During each year of the study period (2023‐2025), the festival was held over a single mid-May weekend at a large outdoor venue within the same sewershed as the casino-resort corridor known as the Las Vegas Strip. Off-site promotional events catering to attendees began as early as the preceding Tuesday, but the on-site campground opened on Thursday. The on-site festival was then held nightly on Friday, Saturday, and Sunday, closing at 5:30 AM each morning. The festival concluded on Monday morning, with the campground closing later that day.

Target analytes in this study included 24 parent compounds and metabolites (Table S1 in Multimedia Appendix 2). As in our concurrent campus-based wastewater monitoring campaign [17], nonopioid substances included cocaine and its metabolites (benzoylecgonine [BZE], ecgonine methyl ester [EME], ecgonine, and norcocaine), methamphetamine (and amphetamine), MDMA (and 3,4-methylenedioxyamphetamine [MDA]), Δ-9-tetrahydrocannabinol (THC; also 11-hydroxy-Δ-9-tetrahydrocannabinol [THC-OH] and 11-nor-9-carboxy-Δ-9-tetrahydrocannabinol [THC-COOH]), and the tranquilizer xylazine. Natural (heroin, morphine, and codeine), semisynthetic (hydrocodone and oxycodone), and synthetic (fentanyl, methadone, and tramadol) opioid targets were also included. Relevant opioid metabolites included 6-acetylmorphine for heroin, norfentanyl for fentanyl, and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) for methadone. Fentanyl and xylazine were added to the analyte panel in 2025.

Analyses involved direct injection of aqueous samples after a 10-fold dilution to reduce matrix interference; samples were diluted further if concentrations fell outside the calibration range. All compounds were analyzed by liquid chromatography–tandem mass spectrometry (LC-MS/MS) with isotope dilution quantification according to previously published methods [18,19]. Analyses used a CTC Autosampler (CTC Analytics) and a 1260 LC Binary Pump (Agilent). Target compounds and their isotopically labeled analogs were then analyzed using positive electrospray ionization (ESI) in multiple reaction monitoring (MRM) mode on a 6500 QTRAP mass spectrometer (SCIEX). Additional analytical details for all target compounds, including method reporting limits (MRLs) and MRM transitions, are included in Text S1 in Multimedia Appendix 2. Field and analytical blanks (consisting of Milli-Q water treated as samples) were negative for all target compounds.

Wilcoxon signed-rank tests were conducted in R using RStudio. The first analysis compared site-specific analyte concentrations between 2024 and 2025 (ie, festival 2024 vs festival 2025 and WWTP 2024 vs WWTP 2025), and data were limited to sampling days that overlapped in both years. Note that the festival comparison was potentially confounded by the difference in sample type (grab in 2024 and composite in 2025). The second analysis compared same-day concentrations at the festival site and the WWTP across both 2024 and 2025. Any concentrations below the MRL were omitted from the statistical analyses, along with their paired samples.

The University of Nevada, Las Vegas Institutional Review Board reviewed this project and determined it to be exempt from human participant research regulations and from the requirement for informed consent, according to federal regulations and university policy.

This multiyear surveillance study analyzed wastewater from multiple locations to characterize the presence and concentration of substances (and their metabolites) surrounding an annual EDM festival. The findings in 2023 reaffirmed the presence of HRS in raw community wastewater during festival and nonfestival time points, with notable differences by weekends and sample locations. The findings from 2024 to 2025 further illustrated the prevalence of stimulants (cocaine, methamphetamine, and notably MDMA) in festival-site wastewater, although opioid-related analytes were also detected there and in the community. Detections related to THC were not unexpected, given the legality of recreational marijuana products in the state of Nevada and prior literature indicating its frequency in festival-related WBE studies [22].

The study festival (with more than 520,000 annual attendees) is large in comparison to other events included in prior wastewater-based studies of substance use. The 2010 to 2011 Australian festival described by Lai et al [6], for example, had daily attendees between 12,100 and 19,700; Bijlsma et al [9] sampled from 6 European music festivals with a total combined attendance of 465,000. A recent systematic review [22] included 23 WBE studies on HRS, with 6200 to 600,000 attendees; only 2 studies reported attendance in excess of 500,000 people.

Although this study included just 3 sequential years, the results offer a preliminary wastewater-based profile of substances and metabolites present at the festival site, namely stimulants (eg, MDMA, cocaine, and methamphetamine), as well as select opioids. Lai et al [6] similarly noted consistency in festival attendance and wastewater-based HRS profiles each year, in line with findings that event-related WBE findings may vary by attendee characteristics and time of year [23]. Elsewhere, researchers have documented wastewater-based concentrations rising to a peak during the event [23], a pattern also observed here on the final days of the festival and potentially reflecting accumulation and/or intentional discarding at the end of the event [8]. Multiple WBE studies have highlighted wastewater-based profiles of substance use surrounding holidays [23-25], during weekends compared to weekdays [24], and among nightclub, music event, and festival attendees in general [26-28], with some variation by region and music genre [22]. Several results in this study indicated concurrent detections of select analytes (eg, MDMA and norfentanyl at the bars), highlighting another potential opportunity for multifaceted public health responses to address intentional (or unintentional) polydrug use [29] amid the recent wave of polysubstance use and related deaths in the US overdose crisis [30].

Across analytical approaches, geographic locations, and event types, MDMA was the most consistently detected substance in the Cainamisir et al [22] review of 23 wastewater studies of psychoactive substances at music festivals. The maximum MDMA concentrations in this study (1,280,000 ng/L and 1,860,000 ng/L), however, exceeded concentrations reported previously [22], including those from a neighboring Southern Nevada sewershed during a separate music festival in the fall of 2022 (1100 ng/L) [19] and from a festival in Thailand (1267 ng/L) [31]. An analysis of pooled wastewater from a festival in Belgium, however, reported a higher concentration (7580 ng/mL=7,580,000 ng/L) [32]. The accumulation of MDMA over a festival weekend (with a blood half-life of 8 h; 95% clearance within 40 h [33]) may help explain the concentrations in this study. Importantly, prior work has observed that MDMA concentrations tend to be higher on weekends, even in the absence of a large event or festival [34,35]. Recent wastewater findings in Europe also emphasized higher MDMA loads during weekends compared to weekdays (occurring in more than 75% of cities studied), though overall MDMA wastewater loads in European cities decreased by nearly 16% from 2024 to 2025 [36].

Methamphetamine and cocaine (and its metabolites) were consistently present over the festival weekends (with 100% detection frequencies), but the 2023 nonfestival weekend concentrations suggest community use independent of the event. A separate local wastewater surveillance study conducted in 2022 to 2023 [19] similarly concluded that methamphetamine and cocaine were among the highest per capita consumption estimates across all Las Vegas sewersheds. Wastewater detections of cocaine (the parent compound) have been reported in similar festival-based studies [22], albeit with slightly lower concentrations (eg, a maximum of 8914 ng/L in Brazil [8] vs 10,000 ng/L in this study). Here, we used a mass-balance approach to visualize total cocaine equivalents, but focusing on one of the more abundant cocaine metabolites, specifically BZE, may be sufficient for public health and event-related wastewater monitoring of cocaine use (Figure 2). Cainamisir et al [22] highlighted geographic variability in wastewater-based indicators of methamphetamine presence and consumption at festivals elsewhere, though the methamphetamine detections in this study align with the established presence of the substance in Southern Nevada [37].

Although stimulants were detected most frequently in this study, several opioids were also present during the festival week and weekend, both on-site and in the broader community. Norfentanyl, for example, was detected in both festival and WWTP wastewater, though more consistently at the WWTP; this aligns with the prior local findings [19] and could reflect some medical use. Other festival-related WBE studies have reported detections of norfentanyl, though it was not the most common analyte; in one study, norfentanyl was detected at only 2 of 6 festivals sampled [7]. Generally, festival-related opioid concentrations in this study mirrored those at the downstream WWTP, suggesting a less-pronounced event-associated use compared with the stimulants discussed earlier. However, the co-occurrence of some opioids and stimulants in site-specific wastewater samples may reflect intentional or unintentional polysubstance use (ie, through lacing) [29], although this cannot be confirmed with wastewater alone.

The absence of heroin (the parent compound) detections was not unexpected, given previous work suggesting its potential instability in the presence of the quenching agent, ascorbic acid, used here [19]. Heroin and its metabolite acetylmorphine are known to be subject to rapid degradation in wastewater [38], though acetylmorphine has been detected during previous hold-time experiments [19], in other local studies [17], and here in 2023 (n=4 off-site samples and 2 WWTP samples) and in 2024 (n=2 off-site samples and 2 festival-site samples).

Several elements of this study may limit and/or confound the interpretation of results and their broader contextualization in the fields of WBE and public health. First, due to practical and logistical limitations, the wastewater sampling framework varied each year of the study, complicating year-over-year comparisons. Multiday sampling in 2024 and 2025 offered more granular detail about the festival week and weekend, and the limited comparisons between grab and composite samples highlighted the potential effect of sample type on analyte detections (eg, codeine, morphine, oxycodone, and norcocaine) and on concentrations (eg, MDMA). Generally, composite samples are preferred over grab samples in the WBE literature, given concerns about potentially nonrepresentative slugs of wastewater that may underestimate or overestimate the presence of a target analyte in wastewater [39]. However, the results of this study show that even limited grab sampling offers valuable information about HRS presence and use surrounding an event.

Analyte source, consumption, metabolism, excretion, stability, and dilution in wastewater are also key factors that affect interpretation of wastewater results. Others have addressed metabolism with correction factors to back-calculate consumption estimates [19,40]. This would be a valuable addition to future festival-specific WBE studies for HRS, although corresponding flow measurements would be needed to calculate mass loadings. Wastewater analyses also do not distinguish between the various individuals present at a festival, potentially including attendees, artists, vendors, staff, and volunteers. Supplemental analyses to distinguish between licit and illicit consumption of select substances (eg, methamphetamine) have been established [41], but were not performed here. This means, for example, that a portion of the amphetamine and methamphetamine concentrations observed here may be due to medically prescribed use, rather than illicit use. Furthermore, these wastewater detections may partially reflect intentional discarding (ie, dumping) of substances in festival restrooms and subsequent in-sewer degradation during and/or at the end of the event, as has been noted previously [8]. Although wastewater monitoring does not identify specific sources (eg, original user or lacing vs polysubstance use vs intentional discarding), these findings still complement existing public health surveillance activities and indicate the presence of these substances on-site and in the community surrounding the event.

Results were not normalized to wastewater flow rate, a fecal indicator, or another analyte (eg, caffeine or sucralose). As described earlier, normalization would have facilitated consumption calculations and/or captured differences in human urine or fecal load within these samples [19]. Site-specific consumption estimates could be calculated with site-level flow data and corrections for metabolism or excretion or molecular weight equivalence [19,40]. Importantly, upstream flows may be highly variable throughout the day and across the event.

This study included a selection of conventional prescription and recreational substances of public health significance, but a variety of other targets could be included in future work. Several published event-focused studies have used wastewater to characterize the presence of less-regulated novel psychoactive substances (NPS) [7,9]. Wastewater may be particularly valuable in tracking the emergence of new substance use trends, including NPS and misuse of existing substances (eg, xylazine), in tandem with other sources such as online discussion forums [42]. Future wastewater-based efforts might also include medetomidine, naloxone (despite potential instability in wastewater [43]), and ketamine, which may be increasingly common [26] and co-occurring with fentanyl [44]. Others have previously characterized the community wastewater-based occurrence of ketamine in Europe [24]; ketamine was also found in the Las Vegas Wash during the 2022 festival weekend [11].

At least one similar WBE study endeavored to relate wastewater HRS results to clinical outcomes or other datasets, although these analyses were beyond the scope of this study. This previous festival-based work measured wastewater HRS concentrations along with self-reported substance use behaviors [7], but links between festival HRS wastewater data and health outcomes associated with substance use (eg, overdoses, health care encounters, and deaths) are less clear in the literature. This does not, however, diminish the serious health outcomes that have been reported among festivalgoers [4,45], including critical care admissions at a tertiary care hospital in Las Vegas associated with the festival weekend [46,47]. In a 2023 case series of intensive care unit (ICU) admissions, substances reportedly used included MDMA (6/6 patients), cocaine (n=2), fentanyl (n=1), lysergic acid diethylamide (LSD; n=1), THC (n=1), and/or alcohol [1,46]. A recent local retrospective cohort study also found that drug or alcohol overdose patients related to the festival in this study were younger and more likely to be admitted to the ICU than nonfestival-related drug or alcohol overdose patients, although the 2 groups did not differ significantly in other characteristics, drug use patterns, or ultimate health outcomes [47]. In 2025, the Clark County coroner’s office reported that MDMA contributed to 2 deaths in the Las Vegas area during the festival weekend [48].

These results offer valuable insights for public health and clinical situational awareness [22] of substance use during the festival week or weekend, complementing other surveillance activities, emergency responses, and the ultimate reporting of health care use and serious clinical outcomes. Selected results, such as the relatively few heroin-related detections in this study, may reflect an evolving landscape of substance use behaviors among attendees. Palamar et al [26] described changes in substances of choice in New York City nightlife and dance club settings from 2017 to 2022, with survey responses indicating decreased use of heroin, nonmedical prescription opioids, and cocaine (among other stimulants) in the post–COVID-19 era, accompanied by increased use of ketamine, psilocybin, poppers, and other NPS. Together with ongoing surveys, online forum monitoring [42], drug checking, and traditional public health or clinical surveillance, event-based wastewater surveillance may inform continued investment in prevention and future responses or interventions, both on-site and in the community, to support successful events while prioritizing attendee and community health and safety. Text S2 in Multimedia Appendix 2 provides additional discussion and recommendations on how event-based wastewater surveillance can inform on-site and community-level public health activities, including activities that have been implemented specifically for the music festival in question.

Event-related wastewater monitoring also offers insight into expected changes in the composition of influent wastewater flows surrounding the event. Although in-sewer mixing, dilution, and degradation likely explain the lower analyte concentrations in the WWTP samples compared with the festival site, the WWTP still experienced significant increases (10- to 100-fold) in influent concentrations of select analytes (eg, MDMA and MDA) over the event weekend. In addition to changes in flow driven by large influxes of people associated with special events, these compositional changes have implications for wastewater treatment processes, water reuse, and downstream environmental surface water quality [11,22,49], highlighting how wastewater monitoring can be informative and mutually beneficial for utilities and public health entities.

Event-related wastewater monitoring is an evolving subtopic in the field of WBE. Several studies have already explored targeted wastewater monitoring for pathogens such as SARS-CoV-2 at the Olympics [50] and other mass gatherings [15], but WBE has also been applied to substance use at music festivals in countries like Australia and Thailand. In Las Vegas, this approach has now been extended to characterize the use of HRS at the community level and in treated wastewater effluent surrounding events and festivals, as has been done elsewhere [11,14,17,19]. Here, wastewater surveillance surrounding a large EDM festival in 3 sequential years at on-site and off-site locations further explored WBE as a complementary tool for public health monitoring of substance use or misuse. Stimulant concentrations were notably elevated during event weekends, with high peak concentrations of MDMA and cocaine-related analytes, but opioid-related detections were also observed. These findings may inform both public health surveillance and response activities and wastewater utility operations.

Wastewater monitoring should complement, not replace, existing public health surveillance and response structures. Multifaceted public health surveillance systems can integrate wastewater surveillance for both pathogens and HRS to expand event-related preparedness, including interventions to prioritize the health and safety of attendees on-site and in the community. Future research may explore additional HRS targets, other non-EDM festivals and events, trends over time, and alignment with other data sources (including health outcomes). Local public health agencies, community stakeholders (eg, health care systems, nonprofit organizations, and local government), and other tourism and event destinations similarly stand to benefit from data-driven understanding of the public health implications of events and mass gatherings.