The EPA’s Citizen’s Guide to Cleanup Technologies Series 11: Vapor Intrusion Mitigation

Jul 17, 2017

Environmental Enlightenment #212

See our Newsletter, Environmental Enlightenment #200, for introduction to one highly valuable, plainly written series of Citizen’s Guides published by the EPA at

With this issue we introduce the EPA’s Citizen’s Guide to Phytoremediation. You can reach it through the hyperlink under the blue header below.

Intrusion is practiced by intruders who intrude.

To intrude is to come in rudely or inappropriately; enter as an improper or unwanted element.

Copyright: gnurf / 123RF Stock Photo

Intruders are generally not welcome: bugs, pests, noxious vapors, explosive gas, radiation, viruses, and the burglar next door, to name a few.

In Los Angeles, vast developments sit on formations loaded with methane (an odorless, colorless, toxic and explosive gas), and measures are implemented to block the methane from intruding into or accumulating within spaces.

Toxic vapors originate from spills of volatile industrial chemicals into underlying soils. The chemicals emit vapors that travel sideways from the source area.

(DNAPLs stands for Dense Non Aqueous Phase Liquids. It means liquids that are not water and heavier than water. Dry cleaning solvents are one example.)

This image shows a case in which the leaking contaminant has reached groundwater and is migrating far with the movement of the water. Being carried away, it volatilizes (partitions) from the water and moves up toward inhabited structures.

In other cases, the vapors will travel laterally in the form of subterranean “clouds.”

One practical way to know if vapors are intruding is by placing samplers inside the spaces of interest.

This procedure demands multiple sampling episodes because different spaces are ventilated at varying rates of ventilation, because rates of intrusion vary with the seasons and during days and nights, and the integrity of slabs, floors and walls changes over time.

To overcome these variables, scientists developed theories that attempt to predict rates of intrusion by measuring the levels of the volatile chemicals in the subsurface soils and calculating probabilities of intrusion to the inhabited places.

These models are highly theoretical.

Copyright: sararoom / 123RF Stock Photo

Further, the risk models estimate levels of risks in terms of the number of people that are allowed (by the model) to contract cancer over time. For example, one such number for residences is one in a million in 70 years.

The models are theoretical, but in the absence of better tools they are used in the evaluation of the risks to public health and provide thresholds numbers for regulatory demands to perform cleanup projects.

Models of risks and the theories behind them develop over time.

At the end of the day, a practical tool could be direct sampling of the indoor air in multiple events, comparison of the results with levels of same chemicals in the outdoor environments, and implementation of mitigating means where risks are suspected.

Go to the EPA’s Citizen’s Guide to Vapor Intrusion Mitigation at for the rest. It is simple and fun to read.


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