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3. Mitigation

Volume 5: Response activity for the VIAP
3. Mitigation

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Mitigation as defined under Part 201 is an interim response activity [Sec. 20101(1)(y)] and occurs prior to the implementation of a remedial action or maybe necessary during the implementation of a remedial action to prevent, minimize, or mitigate injury to the public health. The objective of mitigation for the VIAP is to reduce indoor air constituents of concern (COCs) below applicable action or screening levels that are present as a result of release. This requires modification of the pathway to reduce or eliminate the mass flux of COCs entering the building and/or to reduce indoor air COC concentrations either by removal of COCs or dilution. Generally, the need for mitigation or lack thereof is determined by the collection and analysis of sub-slab soil gas samples as they are the best indicator of the potential risk to a structure. If COCs are present in sub-slab soil gas samples at concentrations above screening levels, paired indoor air samples are recommended to fully evaluate the exposure. When indoor air samples are not above the AAC, the development of a site-specific criterion may be appropriate. See Volume 6 - Volatilization to Indoor Air Criteria for more information.

As illustrated in ITRC’s 2020 “Flowchart for Vapor Intrusion Mitigation Conceptual Site Model Development,” (available in ITRC’s (2020) Conceptual Site Models for Vapor Intrusion Mitigation Fact Sheet), mitigation technologies can be applied at different points in the VIAP to accomplish the goal of reducing the COCs. Understanding how mitigation technology is modifying the VIAP helps understand:

  1. whether the technology is compatible with the site conditions and stakeholder objectives (e.g., cost, timeliness, sustainability, etc.), and
  2. what information is needed to evaluate the performance of the system over the short and long terms.

Examples of common mitigation technologies applied below the slab (e.g., sub-slab technologies), at the slab (e.g., vapor barriers), and inside the building (e.g., heating, ventilation, and air conditioning [HVAC] controls) are described below, including how they modify the VIAP and information typically needed to evaluate the suitability and performance of the technology at a site. A similar thought process should be applied to other mitigation technologies that are not discussed below but may be considered at a site, such as technologies to address migration into buildings through preferential pathways (e.g., conduits such as sewer lines or utility tunnels).

Mitigation is typically broken down into three distinct types of mitigation:

  • Rapid Response & Ventilation (see Section 3.1)
  • Active Mitigation (see Section 3.2)
  • Passive Mitigation (see Section 3.3)

Multiple factors affecting the suitability and efficacy of a mitigation system should be considered during the design, review, and approval process. Many of these are discussed in ITRC’s (2020) Design Considerations Fact Sheet. The selected technology should be based on a good understanding of the site conditions and able to meet the remedy objectives at the site, whether applying an active system, passive system, ventilation, rapid response, and/or an environmental remediation technology.

 

Note: Sec. 20118(1) identifies that EGLE may only approve a response activity that the department concludes is necessary and proper to protect public health, safety, or welfare, or the environment.

Presumptive mitigation may be proposed due to time and/or budget constraints commonly associated with property redevelopment, which may prevent the investigator from achieving a sufficient level of characterization needed to fully evaluate the risks from the VIAP. However, in general, EGLE will not approve of mitigation using public funds (e.g. grants, loan, TIF, state-funding) unless it is shown with adequate site data to be necessary to protect public health. If presumptive mitigation is identified as the preferred approach for privately funded sites, EGLE approval may be sought, or it may be self-implemented. In most instances, however, it is better to evaluate the site and identify if unacceptable risks are present rather than the installation of a mitigation system that will require a long-term cost and resource burden of system monitoring and maintenance that is unnecessary. Further, in the long term, remedial or removal actions may prove to be the most cost-effective option for the contaminant mass that may pose an unacceptable risk to the VIAP.

3.1 Rapid response and ventilation

Rapid response is an interim VIAP mitigation approach that may be appropriate under certain conditions (e.g., high contaminant concentrations and sensitive populations) before implementing a long-term mitigation strategy or remedial action. A rapid response is designed to be easily implemented and verified within a timescale of days to weeks, depending on the contaminant of concern.

A rapid response should be a self-implemented mitigation strategy that is aimed at preventing exposures while a long-term mitigation strategy is developed. A rapid response is often used before a CSM is fully supported with site data. Acceptable rapid response methods can vary based on site location and building use. A good understanding of building occupant demographics and building use is helpful in evaluating the need for and type of rapid response. For cases where chemicals of concern are detected in indoor air at concentrations exceeding short-term exposure criteria, a rapid response should be implemented immediately. Although indoor air alone does not dictate the need for further action, it is the point of exposure and is valuable for assessing exposure risks at a point in time represented by indoor air sampling.

Rapid response activities can include administrative controls, such as relocating occupants or restricting access to the building, as well as engineering controls that reduce chemical vapor exposure. Engineering controls may involve improving building ventilation, treating indoor air, or physically preventing vapor entry into the building. The ITRC’s (2020) Fact Sheet on Rapid Response & Ventilation for Vapor Intrusion Mitigation provides detailed information on specific rapid response options, including Preferential Pathway Sealing and Ad Hoc Ventilation, Indoor Air Treatment, and HVAC Modifications.

Note: “Emergency” situations (i.e., “call 911” situations) are where combustible, explosive, or oxygen-deficient conditions may exist inside a building. If these conditions are believed to be present, first responders should be contacted immediately.

3.2 Active mitigation

Active mitigation involves the interception, isolation, dilution, or diversion of soil gas entry into a building using mechanical means powered by electricity. The performance of active mitigation systems is quantifiable through measurements of vacuum, area of influence, flow rates, sub-slab soil gas concentrations, and mass flux. Active mitigation for new construction can differ significantly from that for existing buildings due to variations in building components and the control of system installation during construction.

In general, the technologies discussed assume that soil gas entry primarily occurs via advection rather than diffusion. Diffusion through the concrete slab is typically not considered a significant transport pathway and provides a significantly lower rate of mass transfer when compared to advective flow. Active vapor mitigation systems are designed to achieve either depressurization of the sub-slab soil or granular fill relative to inside a building (i.e. create and maintain a vacuum beneath a building) or have some degree of air flow and dilution soil gas concentrations in the sub-slab space. Systems designed for depressurization will also provide some degree of ventilation, and vice versa. These technologies—depressurization and ventilation—are not fully separate and distinct and often use similar equipment. However, the systems have drastically different goals and are monitored using different performance metrics and criteria.

The four most common types of active mitigation are as follows:

  1. Sub-slab depressurization (SSD; see also the ITRC (2020) SSD Technology Information Sheet)
  2. Sub-slab ventilation (SSV; see also the ITRC (2020) SSV Technology Information Sheet)
  3. Sub-membrane depressurization (SMD; see also the ITRC (2020) SMD Technology Information Sheet)
  4. Crawlspace ventilation (CSV; see also the ITRC (2020) CSV Technology Information Sheet)

It is common for new construction to combine an active mitigation strategy with a passive mitigation technology. When these are combined, it can make for a more effective system that will reduce long-term operational costs. More information on Passive Mitigation can be found in Section 3.3. EGLE has developed a table of contents to aid in the design, operation, and documentation of the performance metrics of the active mitigation system. The table of contents is provided on Attachment A.5 of this Volume.

Details on the differences between active mitigation for new construction and existing buildings can be found in ITRC (2020) Design Considerations, Post-Installation Verification, and OM&M/Exit Strategy Fact Sheets.

3.3 Passive mitigation

Passive mitigation of the vapor intrusion (VI) pathway involves limiting, intercepting, diluting, diffusing, or diverting soil gas entry into a structure without using mechanical means. Generally, these systems physically block vapor entry into a building and/or rely on natural mechanisms, such as chemical diffusion, thermal gradients, or wind-induced pressure gradients, to divert volatile organic compounds (VOCs) and soil gas around the building (e.g., to riser pipes). Passive mitigation systems require thorough documentation during installation, as well as methods for establishing and confirming the system’s effectiveness,
such as using surrogates and tracers.

As outlined in the ITRC’s (2020) Conceptual Site Models for VI Mitigation Fact Sheet, the technologies discussed assume that soil gas entry primarily occurs via advection rather than diffusion. There are three general categories of passive mitigation technologies, each with several different types. Below is a list of passive mitigation technologies and their associated ITRC Technology Information Sheets (2020):

EGLE has developed a standard operating procedure (SOP) that identifies how diffusion coefficients for barriers can be established to assess if a proposed liner associated with a passive mitigation technologies would be appropriate for use. In addition, EGLE has also developed a table of contents to aid in the oversight requirements and documentation for liners that should be collected during the installation process to assist in the necessary documentation to support the proper installation of the liner system. The table of contents is provided in Attachment B.5 and the SOP is provided in Volume 2 - Investigation Methods from the Volatilization to the Indoor Air Pathway.

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