C.3.2 Petroleum vapor source in an underground conduit

This section is only relevant when all three conditions exist:

1. Mobile NAPL is in contact with and can directly enter a conduit,
2. The conduit can transport vapor or mobile NAPL within it, and
3. The conduit directly connects to a structure.

Where the CSM and data identify that mobile NAPL is in contact with an underground conduit, an evaluation is necessary to ensure that petroleum vapors are not preferentially migrating directly into a structure.

The initial step in determining whether an underground conduit needs to be evaluated is to understand whether mobile NAPL 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.

The next step is to determine whether the mobile NAPL has entered into the conduit. If the mobile NAPL has entered into a conduit, it is more likely to migrate beyond the extent of the LIZ and may require additional characterization. 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.4 for more information.

An underground conduit is initially screened using a photo ionization detector (PID) to aid the evaluation. Video is often used to evaluate the integrity of the conduit and whether the mobile NAPL 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, such as conduit cleanouts. 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 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 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 site-specific VIAC (SSVIAC) to evaluate risk. Concentrations above the applicable preferential conduit vapor SSVIAC specifically for the entry of mobile NAPL 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 into a conduit and may need to mitigate conduit vapors to prevent entry into structures. If mobile NAPL is not entering a utility conduit, then the prevention of a future infiltration of the mobile NAPL vapor source into a conduit must occur. Further discussion on the response activities can be found in Volume 5 – Response Activity.

Table C-4: Vapor Source Entering an Underground Conduit