Tracing PFAS to the Tap: How the Colorado River and Everyday Products Shape Lake Mead’s Water Chemistry

Deep in the Mojave Desert, Lake Mead quietly supplies drinking water to the Las Vegas Valley and, through the Lower Colorado River Basin, to roughly 25 million people. For decades, concentrations of per- and polyfluoroalkyl substances—the notoriously persistent chemicals known as PFAS—have remained low in the reservoir. But drought-driven declines in lake levels are changing the arithmetic. As the reservoir shrinks, the relative influence of the Las Vegas Wash, the channel that carries treated wastewater and stormwater from the Las Vegas Valley into the lake, grows. A new study published in Frontiers in Environmental Chemistry has now mapped, with unusual precision, exactly where the PFAS in this critical water source come from—and the answers reach from the Rocky Mountain snowpack to the laundry drain.

The research team, led by scientists at the Southern Nevada Water Authority, mounted one of the most comprehensive PFAS source investigations ever attempted for an arid, water-stressed system. They sampled Lake Mead itself at three locations, including the Boulder Basin and the effluent just below Hoover Dam, drawing on monthly monitoring records stretching from July 2021 through May 2025. They collected rainwater on the southeast side of the Las Vegas Valley and snowmelt from Vasquez Creek, 45 miles west-northwest of Denver in the Upper Colorado River watershed. They sampled the Las Vegas Wash during both dry weather—after more than 200 days without significant rain—and during a first-flush storm event. And they went deep into the sewersheds of two wastewater treatment plants, sampling manholes serving residential neighborhoods, the Las Vegas Strip, Nellis Air Force Base, and Harry Reid International Airport, along with commercial laundries, car washes, truck washes, a paper factory, and an electroplating facility.

Nineteen PFAS were quantified using isotope dilution liquid chromatography tandem mass spectrometry following automated solid phase extraction. But the team went further. Some samples were analyzed by non-targeted high resolution mass spectrometry, allowing tentative identification of PFAS compounds absent from the targeted analyte list. Others were processed using the total oxidizable precursor assay, a technique that subjects samples to strong oxidizing conditions—80 millimolar potassium persulfate and 200 millimolar sodium hydroxide at 85 degrees Celsius for six hours—transforming precursor compounds into the more persistent perfluoroalkyl acids they would eventually become in the environment. This matters because a significant portion of PFAS mass entering treatment plants typically exists as precursors that conventional targeted methods miss entirely.

The headline finding is counterintuitive. Total PFAS in the Boulder Basin of Lake Mead measured just 3.84 nanograms per liter, dominated by short-chain perfluorocarboxylic and perfluorosulfonic acids such as perfluoropentanoic acid at 1.1 nanograms per liter and perfluorobutane sulfonic acid at 0.747 nanograms per liter. Despite the Las Vegas Valley’s reputation as a wastewater-dominated system, the Colorado River—which supplies 97 percent of the lake’s flow—turned out to be the primary PFAS source, contributing 65 percent of total loading. The Las Vegas Wash, despite conveying the city’s treated effluent, accounted for only 27 percent. The PFAS fingerprint at Boulder Basin closely resembled that at the Colorado River confluence, confirming that most of the chemistry in the lake arrives from upstream rather than from local discharge.

Precipitation, often theorized as a growing PFAS pathway now that scientists argue the planetary boundary for these chemicals has been exceeded, played a surprisingly minor role. Four rainwater samples contained total PFAS between 0.329 and 1.51 nanograms per liter—low compared with recent measurements in Wyoming rainfall, where total PFAS reached 39 nanograms per liter, or Arizona rainfall at 92 nanograms per liter. Las Vegas rainwater was dominated by long-chain perfluoroalkyl acids, with PFOA and PFOS as recurring detections, while snowmelt from the Upper Colorado River watershed contained only perfluorobutane sulfonic acid at 0.329 nanograms per liter. Given the valley’s meager 4.18 inches of annual rainfall, precipitation is not expected to be a major PFAS loading pathway to the lake, though the researchers note that sampling rainfall in the Upper Colorado River watershed would provide further insight into atmospheric contributions to the river system itself.

Within the Las Vegas Wash, the picture sharpened considerably. During dry weather, effluent from the four wastewater reclamation facilities discharging to the wash accounted for 105 percent of the flow at the Rainbow Gardens sampling location and 56 percent of the PFAS mass flow. When concentrations below method reporting limits were conservatively assumed at half those limits, the facilities plus a stormwater channel near the airport explained 83 percent of the mass flow. The wash was an important secondary source to the lake for specific compounds, alone accounting for 42 percent of PFHxS loading, 37 percent of PFHxA, 20 percent of PFOA, 18 percent of PFOS, and 13 percent of PFBS. Shallow groundwater near Harry Reid International Airport—contaminated by historic use of PFAS-containing aqueous film forming foam in firefighting and training activities—registered extraordinarily high concentrations, up to 4,768 nanograms per liter total PFAS in the Rawhide Channel, but low flows limited its dry-weather contribution.

Non-targeted analysis revealed distinct chemical fingerprints separating dry from wet weather samples. The AFFF-impacted dry weather locations—the airport well, the Paradise Vista Park well, and the Rawhide Channel—showed large peaks for seven perfluorosulfonate and perfluorosulfonamide compounds not on the targeted list, including perfluoropropane sulfonate and perfluorobutane sulfonamide, classes commonly associated with aqueous film forming foam formulations. These samples also carried high PFOS to PFOA ratios and elevated branched-to-linear PFOS isomer ratios, hallmarks of electrochemical fluorination-derived foam plumes. Wet weather samples, by contrast, were dominated by short-chain polyfluorinated compounds, with two substances—4H-perfluoro-1-butene and heptafluoropentanoic acid—tentatively identified in all wet weather runoff samples and also present in rainwater collected from the same storm, suggesting rain itself may be an important source of these short-chain PFAS.

The sewershed investigation produced perhaps the most striking results. Las Vegas is unusual among major cities: its primary industry is hospitality, and it lacks the industrial PFAS sources—fluorotelomer manufacturers, semiconductor plants, carpet cleaning facilities, landfill leachate—that dominate PFAS loading elsewhere. Consequently, domestic wastewater emerged as the primary PFAS contributor to both treatment plants, accounting for 63 to 94 percent of flow and roughly 86 percent of the mass flow to the largest facility’s influent. Residential wastewater samples showed the highest contributions from TOP-quantified precursors, reflecting activities like showering and laundry that peak in the evenings and on weekends. Even the Las Vegas Strip line, conveying flow from casinos and hotels, carried a total PFAS concentration of 0.312 nanomoles per liter, similar to the treatment plant influent at 0.291 nanomoles per liter.

The everyday sources within that domestic wastewater proved remarkably specific. A sample from a household laundry machine washing synthetic sports clothing contained perfluorobutanoic acid at 115 nanograms per liter—83 percent of its total PFAS—along with measurable PFOS. Control samples from an empty machine and from detergent alone showed almost nothing, implicating the synthetic fabrics themselves. If every household in the valley ran six such loads per week, laundry would contribute approximately 11 percent of total influent PFAS loading to the treatment plants. Estimated mass flow contributions from PFAS-containing personal care products added further granularity: shampoo at 9.3 percent and cosmetic foundation at 2.2 percent, while toilet paper and human excretion each contributed less than 1 percent. Commercial laundries, though low-flow, registered total PFAS concentrations as high as 3.24 nanomoles per liter, almost entirely perfluorobutanoic acid, making them significant intermittent sources.

These findings arrive at a regulatory inflection point. In 2024, the United States Environmental Protection Agency established maximum contaminant levels for six PFAS, including 4 nanograms per liter for PFOS and PFOA. Historical monitoring shows a slowly increasing trend for total PFAS in Lake Mead since mid-2021, particularly at the Hoover Dam intake, where average total PFAS reached 8.46 nanograms per liter compared with 6.58 nanograms per liter at Boulder Basin—consistent with the greater recycled water contribution there, as confirmed by the wastewater tracer sucralose. As drought continues to lower the reservoir and the proportional influence of the Las Vegas Wash grows, understanding that the city’s own domestic wastewater—its laundries, shampoos, and foundations—is the dominant local PFAS source gives water managers a concrete target. The chemistry of a desert metropolis’s tap water, it turns out, is written as much in its washing machines and bathrooms as in the river that feeds it.

Subject of Research: New research maps PFAS sources to Lake Mead, finding the Colorado River is the primary contributor while household laundry and personal care products dominate local wastewater loading.

Article Title: Identifying sources of per- and polyfluoroalkyl substances (PFAS) in an arid environment with de facto reuse

Article References: Steigerwald, J., Trenholm, R., Quiñones, O., Vanderford, B. J., & Dickenson, E. (2026). Identifying sources of per- and polyfluoroalkyl substances (PFAS) in an arid environment with de facto reuse. Frontiers in Environmental Chemistry, 7, Article 1694851. https://doi.org/10.3389/fenvc.2026.1694851

Image Credits: AI Generated

DOI: 10.3389/fenvc.2026.1694851

Keywords: PFAS, Lake Mead, Colorado River, Las Vegas Wash, drinking water, wastewater, TOP assay, non-targeted analysis, AFFF, de facto reuse, arid environment, water quality