U.S. EPA Development of the CMAQ-PFAS Model and Potential Impacts to PFAS Evaluation
Posted: June 28th, 2021Authors: Ryan C.
On May 18, 2021, the United States Environmental Protection Agency (U.S. EPA) Office of Research and Development (ORD) hosted a webinar presenting the current state of research on modeling Per- and Polyfluoroalkyl Substances (PFAS). The webinar, part of the Air, Climate, and Energy (ACE) Research Webinar Series, was hosted by Dr. Emma D’Ambro and Dr. Ben Murphy, and provided an overview of their research of the characterization of atmospheric transport and deposition of PFAS emissions.
PFAS are manufactured chemicals that have been produced by a wide range of industries throughout the world and are both persistent in the environment and accumulate over time. PFAS have garnered increased interest as a contaminant of concern in recent years due to their prevalence in consumer goods and its impacts on human health. PFAS are commonly found in household products, such as stain-repellant fabrics, nonstick cooking products, and in polishes, food packaging, fire-fighting foams, and industrial facilities producing electronics and chrome plating. Research has linked PFAS exposure to impacts on the human endocrine and reproduction systems and certain types of cancer. As a result of the awareness of human health impacts, long-chain, or “legacy”, PFAS are no longer manufactured in the United States, although replacement, short-chain PFAS remain in production.
PFAS have been detected in samples of drinking water throughout the United States, with data from the U.S. EPA’s third Unregulated Contaminant Monitoring Rule (UCMR3) program indicating that over 6 million residents are exposed to drinking water with levels exceeding the U.S. EPA health advisory level of 70 ng/L (70 parts per trillion). The actual number of residents exposed to PFAS via the water supply is understood to be much higher, as initial data measured a subset of known PFAS, did not sample private water supplies, and did not include public water supplies serving more than 10,000 people. Increased scrutiny into the origin of drinking water contamination has revealed the contribution from air sources, via wind transport from manufacturing facilities and wet and dry deposition.
In an effort to better understand the physical processes of PFAS transport and deposition, U.S. EPA has applied the Community Multiscale Air Quality (CMAQ) regional air quality model to PFAS research, using available emissions data from a fluoropolymer manufacturing facility in North Carolina. CMAQ is typically used to evaluate long-range ozone and particulate matter transport and allows for evaluation of pollutant concentrations from non-source specific contributions such as mobile, agricultural, and industrial categories. The referenced fluoropolymer facility has been the subject of PFAS air research since 2017, when PFAS were identified in the facility’s neighboring river and in blood samples of local residents. Subsequent research by the North Carolina Department of Environmental Quality (NC DEQ) has been conducted over the years and has confirmed deposition of PFAS originating from the facility through the use of dispersion modeling. Dr. D’Ambro and Dr. Murphy, in collaboration with the facility, have applied CMAQ to actual emissions data from the facility. The data included emissions of 53 PFAS contaminants, which, as a subset of VOC are reported to the National Emissions Inventory (NEI), were based on mass-balance calculations and PFAS stack test information.
The combined CMAQ-PFAS model applied 2017 emissions from the facility to a 1 kilometer by 1 kilometer modeling domain over southeastern North Carolina and South Carolina to compare model-predicted PFAS deposition concentrations to NC DEQ measurements in an attempt to quantify ambient air concentrations and deposition in the vicinity of the facility. Model results indicated PFAS deposition to be elevated in the vicinity of the facility, consistent with prevailing winds and NC DEQ observations. On a regional scale, deposition decreased with distance from the facility, although small levels of PFAS deposited more than 150 kilometers from the facility. The large contribution to deposition far from the facility supported the notion that impacts from PFAS transport are not limited to the near field. The authors indicated that, in sensitivity simulations, there was large variability in deposition based on assumptions related to PFAS speciation and characterization as an aerosol or gas-phase.
PFAS air transport and deposition have also been evaluated using the American Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) dispersion model. Previous evaluations of PFAS deposition from facilities in West Virginia and North Carolina, among others, are often based on assumptions of particle mass and size distribution, despite uncertainties in model performance and representative data. The increased scrutiny of PFAS concerns has prompted U.S. EPA to continue research and evaluation of deposition algorithms in AERMOD. On May 11, 2021, U.S. EPA released an updated version (21112) of AERMOD, which includes two alpha options for deposition. Alpha options are considered non-regulatory and are recommended for research purposes only, and U.S. EPA has acknowledged that AERMOD’s deposition algorithms require additional evaluation. Additional research into these options may prompt future AERMOD changes, including incorporation of deposition algorithms from other models. The CMAQ-PFAS model represents a potential source of new algorithms, as it represents a new state of the science model, with the authors developing the model to include additional semi-volatile and volatile chemistry considerations that are not captured in current air quality models. Users of AERMOD should expect future updates to deposition that may be of use for PFAS modeling applications.
The importance of continued development of PFAS evaluation methods takes on additional significance in the context of environmental justice (EJ) considerations. EJ focuses on the fair treatment of all people, regardless of race, color, national origin, or income, with respect to the implementation and enforcement of environmental laws and policies. Policies aimed at addressing PFAS contamination equitably will rely on multiple evaluation strategies and data collection. As part of an effort to collect additional data on PFAS deposition, the recent Great Lakes Integrated Atmospheric Deposition Network (IADN) project, a monitoring study funded by U.S. EPA, found detectible levels of replacement short-chain PFAS in all collected samples, and that PFAS are a major contaminant to the Great Lakes via deposition. Citing results of the study as an example of the ubiquitous nature of PFAS in the environment, a June 8 statement from the Environmental Working Group (EWG) reiterated the need to apply the “whole-of-government” approach for addressing PFAS pollution, to include the Department of Defense, the Food and Drug Administration, and the Federal Aviation Administration. The Biden administration has made addressing PFAS pollution a priority, with U.S. EPA Administrator Michael S. Regan establishing a new U.S. EPA Council on PFAS, charged with the better understanding of, and reduction to, risks associated with the chemicals. To date, U.S. EPA has only set health advisory levels for two legacy PFAS, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), with neither level enforceable. However, enforceable drinking water limits for both are in development, and both are expected to be designated as Superfund hazardous substances, which would require cleanup actions. With an interagency “whole-of-government” approach and continued calls for action in motion, additional PFAS considerations and regulations should be expected.
The development of the CMAQ-PFAS model serves as an example of the continued research being performed by the U.S. EPA to better understand, and ultimately address, PFAS contamination due to atmospheric deposition. Ongoing development, both of air quality models and enforceable regulations, should be expected to impact industry stakeholders as the call for addressing PFAS pollution continues.
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