New EPA Guidance on Vapor Intrusion

The US Environmental Protection Agency has issued two draft guidance documents concerning vapor intrusion. Vapor intrusion is the migration of volatile chemicals from contaminated soil or groundwater into overlying buildings. The first new report is a general guidance document that supersedes a version published in 2002. By issuing a revised guidance document, EPA appears to be acknowledging the enhanced scrutiny being paid in the environmental community to vapor intrusion issues. EPA also seeks to incorporate new science that has been accumulating rapidly regarding methods for monitoring vapor intrusion, quantifying risk posed by vapor intrusion and assessing the vulnerability of sites to vapor intrusion. One important concept that is emphasized by EPA is spatial and temporal variability of indoor air concentrations:

“Field observations and measurements demonstrate this—that is, indoor air concentrations and soil gas concentrations can exhibit significant temporal variations even for a single building and suggest that the mass flux of vapors via soil gas entry may be highly variable, perhaps even episodic rather than continuous, due to varying driving forces and sub-slab soil gas concentrations.”    (EPA, 2013, OSWER Final Guidance for Assessing and Mitigating the Vapor Intrusion Pathway From Subsurface Sources to Indoor Air, External Review Draft, p. 24).

We were pleased to see EPA acknowledge the importance of temporal variability and the related challenges of designing adequate monitoring protocols that can capture dynamic trends in vapor concentrations due to factors such as changes in soil moisture, atmospheric pressure and building ventilation. Dr. Everett co-authored (with colleagues Mark Kram and Peter Morris) an important paper on this topic in the academic journal, Remediation (Winter 2011, pp. 59-69): “Dynamic Subsurface Explosive Vapor Concentrations: Observations and Implications”.

As noted in an earlier posting, Dr. Everett also chaired the Symposium on Continuous Soil Gas Measurements: Worst Case Risk Parameters sponsored by ASTM International in January 2013. This symposium focused on newly developed capabilities for continuous soil gas monitoring of volatile contaminants for vapor intrusion investigations. Understanding the effects of barometric pressure changes, soil moisture fluctuations, seasonal and diurnal temperature changes and sampling technologies were highlighted in the Symposium.

With the second draft guidance document, EPA addresses for the first time, protocols for assessing and mitigating vapor intrusion from petroleum hydrocarbons. Because gasoline constituents like BTEX (benzene, toluene, ethylbenzene and xylenes) are biodegradable under oxygen-rich conditions, some people have assumed that these compounds do not pose a risk to vapor intrusion. While biodegradation in the vadose zone is an important factor, this uncontrolled process cannot be relied upon to reduce hydrocarbon vapor concentrations sufficiently such that they pose no risk. In fact, the new draft document provides guidance for determining the lateral and vertical dimensions of a zone of concern surrounding subsurface hydrocarbon contamination. Structures within the zone of concern should be evaluated for petroleum vapor intrusion. According to EPA, the principal variable is whether or not LNAPL (light nonaqueous phase liquid) is present in the subsurface. If LNAPL is absent, the zone of concern is smaller; if LNAPL is present, the zone is larger.

Links to the draft documents can be found here.