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Volume 4: Investigative approach for petroleum VIAP

C.1.2 Vapor source in contact with a structure

A vapor source in contact with a structure occurs when there is either 1) NAPL in contact with a structure or 2) there is a shallow groundwater vapor source above applicable unrestricted residential VIAC in contact with or entering a structure. A vapor source is in contact with a structure when, after considering seasonal variation to the depth to groundwater and the capillary fringe, there is not a measurable distance between the vapor source and the structure. The structure is measured from the bottom of the slab; or the depth of footings if they can transport vapor into the structure (poured footings cannot); or the subsurface utilities (including the sump) if they can transport vapor or the vapor source.

When a vapor source is in contact with a structure, direct volatilization is likely and will need to have an appropriate response activity implemented to evaluate the risks and likely to prevent the direct volatilization into the structure (see Section 6). For the evaluation of immediate risks when this scenario occurs, EGLE’s preference is the sampling of indoor air (Section 2). To account for the variations in indoor air, EGLE recommends continuous indoor air monitoring. If the vapor source is being transported via utilities and sumps, EGLE recommends sampling the media in the utilities and sumps, as well as sampling the head space for vapor.

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