June 3, 2019

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Andrea Siefers Coauthored AERMOD Modeling for Health Impact Assessment of Dimethyl Mercury Paper Presented at the Air Quality Modeling Conference

Andrea Siefers, P.E. (North Carolina) coauthored a paper entitled "AERMOD (American Meteorological Society/Environmental Protection Agency Regulatory Model) Modeling for Health Impact Assessment of Dimethyl Mercury" that was presented at the Guideline on Air Quality Models: Planning Ahead Conference from March 19-21, 2019.

Her coauthors were Geosyntec staff members: Grant Walter (Georgia); Chris Saranko, Ph.D. (Georgia); and Kate Graf, P.E. (Pennsylvania). Tapshree Tah also contributed.

Andrea is an Engineer based in North Carolina focused on regulatory experience and air quality, through which she has developed various types of air permit applications and provided environmental compliance assistance and related engineering support to manufacturing facilities and stationary sources. 

Grant is a Senior Staff Engineer based in Georgia who focuses on risk assessment, groundwater hydrology, chemical transport, and wastewater process engineering. He also performs database management for projects within these fields. 

Chris is a Principal Senior Board-Certified Toxicologist with more than 20 years of research and consulting experience in evaluating health effects associated with exposures to chemicals in the environment, the workplace, and consumer products.

Kate is a Principal Engineer with more than 20 years of experience focused on providing air quality services to industrial clients. Her experience spans environmental engineering, including regulatory agency compliance, permitting, and enforcement expertise, and includes management of an air permitting division of a regulatory agency in the southwestern United States.

Planned by the Air and Waste Management Association (A&WMA) in conjunction with the Atmospheric Modeling and Meteorology Committee of the Technical Council, the "2019 Guideline on Air Quality Models: Planning Ahead" conference covered the implementation of the changes, experience gained, and viable solutions.

A&WMA is a nonprofit, nonpartisan professional organization enhancing knowledge and expertise by providing a neutral forum for information exchange, professional development, networking opportunities, public education, and outreach to more than 5,000 environmental professionals in 65 countries.  

The mission of the Atmospheric Modeling and Meteorology Committee is to encourage and facilitate the development, advancement, and use of state-of-the-art methods of meteorological and atmospheric dispersion analysis as a foundation for effective environmental design of air emissions sources and for assessment of environmental impacts of air emissions.


Perma-Fix Northwest (PFNW), located in Richland, Washington, processes and stabilizes low-level radioactive waste and mixed waste.  PFNW is proposing to receive and stabilize supernatant tank waste from the U.S. Department of Energy's Hanford Site that is classified by the U.S. Nuclear Regulatory Commission (NRC) as low-level mixed waste."  Due to the origins of the waste, the Washington State Department of Ecology (Ecology) has expressed concerns that the waste may contain dimethyl mercury (DMM) and that emissions of DMM may occur as a result of processing the waste.

DMM is listed as a toxic air pollutant (TAP) under Ecology regulations for sources of TAPs in Washington Administrative Code (WAC) 173-460.  Ecology and the local air quality agency, Benton Clean Air Agency (BCAA), regulate emissions of TAPs through a series of review processes known as "first, second, and third tier review."  First-tier review includes an analysis of potential TAP emissions and comparison to regulatory emission rate thresholds.  If potential emissions of a TAP exceed the regulatory emission rate threshold, an air dispersion modeling analysis is required to determine if the maximum off-site concentration is above the ambient concentration threshold.  If the maximum off-site concentration threshold is above the ambient concentration threshold, then second-tier review is required.  Second-tier review includes a health impact assessment of the potential emissions.  If health risk or hazard from the potential TAP emissions exceeds the regulatory threshold, third-tier review could be required.  The ambient concentration threshold ("acceptable source impact level," or ASIL) for DMM in WAC 173-460-150 is 1 x 10-99 micrograms per cubic meter (µg/m3), which is essentially a "zero threshold" and represents less than one molecule of DMM.

In the first-tier review, PFNW quantified potential TAP emissions from the processing of the waste, including DMM.  Ecology therefore required PFNW to use conservative assumptions about the concentration of DMM, and the results of the air dispersion modeling analysis showed that the maximum off-site concentration of DMM was above the regulatory threshold.  PFNW was thus required to submit a second-tier review for the potential emissions of DMM.

Ambient air concentrations from the facility were calculated using the air dispersion model AERMOD.  Off-site concentrations were modeled for a discrete receptor grid including the maximally exposed individual, maximally exposed resident, maximally impacted worker, and sensitive populations such as nearby workplaces, residences, schools, retirement facilities, daycares, hospitals, and doctor's offices.  Modeled concentrations for these areas were used to calculate human health risk from multiple exposure pathways.  Results of the health impact assessment showed that the hazard from potential emissions of DMM associated with processing and stabilizing of DMM at PFNW were several orders of magnitude lower than the hazard threshold in all areas analyzed.  PFNW as submitted these technical analyses to the Washington State Department of Ecology for review in support of its air permit application for processing and stabilizing of the waste.

More Information

Learn more about the paper at http://proceedings.awma.org/wp-content/uploads/2019/04/MO34Tah.pdf.
Learn more about A&WMA at https://www.awma.org/.