Volume 3: Investigative approach for VIAP
Attachment C.3 3.0 - Utilities

A vapor source may travel from the vapor source area to a receptor along a preferential pathway such as utility corridors, which could include any utility corridor that intersects the vapor source. The vapor source may also enter the utility itself that will act as an underground conduit that can transport the source and vapor which may allow direct entry into a building. Both scenarios are described in more detail in the following Section C.3 3.1 and Section C.3 3.2.

C.3 3.1 - Vapor source within utility backfill material

Backfill material in utility corridors can be more permeable than the adjacent native soil and, when it is, it may result in a vapor source migrating preferentially along these pathways. The characterization and evaluation for the VIAP associated with a release or migration of a vapor source in utility backfill material around utility corridors is limited to utilities that have all the conditions below:

• Soil types surrounding the utility corridor are less permeable than the utility backfill material that creates preferential migration pathways for the vapor source,
• A vapor source above the appropriate VIAC is directly within the utility backfill, and
• The utility backfill with the vapor source above the appropriate VIAC in it leads to a building.

If the vapor source above the appropriate unrestricted residential VIAC has entered the more permeable utility backfill, it is more likely to migrate beyond the extent of the LIZ that was identified during the traditional investigation approach and may require additional characterization. Care must be taken when conducting an evaluation near utilities and the possibility for exposures and unacceptable risks should be considered prior to collecting data. Vapors beyond the LIZ require evaluation when the backfill contains more permeable fill than the surface or covering material.

A vapor source that is in the utility backfill material and is directly beneath a building is evaluated using the sampling methods described in Section C.3 1.0. This includes the use of sub-slab soil gas samples within the backfill material beneath the building and alongside the path of the utility. The number sampling events is described in Table C.3 – 4.

A vapor source that is in the utility backfill material that is adjacent to a building and within screening distances described in Section 4 is evaluated using either near-slab soil gas or soil gas wells like the approach in Section C.3 2.0. For this scenario, the near-slab soil gas or soil gas wells are placed directly in the backfill between the vapor source and the building. The building can be evaluated using the approach described in Section C.3 1.0. The number of sampling events is described in Table C.3 – 4.

C.3 3.2 - Vapor source in an underground conduit

This section is only relevant when all three conditions exist:

1. Vapor Source is in contact with and can directly enter a conduit,
2. The conduit can transport the vapor source and vapors within it (i.e., it is not pressurized), and
3. The conduit directly connects to a building.

Where the CSM and data identifies that a vapor source is in contact with an underground conduit, an evaluation is necessary to ensure that vapors are not preferentially migrating directly into a building. The most common conduit that this occurs is a sanitary or combined gravity flow sewer.

The initial step in determining whether an underground conduit needs to be evaluated is to understand whether vapor source can or has entered a conduit and if the conduit directly connects to a building. If either of these cannot occur, no further evaluation of the utility is warranted. Many underground utilities that are under pressure (e.g., force main sewer lines, gas lines, or water lines) and will screen out. 

The next step is to determine whether the vapor source has entered the utility which is referred to as a conduit. If the vapor source has entered 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. Because a PID is often inconclusive when evaluating chlorinated compounds, video is used to evaluate the integrity of the conduit and whether the vapor source is in or can enter 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 and should include the upstream location of where the vapor source entered 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.

The number of sampling events is based on whether the vapor source is known to have entered 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.3 – 5.

Concentrations above the applicable preferential conduit vapor SSVIAC or screening levels specifically for the entry of vapor source into a conduit requires that access points along the conduit using manholes are investigated until the extent is defined using the applicable SSVIAC or screening levels. If above the SSVIAC or screening levels, response activities should be immediately implemented to prevent the further entry of vapor source into a conduit and to verify that the building’s plumbing (e.g., p-traps, etc.) is intact to prevent gases from a conduit entering a building. This should be completed for every lateral where the conduit main exceeds SSVIAC. In addition, there may also be a need to flush the conduit and/or mitigate conduit vapors to prevent entry into buildings. If vapor source is not currently entering a conduit but may in the future, then the prevention of a future infiltration of the vapor source into a conduit must occur unless it can be demonstrated through a CSM that the vapor source is unlikely to enter it. Further discussion on the response activities can be found in Volume 5 – Response Activity.

Table C.3 – 5 –Vapor Source entering an Underground Conduit