In June 2015, the EPA produced a document titled Technical Guide For Addressing Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites (EPA PVI Guide – EPA 510-R). This document states that “in contrast to chlorinated solvents, petroleum hydrocarbons generally biodegrade rapidly under aerobic conditions, and if biodegradation is complete, produce only carbon dioxide and water.” Additionally, “the aerobic biodegradability of petroleum hydrocarbons (PHCs) typically reduces the potential for PVI and justifies a different approach for addressing PVI than for vapor intrusion from chlorinated solvents and other non-aerobically biodegradable volatile organic compounds (VOCs).”
To view the full EPA document, click here.
Determining the need to address PVI relies heavily on some key terms defined below:
“Clean soil” is defined by the EPA as soil that has adequate moisture (>2%), does not contain free product, and has enough aerobic activity to support the biological and chemical breakdown of petroleum hydrocarbons. If total petroleum hydrocarbon (TPH) soil data is available, then for fresh gasoline, the TPH data should be less than 100 milligrams per kilogram (mg/Kg) for new releases, or 250 mg/Kg for weathered gasoline or diesel fuel. If the analyte concentrations are below these guidelines, the soils can be considered “clean soil.”
Soils specifically excluded from the “clean soil” designation include:
Coarse sand and gravel with low contents of silt, clay, and organic matter
Fractured, faulted, or jointed consolidated rock
Consolidated rock with solution channels (i.e., karst).
Large areas of soil covered by impervious surfaces (buildings or parking lots) may not have the moisture content or aerobic conditions required for “clean soil” designation.
Aerobic is defined as “with oxygen.” In groundwater, oxygen content should be greater than 1.0 mg/L (milligrams per liter). For soil to be considered aerobic, the oxygen content should be at least 1.0 mg/Kg. Generally, oxygen content between 5 mg/Kg and 10 mg/Kg is considered ideal for aerobic microbial digestion.
Soils that have been heavily compacted or covered with an impervious surface become less aerobic over time. Likewise, indigenous microorganisms that digest petroleum hydrocarbons may deplete the available oxygen downgradient of a petroleum hydrocarbon contaminant source. The converse is also true; soils that have vegetated cover, or trees planted, will have greater aerobic capacity and, therefore, provide greater buffering effect from a contaminant plume to a vapor intrusion pathway.
In south Florida, I often saw relatively shallow groundwater depths (less than 6 feet), and high VOC concentrations in dissolved contaminant plumes near underground storage tank (UST) releases. With very active aerobic conditions, there was rarely a dissolved VOC groundwater concentration greater than 10% of the source area contamination once groundwater had migrated over a 10-foot vegetated buffer. If average groundwater velocity is 10 feet per year, that provides a one-year residence time for the aerobic destruction of 90% of the dissolved hydrocarbon plume.
The EPA advances the idea of “Vertical Separation Distance,” which is defined as the vertical thickness of clean soil above a soil or groundwater contaminant plume.
If more than 6 feet of clean soil exists between a dissolved groundwater plume (benzene concentration less than 5,000 micrograms per liter [mg/L]) and the slab of a building, under aerobic conditions, then generally no additional assessment or mitigation is required.
The vertical separation distance is 15 feet above groundwater when there is light non-aqueous phase liquid (LNAPL), which is defined as 1/8 inch or more of petroleum resting on top of the water table or benzene concentration greater than 5,000 micrograms per liter (ug/L), at or near the building. Over 5,000 ug/L benzene dissolved in groundwater degrades in a similar manner as an LNAPL plume.
The following table illustrates the separation distances in relation to the substrate and analyte concentrations of total petroleum hydrocarbons (TPH) and benzene.
As you can see from the above table, if the benzene concentration in the groundwater near a building is less than 5 mg/L (5,000 ug/L), then a vertical separation distance of 6 feet of clean soil between the top of the water table and the bottom of the building would be protective in most cases, unless a recent release or other mitigating factor exists.
There are a number of factors to consider when determining if petroleum vapor intrusion is an issue to a potential purchaser or a loan underwriter. Hopefully, the methods described in this white paper will help make that determination. Remember, this is not a decision matrix created in a vacuum. If there are sensitive populations, not enough data, or you are unsure of the data, you can’t just ignore a potential problem. Call the professionals at Nova; we can help.
Michael W. Minett
Vice President, Mid-Atlantic Region
Nova Consulting Group Inc.