Vapor Intrusion

Typically, the initial concern of soil and ground water contamination was a subsurface issue. However, many states, including Massachusetts and New Jersey, have recently enacted stricter guidelines for remediating sources of vapor intrusion, which is the transfer of volatile organic compounds from the subsurface into the air. Specifically, environmental agencies are concerned about the potential acute and chronic human health effects from vapor intrusion. Past remediation projects have identified this potential risk and awareness among residential communities. As a result, more individuals are becoming conscious of the hazards associated with vapor intrusion. This in turn is forcing environmental professionals to focus not only on the subsurface investigation but also on potential vapor intrusion pathways.

There are several migration pathways for chemical vapors from the subsurface. The vapors can diffuse from unsaturated zones or shallow ground water as well as be transported through advective or convective flow. There are natural and man-made preferential pathways that vapors use for vertical and horizontal transport. The mitigation of vapor intrusion focuses on controlling the migration and exposure pathways. This is completed by removing the contamination source.

The challenge for the environmental professional is to recognize the major and minor pathways of vapor intrusion at a site, determine which are associated with a subsurface source, and then design an effective remedial action plan to reduce potential health risks.

One effective method for controlling vapor intrusion is in-situ remediation through bioaugmentation. Bioaugmentation introduces high populations of microorganisms to the subsurface to expedite the degradation rates and eliminate the contamination source in weeks or months, rather than years. An inoculation plan that uses specialized aerobic bacteria can effectively and economically reduce vapor intrusion produced by chlorinated solvent and petroleum hydrocarbon sources. Indigenous microorganisms may degrade the source of the potentially harmful vapor, but often at a rate much slower than the vapor formation. In addition, there are daughter products that pose a greater risk due to their toxicity and volatility than their respective parent chemical. Specifically, vinyl chloride is more toxic and volatile than tetrachloroethylene (PCE). The advantage of aerobic bioaugmentation is that PCE is degraded to carbon dioxide and water without the production and accumulation of vinyl chloride as an intermediate. Bioaugmentation can also be successful without the site disruption required by more invasive remediation alternatives.