2,4,6-Trichlorophenol is a well-known carcinogen, aromatic disinfection byproduct (DBP), and anthropogenic contaminant in drinking water, with a high detection frequency and concentration. Natural organic matter (NOM) is considered as its key precursor; however, the formation potential of NOM to 2,4,6-trichlorophenol during chlorination does not explain the μg/L levels observed in disinfected drinking water. In this study, concentrations of 2,4,6-trichlorophenol in drinking water from South Carolina were determined, and important new tert-butylphenol precursors were identified using nontarget and suspect screening analysis with gas chromatography (GC)-mass spectrometry (MS). Concentrations of 2,4,6-trichlorophenol in 12 chlorinated and chloraminated drinking water samples ranged from 9 to 14,966 ng/L (mean: 4805 ng/L), higher than 11 other classes of aromatic DBPs and 4 classes of aliphatic DBPs. During chlorination, yields of 2,4,6-trichlorophenol from Suwannee River NOM were 22 ng/mg-total organic carbon (TOC). In contrast, 2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol, and 2,6-di-tert-butylphenol, found up to 41,037 ng/L in river water as industrial contaminants, showed yields of 737, 711, 1261, and 2939 ng/mg-TOC, respectively. tert-Butylphenols in source water contributed ∼3% of 2,4,6-trichlorophenol in the studied chlorinated river waters. Formation mechanisms were proposed. This study provides insight into uncovering additional unknown precursors of chlorophenols.

Emerging contaminants pose substantial threats to ecosystems and public health. While advances in analytical techniques and chemicals assessment have revealed the impacts of contaminants such as per- and polyfluoroalkyl substances, pharmaceutical residues, and microplastics, knowledge of the vast chemical universe remains limited. This Perspective examines the evolving attention toward emerging contaminants and drivers of their discovery. It identifies key challenges, including methodological limitations in detecting diverse chemical classes, fragmented data landscapes, and delays in translating scientific evidence into regulatory action. We highlight how technological and methodological innovations, particularly advanced analytical technologies, artificial intelligence, and future-oriented assessment frameworks, can shift paradigms in identifying and managing emerging contaminants from reactive to predictive. Critical to this transition is the adoption of open and FAIR principles (Findable, Accessible, Interoperable, and Reusable) and harmonized data standards to enable integration of heterogeneous evidence streams. Furthermore, we advocate coupling early warning systems with "Safe and Sustainable by Design" approaches to prevent regrettable substitution and reduce hazards at the source. Coordinated action across academia, regulators, industry, and funders is essential to establish proactive prevention as the norm in emerging contaminant management.

Due to growing populations and water scarcity, desalination is increasingly used to generate safe drinking water. A concern is the potential formation of more toxic brominated and iodinated disinfection byproducts (DBPs) in the finished drinking water. While desalination technologies remove much of the organic matter and bromide/iodide precursors to DBP formation, lower molecular weight, uncharged compounds and salts are not completely removed. We present a comprehensive nontarget identification of DBPs from desalination, using a broadscreen, comprehensive approach with gas chromatography (GC)-high resolution-mass spectrometry (MS) to identify DBPs produced in desalinated waters treated with chlorine, chlorine dioxide, ozone, chloramine, and UV. We include real desalinated waters from three large desalination plants in the United Arab Emirates, along with controlled laboratory treatments of these waters, allowing a direct comparison of different disinfectants. Mammalian cell cytotoxicity analyses for new desalination DBPs are also reported, along with whole-water toxicology for treated desalinated waters. Fifty-five DBPs were identified, including many reported for the first time in desalinated water, including a new dibromomethyl-pyrazole, along with haloketones, haloaldehydes, halonitromethanes, haloamides, and >2 carbon haloacids. Di- and tribromophenols had greater cytotoxicity compared to corresponding regulated di- and trihaloacetic acids. Reverse osmosis (RO)-treated waters had higher cytotoxicity than distilled waters (multistage flash and multieffect distillation). Of the RO-treated waters, samples treated with ozone-chlorine had highest cytotoxicity, followed by UV, ozone, and chloramine, consistent with increased formation of more toxic bromonitromethanes and dibromoacetic acid with ozone-chlorine. However, DBP levels in the treated desalinated waters and whole-water cytotoxicity were lower than found in typical drinking waters, suggesting that, despite increased formation of brominated DBPs, desalination may produce safer water overall.

Disinfection is one of the greatest public health achievements, effectively controlling the spread of waterborne diseases. However, the formation of disinfection by-products (DBPs) is an unintended consequence of disinfection, arising from reactions between disinfectants (such as chlorine, ozone, chlorine dioxide, UV, or chloramine) and natural organic matter, anthropogenic contaminants, bromide, and iodide. DBPs are of great concern because they are ubiquitous in drinking water and generally occur at higher concentrations and exhibit greater toxicity than most anthropogenic contaminants. Since the discovery of the first DBP (chloroform) in 1974, approximately 1,000 DBPs have been identified in drinking water through the collective efforts of scientists worldwide over the past 50 years. In this review, we summarize five decades of research on the identification, formation, occurrence, and state-of-the-art analytical methods of DBPs in drinking water. Specifically, we present a comprehensive list of DBPs identified under different disinfection conditions, discuss analytical techniques used for DBP identification and quantification, and summarize the occurrence and formation mechanisms of both regulated and emerging unregulated DBPs. In addition, we outline future research directions for DBP studies. This review aims to serve as a valuable reference particularly for new researchers entering the field of DBP research.

Abstract Desalination by reverse osmosis (RO) is an increasingly important source of drinking water in coastal regions facing population growth and water scarcity. Prechlorination of seawater to control membrane biofouling can produce disinfection byproducts (DBPs), yet little is known about their occurrence in finished desalinated water or discharged RO brines. This study quantified 53 DBPs, including trihalomethanes (THMs), iodo-THMs, haloacetonitriles, haloacetamides, haloketones, halonitromethanes, and haloacetaldehydes, using GC–MS at three full-scale U.S. desalination plants (two seawater and one inland brackish groundwater facility). DBPs were detected on both sides of the RO membrane, with 44 compounds in permeate and 45 in brine at total concentrations up to 34.9 and 9.5 μg/L, respectively. Brominated DBPs dominated both matrices, with bromoform the most abundant, indicating that many brominated species readily pass through RO membranes. Low levels of iodinated DBPs were also detected in brine and permeate from one seawater plant. DBPs from all six classes crossed the RO membrane, although permeability varied by compound class, with THMs, haloacetonitriles, and haloketones contributing most to permeate concentrations. At one seawater plant, brines exhibited higher calculated cytotoxicity than permeate, and DBPs detected in source water before chlorination suggested the potential for persistence in coastal environments.

While most drinking water treatment plants use traditional disinfectants such as chlorine or chloramine, we discovered a small plant that for several years had been using bromochlorodimethylhydantoin (BCDMH), a disinfectant typically used for spas. In this plant, serving similar to 3400 residents, solid BCDMH tablets were dissolved in water and injected directly into the groundwater to control iron-reducing bacteria. While BCDMH hydrolyzes to form HOCl and HOBr as disinfectants, 5,5-dimethylhydantoin (DMH) is produced as a byproduct in stoichiometric amounts to the milligrams per liter levels of disinfectant applied. Due to concerns over high levels of DMH that could be released for human consumption, this chemical was quantified for the first time in drinking water and the well water feed, as well as a broad group of 51 chloro-, bromo-, and iodo-disinfection byproducts (DBPs) from five different classes. Nontarget analysis utilizing gas chromatography-high-resolution electron ionization mass spectrometry was also conducted to screen for other potential DBPs and contaminants. Results revealed 5.3 mg/L levels of DMH in the BCDMH-treated well and 24 mu g/L in the finished drinking water (diluted by blending with chlorinated water from three other wells) as well as unique brominated DBPs, including brominated hydrocarbons.

Bottled water consumption has increased globally at a tremendous rate, however, little is known about levels of disinfection by-products (DBPs) in bottled water beyond the regulated trihalomethanes (THMs), haloacetic acids (HAAs), and bromate. This is important because many bottled waters are sourced from tap water. Using gas chromatography-mass spectrometry, we quantified 64 regulated and priority, unregulated DBPs in 10 popular brands of bottled water, including 50 toxicologically important, unregulated DBPs not measured in previous studies. Several priority, unregulated DBPs were found in bottled water for the first time, including dibromoacetonitrile, chloroacetonitrile, dichloroacetamide, trichloronitromethane, dichloroacetaldehyde, 1,1-dibromopropanone, and 1,1,1-trichloropropanone. Dibromoacetonitrile was present in two Grocery Brand bottled waters, up to 0.2 μg/L, and is particularly important because it is highly cytotoxic, genotoxic, and carcinogenic. Total organic halogen (TOX) was also measured in bottled waters for the first time. All bottled waters contained measurable TOX, with highest TOCl levels in Grocery/Name Brands (up to 31.4 μg/L) and highest TOBr levels in a Name Brand water (up to 7.0 μg/L). A surprising finding was the presence of HAAs and chloroform in one of the Designer Brands, which is supposed to be treated with UV disinfection only. THMs and HAAs were found at the highest levels (0.01-12.4 µg/L), with other priority, unregulated DBPs present at ng/L levels when detected. However, while all bottled waters (even untreated spring waters) contained DBPs, they were present at levels significantly lower than typically found in tap water. For example, total DBP levels ranged from 0.01-22.4 µg/L (mean of 2.6 µg/L) in bottled water, compared to 47.3 µg/L and 52.2 µg/L for chloraminated tap water and other tap waters treated with chlorine and other disinfectants, respectively. Also, fewer DBPs were detected in the bottled waters (3 on average vs. 37 in the tap water). The rank order of lowest to highest mean DBP levels followed: Designer Brands < Grocery Brands < Name Brands (0.6, 3.1, and 3.7 µg/L, respectively). Grocery Brands and Name Brands had higher numbers of DBPs compared to Designer Brands. Brands sourced from spring water generally showed lower overall DBP levels than those labeled as purified. DBP levels varied dramatically with lot number (from the same brand), with large variability in both the number of DBPs detected and concentrations. Finally, two grocery brands, which were purified tap waters, had significantly higher calculated cytotoxicity than other bottled waters sampled, with 43x and 83x the average levels of other bottled waters, but bottled waters overall had quite low calculated cytotoxicity compared to tap water.

To effectively combat antibiotic resistance, it is critical to understand antibiotic usage patterns and their environmental dissemination. Wastewater treatment plants (WWTPs) are well-documented sources of antibiotics discharged into aquatic environments, but their role in releasing antibiotics via bioaerosols has not previously been investigated. In this study, seasonal air and liquid samples were collected throughout 2019 from a midsize WWTP employing both mechanical surface agitation and fine bubble aeration of activated sludge. Azithromycin and ofloxacin were detected in bioaerosols collected near aeration tanks at concentrations ranging from below detection limits up to 29 pg L-1 air, suggesting that bioaerosols may represent a previously underappreciated route of environmental and occupational antibiotic exposure. Metagenomic analysis confirmed the co-occurrence of antibiotic resistance genes (ARGs) conferring resistance to macrolides and fluoroquinolones in both air and liquid samples. These findings highlight bioaerosols as an important yet overlooked pathway for the dissemination of antibiotics and ARGs, emphasizing the necessity of integrating airborne pathways into environmental antibiotic resistance surveillance programs, especially given the global scale of WWTP operations.

Aromatic disinfection byproducts (DBPs) can account for up to 36% of total organic halogen (TOX) in chlorinated drinking water and may be key drivers of developmental toxicity. However, aromatic DBPs reported to date cannot fully explain the observed TOX or developmental toxicity of drinking water. In this study, 17 chlorobenzenediols were identified in chlor(am)inated drinking water using gas chromatography-high resolution mass spectrometry (GC-HRMS). Total concentrations of chlorobenzenediols in tap water from seven cities ranged from 15 to 116 ng/L, with a mean of 54 ng/L. These levels are higher than previously reported for eight classes of aromatic DBPs and halobenzoquinones. Eleven chlorobenzenediols also formed during chlor(am)ination of Suwannee River natural organic matter (NOM), indicating that NOM is an important precursor. Five chlorobenzenediols have a reported LC50 and/or EC50 of less than 1 mg/L for various organisms, indicating that they are classified as very toxic chemicals and are likely to be among the most developmentally toxic aromatic DBPs discovered. Considering that the chlorobenzenediol concentrations in drinking water were higher than eight classes of reported aromatic DBPs and halobenzoquinones, they are likely to have a much more significant impact on drinking water developmental toxicity than other known aromatic DBPs.

Disinfection byproducts (DBPs) are formed during drinking water treatment from the reaction of chemical disinfectants with natural organic matter (NOM), anthropogenic contaminants, and inorganic bromide and iodide. DBPs are of public concern due to their carcinogenic and genotoxic effects and adverse effects observed in many epidemiologic studies. Formation mechanisms have been studied in order to identify precursors, reaction intermediates, and reaction kinetics and to predict new classes of DBPs. By understanding formation mechanisms, steps can be taken to remove DBP precursors and adjust treatment conditions to minimize DBP formation. This paper presents a critical review of formation mechanisms for eight classes of DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloketones (HKs), haloacetaldehydes (HALs), haloacetonitriles (HANs), haloacetamides (HAMs), halonitromethanes (HNMs), and nitrosamines (NAs) from the popular disinfectants chlorine, chloramine, and ozone. Important precursors in the formation of many of these DBPs include phenolic, β-dicarbonyl, and oxopentadioic acid groups found in humic NOM species. Likewise, amino acids are also precursors for several classes of DBPs. HANs can form by chloramination of aldehydes, and HAMs can form from the hydrolysis of HANs. HNMs can form by the oxidation/halogenation of amines as well as by nitration and halogenation of humic substances in the presence of nitrite. Preozonation followed by chlorine or chloramine can significantly increase HNM formation. Finally, dichloramine and aqueous oxygen are important reactants in the formation of nitrosamines from chloramine. Nitrosamines can also be formed by N,N-dimethylsulfamide. Detailed formation mechanisms for 8 classes of drinking water DBPs formed by chlorine, chloramine, and ozone are presented.

While disinfection byproducts (DBPs) are typically measured at drinking water treatment plants, levels can change dramatically within the distribution system before reaching the consumer. In this study, the spatio-temporal trends of 66 DBPs across 9 different classes were examined in two drinking water distribution systems with similar source waters, but different pretreatments and residual disinfectants. One system uses residual chlorine in the distribution system, and the other uses chloramine, allowing for examination of how DBP concentrations change over time in distribution systems with different residual disinfectants. Four routes were sampled for each system with six time points over three days travel time, collected at increasing distance from the treatment plant to "chase" the same packet of water. On average, total DBP levels increased 32% over time in the chlorine system, but decreased 23% in the chloramine system, indicating that DBP formation in the pipes outpaced DBP degradation in the chlorine system and vice-versa in the chloramine system. Among emerging DBP classes, iodo-DBPs decreased overall by 64% (but dichloroiodomethane maintained steady levels) in the chlorine distribution system, while haloacetic acids and haloacetaldehydes reached maximum concentrations in the middle of the chloramine distribution system before decreasing by an average of 29% and 54%, respectively, by the end of the system. Comparatively, I-DBPs degrade slightly over time in the chloramine DSs, but 0.5-1.9 µg/L total levels were still present in the tap water after 72 h. Haloacetonitriles were detected at similar levels in both distribution systems and followed similar trends, with an initial maximum in the finished water or at 2 h, then a decrease in concentration at medium detention times, followed by an increase to a second maximum at 48 h or 72 h. Haloacetamides decreased by an average of 26% from finished water to 72 h in the chloramine DS and were detected in only one route of the chlorine DS2 at low levels, up to 8.6 µg/L total. Calculated cytotoxicity based on measured DBPs decreased by 41% on average in the chlorine system and by 33% in the chloramine system from finished water to the end of the distribution system. The decrease in calculated cytotoxicity in the chloramine system corresponds with a universal decrease of all measured DBP classes from finished water to 72 h, meanwhile the decrease in calculated cytotoxicity in the chlorine system was driven by a shift towards more chlorinated DBPs vs. brominated DBPs.

Wildland-urban interface wildfires are increasing in frequency worldwide with negative implications for drinking waters due to alteration of organic matter, which can enter waterways following rain events, flow downstream to water treatment plants, and form disinfection byproducts (DBPs). Previous research has focused on vegetative wildfires with nothing known about DBPs from wildland-urban interface wildfires. In this study, the water-extractable fraction of seven structural ashes and two vegetation ashes from the 2020 LNU Lightning Complex Fire in California were treated with chlorine and chloramine. Formation of 65 DBPs from seven different classes was measured by gas chromatography-mass spectrometry (GC-MS). Chlorination of the water-soluble fraction of structural ashes caused formation of higher levels of brominated, iodinated, and nitrogenous DBPs than the vegetation ashes due to structural ashes containing greater levels of water-soluble bromide and iodide. Chloramination reduced total DBP formation and calculated cytotoxicity vs chlorination, except in two structural ashes where iodinated and nitrogenous DBP formation was high. Structural ash leachates had lower dissolved organic carbon levels than vegetation ash and contribute lower levels of DBP precursors. However, these precursors may increase the toxicity of drinking water due to a shift toward more toxic DBP species.

Frequent and severe occurrences of harmful algal blooms increasingly threaten human health by the release of microcystins (MCs). Urgent attention is directed toward managing MCs, as evidenced by rising HAB-related do not drink/do not boil advisories due to unsafe MC levels in drinking water. UV/chlorine treatment, in which UV light is applied simultaneously with chlorine, showed early promise for effectively degrading MC-LR to values below the World Health Organization's guideline limits. Still, much is unknown regarding potential disinfection byproduct formation and associated toxicity, which can occur from the reaction of chlorine and other reactive species with MCs and algal and natural organic matter. To ensure UV/chlorine guarding drinking water for human consumption, the degradation and detoxification of four of the most problematic MC variants, namely, MC-LR, -RR, -YR, and -LA, which differ in amino acid substituents, were evaluated using UV/chlorine and compared to results from chlorination. Overall, UV/chlorine effectively enhanced MC degradation kinetics and generated less halogenated disinfection byproducts in the target analysis of 11 types of DBPs_C1-3 from 7 classes, total organic chlorine, and nontarget analysis revealing 35 higher molecular weight DBPs_C46-52, which maintained the MC structures. Reactivity and cytotoxicity changes varied based on the individual amino acid moieties within the cyclic heptapeptide structure common to all MCs. Analogous trends in MC reactivity were observed in degradation kinetics and mixed MC competition reactions, aligning with individual amino acid structure-reactivity. Cytotoxicity results indicated no significant unintended toxic consequences from MC_DBPs. Our results suggest that UV/chlorine treatment offers an efficient strategy for treating MCs in drinking water.

Forested watersheds are instrumental in providing purified and reliable water to millions of people worldwide. The changing climate has increased the frequency and severity of global fire events. Forested watersheds and their ecosystem functions are greatly disrupted during fire activity. Postfire concerns in forested watersheds include unpredictable and potentially simultaneous alterations in source water quality and hydro-biogeochemical processes. The degree of fire severity can complexly modify water quality through the production of fire-transformed constituents on the burned forest floor (i.e., nutrients, metal(loid)s, dissolved organic matter, and the formation of disinfection byproducts). Correspondingly, fire severity and postfire rainfall patterns can refine hydro-biogeochemical processes that influence the transport of the fire-transformed constituents (i.e., vegetation function, soil structure, hydrological pathways, and microbial communities). Postfire alterations to water quality and hydro-biogeochemical processes introduce further complexity with varying temporal influence, which ranges from months to decades. As postfire water quality and watershed response research progresses, it is essential to homogenize interdisciplinary expertise to bridge knowledge gaps between fields ranging from forest ecology, hydrology, microbiology, and geochemistry. A multidisciplinary approach in wildfire research will facilitate a comprehensive perception of the diverse water quality risks associated with fire activity and mitigate fire concerns on a global level.

Although >700 disinfection by-products (DBPs) have been identified to date, most DBPs in drinking water are still unknown. Identifying unknown DBPs is an important step for improving drinking water quality because known DBPs do not fully account for the adverse health effects noted in epidemiologic studies. Using gas chromatography high-resolution mass spectrometry, six chloro- and bromo-halocyclopentadienes (HCPDs) were identified in chlorinated and chloraminated drinking water via non-target analysis; five HCPDs are reported for the first time as new alicyclic DBPs. Formation pathways were also proposed. Simulated disinfection experiments with Suwannee River natural organic matter (NOM) confirm that NOM is a precursor for these new DBPs. Further, HCPDs are more abundant in chlorinated drinking water (real and simulated) when compared to chloraminated drinking water due to the higher reactivity of chlorine. Of these new DBPs, 1,2,3,4,5,5-hexachloro-1,3-cyclopentadiene is approximately 100,000× more toxic (in vivo) than regulated trihalomethanes (THMs) and haloacetic acids (HAAs) and 20-2000× more toxic than halobenzoquinones, halophenols, and halogenated pyridinols using the available median lethal dose (LD50) and concentration for 50% of maximal effective concentration (EC50) of DBPs to aquatic organisms. The predicted bioconcentration factors of these HCPDs range from 384 to 3980, which are 2-3 orders of magnitude higher than those for regulated and priority DBPs (including THMs, HAAs, halobenzoquinones, haloacetonitriles, haloacetamides, halonitromethanes, haloacetaldehydes, iodo-THMs, and iodo-HAAs). Thus, HCPDs are an important emerging class of DBPs that should be studied to better understand their impact on drinking water quality and long-term human health exposure.