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Volume 4: Investigative approach for petroleum VIAP

C.3.1 Petroleum vapor source within 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 petroleum vapor source in 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,
  • Mobile NAPL or contaminated groundwater above the appropriate VIAC are directly within the backfill, and
  • The utility backfill with the mobile NAPL or contaminated groundwater above the appropriate VIAC in it leads to a structure.

If the mobile NAPL or contaminated groundwater above the appropriate unrestricted residential VIAC has entered into the more permeable utility backfill, it is more likely to migrate beyond the extent of the LIZ and may require additional characterization.

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

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

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