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Volume 4: Investigative approach for petroleum VIAP

C.4.0 Structures are not currently present

Areas where structures are not currently present pose unique and difficult challenges for the VIAP especially when the vapor source is located less than 10-feet below the ground surface. When the vapor source is less than 10-feet below ground, a representative soil gas sample can be collected; however, reasonable future property uses must be considered. Whether or not an unacceptable risk for the VIAP will occur in a future structure requires a thorough understanding of the vapor source, what the soil gas concentrations are near the vapor source, and the actual or potential structure that will be constructed. Modeling done by USEPA (2012) provides simplified simulation models to illustrate graphically how subsurface conditions with NAPL as a vapor source and building-specific characteristics, such as the presence or absence of a structure, is likely to impact the concentrations detected. In addition, a change in land use could alter the exposure scenario changing the vapor source from not in contact to in contact. Therefore, in many situations with shallow vapor sources, even when an evaluation is made, there still remains a need to place a land or use restriction on the property to ensure the exposure scenario that was evaluated remains the same.

However, where there is no structure present, the shallow vapor source has been addressed or only a deeper-vapor source of petroleum remains greater than approximately 10-feet below ground, appropriately placed and monitored soil gas samples may allow for an evaluation of current and reasonable future land uses. The soil gas sampling allows for a determination that either the NAPL or groundwater vapor source does not pose a vapor intrusion risk per Rule 299.14(5) and Rule 299.24(5). If there is a vapor source still present, it is unlikely to pose an unacceptable risk for all current and reasonable future land uses. Information on this process, when a land or resource use restriction (IC) is required, and other site management strategies is described in Section 6, Attachment D, and further detailed Volume 5 – Response Activity.

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