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

Lake Mead is the primary drinking water source for the Las Vegas Valley and supplies water to 25 million people in the Lower Colorado River Basin. Historically, Lake Mead per- and polyfluoroalkyl substances (PFAS) concentrations have been low; however, decreasing lake levels from drought may result in increased impact from the Las Vegas Wash (LVW) leading to increased PFAS levels. Thus, there is a need to better map Lake Mead PFAS sources. Herein, samples were collected from (1) Lake Mead, rainwater, and snowmelt; (2) wastewater, groundwater, and stormwater sources to the LVW; and (3) sewershed sampling for two wastewater treatment plants (WWTP) in the Las Vegas Valley. Nineteen PFAS were quantified via liquid chromatography tandem mass spectrometry. Additionally, some samples were either analyzed using non-targeted high resolution mass spectrometry or processed using the total oxidizable precursor (TOP) assay method. Total PFAS in the Boulder Basin area of Lake Mead was 3.84 ng/L and was dominated by short-chain perfluorocarboxylic and perfluorosulfonic acids. The Colorado River was the primary source of PFAS to Lake Mead (65% of total PFAS loading) and the LVW was an important secondary source (27%). Total PFAS in precipitation samples were low (0.329–1.51 ng/L) with rainwater dominated by long-chain perfluoroalkyl acids while only perfluorobutane sulfonic acid (PFBS) was detected in snowmelt. Domestic wastewater was the primary WWTP PFAS source due to high residential and domestic flow contributions (63%–94%) and lack of industry, while laundry and PFAS-containing cosmetics are significant PFAS sources in residential wastewater (i.e., estimated mass flow contributions of 11%, 9.3% and 2.2% for household laundry, shampoo and cosmetic cream, respectively). These results provide a better understanding of the PFAS sources to Lake Mead, and similar systems, and will help inform future efforts to manage PFAS flows to this important drinking water source.

1 Introduction

Wastewater has been identified as an important contributor of per- and polyfluoroalkyl substances (PFAS) to the environment and drinking water sources (Appleman et al., 2014; Furl et al., 2011; Houtz et al., 2016; Lenka et al., 2021; Phong Vo et al., 2020; Schaefer et al., 2023; Thompson et al., 2024; Thompson et al., 2022). This is of particular importance in areas practicing intentional or de facto reuse, for example, Lake Mead and the Lower Colorado River system. Lake Mead is an important water resource for over 25 million people, serving as a drinking water, irrigation, and recreational resource for both the Las Vegas Valley and the Lower Colorado River Basin (Holdren and Turner, 2010). Lake Mead receives the majority of its flow (97%) from the Colorado River (Hannoun and Tietjen, 2022; Holdren and Turner, 2010) with the remaining flow coming from the Virgin and Muddy Rivers (around 0.9%) and the Las Vegas Wash (around 2.2%) (Hannoun et al., 2021; Holdren and Turner, 2010). PFAS have been continually monitored in Lake Mead and the Las Vegas Wash (which conveys stormwater and treated wastewater from the Las Vegas Valley to Lake Mead) since the early 2000s (Bai and Son, 2021; Quiñones and Snyder, 2009; Thompson et al., 2024). Historically, PFAS concentrations in Lake Mead have been low with concentrations of regulated PFAS below their reporting limits (Bai and Son, 2021; Thompson et al., 2024); however, decreasing lake water levels due to drought may result in increased impact from the Las Vegas Wash on Lake Mead water quality (Hannoun and Tietjen, 2022), potentially leading to increased PFAS levels in the lake. Additionally, several recent studies have theorized that we have exceeded the “planetary boundary” for PFAS (Cousins et al., 2022) resulting in increased concentrations of PFAS in rainwater (Coates and Harrington, 2024) and snow (Hartz et al., 2024; Kim and Kannan, 2007; MacInnis et al., 2019) globally as PFAS are transported through the water cycle. Thus, precipitation in both the upper Colorado River watershed and the Lake Mead drainage area, including the Las Vegas Valley, may be contributing to increased PFAS loading to Lake Mead.

In 2024 the United States Environmental Protection Agency (US EPA) published maximum contaminant levels (MCLs) for six PFAS: perfluorooctanesulfonic acid (PFOS; 4 ng/L), perfluorooctanoic acid (PFOA; 4 ng/L), perfluorononanoic acid [PFNA; 10 ng/L and combined hazard index (HI)], perfluorohexanesulfonic acid (PFHxS; 10 ng/L and HI), hexafluoropropylene oxide dimer acid (HFPO-DA or Gen-X; 10 ng/L and HI), and perfluorobutanesulfonic acid (PFBS; HI). The HI is a method of calculating combined toxicity for two or more compounds wherein first a ratio is calculated for each compound in the HI by dividing the measured concentration by the Health Based Water Concentration (HBWC, i.e., the concentration at below which adverse health effects are not expected) and then those ratios are summed for all compounds in the HI. For these four PFAS, an HI of greater than one (1) indicates the potential for adverse health effects. These recent PFAS regulatory decisions and improved understanding of PFAS toxicity make it increasingly important to understand the primary sources of PFAS to Lake Mead and their relative contributions of key PFAS. Thompson et al. (2024) investigated sources of PFAS to the Las Vegas Wash during both dry weather and wet weather events. Four wastewater reclamation facilities were found to contribute a combined 97% of the PFAS loading during dry weather sampling, comprising the majority of the PFAS found in the Las Vegas Wash. Shallow groundwater near Harry Reid International Airport was also determined to be a significant point source of PFAS (i.e., approximately 1.2 g/day) via dewatering to stormwater channels (Thompson et al., 2024). PFAS in the shallow groundwater was attributed to historic application of PFAS-containing aqueous film forming foam (AFFF) associated with firefighting and training activities at the airport. During wet weather events several stormwater channels with proximity to and drainage from the airport contributed approximately 13% of PFAS loading to the Las Vegas Wash combined (Thompson et al., 2024). During longer periods of dry weather, flow (and PFAS) from these shallow groundwater dewatering sources do not consistently make it all the way to the Las Vegas Wash due to evaporative losses.

Prior studies have identified AFFF discharges, industrial wastewater from manufacturing facilities producing fluorotelomer products, laundry and carpet cleaning facility wastewater, semiconductor wastewater, and landfill leachate as significant contributors of PFAS to municipal wastewater (Dasu et al., 2022; Masoner et al., 2020; Nguyen et al., 2022; Payne et al., 2024; Thompson et al., 2022). The Las Vegas Valley is somewhat unique in that it is a major city where the primary industry is hospitality, it does not have industrial wastewater contributions from any of the sectors previously identified as major PFAS sources, and the wastewater treatment plants do not receive landfill leachates (another important source of PFAS to some wastewaters). Thus, smaller commercial sources (i.e., commercial laundry, car washes, recycling facility) and domestic wastewater are expected to be the primary sources of PFAS in raw wastewater (Salawu et al., 2025). Human excrement (Hartmann et al., 2017) and certain toilet paper (Thompson et al., 2023), detergents (Guo et al., 2009), clothing (Danish EPA, 2015; Guo et al., 2009; Schellenberger et al., 2019), and cosmetics (Brinch et al., 2018; Dasu et al., 2022; Schultes et al., 2018; Whitehead et al., 2021) have all been shown to contain significant concentrations of PFAS, contributing to PFAS loading in domestic wastewater. Additionally, in the Las Vegas Valley, shallow groundwater infiltration to the sewer system in areas with known PFAS plumes may contribute to PFAS to the largest wastewater reclamation facility.

A significant portion of the PFAS mass loading to wastewater treatment plants is typically from precursor compounds which can transform during wastewater treatment to more persistent perfluoroalkyl acids (PFAAs) (Thompson et al., 2022). Several options exist for bulk quantification of PFAS precursors including total organofluorine (TOF), absorbable and extractable organic fluorine (AOF/EOF) methods, and the total oxidizable precursor (TOP) assay method (Dixit et al., 2024). The TOP assay method in particular is useful for quantification of precursors in wastewater since it is a measure of precursors which transform to PFAAs under strongly oxidizing conditions (Houtz and Sedlak, 2012). Although the TOP assay method was originally developed for stormwater (Houtz and Sedlak, 2012), prior studies have successfully adapted it to wastewater influent and effluent samples (Glover et al., 2018; Gonzalez et al., 2021; Houtz et al., 2018; Schaefer et al., 2023; Tavasoli et al., 2021; Tsou et al., 2023).

The objective of this study was to investigate sources of PFAS to Lake Mead and evaluate their relative contribution of key PFAS. This was achieved through three sampling efforts. (1) targeted analysis of PFAS in surface water samples from Lake Mead, rainwater, and snow melt from the upper Colorado River watershed near Denver, CO; (2) wet and dry weather sampling of wastewater, groundwater, and stormwater sources to the Las Vegas Wash; and (3) sewershed sampling to evaluate PFAS loading to two of the wastewater treatment plants in the Las Vegas Valley. PFAS loading to Lake Mead is hypothesized to be predominantly from Colorado River inflows and de facto reuse via the Las Vegas Wash with local precipitation and stormwater runoff as secondary contributors.

2 Materials and methods

2.1 Sample collection

All sample locations are indicated in Figure 1, with insets included providing a more detailed overview of certain sample locations. All samples were grab samples (unless otherwise stated) collected in 1 L HDPE bottles. When chlorine residual was considered, samples were quenched with 50 mg/L ascorbic acid. Samples were analyzed for nineteen PFAS listed in Supplementary Table S3 unless otherwise noted. Analytical details are described further in Sections 2.2, 2.3 and Supplementary Sections S3 and S4.

Figure 1

Figure 1. Sample location map. Callouts show samples collected (1) in the Las Vegas Valley, (2) from Lake Mead and rainfall, and (3) from the Upper Colorado River watershed.

Samples were collected from six locations to evaluate PFAS concentrations in Lake Mead and in direct sources to Lake Mead. Three surface water sample locations were in Lake Mead: one near the confluence of the Colorado River and Lake Mead (CR-LM) with samples collected in February, May, June, July, October, November, and December of 2025; one from the effluent of Lake Mead at Hoover Dam (LM-HD) collected just below the dam in August of 2024; and one from the Boulder Basin area in Lake Mead (LM-BB) collected in August of 2024. Additionally, samples were collected at the LM-HD and LM-BB locations monthly from July 2021 through April 2025 (for LM-HD) or May 2025 (for LM-BB) and analyzed for a shortened list of PFAS analytes (i.e., the ten PFAAs listed in Supplementary Table S3). Two samples were collected from the Las Vegas Wash at Rainbow Gardens (LVWRG). The first sample was collected in September 2024 during dry weather (over 200 days since the last significant rain event) and was identified as LVWRG-1. The second sample (LVWRG-2) was collected in February 2025 during a 0.43-inch rain event. This sample was collected as a flow-based composite for the duration of the storm using an ISCO 6712 Portable Sampler equipped with EZ-FLO clear vinyl tubing and a Nalgene polypropylene carboy for sample compositing. A direct snowmelt sample (CR-Snow) was collected in June 2024 from Vasquez Creek located 45 miles WNW of Denver, CO in the Upper Colorado River watershed. Four rainwater samples (LM-Rain) were collected in February, May, and November of 2025 at the River Mountains Water Treatment Facility in Henderson, NV. The number of rainwater sample collection events was limited due to the lack of adequate precipitation events within the study period (at least 0.2-inches of rainfall were required for sample collection). Additional details about the rainwater sample collection are provided in the Supplementary Section S1.

PFAS contributions to the Las Vegas Wash (LVW) were evaluated from known or suspected sources during both dry weather (September 2024) and wet weather (February 2025). Dry weather samples were collected from final effluents of each of the four wastewater reclamation facilities discharging to the Las Vegas Wash (WWTP 1 through 4; operational details in Supplementary Table S1) and from rawhide channel (RC-1), a stormwater conveyance channel located near the Harry Reid International Airport which is known to receive dewatering from shallow groundwater (Thompson et al., 2024). Samples were also collected from two shallow groundwater wells [airport well and Paradise Vista Park (PVP) well] located near the airport which have previously been identified as having high PFAS (Thompson et al., 2024). Details on well location, depth and construction can be found in the supplemental materials of Thompson et al. (2024). Shallow groundwater may contribute PFAS to the LVW via either seepage or dewatering into nearby stormwater channels. Wet weather samples were collected from a first flush storm event following over 200 days without rain. Samples were collected from two locations within Sloan channel which receives runoff flow from Nellis Air Force Base (NAFB): the upper Sloan channel (USC) sample was collected above NAFB and the lower Sloan channel (LSC) sample was collected just below NAFB. Samples were also collected from three locations receiving runoff from the Harry Reid Airport: rawhide channel (RC-2), rawhide channel source (RCS) and a tributary to Tropicana Wash (TWT).

Wastewater samples were collected in three sampling events from two sewersheds in the Las Vegas Valley. Where available, flow rate data from sewershed sample locations is provided in Supplementary Table S2. Wastewater source sample event one was conducted on a Wednesday in November 2024 with samples collected from WWTP 3 and its sewershed. Four samples were collected from the discharge of industrial locations identified as potential PFAS sources: a truck wash, a car wash, a commercial laundry facility, and an electroplating facility. Two samples were collected in the afternoon from manholes within two neighborhoods receiving exclusively domestic wastewater (i.e., Neighborhoods 1 and 2). Three samples were collected from locations where nearby shallow groundwater is known to have high PFAS concentrations: a manhole receiving water exclusively from the Harry Reid Airport (Commercial Airport), a manhole receiving flow from Nellis Air Force Base (Nellis AFB), and a manhole receiving flow from the Las Vegas Strip (LV Strip). Two samples were collected from the solids dewatering stream within WWTP 3 (one in the morning and one in the afternoon, called dewatering 1 and 2, respectively). Flow-based 24-h composite samples were collected from the influent and final effluent of WWTP 3 using a HACH All-Weather SD 950 autosampler with silicone pickup tubing and vinyl pump tubing. An equipment blank was collected from the tubing; results are presented in the Supplementary Material. These samples were called WWTP 3B inf and eff, respectively.

Wastewater source sample event two was conducted in February 2025. One sample was collected from the wash cycle of a household laundry system washing synthetic (sports) clothing with BioKleen (Gurnee, IL) citrus essence 3X concentrated laundry liquid in a Kenmore (Hoffman Estates, IL) 600 series top-loading washer. Two control samples were collected for the household laundry as well: (1) a sample from the wash cycle when no clothing or detergent was added and (2) a detergent only sample diluted to the same concentration used during the wash cycle (i.e., 1 tablespoon for 52 L of water, or 0.028% v/v). Two additional samples were collected on a Wednesday from the WWTP 3 sewershed: the two neighborhoods sampled in November (Neighborhoods 1 and 2) were re-sampled in the early morning (i.e., around 8 a.m.) to capture peak morning flow. The remaining samples were collected on a Tuesday from WWTP 1 and its sewershed. Five samples were collected from the discharge of industrial locations identified as potential PFAS sources: a bus wash, a paper factory (Thompson et al., 2024), a garbage truck wash, a dumpster wash station, and a commercial laundry facility. One sample was collected from a neighborhood manhole receiving exclusively domestic wastewater (i.e., Neighborhood 3). This sample was collected in the early morning (i.e., around 8 a.m.) to capture peak morning household flows. Finally, samples were collected from the influent and effluent of WWTP 1 (WWTP 1B inf and eff, respectively).

Wastewater source sample event three was conducted in August 2025. Two flow based composite samples were collected from a pumping lift station in the WWTP 3 sewershed which receives wastewater solely from residential sources (Neighborhood 4). The first sample was a weekend composite with collection starting Friday morning and finishing Monday morning. The second sample was a weekday composite with collection starting Monday morning and finishing Tuesday morning. These residential wastewater composite samples intended to better capture flow from suspected residential PFAS sources, including laundry and showering wastewater, when compared with earlier grab sampling. A summary of sample locations and descriptions is provided in Supplementary Table S7.

2.2 TOP assay sample prep

The total oxidizable precursor (TOP) assay method was employed to quantify PFAS precursors capable of transformation to PFAAs in wastewater influent, effluent, and sewershed samples. A modified method adapted from Gonzalez et al. (2021), Houtz et al. (2018), Schaefer et al. (2023) and Tsou et al. (2023) was employed. Briefly, two 500-mL samples were collected for targeted PFAS analysis prior to and following TOP assay sample processing, as described below. Two 250-mL aliquots of sample were transferred to 250-mL HDPE bottles and 12.5 ng of mass labeled perfluorooctanesulfonamide (i.e., ¹³C₈-FOSA) was added to each bottle. This ¹³C₈-FOSA was expected to transform completely to mass labeled PFOA (i.e., ¹³C₈-PFOA) during TOP processing (if TOP processing proceeded to completion) and thus was used to confirm sufficient oxidant addition in each sample given the diverse range of sewershed sample matrices (Tsou et al., 2023). Yields of mass labeled ¹³C₈-PFOA for each TOP sample generally ranged from 45% to 97% (except for several samples with apparent high organic loading) and are detailed in Supplementary Table S11. Next, 5.4 g of potassium persulfate and 5 mL of 10 N sodium hydroxide were added to each bottle to achieve concentrations of 80 mM and 200 mM, respectively. Bottles were then placed in a water bath at 85 °C for 6 h. The pH was checked every 2 h and additional volume of 10 N sodium hydroxide was added if needed to maintain a pH above 12. Finally, samples were cooled to room temperature in a tap water bath and acidified to pH 5–9 with concentrated hydrochloric acid. Samples were stored at 4 °C until analysis.

Several quality assurance/quality control (QA/QC) TOP samples were run. First, prior to running any wastewater samples, confirmation TOP samples were run in ultrapure water to verify (1) the performance of this adapted TOP method and (2) confirm conversion of mass labeled FOSA to mass labeled PFOA. Briefly, ultrapure water samples were spiked with 500 ng/L of 8:2-fluorotelomeresulfonate (8:2-FTS), N-methylperfluorosulfonamidoacetic acid (N-MeFOSAA), or FOSA individually to obtain duplicate samples for each analyte. Targeted PFAS analysis was performed before and after TOP processing, as described above. Results from method development samples in ultrapure water agreed well with those obtained in previous studies (Houtz and Sedlak, 2012) with total molar yields ranging from 71% for the 8:2-FTS spikes to 80% for the N-MeFOSAA spkes and 91% for the FOSA spike (Supplementary Table S10). Incomplete recovery from the 8:2-FTS spikes in particular is likely due to oxidation of the 8:2-FTS precursor to ultrashort-chain perfluorocarboxylic acids (Ateia et al., 2023) which were not quantified with the targeted analytical method used for this study. Additionally, 25% of wastewater TOP samples were run as duplicates. A method blank was run to confirm the absence of PFAS contamination during the process. Finally, duplicate spike samples with both 8:2-FTS and N-Me-FOSAA (500 ng/L) were run in an influent wastewater matrix. Total molar yields from the 8:2-FTS matrix spike was around 95%, slightly higher than that observed in the method development samples. Molar yield from the N-MeFOSAA matrix spike was quite high at around 282%, which was attributed to low recovery of spiked N-MeFOSAA in the pre-TOP sample, likely due to sorption to the organic fraction of suspended solids that were filtered out during sample processing. Detailed results from QA/QC samples are presented in the Supplementary Table S10.

2.3 PFAS analysis via LC-MS/MS and high resolution MS

Samples were analyzed for 19 PFAS (Supplementary Table S3) via a targeted method which employed isotope dilution liquid chromatography tandem mass spectrometry (LC-MS/MS) following an automated solid phase extraction (ASPE) cleanup and concentration as described by Gonzalez et al. (2021). Non-targeted analysis of PFAS compounds not quantified in the LC-MS/MS method was performed on a subset of samples. These samples for non-targeted PFAS analysis were collected and processed in the same manner as samples for targeted PFAS analysis with the exception of the isotopically labeled PFAS spike which was not added to non-targeted analysis samples prior to ASPE. Briefly, a 250 mL sample volume was extracted for each sample. Laboratory reagent blanks (LRB), laboratory fortified blanks (LFB), and laboratory fortified matrix (LFM) spikes were extracted alongside samples. Extracts were evaporated to 0.5 mL under nitrogen gas and then brought up to a final volume of 1 mL with ultrapure water. Additionally, aliquots of the ASPE spike mix (i.e., 19 PFAS isotope analogs used for quantitation which are listed in Supplementary Table S3) were diluted to obtain both a 10 μg/L and a 1.0 μg/L standard solution. These were used as representative standards to confirm retention times of targeted PFAS compounds and instrument performance. Details on LC and high resolution mass spectrometry (HRMS) instrumentation and operational parameters are provided in the Supplementary Section S4.

Non-targeted data processing was performed using Compound Discover software version 3.3.3 from Thermo Scientific (Bremen, Germany). A modified version of the “PFAS Unknown ID with Database Searches” workflow was used to process data files. Briefly, the workflow used the following nodes to generate the peak lists: Input (Input Files) → Spectrum Processing (Select Spectra, Align Retention Times) → Compound Detection (Detect Compounds, Group Compounds, Fill Gaps) → Peak Area Refinement (Mark Background) → Compound Identification (Search Mass Lists, Search Chemspider, Predict Compositions, Search m/zCloud, Search m/zVault, Assign Compound Annotations) → Compound Scoring (Calculate Mass Defect, Compound Class Scoring, Search Neutral Losses) → Post-Processing (Scripting Node using R script). Potentially identified PFAS related compounds with high confidence were compounds with: matching retention times (if included in the ASPE spike mix), chemical formulas and masses matching compounds found in various PFAS Mass List databases, ChemSpider matches, and triggered MS2 spectrum matching spectra found in the m/zCloud or m/zVault spectra libraries (>50% score), and supportive predictive MS2 fragments using Compound Class Scoring. Compounds reported as low or medium confidence were compounds in which: identification may have been only from MS1 information (accurate mass and possible formulas), MS2 spectra did not match any spectra from the m/zCloud or m/zVault libraries or had slight differences, only had MS2 spectrum and fragments supported by Compound Class matches, several possible isomers were identified, or detected compounds were matched to possible, un-named compounds from PFAS databases which were only identified by PFAS class. Compounds identified as low or medium may be the subject of future research, where possible. Examples of MS2 spectra used to support high and medium confidence identification are provided in the Supplementary Figure S2 through S6). The high confidence level classification used herein is similar to confidence level 2a described by Charbonnet et al. (2022) and level 2 described by Schymanski et al. (2014) while the medium and low confidence levels herein fall within the range of confidence levels 3a through 3d from Charbonnet et al. (2022) and level 3 from Schymanski et al. (2014). Although the classifications described in prior works were used as a guideline for this work, direct correlation between confidence levels described here and those from Charbonnet et al. (2022) or Schymanski et al. (2014) are not possible due to the subjectivity of non-targeted analysis work and slight differences in classification criteria. All tentatively identified non-targeted PFAS compounds in water samples would need to be compared to a standard to ensure matching retention time and MS2 spectrum for definitive confirmation. Non-targeted PFAS results are limited to compounds suitable to the described LC-HRMS negative mode method.

3 Results and discussion

3.1 PFAS mass flow to Lake Mead is dominated by the Colorado River influent

Sampling in Lake Mead (LM) and from the Las Vegas Wash (LVW) indicated the Colorado River, which provides 97% of the flow to Lake Mead (Holdren and Turner, 2010), contributes 65% of the total PFAS loading to the lake (Figure 2A), making it the primary source of PFAS to Lake Mead. Additionally, the PFAS fingerprint at LM-BB looks very similar to that at CR-LM (Figure 2B). PFAS loading to Lake Mead was calculated using results from the LM-BB samples to represent Lake Mead concentrations (since this was the only sample location within the main body of the lake) and results from CR-LM to represent Colorado River concentrations. Results indicate total PFAS at LM-BB was predominantly comprised of short-chain perfluorocarboxylic acids [PFCAs; i.e., perfluoropentanoic acid (PFPeA; 1.1 ± 0.321 ng/L) and PFHxA (0.591 ± 0.216 ng/L)] and lower but still significant contributions from longer chain PFCAs and perfluorosulfonic acids [PFSAs; i.e., perfluoroheptanoic acid (PFHpA; 0.256 ± 0.039 ng/L), PFOA (0.501 ± 0.211 ng/L), PFNA (0.262 ± 0.057 ng/L), PFBS (0.747 ± 0.135 ng/L), PFHxS (0.138 ± 0.040 ng/L), and PFOS (0.236 ± 0.110 ng/L)] (Figure 2B). PFHxA, PFHpA, PFOA, PFNA, PFBS, PFHxS and PFOS appeared to primarily come from upstream sources (i.e., greater than 70% of loading to Lake Mead coming from the Colorado River). Percent contributions for individual and total PFAS were calculated with a flow-weighted mass balance using the flow contributions identified by Holdren and Turner (2010): ${\%PFASContribution}_{i,j}=\frac{{Q_j*\left[PFAS\right]}_{i,j}}{{\left[PFAS\right]}_{i,LMBB}}*100\%$ where i is the PFAS analyte, j is the PFAS source (i.e., either CR LM or LVW) and Q is the fractional flow contribution (i.e., 0.97 for CR LM or 0.022 for LVW). PFBA was excluded from this analysis and discussion because of its low detection frequency (only one detection at CR LM and none at LM BB) and variable MRL (see Supplementary Table S8). PFBS was also the only analyte detected in the snow melt sample collected from the upper Colorado River watershed at 0.281 ng/L (around 50% of the average concentration of 0.657 ± 0.058 ng/L found in the CR-LM samples). When evaluating mass loading (Figure 2A) and source attribution (Figure 2B) PFAS results below the MRL (Supplementary Table S8) were assumed to be either 0.5*MRL for analytes with more than one detection (at sites with multiple samples) or zero for analytes with one or fewer detections. For individual PFAS detected in either the CR-LM or LVWRG-1 sample but not the LM-BB samples [i.e., perfluorodecanoic acid (PFDA)], concentrations in LM-BB were assumed to be 100% attributable to measured concentrations at CR-LM and LVWRG-1 (i.e., ${\left[PFAS\right]}_{i,LM-BB}={0.97\ast \left[PFAS\right]}_{i,CR-LM}+{0.022*\left[PFAS\right]}_{i,LVWRG-1}$). Spatial and temporal differences in PFAS concentrations at CR-LM and LM-BB can be attributed in part to concentrations close to the method reporting limits where a small decrease in concentration would yield a non-detect result. Regardless, results indicate the Colorado River is the primary source of PFAS loading to Lake Mead.

Figure 2

Figure 2. (A) Percent of targeted PFAS mass loading at Lake Mead Boulder Basin (LM-BB) attributed to flows from the Colorado River (CR LM) and the Las Vegas Wash during dry weather flows (LVWRG-1). Percent contribution is given for PFAS detected via targeted LC-MS/MS in either CR-LM or LVWRG-1 and for total PFAS. PFAS analytes which were non-detect in LM-BB only were assumed to be 100% attributable to measured concentrations in CR-LM and LVWRG-1. (B) Fractional contribution by mass of targeted PFAS to the total PFAS concentration measured in the samples collected either from Lake Mead or direct sources to Lake Mead. Total PFAS concentrations for each sample are indicated with red diamonds and called out in black text.

The samples collected at Hoover Deck (LM-HD) had a similar PFAS fingerprint to the samples collected at LM-BB and CR-LM, although historical sample results (i.e., collected monthly from July 2021 through May 2025 and shown in Supplementary Figures S7 and S8) indicate concentrations at LM-HD are both higher and have greater variability when compared to results from LM-BB for several PFAS compounds. For example, average total PFAS concentrations were 8.46 ± 2.08 ng/L and 6.58 ± 0.788 ng/L, at LM-HD and LM-BB respectively. Similarly, average concentrations of PFOS were 0.606 ± 0.754 ng/L and 0.236 ± 0.110 ng/L at LM-HD and LM-BB, respectively, and average concentrations of PFOA were 0.908 ± 0.364 ng/L and 0.501 ± 0.211 ng/L, at LM-HD and LM-BB, respectively. This may be attributed to the greater contribution from recycled water flow at Hoover Deck (coming from either the LVW or wastewater discharge at the Hoover Dam) compared to that at Boulder Basin. Sucralose has been shown to be a strong indicator for percent contribution from recycled water flow (i.e., percent de facto reuse; %DFR) given its resilience to conventional wastewater treatment and its persistence in the environment (Islam et al., 2023; Li et al., 2023; Mawhinney et al., 2011). Historic PFAS and sucralose concentrations from LM-BB and LM-HD (Supplementary Figure S7) show higher concentrations of both total PFAS and sucralose are consistently found at LM-HD. Interestingly, the correlation between total PFAS and sucralose concentrations in Lake Mead (R² = 0.46) was not as strong as had been previously shown in the Trinity River (i.e., R² = 0.78); however, this makes sense when the Colorado River is considered as the primary source of PFAS to Lake Mead (as described in the previous paragraph) since upstream sources of PFAS to the river are likely to be only partially wastewater-derived. Additionally, historical SNWA sample results (Supplementary Figure S8) show a slowly increasing trend for total PFAS concentrations in Lake Mead since mid-2021, particularly at the Hoover Dam sample location.

The LVW, by contrast, contributes only 2.2% of the flow to Lake Mead (Holdren and Turner, 2010) and around 27% of the total PFAS loading during dry weather sampling (Figure 2A). The LVWRG PFAS fingerprint (Figure 2B) has a strong similarity to that of the LM BB sample (and thus similar to CR LM and LM HD as well). LVWRG alone accounted for 42% of the PFHxS (2.65 ng/L), 37% of PFHxA (9.94 ng/L), 20% of PFOA (4.46 ng/L), 18% of PFOS (1.89 mg/L), and 13% of PFBS (4.35 ng/L) loading to Lake Mead. PFPeA and PFDA were detected in the LVWRG sample but not in the 2024 LM-BB sample (Supplementary Table S8), although historic sample results reveal infrequent PFPeA detections at LM-BB (i.e., detection frequency of 0.09). It is important to note that Lake Mead concentrations from LVWRG loading were expected to be below detection limits for these analytes given dilution. For example, PFPeA had the highest PFAS concentration in the LVWRG sample (20 ng/L); however, after accounting for dilution this would contribute 0.4 ng/L PFPeA to Lake Mead which is well below the detection limit of 2 ng/L for the LM-BB sample. The LVW is an important secondary source of PFAS to Lake Mead, particularly for PFPeA, PFHxA, PFDA and PFHxS.

Total PFAS concentrations in both precipitation samples were significantly lower than in the Lake Mead samples (Figure 2B). Four rainwater samples collected on the southeast side of the Las Vegas Valley contained measurable concentrations of eight short- and long-chain PFAS. The sample collected in February (LM-Rain 1) contained primarily PFOA (0.275 ng/L) and some PFOS (0.132 ng/L) while the sample collected in May (LM-Rain 2) from the same location contained slightly higher concentrations of PFOA (0.341 ng/L) and PFOS (0.325 ng/L) as well as PFDA (0.369). By contrast, the sample collected from the first half of a multi-day rain event in November (LM-Rain 3) contained two short-chain PFAS, PFHxA (0.391 ng/L) and PFBS (0.179 ng/L), and two polyfluorinated substances, N-MeFOSAA (0.150 ng/L) and N-ethylperfluorooctane sulfonamidoacetic acid (N-EtFOSAA; 0.137 ng/L), in addition to PFOA (0.362 ng/L). The sample collected from the second half of the same November rain event (LM-Rain 4) had detectable levels only of PFOS (0.218 ng/L). PFAS concentrations measured in Las Vegas rainwater are on the low end of concentrations measured in recent studies in North America. For example (Pike et al., 2021), measured 2.00 ng/L PFOA, 20.0 ng/L PFOS, 7.00 ng/L PFDA, and 39.0 ng/L total PFAS in Wyoming rainfall while (Villagómez-Márquez et al., 2023) measured 31.1 ng/L PFOA, 36.8 ng/L PFOS, and 92.42 ng/L total PFAS in Arizona rainfall. By contrast, a recent study in Florida measured similar PFAS concentrations (e.g., 0.24 ng/L and 0.32 ng/L of PFOS and PFOA, respectively) (Guerra de Navarro et al., 2024). Additionally, the Las Vegas rainfall samples differ from prior study results with no detections of short chain PFAAs or common precursors such as 6:2-FTSA and 6:2-FTS (Guerra de Navarro et al., 2024; Olney et al., 2023; Pfotenhauer et al., 2022). Due to the low annual rainfall in the Las Vegas Valley [4.18 in. (NOAA, 2021)], precipitation in this area is not expected to be a major source of PFAS to Lake Mead. Sampling and analysis of rainfall in the upper Colorado River watershed would give further insight into the contribution of PFAS from rainfall to the Colorado River system, and thus to Lake Mead, but was outside the scope of this work. By contrast, snowmelt collected in June from the upper Colorado River basin contained measurable concentrations only of the short chain PFBS (0.329 ng/L). Neither sample site is located in close proximity to industry expected to produce air emissions of PFAS, so PFAS in these samples is expected to be the result of transport through the water cycle (Cousins et al., 2022; Hartz et al., 2024; MacInnis et al., 2019). Differences in the PFAS fingerprint between these precipitation samples could be due to geographical variability; however, additional research is needed to fully understand the impact of precipitation on PFAS loading in the Colorado River system.

3.2 Wastewater effluent is the primary PFAS source to the Las Vegas Wash

Dry weather PFAS (and flow) loading to the LVW is dominated by effluent from the four WWTPs in the Las Vegas Valley which discharge to the LVW (Figure 1). Combined WWTP effluents accounted for 105% of the flow at Rainbow Gardens on the LVW on the day sampled and 56% of the PFAS mass flow (Figure 3). Similar to findings in Thompson et al., 2024, WWTP 3 had the highest PFAS loading (25%) due to its high flow (60% of LVWRG flow). While WWTP 1 had a lower flow (10% of LVWRG flow), the total PFAS concentration of 64 ng/L resulted in the second highest PFAS loading (at 14%). WWTP flows and PFAS concentrations are detailed in Supplementary Tables S1 and S8, respectively. Flow at LVWRG during dry weather sampling (LVWRG-1) was the average flow from the sample day (712 ML/day) calculated from the USGS stream gauge data at that location (USGS, 2025). Flow at the rawhide channel sample location was assumed to be 0.53 ML/day during dry weather sampling (RC-1), as calculated in Thompson et al. (2024). Targeted analysis results show detections of only PFAAs in all four WWTP effluents; however, post-TOP results (discussed in more detail in Section 3.4 below) indicated the presence of other polyfluorinated PFAA precursors in both WWTP 1 and 3. Rawhide channel (RC-1) and two shallow groundwater wells near the airport had high PFAS concentrations (4,768, 1,801 and 296 ng/L total PFAS from RC, Airport well and PVP well, respectively; Figure 4), but are not expected to be major sources of PFAS to the LVW during dry weather conditions because of the low flow from these sources. Water from the shallow groundwater wells in this area is thought to be contaminated with AFFF and is primarily expected to enter the LVW through dewatering activities which in part contribute flow to RC (Thompson et al., 2024). These samples contain high PFSA concentrations compared to the LVWRG, with RC contributing 31% of PFHxS and 42% of PFOS to the LVWRG. Most of the gap in the PFAS contribution to LVWRG can be attributed to shorter chain PFCAs (i.e., PFPeA, PFHxA and PFHpA) which may indicate transformation of precursor compounds occurring farther upstream in the wash (Houtz and Sedlak, 2012). Additionally, method reporting limit (MRL) limitations for short chain PFAS in some WWTP samples may help explain the gap in loading of these compounds to the LVWRG. PFPeA and PFBS contributed 43% and 10%, respectively to the PFAS in the LVWRG sample. If concentrations of PFBS and PFPeA are assumed to be half the MRL (rather than zero), PFAS mass flows from the four wastewater reclamation facilities plus rawhide channel account for 83% of the mass flow at LVWRG (Supplementary Figure S9). Thus, during normal, dry weather conditions, effluent from the four WWTP discharging to the LVW are expected to be the primary PFAS contributors to the LVW.

Figure 3

Figure 3. Percent of targeted PFAS mass loading in the Las Vegas Wash at Rainbow Garden during dry weather (LVWRG-1) attributed to flows from four wastewater treatment plant effluents and rawhide channel (RC). Percent loading is given for individual PFAS detected via targeted LC-MS/MS in the LVWRG-1 sample and total PFAS. Mass loading for LVWRG-1 was calculated using the average flow from the day sampled (712 ML/day) as measured by the USGS stream gauge at Rainbow Gardens (USGS, 2025). Flow at RC is assumed to be 0.53 ML/day, the dry weather flow measured by Thompson et al. (2024).

Figure 4

Figure 4. Fractional contribution by mass of individual PFAS detected via targeted LC-MS/MS to the total targeted PFAS concentration measured in the ten samples collected from either stormwater channels throughout the Las Vegas Valley (i.e., UCS, LCS, RCS, RC and TWT), shallow groundwater near the major commercial airport (i.e., Airport Well and PVP Well), or the Las Vegas Wash at Rainbow Gardens (i.e., LVWRG). Four samples were collected during dry weather (i.e., RC-1, Airport Well, PVP Well and LVWRG-1) while the remaining samples were collected during a wet weather event. The total PFAS concentration for each sample is indicated with a red diamond.

First-flush stormwater samples collected from several washes around the Las Vegas Valley where prior AFFF use is known or suspected revealed higher contributions from long chain PFCAs and polyflouorinated compounds compared to the dry weather samples and WWTP effluents described above. In particular, 6:2-FTS and 6:2-fluorotelomer unsaturated carboxylic acid (6:2-FTUCA) were detected in all five of the wet weather samples collected around the Valley (Figure 4), at concentrations ranging from 1.88 to 125 ng/L for 6:2-FTS and from 5.19 to 9.02 ng/L for 6:2-FTUCA; however, these two PFAS still comprised less than 10% of the total PFAS concentration at any of these locations. Interestingly, the two samples collected from Sloan Channel had similar PFAS fingerprints and total PFAS concentrations (202 and 206 ng/L above and below NAFB, respectively), indicating PFAS sources in this wash are likely from urban runoff (e.g., roadways, residential areas) rather than runoff from AFFF-impacted areas within NAFB. The wet weather LVWRG-2 sample had higher total PFAS compared to the LVWRG-1 sample (i.e., 93.4 ng/L and 47 ng/L for wet and dry sample events, respectively) but did not have detectable concentrations of any of the polyfluorinated PFAS, PFNA or PFDA, likely due both to dilution from runoff from other areas and higher reporting limits for several analytes for that sample. PFAS mass flows were not calculated for the stormwater tributaries since samples were collected from the first flush and thus did not represent peak flow; however, the total PFAS mass flow at LVWRG for the sampled 3-h storm event was calculated to be 67 g, compared to an estimated mass flow of 33 g/day during dry weather. Mass loading at LVWRG was calculated from flow data collected from the USGS LVWRG stream gauge at 15-min intervals (i.e., 712 ML/day on 9/9/2025 during the dry weather sampling and 721 ML for the duration of the storm sampled on 2/14/2025) and total PFAS data collected during each sample event (USGS, 2025). Thus, while storm events are important intermittent contributors of PFAS to Lake Mead, their overall contribution is expected to be low given the infrequent nature of precipitation events in the Las Vegas Valley (NOAA, 2021).

3.3 Wet and dry weather samples had distinct PFAS fingerprints from non-targeted analysis

Non-targeted analysis revealed distinct fingerprints unique to the dry weather and wet weather samples (Table 1). Relatively large peaks were tentatively identified in the three dry weather samples (i.e., airport well, PVP well and RC-1) for seven suspected perfluorosulfonate and perfluorosulfonamide compounds not included in the targeted analyte list [namely, perfluoropropane sulfonate (PFPrS), perfluoropentane sulfonate (PFPeS), perfluoroheptane sulfonate (PFHpS), perfluoroethane sulfonamide (FEtSA), perfluoropropane sulfonamide (FPrSA), perfluorobutane sulfonamide (FBSA), and perfluorohexane sulfonamide (FHxSA)]. With the exception of FEtSA (medium confidence), these seven compounds were identified with high confidence as described in Section 2.3. Representative MS2 spectra have been included in Supplementary Section S4 along with additional details on identification and selection of confidence level. All three of these dry weather sample locations (airport well, PVP well and RC) have suspected AFFF influence due to (1) their proximity to a commercial airport (Figure 1) where historic AFFF may have occurred and (2) high PFAS concentrations found in samples previously collected at these locations (Thompson et al., 2024). Additionally, the perfluorosulfonate and perfluorosulfonamide compounds tentatively identified in these samples are commonly associated with a number of AFFF formulations (Place and Field, 2012; Wanzek et al., 2024). Generally, peak areas for individual compounds were largest in the RC-1 sample > Airport well > PVP well. The airport well, PVP well and RC-1 samples also contained both a high ratio of PFOS:PFOA (i.e., 0.7, 1.1 and 1.5, respectively) and high percentage of branched PFOS isomer (i.e., branched to linear PFOS ratios of approximately 0.46, 0.53 and 0.46, respectively; discussed further in Supplementary Section S7), which are characteristic of PFAS originating from electrochemical fluorination AFFF groundwater plumes (Adamson et al., 2022). A number of other, less well studied PFAS were also tentatively identified with significant peak areas, primarily in the airport well and RC-1 samples (Table 1). Six tentatively identified compounds (medium and low confidence) were in classes previously discussed by Barzen-Hanson et al. (2017): H-PFAS, O-PFAS, K-PFAS, and PFASi. One PFAS of particular interest is 1-hydroperfluoroheptane which was tentatively identified (medium confidence) with high peak areas in all three samples. This is a non-polar PFAS which can be a decomposition product of PFOA (Wang et al., 2024). Interestingly, many of tentatively identified PFAS in the RC-1 and airport well samples were short- or ultrashort-chain PFAS (i.e., fewer than six fluorinated carbons, for example, PFPrS) or fully fluorinated PFAS (i.e., PFSAs and perfluorosulfonamides). This indicates either the sample points are farther from the source zone with precursor concentrations low due to upgradient transformation and retention on soil-bound organics or the PFAS source is older and precursor transformation has occurred both at the source and farther out in the plume (Adamson et al., 2022; 2020). Overall, non-targeted analysis of dry weather samples provides support for the hypothesis that these three sample locations are impacted by an AFFF PFAS source.

Table 1

Table 1. Relative abundance of PFAS compounds tentatively identified via non-targeted analysis which were not part of the targeted analytical method.

By contrast, wet weather samples (i.e., USC, LSC, RCS, RC-2, TWT and LVWRG-2) had significant contributions from only a few short chain polyfluorinated compounds (C2 – C4; see Table 1) including tentative identification of two sulfonamides (high confidence) and one fluorotelomer carboxylic acid (FTCA; medium confidence). One sulfonamide (FBSA) was found in both the wet and dry weather samples. The RCS, RC-2 and TWT samples (all three of which are expected to receive stormwater runoff from the commercial airport) also contained four additional PFAS (PFPeS, PFHpS, FHxSA, and compound 118; compound identification tentative) with similar peak areas to those found at RC during the dry weather sampling event. This indicates some similarity in PFAS source between the two sampling events (e.g., AFFF impacted runoff and shallow groundwater); however, the overall reduction in the relative peak area and type of PFAS detected during wet weather sampling compared to dry weather sampling may be partially the result of dilution from rainwater, reducing the apparent impact of the suspected AFFF PFAS source on wet weather samples. Additionally, the ratio of branched to linear PFOS decreased from the RC-1 sample (dry weather; branched to linear ratio of approximately 0.46) to the RC-2 sample (sampled during a storm event; branched to linear ratio of approximately 0.36) indicating the presence of a secondary PFAS source contributing more linear PFOS isomer which could be the result of either a different PFAS generation method (e.g., urban runoff containing PFOS generated via fluorotelomerization rather than electrochemical fluorination) or an older PFOS source (e.g., an older AFFF spill which primarily reaches this sample point during runoff events) (Adamson et al., 2020). Two of the PFAS tentatively identified (medium confidence) in all of the wet weather samples [i.e., 4-H-perfluoro-1-butene and hepafluoropentanioc acid] were also identified in the rainwater sample collected from the same storm event Table 1), indicating rainwater may be an important source of these two short chain PFAS. Longer chain FTCAs have been identified in rainwater in Canada where they were attributed to oxidative transformation of fluorotelomer alcohols present in the atmosphere (Loewen et al., 2005). 4-H-perfluoro-1-butene is a less commonly identified, neutral PFAS. The two Sloan Channel samples had similar PFAS fingerprints, indicating PFAS likely came from an upstream source, such as urban runoff, rather than from NAFB. It is important to note this was a first-flush storm event with more than 200 days since the prior significant rain event, so urban runoff sources of PFAS were more pronounced than would be expected in subsequent rain events. Overall, results indicate PFAS contributions from several sources (rain, AFFF impacted waters and urban runoff) were important during this wet weather event.

3.4 Domestic wastewater is the primary PFAS contributor for two Las Vegas WWTP

Sampling results from WWTP 3 (the largest WWTP in the Las Vegas Valley) and it’s sewershed indicate domestic wastewater is the primary PFAS contributor to that WWTP despite much higher total PFAS concentrations from other, low flow sources (Figure 5). The LV Strip sample which receives flow from primarily domestic sources (i.e., hotels, casinos, and residential areas in the vicinity of the LV strip) had a total PFAS concentration (sum of targeted PFAS and precursor PFAS obtained from TOP analysis) of 0.312 nmol/L. This was similar to the total PFAS concentration in the WWTP 3 influent of 0.291 nmol/L. On average, the LV strip line conveys 26% of the WWTP 3 flow and 27% of the PFAS mass flow. Residential wastewater samples collected from the WWTP 3 sewershed showed variable results based on sampling method (grab versus composite) and time (weekday versus weekend). Grab samples collected from neighborhoods one and two had total PFAS concentrations of only 0.049 nmol/L and 0.131 nmol/L, respectively (see Supplementary Figure S14). By contrast, the weekday composite sample collected from neighborhood 4 had a total PFAS concentration of 0.272 nmol/L while the weekend composite sample from neighborhood 4 had a total PFAS concentration of 0.473 nmol/L (Supplementary Figure S14). These differences can be explained by the timing of sample collection compared to when residential PFAS-source activities (e.g., showering and laundry) are expected to occur with greatest frequency (i.e., evenings and weekends). If the total PFAS concentration from the weekday composite for neighborhood 4 (which is anticipated to best represent flow to the WWTP during the weekday influent composite sampling) are assumed to apply to 63% of the WWTP 3 flow (based on conversations with treatment plant staff), residential wastewater sources would account for 59% of the PFAS mass flow to the WWTP 3 influent (calculated on a molar basis; see Supplementary Section S8 for calculation details). Interestingly, despite high total PFAS concentrations at some industry locations, such as the truck wash (0.335 nmol/L), commercial laundry (0.373 nmol/L), and commercial airport (0.973 nmol/L), relatively low flows at these locations resulted in PFAS mass contributions of less than 1%. Similarly, total PFAS concentrations in two dewatering samples (0.271 and 0.262 nmol/L) within WWTP 3 indicate this is not a significant source of PFAS recycle within WWTP 3. Thus, residential and other domestic wastewater PFAS sources (discussed in more detail below) appear to be the primary source of PFAS in this sewershed comprising approximately 89% of the flow and 86% of the mass flow to the WWTP 3 influent (Figure 5).

Figure 5

Figure 5. Percent of total PFAS (targeted + TOP) mass flow in WWTP 3 influent that (1) can be attributed to each of seven sewershed source categories and (2) remains in the WWTP 3 effluent. Residential wastewater was assumed to comprise 63% of the influent flow at WWTP 3. All other flows are given in Supplementary Table S3. Total PFAS concentration [in nano moles per liter (nmol/L)] is indicated with red triangles with values on the right y-axis.

Results from sampling in the WWTP 1 sewershed (Supplementary Table S8), which has a greater industrial presence, tell a similar story although high perfluorobutanoic acid (PFBA) detections in some samples made interpretations of PFAS mass flow more challenging. The total PFAS concentration at the neighborhood 3 location (domestic wastewater only) was 0.078 nmol/L which was similar to results from the WWTP 3 sewershed. Domestic wastewater is expected to account for 94% of the average daily flow to WWTP 1 (from discussions with treatment plant staff); however, both total flow and flow contributions fluctuate throughout the day (and week) with domestic flow contributions being greatest during the mornings, evenings and weekends as is typical for municipal WWTP (Metcalf & Eddy, 2014). High total PFAS concentrations were measured at several industrial sites in this sewershed as well, for example, the bus wash (0.232 nmol/L), garbage truck wash (0.786 nmol/L) and dumpster wash (0.352 nmol/L); however, the low flows from each of these locations (less than 1% of the treatment plant flow rate) make their overall PFAS mass flow contributions relatively low. Interestingly, the commercial laundry sample collected from this sewershed had a much higher total PFAS concentration (3.24 nmol/L) than that from the WWTP 3 sewershed. The difference in total PFAS concentration between the two commercial laundry samples is primarily attributed to PFBA (Figure 6) which comprises 99% of the total PFAS (molar basis) for the Laundry 2 sample (i.e., 460 ng/L PFBA) collected from the WWTP 1 sewershed but was below the reporting limit (i.e., <125 ng/L) in the Laundry sample collected from the WWTP 3 sewershed. This difference between the laundry samples at different locations may be due to differences in customer base and types of laundry processed at each location. High PFBA concentrations were also noted in the WWTP 1 influent (282 ng/L) and effluent (137 ng/L) which contributed to high total PFAS concentrations during this sampling event (1.36 and 0.839 nmol/L, respectively). Previous sampling of the WWTP 1 effluent in this study measured a much lower total PFAS concentration (targeted analysis only) of 0.222 nmol/L with PFBA below the MRL of 5 ng/L. Thompson et al. measured a slightly higher total PFAS (targeted analysis only) of 0.448 nmol/L in 2023 with PFBA below the MRL of 100 ng/L (Thompson et al., 2024). This variability highlights the challenges inherent to grab sampling and thus made mass flow calculations in this sewershed challenging; however, results do indicate commercial laundry wastewater is likely an important intermittent source of PFAS (particularly PFBA) to WWTP influent.

Figure 6

Figure 6. Fractional contribution by moles of individual targeted PFAS and other PFAS precursors measured via TOP assay to the total PFAS concentration (targeted + TOP) measured in the sewershed and treatment plant samples collected from WWTP 1 and 3. Samples are grouped into five general source categories (listed across the top of the figure) with relatively distinct PFAS fingerprints. Total PFAS (in nano moles) is indicated with red diamonds with values on the right y-axis.

A comparison of the fractional contribution of different PFAS in WWTP and sewershed samples from WWTP 1 and WWTP 3 (Figure 6) reveals several interesting categories of PFAS sources with distinct fingerprints. The primarily domestic wastewater samples (residential wastewater, LV Strip and Nellis AFB) had medium to high contribution (i.e., 50%–94%) from other (TOP) precursor PFAS. The WWTP 3 influent had a similarly high contribution from other precursors (95%), reflecting the importance of domestic wastewater as a PFAS source. The residential wastewater samples also had significant contributions from PFHxA (18%), PFHxS (14%), PFBS (8%) and PFOA (7%). The LV Strip sample (collected from a large sewer main conveying flow from casinos along the LV strip, several neighborhoods to the west of the strip, and the commercial airport wastewater) had high contribution from other precursors (93%). This sample also had a significant contribution from PFOA (1.5%), PFHxA (3.5%) and 6:2-fluorotelomersulfonic acid (6:2-FTS; 1.7%). The higher concentration of 6:2-FTS in this sample compared to those that were predominantly domestic wastewater likely reflects contributions from other hospitality and commercial sources. Another possible source of the 6:2-FTS is infiltration of shallow groundwater contaminated with AFFF from a nearby plume. A portion of this sewer line above where the sample was collected runs along the west side of the runway for the Harry Reid Airport where an AFFF plume is known to exist. Both the commercial airport and Nellis AFB samples had high PFOS:PFOA mass ratios (i.e., 1.6 and 1.3, respectively), comparable to that of the shallow groundwater and RC-1 samples (Supplementary Table S8). This may indicate electrochemical fluorination derived AFFF contribution to PFAS at these locations, potentially from shallow groundwater infiltration to sewer lines. Both locations had around 2%–3% contribution from 6:2-FTS which is commonly associated with fluorotelomer derived AFFF. Interestingly, the commercial airport sample appeared to have little to no contribution from other PFAS precursors (i.e., precursors not a part of the targeted method analyte list in Supplementary Table S3) following TOP processing while the Nellis AFB sample had around 94% contribution from other PFAS precursors (Figure 6). This indicates infiltration from AFFF-contaminated shallow groundwater likely occurred far from the source at the commercial airport and closer to the source at Nellis AFB (Adamson et al., 2022; Adamson et al., 2020). Interestingly, the laundry samples did not have a uniform fingerprint across the three samples. One commercial laundry sample had high contributions from other PFAS precursors (93% of post-TOP total PFAS; Figure 6) with the remaining PFAS coming primarily from PFOA (4.2%) and PFHxA (2.4%). By contrast, the second commercial laundry sample (collected from a second facility of the same commercial laundry chain) and the household laundry sample had high contributions from PFBA (98.6% and 83%, respectively; Figure 6) with only low concentrations of other targeted PFAS and some other precursors (12.6%) in the household laundry sample. This difference between laundry samples may be due to differences in the items being washed or the timing of the sample collection during the washing process at each facility (since both were grab samples). Interestingly, the WWTP 1 samples collected on the same day had a similar fingerprint to the laundry sample. Five samples were collected from vehicle or garbage washing sites, and all had lower contributions from precursors compared to domestic samples (i.e., 54%–81%). These wash samples also had significant contributions from long-chain PFCAs (>C6) and four of five samples had significant 6:2-FTS contributions (i.e., 3%–8%). Two other industry samples were also analyzed: a paper factory and an electroplating facility. Both had lower precursor contributions (0%–50%) and higher contributions from long chain PFCAs (>C6) and PFBS (i.e., 20% and 32%, respectively). The dewatering samples from WWTP 3 had relatively low contributions from other PFAS precursors (37%). Similarly, the WWTP 3 effluent sample had low contribution from other PFAS precursors (18%). Prior research has shown conversion of precursors occurs through the wastewater treatment train due to biotransformation and partitioning to solids (Schaefer et al., 2023; Thompson et al., 2022).

3.5 Laundry and personal care products are potentially significant PFAS sources in domestic wastewater

Results from this study and prior work indicate household laundry water (particularly from washing synthetic clothes) and use of personal care products containing PFAS may be significant sources of PFAS to domestic wastewater. Some synthetic clothing (e.g., quick dry and sweat-stain-resistant sports clothing, uniforms made with synthetic fabrics, and stain-resistant children’s clothing) have been shown to contain measurable levels of PFAS (Danish EPA, 2015; Guo et al., 2009; Schellenberger et al., 2019). In this study, a sample collected from the wash drain cycle (52 L total) of a household laundry machine after washing an assortment of synthetic sports clothing contained high concentrations of PFBA (115 ng/L, 83% of total PFAS), other precursors (13%), and PFOS (4.83 ng/L, 1.5%) as well as measurable concentrations of several other PFAAs (Figure 6). If each household in the Las Vegas Valley were to run six loads of laundry per week having a similar PFAS composition, this would contribute approximately 11% of the total influent PFAS loading to these WWTPs. Control samples from the wash cycle when the machine was empty and from the laundry detergent alone showed little to no PFAS (0.011 and 0.006 nmol/L total PFAS, respectively). Some personal care products including certain lotions, face cream, foundation, lipstick/lip balm, and shampoos have also been shown to contain high concentrations of PFAS (Table 2). An estimated daily usage and percent contribution to WWTP PFAS loading in the Las Vegas Valley is presented in Table 2 (calculations described in Supplementary Section S9). This limited dataset indicates shampoos and foundation products containing PFAS may be significant contributors to PFAS loading (i.e., 9.3% and 2.2%, respectively); however, more research is needed in this area. Toilet paper and human elimination are expected to contribute less than 1% of PFAS loading to the WWTPs. Calculations for these categories are provided in Thompson et al. (2023) and Supplementary Section S9, respectively. More research is needed to further improve our understanding of the primary PFAS sources to domestic wastewater.

Table 2

Table 2. PFAS contributions to domestic wastewater from selected personal care products and bodily excretion (i.e., urine and feces).

4 Conclusion

• Flow from the Colorado River is the main source of PFAS to Lake Mead (65% of total PFAS mass loading) while the LVW is an important secondary source (27% of total PFAS mass loading), particularly for PFPeA, PFHxA, PFDA and PFHxS.

• Rainwater collected in the Las Vegas Valley contained low concentrations of two short-chain PFAS (i.e., PFBS and PFHxA), four long-chain PFAS (i.e., PFOS, PFHpA, PFOA and PFDA), and two polyfluorinated substances (i.e., N-MeFOSAA and N-EtFOSAA) across four sampling events.

• The four WWTP sending effluent to the LVW are the primary source of PFAS to the LVW; however, dewatering from shallow groundwater near the commercial airport is an important secondary source during both dry and wet weather due to an AFFF impacted plume of groundwater there.

• Non-targeted analysis of groundwater and runoff samples revealed distinct fingerprints in PFAS identified during dry weather compared to wet weather. High peak areas of PFSAs and sulfonamides were detected in AFFF-impacted dry weather samples. Hepafluoropentanioc acid was detected in all wet weather runoff samples with large peak areas.

• Residential wastewater and other domestic wastewater sources are expected to be the primary contributor of PFAS to WWTP influent in the Las Vegas Valley due to the lack of industries which use or produce PFAS. As such, laundry and PFAS-containing personal care products may be important sources of PFAS to these WWTP. Further research is needed to better understand PFAS sources to domestic and residential wastewater.

• Several categories of WW samples (i.e., primarily domestic wastewater, laundry, vehicle/garbage wash, other industrial, and wastewater treatment plant effluent) that were collected from the sewersheds of two WWTPs in the Las Vegas Valley showed distinct PFAS fingerprints with domestic wastewater samples having the highest contribution from PFAS precursors quantified using the TOP assay method.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

JS: Conceptualization, Formal Analysis, Investigation, Visualization, Writing – original draft, Writing – review and editing. RT: Data curation, Formal Analysis, Methodology, Writing – original draft, Writing – review and editing. OQ: Data curation, Formal Analysis, Methodology, Writing – review and editing. BV: Data curation, Resources, Writing – review and editing. ED: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Acknowledgements

The authors would like to thank: Southern Nevada Water Authority staff who assisted with this project including Brittney Stipanov and Janie Holady for their assistance with sample preparation and analysis; Ceonie Washington, Kers Ung-Watson, and Ahdee Zeidman for their assistance with wastewater and stormwater sample collection; Dan Gerrity for providing knowledge and insight into sewershed sampling and for his editorial review; Eric Dano for his assistance with collection of shallow groundwater well and stormwater samples; Todd Tietjen for providing knowledge and expertise related to Hoover Dam operations; Carol Lane and the SNWA Compliance Laboratory sampling staff for assistance with Lake Mead sample collection; and Heath Szymanski for his assistance making a wastewater manhole sampler. The authors would also like to thank Alisson Boeing (HRD) and Josh Coffey (Brown and Caldwell) for assistance with Las Vegas Wash stormwater sampling efforts; staff at WWTPs 1 - 4 for their expertise and assistance with sample collection; and Stephanie Riley (Denver Water) for assistance with the snow melt sample collection.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvc.2026.1694851/full#supplementary-material

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