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Attachment E.3 - Conditions and CSMS that may be encountered and site-specific evaluation approaches

Volume 3: Investigative approach for VIAP
Attachment E.3 - Conditions and CSMS that may be encountered and site-specific evaluation approaches

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There are many alternate approaches that may be utilized for the evaluation of the potential risks associated with Volatilization to Indoor Air Pathway (VIAP) other than what is described in this volume. An alternate approach that may be more representative of the actual sites risks or more cost/time efficient than the linear screening approach described in Sections 3 through 6 of Volume 3 could be considered, especially if the site does not screen out.

Examples of different site conditions that may warrant an alternative approach include:  

  • VIAC exceedances with Limited Spatial Extent – Exceedances of an applicable VIAC with limited spatial extent may indicate that a more site-specific approach based on actual site information is better to evaluate the potential risks the VIAP may pose to human health.Information to be considered could include the exposure assumptions in the VIAC, the mass of contaminants, exposure domain or area over which a building is or may be located, degradation rates and source zone depletion rates of the contaminants present, etc.
  • Lithology – The less permeable the lithology, the lower the likelihood of significant mass flux from the lithology. With low permeability of the lithology, the flux from the subsurface becomes the limiting factor for the advective transport, thereby reducing the potential risks and data needs. In essence, the lower the soil permeability, the slower the transport of VOCs to indoor air and greater potential to encounter lower COC concentrations in indoor air because of mixing and dilution. Pneumatic testing can aid in the evaluation of permeability and ability to collect representative soil gas – see Volume 2 – Investigation Methods for the VIAP for more information.
  • Buildings with Natural Ventilation or High Ventilation Rates – Air exchange rates identified by EGLE to represent the best available information in VIAC development may be conservative for buildings with consistent high air exchange rates or that are naturally ventilated and freely exchange ambient air. For buildings with high air exchange rates, SSVIAC can be developed to evaluate VIAP risks for the specific building. See Volume 6 for more information.

Models

Alternate approaches commonly use models that evaluate and incorporate aerobic degradation or other unique site-specific conditions. The use of models can be applied as a line of evidence in the investigation process but must be field verified with data.

The complexity of modeling applications can vary depending on the objectives of the modeling and availability of project-specific information. The steps indicated in Figure E.3-1 should be followed and documented in a Response Activity Plan submitted for EGLE review and approval. 

Figure E.3-1

Steps in developing and evaluating a model for the volatilization to the indoor air pathway

Define problem and modeling objectives

Review conceptual site model (CSM)

Select model (consistent with CSM)

Determine model inputs

Compare model predictions to available data

Conduct model sensitivity analysis

Each of these steps, as well as different models that may be appropriate for VI, are further described in ITRC (2026). It is important to note that though various models are described in ITRC (2026), the toxicological and chemical specific parameters utilized in the model must reflect best available information compared with generic criteria. If the goal and result of the modeling is site-specific VIAC, then the review, approval, and application of the model is done with EGLE’s toxicologist pursuant to Section 20120b. If the result of the model is to support sampling, fate, and transport of the vapors and does not result in risk based VIAC, then the review and approval is done with the typical submittal process to EGLE. All other input parameters collected should be based on site-specific data and input parameters that is collected from the facility. Information on key input parameters for biodegradation modeling is provided in ITRC (2026).

If modeling is to be utilized, the documentation should include an evaluation and justification of the relevant model parameters, ranges, and parameter sensitivities, especially those that are moderate to high, and how the data supports that the model inputs are representative of the actual facility conditions and more representative than the generic criteria established by Part 201.

Lines of Evidence

The ideal outcome from collecting multiple lines of evidence is a concordant set of site-specific information that supports decisions that can be made and increases confidence in the decisions. However, based upon observations presented to EGLE, the buildings where all available information agrees is typically the exception rather than the rule. Multiple lines of evidence, when used, can be data intensive efforts in making an appropriate demonstration. While it is not necessary that all data agree, multiple lines of evidence supporting a single conclusion can provide confidence in proposed approaches and site-specific evaluations.

Individual grab indoor air sampling alone is not a reliable means to demonstrate a risk from the VIAP will not occur and is therefore not sufficient to support that a response activity is not warranted, however, indoor air data does provide valuable information for the point of exposure at the moment in time when sampled. The use of indoor air samples requires repeated indoor air sampling events over multiple months and HVAC changes that support the site-specific attenuation of the vapor to indoor air.  When evaluating indoor air, the samples should be only collected when the pressure inside the building is less than the slab below so that the advective flow of vapors is from the subsurface into the building. Some lines of evidence may not be definitive (e.g., indoor air varies significantly temporally). Some lines of evidence may be inconsistent with other lines of evidence and should be closely evaluated for weight of evidence when identified. When typical lines of evidence that are collected are not concordant, and the weight of evidence does not support a confident decision, it may be appropriate to collect additional lines of evidence, which may include additional samples depending upon the CSM. For example:

  • Appropriate site-specific testing can be conducted to assess the contribution of background sources of vapor-forming chemicals, including comparisons among chemicals of their relative concentrations in indoor air, outdoor air, and soil gas. Background sources of vapor-forming chemicals may help to explain situations where the indoor air concentration is higher than would be expected given the subsurface vapor source or the sub-slab soil gas data.
  • Diagnostic testing of indoor air, building condition assessments or utility surveys, or supplemental hydrogeologic characterization can be used to investigate the effect of preferential migration routes. Such investigations may help to explain situations where the sub-slab or indoor air concentration appears to reflect unattenuated vapor transport from the subsurface vapor source.
  • Building susceptibility to vapor intrusion can be tested through building pressure control testing. When the building is intentionally placed under a greater vacuum than normal for a period of time while monitoring lines of evidence, the indoor air sample collected during this period can be used to evaluate the potential worst case for the VIAP.The building pressure control testing may also be used help to explain situations where the indoor air concentration is lower than expected based upon the sub-slab soil gas data. See Volume 2 for an SOP on Building Pressure Control testing.

Vapor migration in the vadose zone can be further characterized to identify impedances to vapor migration; appropriate facility-specific attenuation factors can be considered to investigate facility-specific vapor attenuation. In some of these situations, the volatilization to indoor air pathway may be impeded due to geologic or hydrologic characteristics in the vadose zone.

Examples of different lines of evidence that may be appropriate to use for an alternate VIAP evaluation depending on the CSM and the facility conditions include:

  • Data on facility geology and hydrology (e.g., soil type, soil moisture, and porosity) to support the interpretation of soil gas profiles, the characterization of vadose zone permeability, and the identification of anticipated soil gas migration routes in the vadose zone or the identification and characterization of impeded migration.
  • Utility corridor assessment to identify preferential migration routes, if any, that facilitate subsurface vapor source migration between sources and buildings.
  • Building construction and current conditions, including utility conduits or other preferential routes that a vapor source can enter and that can directly volatilize or transport vapor, openings for soil gas entry, heating and cooling systems in use, and any segmentation of ventilation and air handling, including instrumental (e.g., PID) readings to locate and identify potential openings for soil gas entry into buildings.
  • Pressure differential data to assess the driving force for soil gas entry into building(s) via advection.
  • Tracer-release data to verify openings in building foundations for soil gas entry or assess fresh air exchange within buildings.
  • Indoor air sampling data to assess the presence of subsurface contaminants in indoor air.
  • Building-specific indoor sources of volatile chemicals.
  • Concurrent outdoor air data to assess potential contributions of ambient air to indoor air concentrations.
  • Comparative evaluations of indoor air and sub-slab soil gas data, including calculation and comparison of building-specific, empirical attenuation factors to assess their consistency among subsurface contaminants to assist in identifying indoor vapors arising from vapor intrusion and the results of statistical analyses (e.g., data trends, contaminant ratios) to support data interpretation.
  • Results of mathematical modeling that rely upon site-specific inputs. The relative utility of these and other lines of evidence will depend on site-specific factors, as described and documented in the CSM, and the objectives of the investigation.
  • For an industrial building, indoor air testing while the Heating, Ventilation, and Air Conditioning (HVAC) system is not operating (see Section 5.3.3) could be useful for diagnosing vapor intrusion. On the other hand, single family detached homes can generally be presumed susceptible to soil gas entry when the HVAC systems are operating.
  • Sub-slab and indoor air sampling conducted when VI is most likely to occur, i.e., there is a higher pressure in the sub-slab than in the indoor air. This could potentially even be done by using the HVAC system to encourage flow into a building by creating a pressure differential to have advective flow into the building.
    Any use of multiple lines of evidence requires the collection of a sufficient number of lines of evidence that support or provide evidence to the conclusion being made. It is the submitter’s obligation to complete the analysis of the lines of evidence and provide an initial analysis on how the information supports the conclusion being made.
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