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ABS\uff1aThis chapter describes a method for PVI screening based on the vertical distance from a petroleum vapor source to a building foundation (vertical separation distance). Application of the method is expected to improve PVI screening and reduce unnecessary data collection at numerous petroleum release sites.<\/p>\n
This chapter describes a method for PVI screening based on the vertical distance from a petroleum vapor source to a building foundation (vertical separation distance). Application of the method is expected to improve PVI screening and reduce unnecessary data collection at numerous petroleum release sites.<\/p>\n
The vertical screening distance is the minimum thickness of soil between a petroleum vapor source and building foundation necessary to effectively biodegrade hydrocarbons below a level of concern for PVI (see Figure 1).<\/p>\n
![\"\"](\"https:\/\/www.envguide.com\/wp-content\/uploads\/2018\/07\/p22.png\")
\n Figure 1. Conceptual model of petroleum vapor transport for an LNAPL source and a dissolved phase source.<\/b><\/p>\n
The PVI screening process is divided into three steps (see Figure 2)<\/p>\n
Step 1: Develop a Preliminary CSM:<\/p>\n\t\t\t\t\t<\/div>The preliminary CSM is based on critical data obtained from an initial site investigation or valid preexisting data. If COC concentrations in soil and groundwater are below existing PVI regulatory criteria, the development of a preliminary CSM and use of a vertical screening distance are not required and no further PVI evaluation is needed.<\/p>\n
Step 2: Evaluate Site for Precluding Factors and Lateral Inclusion:<\/p>\n\t\t\t\t\t<\/div>If there are no precluding factors, evaluate whether current or future buildings are located within the lateral inclusion zone. If buildings are located within the lateral inclusion zone, continue to Step 3 for further screening. Buildings outside of the lateral inclusion zone require no further PVI evaluation.<\/p>\n
Step 3: Screen Building Using Vertical Separation Distance:<\/p>\n\t\t\t\t\t<\/div>Assess whether further site investigation is necessary based on the measured vertical separation distance between the building foundation and the top of the petroleum vapor source in soil or groundwater.<\/p>\n
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Figure 3-2. Flowchart for PVI screening application Step 1 – Develop Preliminary CSM<\/p>\n\t\t\t\t\t<\/div>\n The preliminary CSM is developed by collecting soil and groundwater data as part of routine initial site investigations. The CSM necessary for PVI screening has the following components:<\/p>\n\t \n\t\t Site Type<\/p>\n\t\t\t\t\t<\/div>\n USEPA has published empirical studies that reported different LNAPL screening distances based on facility type:<\/p>\n\t\t\t\t\t<\/div>\n\t \n\t\t Differences in the vertical screening distances according to site type may relate to the volume of the LNAPL release or extent of the LNAPL plume.<\/p>\n Petroleum Vapor Source<\/p>\n\t\t\t\t\t<\/div>\n The empirical studies found that vertical screening distances are longer for LNAPL sources than for dissolved-phase sources because LNAPL sources can generate higher concentrations of vapors.<\/p>\n The LNAPL may not be readily apparent unless there is measurable thickness of LNAPL in a nearby groundwater monitoring well. In these cases, a multiple-lines-of-evidence approach can be used for LNAPL identification (see Table 3-1).<\/p>\n Table 1. General LNAPL indicators for PVI screening Extent of Source<\/p>\n\t\t\t\t\t<\/div>\nProper delineation of the petroleum vapor source in soil or groundwater is critical in determining which buildings are within the lateral inclusion zone (see Section 3.1.5) and the vertical separation distance (Section 3.1.6). There is a higher probability of encountering LNAPL in the vadose zone near and below former or current UST\/AST equipment, in known or suspected release areas, along perching units or low-permeability soil layers, or in a smear zone near the water table. Soil sampling and soil screening using a photoionization detector (PID) or flame ionization detector (FID) are recommended for delineating vadose zone sources.<\/p>\n Precluding Factors<\/p>\n\t\t\t\t\t<\/div>\nTypical precluding factors include the following: Figure 3-3. Precluding factor: conduit intersecting source and entering building<\/p>\n Lateral Inclusion Zone<\/p>\n\t\t\t\t\t<\/div>\nThe lateral inclusion zone is used to determine whether a building is close enough to the lateral edge of a petroleum vapor source to be considered for PVI screening. This method is based on interpolation of contaminant concentrations between monitoring locations. Vertical Separation Distance<\/p>\n\t\t\t\t\t<\/div>\nVertical separation distance should be determined from the top of the source closest to the building foundation where these additional (secondary) sources are identified. Step 2 – Evaluate Building for Precluding Factors and Lateral Inclusion<\/p>\n\t\t\t\t\t<\/div>\n If no precluding factors are present, then determine whether the edge of the building foundation is within the lateral inclusion zone that extends 30 feet from the edge of the petroleum vapor source.<\/p>\n Step 3 – Conduct Screening with Vertical Separation Distance<\/p>\n\t\t\t\t\t<\/div>\n Several empirical studies have defined vertical screening distances for LNAPL and dissolved phase sources. Although the values derived for dissolved-phase and LNAPL sources vary slightly among the studies, they can be conservatively defined as:<\/p>\n\t \n\t\t
\n <\/b><\/p>\n\n\t\t\t\t\t\t\n\t\t\n
\n\t\t\t\t\t\t\n\t\t\t\t\t\t\n\t\t\n
\n\t\t\t\t\t\t
\n![\"\"](\"https:\/\/www.envguide.com\/wp-content\/uploads\/2018\/07\/t1.jpg\")
<\/p>\n\n\t\t\t\t\t\t\n\t\t\t\t\t\t
\n1. Preferential pathways that intercept both the source (either LNAPL or dissolved phase) and building foundations (see Figure 3-3).
\n2. Ongoing releases of petroleum products that result in expanding or mobile contaminant plumes.
\n3. Certain fuel types has insufficient data to draw confident conclusions for these fuel types. These fuel types include:
\n\t \n\t\t\n\t\t\n
\n\t \n\t\t\n\t\t\n
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\nLateral inclusion distances and vertical screening distances should be approximately the same. The vertical screening distances are expected to apply laterally in the absence of preferential pathways or significant hydrogeologic barriers.
\nA conservative, 30-foot lateral inclusion distance may be considered appropriate to incorporate the uncertainty.<\/p>\n\n\t\t\t\t\t\t
\nThe soil within the vertical separation distance may contain detectable concentrations of PHCs, which typically do not interfere with biodegradation (because the soil does not contain LNAPL source material).<\/p>\n\n\t\t\t\t\t\t\n\t\t\t\t\t\t
![\"\"](\"https:\/\/www.envguide.com\/wp-content\/uploads\/2018\/07\/p44.png\")
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