5. Evaluate within the extent of the lateral inclusion zone – Steps 3 through 6

Any structure or property within the LIZ (including those above a vapor source) require an evaluation of the potential for an unacceptable risk to human health. In some cases, it may not be possible to determine a final LIZ until the investigation is complete. The evaluation and screening process identified below should begin immediately for structures within the preliminary or assumed LIZ.

5.1. Identify properties, structures, and utilities – Step 3

All properties, buildings, utilities, and preferential pathways within the LIZ must be identified. The utilities consist of any conduits that NAPL can enter, and the preferential pathways consists of any utility backfill that is more permeable than the surrounding materials that may alter the migration of the vapor source. Details on what information should be obtained is provided in Table 5-1.

NOTE: The evaluation of utilities and preferential pathways is limited to the vapor source migration. The migration of PHC vapors in utility backfill or the migration of vapors to a utility doesn’t warrant additional consideration for the risk assessment as the PHC vapors will rapidly degrade as outlined in Section 1.3.

Table 5-1: Information on the Buildings, Properties, and Utilities Required

5.2. Initiating the screening process – Step 4

The screening process should begin as soon as a structure is identified within the LIZ (see Section 5.1 above), even if the LIZ is not completely established.

The screening process is implemented by first evaluating if there is sufficient data to determine if the vertical screening distances may be applied to a structure in the LIZ. In order to complete this evaluation, data that was collected in Section 4.0 and 5.1 is evaluated to confirm that the following questions can be answered:

• Is the depth of groundwater known?
• What is the depth of the building foundation below grade?
• Is the location of the vapor source known?
• Is the distance between the vapor source and the structure known?
• Where are conduits located in relation to the mobile NAPL (if present)?
• Can the mobile NAPL enter the conduit?

If the questions cannot be answered, additional data collection is warranted. If the questions can be answered, a person should then proceed to Step 5 and screen the building(s) using the vertical separation distances discussed in Section 5.3 below. Sampling methods are further described in Section 5.4 and Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP).

Additional Considerations

The VIAP can only pose an unacceptable risk if there is a structure present. Therefore, a person implementing their due care obligations under Sec. 20107a should focus on any current or planned structure when evaluating the site. However, when closure or a no further action is sought, future use must also be considered, which could include a future development or changes to the current structure. See Attachment D.4 for when land and resource use restrictions may be needed.

NOTE: If a party is using the site-specific VIAC provided by EGLE: Under Sec. 20120b, alternate site specific evaluations that better reflect the best available information concerning the toxicity, exposure risk posed by the hazardous substance, or other factors that support an unacceptable risk will not occur may be proposed for review and approval to EGLE.

Petroleum vapor sources that remain and are deeper than the bottom of a future structure and greater than 5 feet below grade are typically able to be assessed. However, shallow vapor sources (<5 feet) are often difficult to assess.  Modeling done by the USEPA (2012b and 2013) suggest that if a sufficient mass of contaminants remain and soil gas concentrations are high, the construction of a building can change the soil gas concentrations or create an oxygen shadow and may cause an unacceptable risk to the VIAP, even though soil gas sampling data prior to construction may not indicate it. In areas where a structure is not present, but a shallow vapor source remains, a response action that uses a restricted closure or no further action may use a land or use restriction in lieu of further assessment.

5.3. Screen building using vertical separation distance – Step 5

Assess whether further investigation is necessary (in Section 5.4) based on the measured vertical separation distance between the building foundation (including the slab and the depth of any sumps that may allow for the direct volatilization to occur) and the top of the groundwater and/or NAPL vapor source. Vertical separation distances can be used to screen out a facility without any further vapor sampling. This evaluation may need to be reviewed on a seasonal basis to confirm the initial findings, especially when a CSM has required modification based on new data, the NAPL body isn’t stable (see EGLE 2023 Petroleum NAPL guidance), dissolved vapor source in the groundwater that has large elevational fluctuations, or vapor sources beneath the groundwater become exposed with groundwater fluctuations. The vertical separation distance should be based on the top of the vapor source and the bottom of the structure foundation – this distance should not be estimated.

For a facility with a petroleum release, the following initial screening distances may be applied:

• 15 feet for NAPL
• 5 feet for groundwater contamination (i.e., dissolved-phase sources)

If these vertical separation distances are met, then no additional data collection is warranted for the VIAP. If the structure cannot screen out, or if there are utilities that need to be further evaluated, a person should see Section 5.4 for sampling or Section 7 to consider an alternate method for evaluating the VIAP.

5.4. Data collection and evaluation – Step 6

If a structure or property is not able to be screened out using the vertical separation distances identified in Section 5.3 or if there are utilities that may directly transport vapors to a structure, then representative data will need to be collected. The data collection should be based on where the NAPL and/or groundwater vapor source is in relation to the structure. 

Additional information on data collection for the following scenarios is provided in Attachment C.4:

• Structure Over a Vapor Source
  • Vapor Source Not in Contact with a Structure
  • Vapor Source in Contact with a Structure
• Structure Adjacent to the Vapor Source
• Utilities
  • Petroleum Vapor Source within Utility Backfill Material
  • Petroleum Vapor Source in an Underground Conduit
• Structures are Not Currently Present