Site Screening Using Vertical Screening Distance
ABS：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.
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.
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).
Figure 1. Conceptual model of petroleum vapor transport for an LNAPL source and a dissolved phase source.
The PVI screening process is divided into three steps (see Figure 2)
Step 1: Develop a Preliminary CSM:
Step 2: Evaluate Site for Precluding Factors and Lateral Inclusion:
Step 3: Screen Building Using Vertical Separation Distance:
Figure 3-2. Flowchart for PVI screening application
Step 1 – Develop Preliminary CSM
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:
- site type
- petroleum vapor source
- extent of source
- lateral inclusion zone
- vertical separation distance
- precluding factors
USEPA has published empirical studies that reported different LNAPL screening distances based on facility type:
- LNAPL sources at petroleum UST/AST sites which generally include facilities used for vehicle fueling and commercial/home heating oil tanks. Fuel at these sites is typically stored in USTs
- LNAPL sources at Petroleum industrial sites include: (a) bulk fuel terminals; (b)refineries; (c) exploration and production sites; (d) crude oil and product pipelines; (e)former MGPs
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.
Petroleum Vapor Source
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.
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).
Table 1. General LNAPL indicators for PVI screening
Extent of Source
1. Preferential pathways that intercept both the source (either LNAPL or dissolved phase) and building foundations (see Figure 3-3).
2. Ongoing releases of petroleum products that result in expanding or mobile contaminant plumes.
3. Certain fuel types has insufficient data to draw confident conclusions for these fuel types. These fuel types include:
- gasoline containing lead scavengers
- gasoline containing greater than 10% vol/vol ethanol
- Soil organic carbon greater than 4% w/w as a possible indication of high soil oxygen demand
- Excessively dry soils (less than 2% by volume or 1.2% by weight moisture) may have insufficient moisture to support biodegradation
Figure 3-3. Precluding factor: conduit intersecting source and entering building
Lateral Inclusion Zone
Lateral 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.
A conservative, 30-foot lateral inclusion distance may be considered appropriate to incorporate the uncertainty.
Vertical Separation Distance
The 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).
Step 2 – Evaluate Building for Precluding Factors and Lateral Inclusion
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.
Step 3 – Conduct Screening with Vertical Separation Distance
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:
- 5 feet: dissolved-phase sources
- 15 feet: LNAPL sources (petroleum UST/AST sites)
- 18 feet: LNAPL sources (petroleum industrial sites)
Determine whether the vertical separation distance between the top of the petroleum vapor source and the bottom of the building foundation exceeds these vertical screening distances for either an LNAPL source (see Figure 5) or a dissolved-phase source (see Figure 6). If so, then no further evaluation of the PVI pathway is necessary. If not, then further site investigation or vapor control and site management is necessary.
Figure 5. Vertical screening distances for LNAPL source
Figure 6. Vertical screening distances for dissolved-phase source