Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.14


Refer to the HEER Office Environmental Hazard Evaluation (EHE) document for guidance on the comparison of soil vapor and indoor air data to published action levels. The interpretation of data is a key element in planning the project as the data will drive the decision. When planning an investigation, project planners should agree on interpretation of the data before the samples are collected. Although not required, at least initial comparison of the data to HEER Office action levels will significantly expedite data evaluation and decision making.

7.14.1 Soil Vapor Sample Evaluation

The HEER Office EHE document provides risk-based, soil vapor action levels and additional guidance that can be used to screen sites for potential vapor intrusion hazards (HDOH 2017a; see also Sections 7.1 through 7.3). The collection of soil vapor data is recommended when the concentration of a VOC exceeds its action level for groundwater, or when a significant source of VOCs is suspected in vadose-zone soils (Section 13.2). Vapor intrusion action levels for soil are also provided in the HDOH EHE guidance document. Extrapolation of the concentration of a VOC in soil vapor from soil data is considered highly unreliable, however, due to the complexities of soil chemistry.

LVP subslab vapor data are directly comparable to HDOH (2017a) action levels for vapor intrusion risk. Comparison of small-volume soil vapor data, or data representative of only small-volumes of vapor collected at individual sample points, to soil vapor action levels should be done with caution and used in conjunction with other lines of evidence to assess potential vapor intrusion risk, especially if LVP data are not available. Be aware that random, small-scale variability in VOC concentrations within a vapor plume can hinder the interpretation of small-volume sample data (Section 7.5). Reliance on small-volume vapor sample data to estimate the approximate large-scale boundaries of a vapor plume is currently necessary, however.

Groundwater and soil vapor action levels for vapor intrusion are intended to be paired (HDOH 2017a). Empirical data and corresponding models suggest that the concentration of a VOC will not exceed the soil vapor action level at a distance of ten feet (three meters) from the top of the water table. Soil vapor data are therefore not normally necessary if vapor intrusion action levels for groundwater are not exceeded and groundwater data can reasonably be assumed to be representative of potential vapor emissions. Unexpectedly high concentrations of VOCs in soil vapor samples collected ten or more feet from groundwater can usually be attributed to some combination of the following scenarios:

  1. Unidentified source in nearby, vadose-zone soils (most common);
  2. Chemical present primarily in vapor phase (e.g., PCE vapors in dry soil beneath slab of a dry cleaner);
  3. Groundwater source area closer than ten feet from soil vapor sample point (default depth to water table used in models);
  4. Non-representative soil data (reliability of most soil VOC data from discrete samples is very low; see Section 4);
  5. Non-representative groundwater data (e.g., heterogeneous plume with isolated “hot spots” nearby); or
  6. Relict vapor plume associated with earlier migration of more heavily contaminated groundwater through area in past or post remediation of groundwater contamination.

The heterogeneity of contaminants in groundwater plumes has not been studied in detail. Heterogeneity can be expected to be significantly greater in sources areas in comparison to down-plume areas, although the latter could be characterized by discontinuous plugs of heavier contamination that reflect variability in source area releases over time.

As discussed in the EHE guidance document, soil vapor data may not be sufficient as a stand-alone tool to determine if a vapor intrusion hazard is present or absent. A “multiple lines of evidence” approach should be used to evaluate the vapor intrusion pathway. This includes consideration of the following factors, among others:

  • Source area size and volume (e.g., free product on groundwater >100m2 in area and/or >10m3 contaminated soil present; refer to HDOH, 2007c);
  • Mass of VOCs present in the source media (e.g., soil or groundwater) and associated volume of contaminated soil necessary to sustain long-term, vapor emissions over the assumed exposure duration (e.g., six to thirty years; see Section 7.5; can include use of mass-balanced vapor intrusion models);
  • Design of potentially affected buildings and the completeness of possible vapor intrusion pathways (e.g., cracks, or gaps in the floor around utilities), including nature of the building ventilation system and the potential for the building to be consistently under-pressured, and thus more susceptible to subsurface vapors;
  • Potential for intruding vapors to impact indoor air above known or anticipated background concentrations of targeted VOCs, due to emissions from unrelated, indoor or outdoor sources (note that this may not necessarily negate the need for remedial actions);
  • Comparison of indoor air data, if collected, to anticipated, background levels of targeted VOCs.

The first two factors are sometimes referred to as “source strength.” For a long-term, vapor intrusion risk to be present, the source strength must be significant enough to sustain an average vapor flux rate above soil gas action levels for the assumed exposure duration (e.g., six to thirty years; see Section 7.5). The use of a multiple-lines-of-evidence approach allows investigators to more accurately assess the current or future completeness of the vapor intrusion pathway on a site-specific basis and determine if long-term, adverse impacts to indoor air are likely. Currently HEER Office guidance recommends a focus on subslab soil gas data for final decisions regarding potential vapor intrusion risks from multiple compounds, such as chlorinated solvents and petroleum. This is intended to target vapors at the point they could enter a building. This also takes into account attenuation from the source area and/or biodegradation.

Soil vapor sample analytical results should be initially compared to Shallow Soil Gas action levels for evaluation of potential vapor intrusion concerns, published in the EHE document (HDOH, 2016, Table C-2 in Appendix 1). The collection of Large Volume Purge (LVP) sample data immediately beneath the building slab is recommended (Section 7.4).At sites where the EALs for shallow soil gas are approached or exceeded, the need for the collection of additional soil vapor samples and a more thorough evaluation of potential vapor intrusion pathways should be evaluated. Indoor air samples may need to be collected if subslab data or other information suggests potential impact above anticipated background (see Section 7.7).

Based on past experience, scenarios where subslab soil vapor data does not suggest a potentially significant impact to indoor air (e.g., above indoor air action levels) but vapor sample data from relatively shallow source areas exceed action levels is fairly common, especially for petroleum. In these cases, sealing of gaps and cracks in floors and an evaluation of the adequacy of the building ventilation system is recommended as a precautionary measure, although not necessarily required.

As discussed in Section 7.7, the collection of indoor air samples is only recommended when concentrations of VOCs in subslab soil vapor or other information suggest that indoor air could be impacted above anticipated, background levels. As a general guide, testing of indoor air to evaluate potential vapor intrusion impacts is only recommended when concentrations of targeted chemicals in subslab soil vapor are more than one-thousand times typical indoor air concentrations for residences and two-thousand times typical indoor air concentrations for commercial/industrial buildings (assumed indoor air:subslab soil gas attenuation factors; see Section 13.2 and Table 7-2; see also HDOH, 2016).

7.14.2 Indoor Air Sample Evaluation

Determining the source of VOCs identified in indoor air can be challenging, if not impossible, unless reported concentrations are significantly above anticipated background levels, significant VOC levels have been documented in soil vapor samples collected immediately beneath the building slab, and clear entry points have been identified. If collected, indoor air data should be compared to both risk-based action levels and typical background concentrations (e.g., USEPA 2011d). A summary of action levels and typical background concentrations of common VOCs is provided in Table 7-2 in Section 7.7.1.

Data from air samples taken in various parts of a building can be reviewed and compared to help identify contaminant concentration gradients or specific vapor intrusion points. For example, data for basements, bathrooms, kitchens, utility rooms or elevator shafts that suggest VOC concentrations above anticipated background with decreasing concentrations higher in the building are suggestive of a subsurface source. However, the building should be inspected prior to sampling to eliminate the presence of other indoor sources, such as stored chemicals in a basement (see Section 7.7).

If impacts to indoor air above anticipated background are identified and subslab soil vapor data as well as other lines of evidence suggest a likely subsurface source, then actions will be required. Potential actions are briefly discussed in the next section.

If impacts to indoor air above anticipated background are not identified but subslab soil vapor concentrations exceed action levels, then measures to avoid potential future impacts to indoor air may be recommended, although not formally required. This will depend on site-specific circumstances. For example, sealing of floor cracks and gaps and a check of the building ventilation system may simply be recommended in cases where subsurface vapors are associated with a relatively small source area of petroleum-contaminated soil or groundwater. In contrast, measures to eliminate potential vapor pathways might be required at a site where elevated concentrations of chlorinated solvents in soil vapors associated with a large source area are identified immediately beneath a building slab or in nearby, shallow soil vapor, even though adverse impacts to indoor air have not been specifically identified.

If impacts to indoor air above anticipated background are not identified and subslab soil vapor concentrations do not exceed action levels, then no further action will generally be required with respect to the subject home building. If subsurface data indicate a potentially significant vapor plume, however, then sealing of cracks and utility gaps in floors and an evaluation of the building ventilation system is recommended as a precautionary measure. Note that remediation of the source area may still be necessary regardless of the absence of clear impacts to existing buildings if source area soil, groundwater and/or soil vapor data suggest potential future vapor intrusion risks or other environmental hazards. Refer also to the HDOH technical memorandum Long-Term Management of Petroleum-Contaminated Soil and Groundwater (HDOH, 2007c).

As a general rule a home or building should not be flagged for potential vapor intrusion hazards unless this is supported by multiple lines of evidence, including indoor air data well above anticipated, background levels. Doing so could cause significant legal and financial problems for the property owner, even though no impact has been demonstrated. In such cases, it is more appropriate and responsible to state that “Conclusive evidence of adverse, vapor intrusion has not been documented” than an open-ended statement such as “Vapor intrusion into the home (or building) could not be discounted.” Due to the sensitivity of testing indoor air in private residences and buildings, and the challenges posed by distinguishing indoor or outdoor sources of VOCs from subsurface sources, an “innocent until proven guilty” approach for the investigation of potential vapor intrusion hazards is recommended.

Precautionary measures are recommended, however, for sites where significant subsurface source exists even though adverse, vapor intrusion impacts have not been identified. As discussed above, this will typically include sealing of cracks and utility gaps in floors as well as an evaluation of building ventilation adequacy.

7.14.3 Additional Evaluation and Remedial Actions

Assuming that the data are representative of long-term site conditions, and within the limitations described in the EHE document (HDOH, 2016), VOCs in groundwater or soil vapor below the corresponding Tier 1 EALs can be assumed to not pose a significant vapor intrusion threat.

If multiple lines of evidence such as those noted above indicate significant impacts to indoor air of existing or future buildings, then additional evaluation or remedial actions will be warranted. This will typically include the removal of vapor intrusion pathways for existing buildings (e.g., sealing of cracks and gaps in floors, etc.) and remediation of contamination in the source area to reduce soil vapor levels to below levels of potential concern. An evaluation of the adequacy of the building ventilation should also be carried out. Example guidance includes:

  • Building Air Quality, A Guide for Building owners and Facility Managers (USEPA 1991d);
  • The Inside Story: A Guide to Indoor Air Quality (USEPA 1995f);
  • Building Indoor Air Quality Action Plan (USEPA 1998f);
  • Indoor Air Quality Building Education and Assessment Model (USEPA 2008d).

A detailed review of site-specific vapor intrusion risks can also be carried out if desired and can include the preparation of site-specific human health risk assessments, vapor intrusion models and alternative action levels. This level of effort is unlikely to be necessary or cost-beneficial for typical, small sites, however.

A detailed discussion of source area remediation and vapor mitigation is beyond the scope of this section but will be included in future updates to the TGM. Proposed mitigation measures should be discussed with the HEER Office on a site-by-site basis. The extent and nature of source area remediation is dependent in part on extent and location of the contamination. For example, removal of the floor and excavation of contaminated soil might be the most cost- and time-effect means to address a localized area of solvent-contaminated soil beneath the floor of a former dry cleaner. Some combination of excavation, soil vapor extraction, in situ injections, or thermal treatment might be required for a site with extensive contamination.

At some point full remediation of a source area may not be practical from a cost or technical standpoint and engineered and/or institutional controls may be needed for existing or future buildings. Sealing of floors and/or improved ventilation may be required for existing buildings. In some cases the installation of a subslab ventilation system could be required. A vapor mitigation system for a new structure might include one or more of the following components (e.g., refer to USEPA 2008c, CalEPA 2011b):

  • Base of permeable fill with collection system of perforated pipes and risers;
  • Impermeable membrane beneath slab;
  • Passive or active venting system above ground, or passive system with the ability to switch to active as needed (e.g., risers with wind-activated turbine vents and option for blowers, etc.); and
  • Permanent soil vapor monitoring points through slab and within collection system.

Passive systems may need to be switched to active to address methane hazards or the buildup of very high levels of solvent or petroleum vapors beneath the slab. Monitoring points within the slab and collection system can be used to evaluate the effectiveness of remedial actions and natural attenuation, as well as to ensure flow in risers, and to support proposals to cease mitigation effects due to a reduced vapor intrusion risk.