Tuesday - March 14, 2023

 6:00 to 8:00 p.m. PST

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"PFAS Sources in the Las Vegas Wash"

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By:  Dr. Hannah Ray,

Process Systems Engineer

Southern Nevada Water Authority

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Abstract:

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Per- and polyfluoroalkyl substances (PFAS) are a class of chemicals that have become notorious for their persistence, bioaccumulation, and toxicity. There are many avenues PFAS can enter surface waters from point sources, such as municipal wastewater treatment plants, Air Force bases, airports, and PFAS-producing and -using industries. PFAS can also move from one water type to another through de facto reuse or leaching from biosolids to groundwater. Two well-known PFAS are perfluorooctanoic acid (PFOA) and perfluorosulfonic acid (PFOS). Based on finished water samples collected for the third Unregulated Contaminant Monitoring Rule (UCMR3), 66 water systems serving an estimated six million U.S. residents had PFOA and PFOS above the EPA health advisory level (HAL) of 70 ng/L combined.

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Since the EPA announced the HAL for PFOA & PFOS in 2016, many states have enacted lower guidelines or standards. Furthermore, recent epidemiological studies have identified additional adverse health effects, such as decreased vaccine effectiveness. Several states have initiated statewide PFAS monitoring, and more nationwide monitoring is planned for UCMR5. The EPA’s 2021 PFAS Strategic Roadmap calls not only for PFOA & PFOS MCLs, but also HALs for other PFAS; ambient water quality criteria; and a risk assessment in biosolids. Utilities are bracing for these upcoming developments and exploring the best approaches for monitoring, managing, and treating PFAS.

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The Las Vegas Wash (LVW) conveys wastewater effluent from the Las Vegas area to Lake Mead, a drinking water source for millions of people. PFAS have previously been detected in the LVW despite few known potential PFAS industrial sources in the area. However, PFAS sources were not identified in the previous studies. In this study, tributaries and wastewater treatment plants discharging to the LVW were sampled in duplicate campaigns to investigate possible PFAS sources. Short-chain PFAS—perfluoropentanoic acid (PFPeA) and perfluorohexanoic acid (PFHxA)—were the majority of PFAS loading measured in the LVW. GenX was not detected in any samples. Among seven polyfluorinated compounds analyzed, none were detected in the first campaign, while four were detected in the second campaign, but at levels below 2 ng/L. On a mass loading basis, the majority of total measured PFAS and at least 48% of each specific PFAS in the LVW could be explained by the municipal wastewater effluents. Harry Reid International Airport was confirmed to be a source of perfluorosulfonic acids, but due to dilution—and likely sorption to sediments—contributed at most 15% of any specific PFAS to the mainstem of the LVW. 

Bio: 

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Hannah Ray received her bachelor’s in environmental engineering from the University of Florida and her master’s and PhD in environmental engineering from Arizona State University where her graduate work focused on membrane treatment of wastewater. She has been a postdoc researcher at Southern Nevada Water Authority (SNWA) working for Eric Dickenson since 2020 where her research has focused on potable water reuse. She recently accepted a position as a process systems engineer at SNWA working for Ariel Atkinson.

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