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Fostering the Remediation & Redevelopment of
Manufactured Gas Plant Sites

Timothy R. Minnich


Presentation Title

Using an AMS/EPA Regulatory Model (AERMOD) for Assessing Community Exposure During MGP Site Cleanups

Timothy R. Minnich, Principal, Minnich and Scotto, Inc.

Co-Author: Robert L. Scotto, Principal, Minnich and Scotto, Inc.


An air pathway analysis (APA) is a process which systematically evaluates the potential or actual air quality effects upon humans or the environment, downwind of an emission source.  For any hazardous waste site cleanup, an APA: (a) characterizes the potential source of contaminant emissions; (b) determines the effects of dispersion and transport upon the contaminant plume; and (c) evaluates the exposure potential across the downwind community.

Although an APA is integral to the Feasibility Study process under CERCLA, this fact is often ignored (or not understood) by many state agencies during remedy-selection activities.  The National Contingency Plan explicitly states that for any remedial alternative to be considered potentially viable, it must be demonstrated, in advance, that airborne emissions from the activity itself will not adversely impact the downwind community.

This presentation describes a real-time approach using AERMOD to predict, consistent with the APA process, community exposure during MGP site cleanups.

The first step in this exposure assessment is the generation of an activity emission factor (units of mass per unit-process-rate per time) during the remedy-selection process, i.e., during pilot-scale activities.  This is accomplished by applying AERMOD – in its “inverse mode” – together with a cross-plume, path-integrated concentration measured downwind of the site and employment of appropriate meteorological instrumentation.  Inverse modeling simply refers to a class of methods for calculating an emission rate from measurements made downwind of a source.  AERMOD is then applied in its usual forward mode to predict a concentration at the nearest downwind residence. Finally, this short-term “impact factor” is scaled-up in order to maximize the rate of the full-scale cleanup, such that the community exposure never exceeds a pre-determined safe level.

This same approach can be used during the full-scale cleanup to document safe-level compliance, thus providing defense against frivolous lawsuits alleging unacceptable inhalation exposure.


Mr. Minnich is a partner in the air quality consulting firm, Minnich and Scotto, Inc.  He holds BS and MS degrees in meteorology, and has 40 years of experience in designing and managing a wide range of ambient air-related projects.  Prior to co-founding Minnich and Scotto in 1996, Tim held positions such as Vice President, Blasland, Bouck & Lee (now Arcadis), and Assistant Office Manager for NUS Corporation, Field Investigation Team (FIT) Superfund contractor for U.S. EPA, Region 2.  He has designed and managed numerous air monitoring investigations, including one of the largest such studies conducted to date under Superfund.  A recognized expert in the application of optical remote sensing (ORS) for assessing human exposure from hazardous waste site remediations – including MGP cleanups – Tim’s work has been extensively published.  He has also taught several courses over the years on this same material for organizations such as the Air & Waste Management Association.