Under Part 201, once the appropriate VIAC have been established, the extent of the vapor source should be defined to the unrestricted criterion using appropriate sampling methodology.
Note: If an evaluation is made in accordance with Rule 299.14(5) or Rule 299.24(5) using more representative data such as soil gas, it is possible to show compliance with the soil and groundwater criteria or screening levels, which aids in and limits the delineation of the vapor source. If soil gas is used to show compliance with soil and groundwater criteria or screening levels, the soil or groundwater media are no longer considered a vapor source, regardless of concentrations. Soil gas data may also be used to show that the vapor source does not represent an unacceptable vapor intrusion risk for a current or future building. The need for land or use restrictions will be highly dependent on-site conditions, how and where the soil gas samples were collected in relation to the vapor source and buildings (current and future). See Attachment D for more details.
Investigation methods are further described in this volume and in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway; however, key considerations are described below.
Analytical
The chemicals selected for analysis at a facility depend on the product released, type of contamination, degradation products, as well as the objectives of the investigation.
Geology and Hydrogeology
Geologic and hydrogeologic information should be collected throughout the investigative process and incorporated into the CSM until there is an adequate understanding of the facility conditions, where the vapor source is located, and how vapors may be transported from the subsurface into indoor air. The processes, methodology, and sampling density necessary to understand a facility is not discussed as part of this document. Each of these items are highly dependent upon the actual facility conditions, the variability that is present within the geology and hydrogeology in the area where the release occurred, and the potential for vapors or a vapor source to enter a building (e.g., the greater the potential for risks, the more data needed for evaluation). The key for evaluating the VIAP is utilizing the data that is collected and evaluating how that data changes the CSM, impacts the need for additional information or evaluation, or may impact the way vapors from a vapor source migrate to and into a building, as well as evaluating what potential risks may be present currently or in the future.
Vertical Distribution of Contaminant Concentrations Below the Water Table
Groundwater or NAPL, as a subsurface vapor source, will be influenced by the vertical distribution of contaminant concentrations in the upper reaches of the aquifer and by seasonal fluctuations in the groundwater table (McAlary et al., 2011; ITRC’s PVI-1, 2014; and USEPA, 2015a and 2015b). Vapor sources that are beneath the water table or contaminated groundwater plumes that are migrating below the water table do not need further evaluation for the VIAP unless they intersect a conduit. The need for a land or use restriction may be applicable if land use changes could result in an unacceptable risk or an evaluation cannot be made for future land uses. This is further discussed in Volume 5 – Response Activity.
Utility Backfill and Conduits that can Transmit Vapor or a Vapor Source Considerations
The utilities and utility corridor must intersect the vapor source for there to be alterations in migration routes of the vapor source or direct transport. For the utility corridors, it is important when defining the vapor source to identify utility corridors that may have more permeable material than the adjacent native soil as well as a vapor source that can migrate (e.g., mobile NAPL and dissolved phase). For the direct transport evaluation, it is important to identify the utilities where the vapor source can directly enter (e.g., mobile NAPL or contaminated groundwater), the utility and serve as a conduit to directly transport a vapor source or vapors into a building.
Note: Utilities that are pressurized (e.g., water and gas lines), or that are documented to not connect to buildings (e.g., dedicated storm sewers), are not necessary to evaluate beyond supplying supporting documentation. Supporting documentation should include the type and location of the utility.
Consideration of the backfill material as a preferential pathway for the vapor source is limited to facilities that have site soils that consist of finer grain soils materials (e.g., silts and clays) that were backfilled during the installation of the utility with coarser grain materials (e.g., gravels and sands). For the backfill to act as a preferential pathway, the groundwater vapor source must be able to enter the backfill material and then migrate along the utility corridor.
Note: Backfill in a utility corridor that is similar in grain size and permeability to the native material are not necessary to evaluate beyond supplying supporting documentation commonly obtained through standard soil sampling techniques and documentation.