Volume 3: Investigative approach for VIAP
1. Introduction

1.2 Vapor intrusion (VI)

VI is a human exposure via inhalation of indoor air that results from the migration of vapors from soil, groundwater or NAPL in the subsurface that contain VOCs into overlying buildings. A vapor source, migration route, and an occupied building (human receptor) must be present for the pathway to potentially pose an unacceptable health risk. In addition, the source of chemicals must have sufficient mass, be sufficiently volatile and toxic to cause a risk or potential harm to human health.

Figure 1-2 provides a simplified schematic illustration of VI for a source of vapors. As shown, VOCs in the soil and dissolved in the groundwater can volatilize and enter the vapor phase. These vapor phase compounds can then migrate both vertically or laterally within the subsurface and move through diffusion and advection into an overlying or adjacent building. Vapor or soil gas migration can occur in any direction due to pressure gradients, variations in soil type, permeability, and moisture content. Pressure gradients near a building will influence vapor migration directly into or out of a building and can be the result of barometric pressure changes or pressure differences between a building's interior and the subsurface. Such gradients can cause vapor near a building to enter through foundation cracks, sumps, or other preferential pathways (ITRC, 2007). VI can also be influenced by the advective air movement within a building caused by thermal and air density variations between the building interior and the outside air. This creates vertical airflow through the building (i.e., via the chimney or other openings) and is known as the building stack effect. Advective flow occurs both into and out of a building.

Note: It is common to confuse direct volatilization with VI for at least three reasons:
1) direct volatilization is sometimes discovered because of a VI investigation; 2) some buildings experience both VI and direct volatilization; and 3) direct volatilization site contaminants may enter living spaces as vapors. Regardless of the terminology used, it is important to explain that direct volatilization requires a different approach to mitigation than VI.

1.4 Conceptual Site Model (CSM)

The value of a CSM in evaluating the potential risks associated with the VIAP is well established by sources such as ITRC (ITRC, 2007; ITRC, 2014; ITRC, 2020, ITRC 2026), USEPA (USEPA, 2015b), and DOD (USDOD, 2009). The CSM helps assess the potential risk, identify data gaps, and communicate findings. It plays a key role in understanding how to design a sampling program. A CSM plays an equally critical role when evaluating whether direct volatilization to indoor air is a potential exposure pathway instead of, or in addition to, traditional vapor intrusion. Given the lack of degradation with recalcitrant VOCs, if there are receptors nearby the initial CSM should be there is an unacceptable risk and should be evaluated with sufficient data to confirm or refute the initial CSM.

A CSM provides an iterative representation of the site data and information collected from the property, or properties, and guides the decision-making process. The CSM should be refined throughout the life of the project as new information is acquired. Additional information on the development of a CSM can be found in ASTM International E1689-95 (2008) Standard Guide for Developing Conceptual Site Models for Contaminated Sites.

Vapor source for a release

CSMs assist in defining and depicting the nature and extent of the vapor source and identifying where a potentially unacceptable risk for the VIAP may occur to guide further evaluation or response actions. The vapor source is key to understanding and identifying where potentially unacceptable risks for the VIAP may occur and is generally associated with exceedances of the applicable unrestricted criteria. The applicable unrestricted Volatilization to Indoor Air Criterion (VIAC) can also be used to understand the full extent of a person’s obligations that exist under Part 201. See Volume 6 – Volatilization to the Indoor Air Criteria for more information.

Note: If an evaluation is made in accordance with Rule 299.14(5) and Rule 299.24(5) using more representative data such as soil gas, the soil gas data can be used to show compliance with the soil and groundwater VIAC (does not represent a vapor source) and that an unacceptable risk will not occur for a specific building. The need for land or resource use restrictions will be highly dependent on-site conditions. How and where the soil gas and vapor samples were collected in relation to the vapor source is crucial to determine if future land uses can be evaluated with representative soil gas samples. See Section 6 and Attachment D for more details.

Criteria under Part 201

A CSM will aid in identifying and establishing the appropriate criteria under Part 201. More information on how to establish the criteria under Part 201 is provided in Volume 6 – Volatilization to the Indoor Air Criteria. However, the key considerations for criteria include the following:

Unrestricted residential criteria

The Johnson and Ettinger (J&E) Model is the fate and transport model used by EGLE for development of the Part 201 generic GVIIC, SVIIC, and Part 213 RBSLs. These criteria were developed to address human health risks resulting from VOCs migrating into the indoor air from the groundwater and soil. The GVIIC and the SVIIC were originally developed by the Michigan Department of Environmental Quality in 1998, and the methodology (including the J&E Model inputs and exposure assumptions) and resulting generic criteria were promulgated as part of the 2002 Part 201 Administrative Rules.

Several assumptions used in EGLE’s application of the J&E Model are not applicable for all sites; therefore, it is critical to evaluate whether use of the generic criteria is appropriate for assessing the potential risk for VI at a site when actual site conditions do not match the assumptions utilized in the model. To ensure the consideration of critical J&E Model assumptions, Rule 299.14(2) and 299.24(2) of the Part 201 Administrative Rules identify conditions when the applicability of the GVIIC and the SVIIC is not appropriate. It has been found that the J&E model tends to overpredict indoor air concentrations for aerobically biodegradable compounds but may under predict indoor air concentrations for recalcitrant compounds; therefore, generally, the SVIIC and GVIIC are more applicable to aerobically biodegradable compounds (e.g. petroleum hydrocarbons) than recalcitrant compounds (e.g. trichloroethylene or tetrachloroethylene).

Site-specific criteria

A site-specific evaluation of the VI pathway considers and evaluates the actual site conditions rather than using generic assumptions. Volume 6 – Volatilization to the Indoor Air Criteria identifies generic screening values that may be used as criteria when applicable. Other site-specific approaches can be utilized when appropriate and supported by documentation.

Note: EGLE generally recommends that a detailed site-specific evaluation be completed on any site or property that exceeds the identified generic screening values in Volume 6 – Volatilization to the Indoor Air Criteria if the exceedance is less than ten times or the CSM differs significantly from how the values were established.

Occupied building

20120f(2) identifies that the VIAP is not a reasonable and relevant pathway if there is not a current or planned occupied building. An occupied building is a structure intended for routine human occupancy and use, or for which human occupancy and use is reasonably anticipated based on current or planned land use, zoning, permitting, or similar documentation. Occupied buildings include, but are not limited to, residential, commercial, institutional, and industrial buildings such as homes, restaurants, schools, offices, warehouses, and retail facilities. Occupied buildings do not include sheds, carports, pump houses, or other structures not intended for routine human occupancy, except for brief or incidental access (e.g., maintenance). Non-occupied structures may be naturally ventilated and may lack building systems typically associated with human occupancy (e.g., HVAC or plumbing); however, the presence or absence of such systems alone does not determine occupancy status.

If the structure is not an occupied building, the VIAP pathway is not relevant, and no further evaluation is needed. If the structure is determined to be an occupied building, but the exposure assumptions do not match those included in the development of criteria for residential or non-residential use, site-specific criteria should be developed based on the specific use of the building.  See Volume 6 for more information on the development of site-specific criteria.

Table 1-1 provides a decision matrix to identify when a structure is an “Occupied Building” for the VIAP.

Occupied Building Screening Matrix

Table 1-1A: Step 1 – Determine if the structure qualifies an “occupied building” for VIAP relevance

Table 1-1B: Step 2 – Determine whether the VI pathway is relevant based on actual or expected use

Table 1-1C: Step 3 – Determine whether generic of site-specific criteria apply based on actual exposure potential