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Volume 4: Investigative approach for petroleum VIAP

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

NOTE: Though a LIZ extends beyond a vapor source it also includes the footprint of the vapor source. 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 may be possible to show that the NAPL or petroleum-contaminated groundwater does not pose a VIAP risk to the current structure.

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 preliminary screening process identified below should begin immediately for structures within the preliminary or assumed LIZ. The screening process discussed below includes an approach based on ITRC’s vertical screening distances described in ITRC (2014).

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

All properties and buildings must be identified within the LIZ. This also includes any utilities that can transport vapor or the backfill surrounding the utilities that are able to transport the vapor source. Details on what information should be obtained is provided in Table 5-1.

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

  • Lot lines
  • Current use or property restrictions
  • Depth below grade of current structure
  • Construction materials and methods
  • Foundation
  • Presence of the sump or other features that may allow for the direct volatilization to occur
  • Location
  • Depth below ground
  • Ability to transport vapors or have a vapor source enter into it
Backfill surrounding the Utilities
  • Type of soil found at the property and throughout the facility

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 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 structure below grade?
  • Is the distance between the vapor source and the structure known?
  • Where are utilities located in relation to the mobile NAPL?
  • Can the mobile NAPL or dissolved groundwater enter the utilities?

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 next. 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 as part of a Brownfield Development Project. 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

NOTE: If a party has not developed their own site-specific VIAC and are 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 or exposure risk posed by the hazardous substance or other factors may be proposed for review and approval to EGLE that supports an unacceptable risk will not occur.

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 as modeling done by the USEPA (2012b) and sampling by EGLE confirm that the future construction of a building will change the soil gas concentrations 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, 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:

  • Structure over a Vapor Source
    • Vapor Source not in Contact with a Structure
    • Vapor Source in Contact with a Structure
  • Structure adjacent to a Vapor Source
  • Conduit in Contact with mobile NAPL
  • Structures are Not Currently Present
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