Before natural gas was widely available, manufactured gas plants (MGPs) used coal and oil to produce gas for heating, lighting and cooking. While most of these facilities were closed in the 1950s, contamination from MGPs still presents an environmental and public health concern today. For new and existing buildings located on property where an MGP once operated, a common concern is the potential for intrusion of contaminant vapors from the subsurface into indoor air, or vapor intrusion (VI).

Burns & McDonnell has investigated this concern at dozens of former MGP sites, and the conclusion has generally been the same: At MGP sites, VI rarely poses an unacceptable risk. Given the relative toxicity and biodegradability of the chemicals of concern (commonly including benzene and naphthalene), this is not surprising. Yet again and again, the owners and managers of former plant sites find themselves required to expend time and resources on collection and evaluation of soil gas and indoor air samples for formal VI evaluations.

Since 2012, we have been aggregating a database of MGP VI sample results and information to evaluate the existing VI guidance relative to sites, find patterns, and make recommendations for future efforts.

Recent Petroleum VI Guidance May Be Overly Conservative for MGPs

Two guidance documents from recent years (one published by the U.S. Environmental Protection Agency (EPA) Office of Underground Storage Tanks and one by the Interstate Technology Regulatory Council) suggest exclusion criteria that can be used to “screen out” petroleum hydrocarbon-contaminated sites where vapor intrusion is unlikely to be of concern, based on the results of soil and groundwater sampling. These criteria are based on separation distances between buildings and contaminated soil and groundwater. Since MGP sites are impacted by many of the same contaminants as petroleum sites, we studied our database to determine the usefulness of the petroleum VI guidance for managing VI risk at MGP sites.

Our evaluation of these documents indicates that while the screening criteria are adequately protective for MGP sites, they may be overly conservative. The screening criteria based on separation from soil and groundwater contamination frequently failed to screen out sites at which sampling of soil gas and/or indoor air had ultimately demonstrated there was no unacceptable risk. This suggests that site owners following the petroleum VI guidance may still be stuck performing unnecessary VI investigations.

Finding the Right Balance

As we continued our study of the available data, it became apparent that better screening criteria for sites is possible. These include use of a shorter separation distance, an increased focus on naphthalene (rather than benzene) as an indicator compound, and the need for a better definition of dissolved-phase contamination.

These study efforts are an example of how an experienced investigation and remediation team should be familiar with the most current science, the relevant guidance and regulatory requirements, and the constraints regulators face in granting site closures. Experience and data can facilitate communication between all parties and generate creative solutions for risk management and regulatory MGP site closure in reduced timeframes, with reduced cost.

 

Experiments are finding new pathways to closure of MGP sites. See how in our white paper.

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by
Aaron Christensen is a senior geological engineer and project manager at Burns & McDonnell. He specializes in investigation and remediation of former manufactured gas plants (MGPs) and other industrial sites.