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Volume 4: Investigative approach for petroleum VIAP

4. Apply a lateral screening distance – Step 2

NOTE: Structures and properties outside of the LIZ require no further evaluation. Utilities outside of the LIZ only require evaluation if they are found to be transporting vapors.

Once the delineation of the vapor source has been established to applicable unrestricted residential groundwater VIAC and the NAPL body, a LIZ can be applied.

The LIZ is used to decide whether a structure or a property is close enough to a vapor source to be considered a potential risk to the VIAP. 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. The lateral inclusion distances and vertical screening distances should be the same because the physical processes acting on PHC vapor sources (mass flux from the source, oxygen demand, and biodegradation) are the same in the vertical and horizontal directions (ITRC 2014). The vertical screening distances are expected to apply 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.

The LIZ should be based on the delineation of the groundwater in exceedance of applicable unrestricted residential VIAC and/or the NAPL vapor source. 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 drawn from the edge of the delineated groundwater plume

For a petroleum facility, a 30-feet 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 that the spacing of the borings is a long distance 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 soil and groundwater samples should be collected to complete the characterization 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 for petroleum to reduce the LIZ to the extent of 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, 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 lateral inclusion may be established. However, 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.

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