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 and 2013) 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 and create oxygen shadows. 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 or requires additional evaluation. The most common scenario where this occurs is where shallow soil gas samples are collected less than 4 feet below the ground due to the presence of shallow groundwater at concentrations above the applicable VIAC and where there currently is not a structure. Vapor data is likely to indicate that the VIAP is unlikely to pose an unacceptable risk; however, to ensure that a future property use doesn’t alter the exposure scenario (e.g. changing it from VIAP to direct volatilization) institutional controls are often necessary.

However, where there is no structure present, the shallow vapor source has been addressed via remediation or other means 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 or per Rule 299.14(5) soil gas sampling data can be used to show compliance with the groundwater VIAC. Even if there is a vapor source still present, if greater than 10-feet below ground and soil gas verifies that it is unlikely to pose an unacceptable risk for all current and reasonable future land uses, a land or resource use restriction would not be necessary. Information on this process, when a land or resource use restriction is required, and other site management strategies is described in Section 6, Attachment D.4, and further detailed in Volume 5 – Response Activity.