a property.

  • In order to evaluate a property for environmental risks, a preliminary screen of potential VI impacts should be done. This will usually involve a file review, Site walk, and an evaluation of historic and current usage of the Site.
  • As part of this screening process, nearby sites will also be reviewed. The goal of this step is to separate Site-related conditions from unrelated background conditions.
  • Based on the initial screening effort, additional sampling may be recommended at the Site. If the Site is considered at-risk for VI, samples may be collected from indoor and outdoor air, sub-slab vapor, or soil vapor. The results will be compared to the most updated vapor intrusion guidance. This information will be extremely valuable in the decision-making process.
  • Indoor air sampling tests for compounds at very low concentrations. As such, indoor air samples are subject to influence from a variety of possible sources, not related to the subsurface release. These sources may include background outdoor air, nearby industrial operations, and various chemicals stored inside the property. As the indoor air results might be biased from a number of sources, the data interpretations are likely to vary; this may result in conflicts between property owners and tenants. In some cases, the installation of a vapor mitigation system may be adequate to address the VI potential.

Vapor Mitigation Systems

There may be vapor impacts at the Site which are relatively low, but still present a potential risk to current occupants of the property, based on the new California vapor intrusion guidance. In that case, a vapor mitigation system may be recommended. The design of the mitigation system will be based (among other factors) on the level of VI risk and the current Site layout.

Installation of a venting layer beneath a building foundation.

  • Passive vapor mitigation systems are often sufficient to prevent a potential VI risk. This type of system is most easily installed during construction, before a building is finished. They are often preferred due to the comparatively low impact and cost over time.
    • For the most part, once a passive mitigation system is installed, there are no on-going operational or maintenance costs.  However, care must be taken to protect/restore the barrier during future property improvements.
    • One common method is to install a vapor barrier beneath the foundation. A vapor barrier often consists of a membrane made of flexible, low-permeable material, such as high density polyethylene (HDPE) or heavy duty plastic lining.
    • Another passive mitigation option is to install a venting layer beneath the slab. This is done using a series of screened PVC lines, which are routed to the roof line and vented to the atmosphere. This helps to reroute sub-slab vapors outside the building, rather than into indoor spaces. A venting layer is often installed in conjunction with a vapor barrier.
    • If there is an existing building at the Site, a third option is to install an epoxy as a seal coat over cracks, gaps and around pipes. Any exposed dirt floor should also be covered with concrete.
  • Active vapor mitigation systems involve creating a pressure differential between the indoor space and the soil below the surface, which prevents vapors from entering indoor air.
    • Active systems are commonly installed at existing buildings. They may also be required by an agency, depending on the VI potential at the Site.
    • One common option is a sub-slab depressurization (SSD) system. This option uses a fan or blower to create a negative pressure (vacuum) beneath the foundation. Extracted vapors are pulled through the blower and evacuated at the roof line.  The blower typically utilizes a low level of energy (roughly equivalent to that of a single 110V-based lamp).  Under certain conditions, SSDs can be exempt from air quality permitting requirements.
    • Another option is building over-pressurization. This involves the use of an HVAC system to create a positive pressure within the building, forcing vapors out into the atmosphere. This technique is commonly used at office buildings; however, it often results in a higher energy footprint than SSD.

Soil Vapor Extraction

If there are more significant impacts in soil gas at the Site, a more active remedial strategy such as soil vapor extraction (SVE) may be recommended.

Soil vapor extraction systems can be highly effective at removing volatile compounds from deeper soil horizons.

  • SVE systems are very effective and widely used to reduce and/or eliminate volatile mass in subsurface soil vapor.
  • This technique utilizes a high-powered blower to create a vacuum in the subsurface. Vapors are then pulled into the SVE system through a series of screened wells. The extracted vapor is routed through a series of activated carbon tanks to “scrub” the air of volatile contaminants; the cleaned air is vented via an effluent stack to the atmosphere.
  • SVE generally continues until a steady-state level of mass recovery is reached. At that point, the system can be run intermittently until the desired reduction in soil vapor concentrations is achieved.
  • This option is more costly than a vapor mitigation system, since it involves the installation of multiple wells below ground surface. In addition, operation of an SVE system may continue between 6 to 18 months, or longer. An air quality permit is also generally required. A permit typically requires weekly visits to the system for data collection, as well as regular operation and maintenance.

Final Thoughts

The above list contains only a few suggested options. It is not intended to be a comprehensive list of methods to address potential VI issues based on the new California VI guidance.

It is also important to remember that every Site is unique. The techniques listed above may or may not be recommended at a particular site. For example, at sites that contain multiple sources of contamination (such as soil, groundwater, or ambient air pollution), remediating the other sources of contamination may also lead to a significant reduction in the VI risk. In addition, some sites may be eligible for State funding based on financial disadvantages. For instance, the Site Cleanup Subaccount Program (SCAP) is a State Water Board program that provides financial assistance to certain qualified Sites with impacted surface water and/or groundwater.

Bowyer Environmental Consulting has extensive experience in assessing and remediating various properties affected by VI due to soil gas, sub-slab and indoor air impacts. In addition, BEC has over 30 years of experience working with State regulators on behalf of a wide variety of residential and commercial properties, to help clean up the environment while minimizing impacts to existing sites. For further information or technical consulting, feel free to contact us at (877) 232-4620.

Further information:

https://dtsc.ca.gov/vapor-intrusion/