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Volume 4: Investigative approach for petroleum VIAP

Attachment B - Vapor source delineation for petroleum

Under Part 201, NAPL, the presence of shallow groundwater, and certain foundation types prevent the use of the generic groundwater volatilization to indoor air inhalation criteria and soil volatilization to indoor air inhalation criteria for the evaluation of the VIAP. Therefore, if these conditions exist, the determination of the extent of a vapor source requires the development of site-specific criteria and are referred to as the VIAC. Once the criteria have been established, the extent of the vapor source should be defined using appropriate sampling methodology.

For petroleum releases, due in part to aerobic biodegradation in clean biologically active soil, as discussed in ITRC’s PVI-1 (2014) and USEPA (2015a and 2015b), the vapor source is limited to where the NAPL is located (i.e., residual, mobile, and migrating) and the location where the dissolved phase groundwater contamination is present above the applicable unrestricted residential VIAC. The extent of the NAPL body will be determined using multiple lines of evidence as discussed in EGLE’s June 2023 Non-Aqueous Phase Liquid – Petroleum Releases Characterization, Remediation, and Management Guidance, which should include the chemical analysis of soil samples. Investigation methods are further described in Section 5 and Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP).

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, it is possible to show that the NAPL or petroleum-contaminated groundwater does not represent a vapor intrusion risk for a current structure. For future structures, the need for land or use restrictions will be highly dependent on-site conditions, how or where the soil gas samples were collected in relation to the vapor source, and if future structures can be evaluated with the soil gas samples. See Attachment D for more details.

For NAPL and dissolved-phase petroleum sources in the groundwater, understanding the NAPL body and groundwater plume stability is a key factor in establishing the extent of the vapor source. This document is not intended to fully address how stability is established; however, in general stability is determined by:

  • Stable footprint over time,
  • Stable or decreasing concentrations in the groundwater, and
  • Residual NAPL located beyond where mobile NAPL is located in the NAPL body.

Additional considerations for the vapor source delineation are provided next.

The chemicals selected for analysis at a potential PVI facility depend on the source and type of contamination, as well as the objectives of the investigation. A list of the recommended parameters for petroleum products can be found in Appendix B of the Application of Target Detection Limits and Designated Analytical Methods RRD Resource Materials, 2016.


If vapor samples are collected, an assessment of biodegradation should be conducted and include the analysis of O2, CO2, and methane. After O2 is depleted, methanogenic bacteria convert petroleum hydrocarbons to methane and carbon dioxide. If methane is above 1% by volume, then conditions are anaerobic and is indicative that the sampling is likely near a petroleum NAPL source. The combined sum of CO2 and O2 is typically around 21% by volume in the atmosphere and with aerobic degradation. If there is an excess of CO2, then anaerobic biodegradation is likely occurring, and methane is likely being oxidized to CO2 under aerobic conditions. Additionally, nitrogen may be collected as an indicator as to whether there is replenishment of atmospheric air or an advective flow of soil gas that flushes out the air. If nitrogen is displaced (much less than 79% by volume), then either the bulk soil gas is migrating or the sample was collected under a vacuum. (ITRC, 2014).

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 and where the vapor source is located. 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 petroleum release occurred, and the potential risks being evaluated. 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 into a structure, 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 water table 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 at depths greater than 5-feet below the seasonal low-water elevation (10-feet may be assumed if the water table variation is unknown) do not need further evaluation for the VIAP. Seasonal evaluations will need to be made for NAPL bodies that are beneath the groundwater and could be exposed to the vadose zone with elevational changes to the groundwater surface. The need for a land or use restriction may be applicable if land use changes could result in an unacceptable exposure 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 porous and 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 utilities, it is important to identify the utilities where the vapor source can directly enter, and the utility then may serve as a conduit to directly transport vapors into a structure.

Conduits that can transmit vapor into a structure that must be evaluated must directly connect a vapor source to a structure and be able to transport vapors. Utilities that are pressurized (e.g., water and gas lines), or that are documented to not connect to structures (e.g., dedicated storm sewers) are not necessary to evaluate beyond supplying supporting documentation.

NOTE: Utilities that are pressurized (e.g., water and gas lines), or that are documented to not connect to structures (e.g., dedicated storm sewers) are not necessary to evaluate beyond supplying supporting documentation.

Consideration of the backfill material 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).

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.
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