4. Apply a lateral screening distance – Step 2

Once the delineation of the groundwater to applicable unrestricted residential VIAC and the NAPL body is complete, a LIZ can be applied. The LIZ is used to evaluate whether a structure or a property is close enough to a vapor source to be considered a potential risk to the VIAP and require further evaluation. For petroleum, the LIZ includes the extent of the groundwater and/or NAPL vapor source as well as the applicable screening distances beyond the delineated extent of the petroleum vapor source.

NOTE: The LIZ distances and vertical screening distances should be the same because the physical processes acting on PHC vapor sources (mass flux from the source, O2 demand, and biodegradation) are the same in the vertical and horizontal directions (ITRC 2014).

The vertical screening distances can be applied laterally in the absence of preferential pathways (natural or man-made). Structures, properties, and utilities located within the distance established by the LIZ (including those structures above a vapor source) require further evaluation as discussed in Section 5. For a well characterized vapor source, the following distances may be applied:

• 15 feet from NAPL beginning at a location where NAPL is not located
• 5 feet for groundwater contamination (i.e., dissolved-phase sources), can be measured or drawn from the edge of the delineated groundwater plume

For a petroleum release, a 30 foot LIZ may be applied for a facility that is not well characterized, when there are physical features or obstructions (e.g., a road) that require long spatial distances between the borings or sampling locations, and the extent of contamination is interpolated. If warranted, the use of statistical methods, such as Kriging, can provide a means for justification of interpolated extent. EGLE recommends that where possible, sufficient delineation is completed to use the smallest LIZ rather than apply the 30-foot LIZ distance.

In some cases, a person may wish to reduce the LIZ distance further than the distances identified above. It may be possible with a petroleum release to reduce the LIZ to the extent of the groundwater and/or NAPL vapor source with supporting data. This can be done through a representative soil gas investigation that shows PHC vapors are not migrating past the sampling location.

Alternatively, if a representative soil gas sample cannot be collected, a demonstration that includes a site-specific evaluation of O₂ in soil gas at the extent of the LIZ can be conducted. If the O₂ in the soil gas is greater than 2%, then there is sufficient O₂ to support rapid aerobic degradation and a reduction in the LIZ may be established. However, the LIZ cannot be less than the extent of the vapor source.

NOTE: The LIZ cannot be less than the extent of the vapor source, which is the NAPL body and the dissolved phase groundwater contamination in exceedance of applicable unrestricted residential VIAC. If an evaluation is made in accordance with Rule 299.14(5) using more representative data such as soil gas, the soil gas data can be used to show compliance with the groundwater VIAC.