November 2021 Vapor Intrusion News

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The Fundamentals of Vapor Intrusion

Petroleum Versus Chlorinated Compounds

By: Henry Stahl, CPG

For vapor intrusion (VI) purposes, volatile organic compounds (VOCs) are often placed into one of two groups – chlorinated VOCs (CVOCs) or petroleum hydrocarbons (PHCs).

Because CVOCs and PHCs behave differently in the subsurface, and because their use and consumption are dominated by different industries, CVOCs and PHCs are often regulated differently.  For example, in April 2013, EPA’s Office of Solid Waste and Emergency Response (OSWER) released the Final Guidance for Assessing And Mitigating the Vapor Intrusion Pathway (EPA VI Guide) while OSWER’s Office of Underground Storage Tanks (OUST) released the Guidance For Addressing Petroleum Vapor Intrusion At Leaking Underground Storage Tank Sites (EPA Petroleum VI Guide).  Both documents apply to PHCs, but some interesting conflicts arise.  For example, the EPA VI Guide, via the Vapor Intrusion Screening Level (VISL) Calculator, has a groundwater screening level for benzene of 31 ug/l (Ohio standards, using 10-5 cancer risk, residential exposure, 11 degrees C), while the EPA PVI Guide has a benzene screening level of 5,000 ug/l.  Which applies?  Should you use one screening level for benzene from a gas station and the other for benzene from a manufacturer?  And what about acetone, which is chemically like a PHC, but generally not associated with fuels?

But first, some background.  CVOCs start out as hydrocarbons.  Through manufacturing, some or all of the hydrogen may be synthetically replaced with chlorine, to reduce flammability and make it less prone to evaporate, among other things.  Other halogens, including fluorine, bromine, and iodine are also used, so the term “halogenated VOC” (HVOC) is more inclusive, but “HVOC” and “CVOC” are used interchangeably.  Commonly encountered CVOCs in the subsurface and indoor air include tetrachloroethene (perchloroethylene, PCE), trichloroethene (TCE), trichloroethane (TCA), chloroform, and carbon tetrachloride.

PHCs are not halogenated and are typically associated with fuels.  According to EPA’s Petroleum VI Guide, “P[etroleum]VI may be associated with three classes of chemicals: 

• PHCs found in gasoline, diesel, and jet fuel (e.g., benzene, trimethylbenzenes),
• Vapor-forming chemicals other than PHCs that may be found in petroleum (e.g., methyl tertiary-butyl ether (MTBE) and other fuel additives), and
• Methane, which can arise from anaerobic biodegradation of PHCs and other constituents of petroleum fuels, especially ethanol.”

Benzene is both carcinogenic and common, making it the risk driver in most PHC assessments.  The other BTEX compounds – toluene, ethylbenzene, and xylene, are also extremely widespread in the subsurface and indoor air.  Another common PHC, hexane is often singled out, but the countless other PHCs, such as heptane, octane, and nonane, may be referenced simply by their number of carbon atoms (in this case, C7, C8, and C9), and discussed collectively as Total Petroleum Hydrocarbon (TPH).  Since most TPH components aren’t identified specifically, and because they tend to have low or unknown toxicity, TPH is often ignored.  But the VI community is starting to realize that, collectively, TPH can be as harmful as benzene.  So when assessing VI from petroleum sites, make sure you understand where your regulators stand on TPH, and make sure the lab reports it, if required.

Getting back to the differences between CVOCs and PHCs, in general:

• CVOCs sink, PHCs float. They migrate differently, and they should be sampled differently;
• CVOCs break down slowly, PHCs break down rapidly;
• CVOCs break down better with low oxygen, PHCs break down better with high oxygen;
• CVOCs don’t burn, PHCs and their byproducts (methane) can burn and explode;
• CVOCs can travel long distances without breaking down, PHCs break down closer to their source; and
• CVOCs are better for Johnson & Ettinger (J&E) modeling than PHCs.

We’ll discuss the differences between CVOCs and PHCs in more detail in next month’s issue of Focus on the Environment.