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Volume 4: Investigative approach for petroleum VIAP

C.3.2 Petroleum vapor source in an underground conduit

This section is only relevant when all three conditions exist:

  1. Mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC is in contact with and can directly enter a conduit,
  2. The conduit can transport vapor, mobile NAPL, or contaminated groundwater above the appropriate unrestricted residential VIAC within it, and
  3. The conduit directly connects to a structure.

Where the CSM and data identifies that mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC is in contact with an underground conduit, an evaluation is necessary to ensure that petroleum vapors are not preferentially migrating directly into a structure. For petroleum, this is limited to situations where a) there is mobile NAPL adjacent and in contact with a conduit or b) the conduit is located beneath the groundwater that is contaminated above applicable unrestricted residential VIAC and a dissolved vapor source enters into it.

The initial step in determining whether an underground conduit needs to be evaluated is to understand whether mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC can enter into a conduit and if the conduit directly connects to a structure. If either of these cannot occur, no further evaluation of the utility is warranted. Though many underground utilities that are under pressure, such as force main sewer lines, gas lines, or water lines, may screen out, others, such as combined sewer lines, sewer lines, and other utilities that have a void space, may screen in. If mobile NAPL enters into a utility, then there is a potential for acute risks at the site that must be evaluated. See Attachment A for more information.

The next step is to determine whether the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC has entered into the conduit. If the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC has entered into a conduit, it is more likely to migrate beyond the extent of the LIZ and may require additional characterization.

An underground conduit is initially screened using photo ionization detector (PID) to aid the evaluation. Video is often used to evaluate the integrity of the conduit and whether the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC is in or can enter into the conduit. Once this evaluation is completed, the next step in the evaluation is the collection of vapor samples from manholes or other direct access points. The analytical sampling should include the same parameters associated with the release (Appendix B in the March 2016, Application of Target Detection Limits and Designated Analytical Methods) and should include location(s) upstream of where the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC entered into the conduit. A standard operating procedure for the collection of vapor samples is provided in Volume 2 – Investigation Methods for the Volatilization to the Indoor Air Pathway (VIAP).

The number of sampling events is based on whether the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC is known to have entered into the conduit or not, and because of the high variability associated with conduits, increased sampling frequency is needed when it is known to be entering. Conduit sampling frequencies are identified in Table C-4. Detections in the conduit necessitates development and approval of applicable preferential conduit vapor SSVIAC to evaluate risk. Concentrations above the applicable preferential conduit vapor SSVIAC specifically for the entry of mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC into a conduit requires that access points are investigated until the extent is defined using the applicable SSVIAC. In addition, the responsible party should immediately implement response activity to prevent the further entry of mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC into a conduit. Further discussion on the response activities can be found in Volume 5 – Response Activity.

Table C-4: Vapor Source entering an Underground Conduit

Known or Suspected Sampling Frequency Response Action
Not in Contact None Not applicable
In contact and determined to not be entering None Response Actions may be needed to ensure vapor source will not enter in the future.
Suspected but unconfirmed
  • Quarterly for 1 year – mobile NAPL
  • One Sampling Event – contaminated groundwater
Any detection above the SSVIAC moves the utility into the known vapor source entering into an underground conduit
Known – Occurring
This occurs through response action to prevent the entry of the vapor source into the conduit
Progress sampling weekly until the vapor source is controlled. Structures connected to the utility should be evaluated for the entry of vapors and explosive conditions. Implement immediate response activity
Known – post corrective or response action Sample monthly for 3 months then quarterly for 3 additional quarters Any detection above the SSVIAC requires sampling to return to the Known – Occurring sampling frequency

The need to prevent the future infiltration of vapor source into a conduit must occur. However, a demonstration that the contribution to a conduit, such as a sewer line that does not add or impact the utility due to the contributions of up-gradient sources, may reduce the need to implement a response activity. This evaluation is typically done by sampling the liquids and vapor in the conduit upstream and downstream from the petroleum vapor source.

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