Drinking water contaminants—reference library

11Cl-PF3OUdS (11-chloroeicosafluoro-3-oxaundecane-1-sulfonic acid)

11Cl-PF3OUdS is a chlorinated ether PFAS compound included in EPA's UCMR5 monitoring list. It belongs to a class of chlorinated polyfluorinated ether sulfonates (Cl-PFESA) used as PFOS replacements primarily in China for the electroplating industry. Its detection in US water reflects the global movement of PFAS through trade and environmental transport.

4:2 FTS (4:2 fluorotelomer sulfonic acid)

4:2 FTS is a four-carbon fluorotelomer sulfonic acid. Fluorotelomer sulfonates are used in firefighting foam (AFFF), surface coatings, and industrial applications. The "4:2" designation refers to four perfluorinated carbons and two non-fluorinated carbons. It is the shortest-chain FTS compound monitored under UCMR5.

6:2 FTS (6:2 fluorotelomer sulfonic acid)

6:2 FTS is the most widely detected fluorotelomer sulfonic acid in drinking water. It is a six-carbon FTS compound used extensively in AFFF firefighting foam and as a surfactant in industrial applications. The "6:2" designation refers to six perfluorinated carbons and two non-fluorinated carbons.

8:2 FTS (8:2 fluorotelomer sulfonic acid)

8:2 FTS is an eight-carbon fluorotelomer sulfonic acid. It is used in AFFF firefighting foam and industrial surface-treatment products. Its longer chain compared to 6:2 FTS results in greater potential for bioaccumulation. It can degrade to PFOA and other long-chain perfluorocarboxylic acids in the environment.

9Cl-PF3ONS (9-chlorohexadecafluoro-3-oxanone-1-sulfonic acid)

9Cl-PF3ONS is a chlorinated ether PFAS compound included in UCMR5 monitoring. Like its analogue 11Cl-PF3OUdS, it belongs to the Cl-PFESA class used as PFOS replacements in electroplating. Its detection in US water systems reflects global PFAS contamination and industrial transport pathways.

ADONA (4,8-dioxa-3H-perfluorononanoic acid)

ADONA is a short-chain ether PFAS used as a replacement for PFOA in the manufacture of fluoropolymers, particularly by 3M under the trade name "Dyneon." Like HFPO-DA (GenX), it contains ether linkages that distinguish it structurally from straight-chain PFAS. It has been detected in water near fluoropolymer manufacturing facilities in Europe and the US.

HFPO-DA / GenX (hexafluoropropylene oxide dimer acid)

HFPO-DA (also known as GenX) is a short-chain PFAS compound used as a replacement for PFOA in the manufacture of non-stick coatings and fluoropolymers. It is structurally different from most other PFAS in having an ether linkage. Contamination has been documented near fluoropolymer manufacturing plants, most notably the Cape Fear River basin in North Carolina.

NEtFOSAA (N-ethyl perfluorooctane sulfonamido acetic acid)

NEtFOSAA is a PFOS precursor compound — an eight-carbon sulfonamido acetic acid used historically in Scotchgard and other 3M fluorochemical products. It degrades to PFOS in the environment and in the body. Included in UCMR5 because its presence in drinking water indicates ongoing PFOS-precursor contamination from legacy products.

NMeFOSAA (N-methyl perfluorooctane sulfonamido acetic acid)

NMeFOSAA is a PFOS precursor compound — the N-methyl analogue of NEtFOSAA, also derived from 3M fluorochemical manufacturing processes. Like NEtFOSAA, it degrades to PFOS in the environment and in biological systems. Included in UCMR5 to capture the full scope of PFOS-precursor contamination in US drinking water.

PFBA (perfluorobutanoic acid)

PFBA is a four-carbon carboxylic acid PFAS — the shortest chain in the common perfluoroalkyl carboxylic acid series. It is extremely water-soluble and mobile in the environment, making it one of the most challenging PFAS to remove with conventional treatment. Detected in water serving 95 million people across 46 states under UCMR5.

PFBS (perfluorobutane sulfonic acid)

PFBS is a four-carbon sulfonic acid PFAS used as a direct replacement for PFOS in stain-resistant coatings, paper and packaging, and firefighting foam formulations. It is among the most mobile PFAS in the environment due to its short chain length and high water solubility. Detected under UCMR5 in water systems serving tens of millions of people across the US.

PFDA (perfluorodecanoic acid)

PFDA is a ten-carbon long-chain carboxylic acid PFAS. Like PFOA and PFOS, its long carbon chain causes it to bioaccumulate in living organisms. It is detected less widely than shorter-chain PFAS in drinking water (5.8 million people across 19 states in UCMR5) but is of elevated concern when detected due to its bioaccumulative properties.

PFDoA (perfluorododecanoic acid)

PFDoA is a twelve-carbon long-chain carboxylic acid PFAS. Its long chain makes it among the most bioaccumulative PFAS compounds, concentrating in blood, liver, and kidney tissue. Detected in UCMR5 monitoring at relatively low frequencies but of high concern when found due to its persistence in the body.

PFHpA (perfluoroheptanoic acid)

PFHpA is a seven-carbon carboxylic acid PFAS detected in water serving 63 million people across 41 states in UCMR5 monitoring. It bridges short-chain and long-chain PFAS in both structure and toxicological properties. PFHpA is one of four PFAS included in EPA's Hazard Index mixture rule alongside PFHxS, PFNA, and HFPO-DA.

PFHpS (perfluoroheptane sulfonic acid)

PFHpS is a seven-carbon sulfonic acid PFAS. It bridges short-chain and long-chain sulfonates in terms of both structure and expected behaviour in the environment. PFHpS is detected in UCMR5 monitoring and shares contamination sources with PFHxS and PFOS.

PFHxA (perfluorohexanoic acid)

PFHxA is a short-chain, six-carbon carboxylic acid PFAS. It is the most widely detected PFAS compound in US drinking water by population exposure, found in systems serving 111 million people across 44 states under UCMR5 monitoring. It has been used as a replacement for longer-chain PFAS in industrial and consumer applications.

PFHxS (perfluorohexane sulfonic acid)

PFHxS is a six-carbon sulfonic acid PFAS compound used as a replacement for PFOS in industrial applications including firefighting foam, metal plating, and surface treatments. It is one of the most widely detected PFAS in US drinking water, found in systems serving 71 million people across 46 states.

PFNA (perfluorononanoic acid)

A long-chain PFAS compound used in food packaging, carpets, and industrial coatings. Less common than PFOA and PFOS but found in some U.S. water supplies.

PFOA (perfluorooctanoic acid)

PFOA is a long-chain, 8-carbon per- and polyfluoroalkyl substance (PFAS) classified by IARC as a Group 2A probable human carcinogen. First regulated by EPA in April 2024 at 4 ppt under the Safe Drinking Water Act PFAS rule, it is found in non-stick manufacturing effluent, AFFF firefighting foam, and contaminated groundwater, and is both persistent and bioaccumulative.

PFOS (perfluorooctane sulfonate)

PFOS is a long-chain, 8-carbon perfluoroalkyl sulfonate — structurally distinct from PFOA's carboxylic acid group but equally persistent and bioaccumulative. Historically used in 3M's Scotchgard fabric and surface protector, it was voluntarily phased out by 3M in 2002. EPA set a 4 ppt MCL in April 2024, and IARC classifies it as a Group 2A probable human carcinogen.

PFPeA (perfluoropentanoic acid)

PFPeA is a five-carbon carboxylic acid PFAS detected in water serving 101 million people across 46 states in UCMR5 monitoring — the second most widely detected PFAS after PFHxA. Like other short-chain PFAS, it has replaced longer-chain compounds in many industrial applications and is highly mobile in water.

PFPeS (perfluoropentane sulfonic acid)

PFPeS is a five-carbon sulfonic acid PFAS. It is one of the less-studied short-chain PFAS sulfonates but was included in UCMR5 monitoring due to its detection in US water supplies. It shares structural and environmental properties with PFBS and PFHxS.

PFTA (perfluorotetradecanoic acid)

PFTA is a fourteen-carbon long-chain carboxylic acid PFAS — among the longest in the UCMR5 monitoring list. Its extremely long chain makes it the most bioaccumulative of the carboxylic acid PFAS monitored under UCMR5. Detected rarely in drinking water but of high concern in contaminated systems.

PFTrDA (perfluorotridecanoic acid)

PFTrDA is a thirteen-carbon long-chain carboxylic acid PFAS. It is among the longest-chain PFAS monitored under UCMR5 and is of elevated concern due to its extreme bioaccumulation potential. Detected at lower frequency than shorter-chain PFAS but present in multiple states.

PFUnA (perfluoroundecanoic acid)

PFUnA is an eleven-carbon long-chain carboxylic acid PFAS. It is among the longer perfluoroalkyl acids monitored in UCMR5 and is detected in water serving 4.3 million people across 17 states. Its long carbon chain makes it among the most bioaccumulative of the UCMR5-monitored PFAS compounds.

Bromate

Bromate is a disinfection byproduct that forms when water containing naturally occurring bromide is treated with ozone. It is a particular concern for utilities that have switched to ozone disinfection to reduce chlorine-based byproducts (TTHMs and HAAs) — ozonation reduces one class of DBPs but creates bromate instead.

Bromodichloromethane

Bromodichloromethane (BDCM) is the second most prevalent trihalomethane compound in chlorinated drinking water, forming when chlorine reacts with naturally occurring organic matter in the presence of bromide ions. It is found at the highest concentrations in utilities that draw from surface water sources containing elevated bromide.

Bromoform (tribromomethane)

Bromoform is a trihalomethane compound that forms primarily in water supplies with very high bromide levels. It is typically the least prevalent of the four TTHM compounds in most US water systems, though it can be significant in coastal or geologically bromide-rich areas.

Chlorate

Chlorate is a disinfection byproduct that forms from the degradation of hypochlorite (liquid bleach) used in water treatment and from the use of chlorine dioxide as a disinfectant. It is one of the most widely detected unregulated contaminants in US drinking water, found in virtually all chlorinated systems.

Chloroform (trichloromethane)

Chloroform is the most abundant of the four trihalomethane (TTHM) compounds, accounting for the majority of total TTHM levels in chlorinated water. It forms when chlorine reacts with naturally occurring organic matter in source water. Higher surface-water content in the source — especially warm-season algae and decaying vegetation — drives higher chloroform levels.

Dibromoacetic acid (DBAA)

Dibromoacetic acid (DBAA) is one of the five HAA5 compounds, distinguished by its two bromine atoms rather than chlorine. It forms in water with elevated bromide concentrations — the same waters that produce higher levels of brominated trihalomethanes. DBAA is often the dominant HAA in water supplies with high natural bromide.

Dibromochloromethane

Dibromochloromethane (DBCM) is a trihalomethane compound that forms when chlorine disinfects water containing higher bromide concentrations. It is typically the third most prevalent TTHM compound, present at lower levels than chloroform and BDCM but still contributing meaningfully to total TTHM exposure.

Dichloroacetic acid (DCA)

Dichloroacetic acid (DCA) is the most health-concerning of the five haloacetic acid compounds in the HAA5 group. It forms when chlorine reacts with natural organic matter in source water. DCA is also the most studied individual HAA for carcinogenicity and reproductive toxicity.

Haloacetic acids (HAA5)

Five regulated haloacetic acid compounds — monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid — formed alongside trihalomethanes during chlorine disinfection.

Haloacetic acids (HAA9)

HAA9 is an expanded measure of haloacetic acid disinfection byproducts that adds four more brominated compounds — bromochloroacetic acid, bromodichloroacetic acid, chlorodibromoacetic acid, and tribromoacetic acid — to the five already measured in HAA5. The nine-acid grouping captures total HAA exposure more completely, particularly in water supplies with elevated bromide where HAA5 undercounts true HAA exposure.

Monobromoacetic acid (MBAA)

Monobromoacetic acid (MBAA) is one of the five HAA5 compounds, containing a single bromine atom. It is typically found at low to moderate levels in chlorinated water supplies with bromide present in source water. MBAA is also an intermediate in the formation of more halogenated HAA compounds.

Monochloroacetic acid (MCAA)

Monochloroacetic acid (MCAA) is one of the five haloacetic acids in the HAA5 group. It is typically the least prevalent HAA5 compound by mass in treated water. MCAA forms from chlorination of natural organic matter and is also a metabolite of trichloroacetic acid in the body.

Total trihalomethanes (TTHMs)

A group of four chemical compounds — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — formed when chlorine reacts with naturally occurring organic matter in source water.

Trichloroacetic acid (TCA)

Trichloroacetic acid (TCA) is one of the five haloacetic acid compounds regulated under the HAA5 standard. It is typically the second most prevalent HAA in chlorinated water after dichloroacetic acid. TCA forms when chlorine reacts with natural organic matter in source water.

Arsenic

A naturally occurring metalloid found in rock and soil. Arsenic dissolves into groundwater as water moves through arsenic-containing geological formations. Also introduced through industrial and agricultural activities.

Copper

A naturally occurring metal that leaches into drinking water primarily from copper household plumbing. At low levels it is an essential nutrient; at high levels it causes gastrointestinal effects.

Lead

A toxic heavy metal with no safe level in drinking water. The EPA's 15 ppb action level (not an MCL) is a threshold for requiring utility action — it is not a health-based limit. The EWG health guideline is zero.

Mercury (inorganic)

Inorganic mercury contamination in drinking water is relatively rare but occurs near industrial sites, mining operations, and areas with historical mercury use. It differs from methylmercury — the bioaccumulative organic form found in fish — which is not typically a drinking water contaminant.

Nitrate

An inorganic compound from agricultural runoff and natural sources — one of the most common contaminants in US public water. At elevated levels, poses acute risk to infants under six months and is associated in research with colorectal cancer and thyroid effects at concentrations well below the EPA MCL.

Nitrite

Nitrite is closely related to nitrate — it is the reduced form of nitrate and is itself oxidised to nitrate in water. Nitrite exposure in drinking water most commonly comes from agricultural runoff, sewage leakage, or from conversion of nitrate by bacterial activity in low-oxygen plumbing. In water treatment, nitrite is also an intermediate in the nitrification of chloramine-disinfected systems.

Radium 226+228

Two radioactive isotopes of radium — radium-226 and radium-228 — that occur naturally in rock and soil and dissolve into groundwater. Combined radium is one of the most commonly detected radionuclides in U.S. drinking water.

Uranium

A naturally occurring radioactive element found in rock, soil, and water. Detected in groundwater supplies throughout the U.S., particularly in the West.

2-Methylisoborneol (2-MIB)

A musty, camphor-like compound produced by cyanobacteria and actinomycetes bacteria. Along with geosmin, it is one of the two most common causes of musty odour complaints in drinking water. Detectable at extremely low concentrations.

Geosmin

A naturally occurring organic compound produced by cyanobacteria (blue-green algae) and certain soil bacteria. Responsible for the earthy, musty smell sometimes noticed in tap water — detectable by the human nose at concentrations as low as 0.000005 ppb.

Manganese

Manganese is a naturally occurring metal that occurs at elevated levels in some groundwater and surface water, particularly where oxygen levels are low (anaerobic conditions). While an essential trace mineral in the diet, excess manganese in drinking water raises health concerns especially for infants and young children.

Selenium

Selenium is a naturally occurring trace mineral that is essential in small amounts but toxic at higher levels. In water, it occurs naturally from selenium-bearing rocks and soils, and from irrigation drainage in western US agricultural areas.

Anionic surfactants

A class of synthetic detergents and cleaning agents that carry a negative charge. Found in household cleaners, laundry detergents, and industrial processes. Enter water supplies through wastewater discharge and agricultural runoff.

Benzene

A naturally occurring component of crude oil and a byproduct of combustion. One of the most studied carcinogens in drinking water. Found near petroleum storage sites, gas stations, and areas affected by fuel spills.

Chromium-6 (hexavalent chromium)

The carcinogenic form of chromium — hexavalent chromium (Cr-VI) — made widely known by the Erin Brockovich case in Hinkley, California. EPA currently regulates total chromium at 100 ppb but has no federal MCL specifically for chromium-6. California set the first US chromium-6 specific MCL (10 ppb) effective July 2024. EWG's health guideline is 0.02 ppb — 500× stricter than California's limit.

Microplastics

Plastic particles smaller than 5mm — often far smaller — detected in tap water globally and in the US. Currently unregulated in US drinking water. Detected in human blood, lung tissue, liver, and placenta. A 2024 NEJM study found an association between microplastics in arterial plaques and elevated cardiovascular events. Research on health effects is early-stage; causal evidence for specific outcomes from drinking water exposure is not yet established.

Perchlorate

Perchlorate is a chemical used in rocket propellant, fireworks, flares, and some fertilisers. It has contaminated groundwater and surface water near military facilities, aerospace sites, and agricultural areas that used Chilean nitrate fertilisers historically.

Phenols

A group of organic compounds containing a hydroxyl group attached to a benzene ring. Phenol and chlorophenols can enter drinking water through industrial discharge and as byproducts of chlorination reacting with naturally occurring organic matter.

Tetrachloroethylene (PCE)

An industrial solvent also known as perchloroethylene (PERC). Widely used in dry cleaning and metal degreasing operations. One of the most common groundwater contaminants at industrial and dry-cleaning sites.

Trichloroethylene (TCE)

An industrial solvent used in metal degreasing and manufacturing. One of the most commonly found volatile organic compounds (VOCs) in U.S. groundwater. Found at many EPA Superfund sites.

Atrazine

One of the most widely used herbicides in the United States, applied to corn, sorghum, and sugarcane. Among the most commonly detected pesticides in U.S. drinking water, particularly in the Corn Belt.

Barium

A naturally occurring metal found in rock and soil. Leaches into groundwater from natural deposits and is also present in some industrial discharge.

Fluoride

A mineral added to most US community water systems at 0.7 mg/L to prevent tooth decay — a practice in place since 1945. Also occurs naturally in some groundwater at higher concentrations. The debate over fluoride has intensified since the 2024 NTP systematic review linking exposure above 1.5 mg/L to lower IQ in children, and the EPA's January 2026 announcement of a new human health toxicity assessment. Approximately 3 in 5 Americans on public water receive fluoridated water.

Lithium

Lithium is a naturally occurring alkali metal that dissolves into groundwater from lithium-bearing mineral deposits including spodumene and lepidolite. It is also released from battery manufacturing, glass and ceramics production, and pharmaceutical waste. EPA's UCMR5 programme (2023–2025) is the first systematic national measurement of lithium in US public water systems. It is detected widely across the country, with higher concentrations in areas with lithium-rich geology such as parts of the Southwest and Mountain West.

Chloramines

A disinfectant formed by combining chlorine with ammonia. Used by more than one in five Americans' water utilities as an alternative to free chlorine — it lasts longer in distribution systems and produces fewer regulated disinfection byproducts. Unlike chlorine, it does not evaporate and requires active filtration to remove.

Chlorine (residual)

Chlorine is added to drinking water as a disinfectant to kill bacteria, viruses, and other pathogens. A residual amount remains in the water as it travels through the distribution system to prevent recontamination.

Turbidity

A measure of water clarity — how much light is scattered by particles suspended in the water. High turbidity indicates the presence of sediment, algae, or other particles that can interfere with disinfection and may indicate treatment problems.