Millions of Americans drink tap water every day without knowing what contaminants may be present. The EPA regulates more than 90 contaminants in public drinking water, setting maximum contaminant levels (MCLs) based on both health risk and treatment feasibility. This guide covers the most common and concerning contaminants found in US water systems, how they enter the water supply, and what the science says about their health effects.
Microbial Contaminants
Microbial contamination is the most immediate threat to drinking water safety. Unlike chemical contaminants that cause harm over years of exposure, pathogenic bacteria and viruses can cause illness within hours or days.
E. coli
Escherichia coli is a bacterium found in the intestines of humans and animals. While most strains are harmless, certain types — particularly E. coli O157:H7 — produce toxins that cause severe illness. The presence of E. coli in drinking water indicates recent fecal contamination.
The EPA’s MCL for E. coli is zero: no E. coli should be detectable in any water sample collected for regulatory compliance. A positive E. coli result triggers immediate action requirements, including public notification.
Health effects: Gastrointestinal illness, including diarrhea (often bloody), stomach cramps, nausea, and vomiting. In severe cases, particularly in young children and the elderly, E. coli O157:H7 can cause hemolytic uremic syndrome (HUS), which can lead to kidney failure.
How it enters water: Fecal contamination from animals or humans, typically through agricultural runoff, aging or cracked distribution pipes, or treatment system failures.
Total Coliform
Total coliform bacteria are a broader group of organisms used as indicators of water quality. Their presence doesn’t necessarily mean the water is directly unsafe, but it signals that contamination pathways may exist.
The MCL for total coliform: no more than 5% of monthly samples can test positive in systems that collect at least 40 samples per month.
Health effects: Gastrointestinal discomfort in healthy individuals. More serious illness in immunocompromised people.
How it enters water: Soil, plant material, or animal waste entering the distribution system through pipe breaks, inadequate treatment, or cross-connections.
Inorganic Chemicals
Inorganic contaminants include metals, metalloids, and ions that occur naturally in geology or enter water through industrial activity, agricultural runoff, and aging infrastructure.
Lead
Lead has no established safe level of exposure, particularly for children. The EPA has set an action level (AL) of 15 parts per billion (ppb) — not a true MCL — meaning that if more than 10% of tap samples exceed this level, utilities must take corrective action. The action level is a regulatory trigger, not a declaration of safety; the EPA acknowledges that no level of lead is without risk.
Health effects: In children, lead exposure causes developmental delays, reduced IQ, learning and behavioral problems, and hearing difficulties. In adults, lead accumulates in bones and is associated with hypertension, kidney damage, and cardiovascular disease. Pregnant women face risks of premature birth and reduced fetal development.
How it enters water: Almost always from plumbing, not the source water. Lead service lines, lead solder used in pipes before 1986, and brass fixtures are the primary culprits. (See our dedicated lead guide for more detail.)
Arsenic
Arsenic is a naturally occurring metalloid that leaches from rock formations into groundwater — making it a particular concern for private wells and utilities drawing from certain aquifers in the western US, New England, and parts of the Midwest.
EPA MCL: 10 ppb (micrograms per liter)
Health effects: Long-term exposure to arsenic in drinking water is associated with bladder, lung, and skin cancer. It also causes skin thickening, cardiovascular disease, peripheral neuropathy, and type 2 diabetes. The EPA classified arsenic as a known human carcinogen.
How it enters water: Natural geological deposits are the primary source. Mining runoff and some agricultural pesticides also contribute.
Nitrate
Nitrate is one of the most common groundwater contaminants in agricultural areas. It moves easily through soil into wells and aquifers.
EPA MCL: 10 mg/L (milligrams per liter, equivalent to 10 ppm)
Health effects: Most notorious for causing methemoglobinemia, or “blue baby syndrome,” in infants under six months. The condition occurs when nitrate reduces the blood’s ability to carry oxygen, turning skin bluish. Infants are particularly vulnerable because their gut bacteria convert nitrate to nitrite more readily than adults. Emerging research also links high nitrate exposure in adults to colorectal cancer.
How it enters water: Agricultural fertilizers are the leading source. Septic system leakage and animal feedlots also contribute significantly.
Fluoride
Fluoride occurs naturally in many water sources and has been added to public water supplies since the 1940s to prevent tooth decay. However, excessive fluoride causes its own health problems.
EPA MCL: 4 mg/L (secondary standard of 2 mg/L for cosmetic effects)
Health effects: At concentrations above 4 mg/L, long-term exposure causes skeletal fluorosis — abnormal hardening of bones and joints that can become debilitating. At 2–4 mg/L, dental fluorosis (mottling of tooth enamel) can occur, particularly in children whose teeth are still developing.
How it enters water: Natural geological sources in some regions; intentionally added at ~0.7 mg/L in fluoridated systems.
Copper
Copper is an essential nutrient at trace levels but toxic in higher concentrations. Like lead, it typically enters water from plumbing rather than the source.
EPA action level: 1.3 mg/L
Health effects: Short-term exposure above 1.3 mg/L causes nausea, vomiting, diarrhea, and stomach cramps. Long-term high exposure can cause liver and kidney damage. Wilson’s disease patients are especially vulnerable.
How it enters water: Corrosion of copper pipes and brass fixtures, accelerated by low pH (acidic water).
Barium
Barium is a naturally occurring alkaline earth metal found in some mineral deposits.
EPA MCL: 2 mg/L
Health effects: Long-term exposure to barium above the MCL increases blood pressure and poses cardiovascular risks.
How it enters water: Leaching from natural deposits; discharge from metal refineries and drilling operations.
Chromium-6 (Hexavalent Chromium)
Chromium-6 gained public attention after the Hinkley, California contamination case (the subject of the Erin Brockovich story). The EPA currently regulates total chromium at 100 ppb — but does not set a specific federal limit for chromium-6. California has set its own limit of 10 ppb.
Health effects: Chromium-6 is a known carcinogen. Drinking water with elevated chromium-6 is associated with stomach and gastrointestinal cancers. The EPA has been working on a specific chromium-6 MCL.
How it enters water: Industrial discharge (electroplating, leather tanning, steel manufacturing), as well as natural geological erosion.
Radium-226 and Radium-228
Radium is a naturally occurring radioactive element found in certain rock types, particularly in the upper Midwest and parts of Texas.
EPA MCL: 5 picocuries per liter (pCi/L) combined for Ra-226 and Ra-228
Health effects: Radium is a bone-seeking element that accumulates in skeletal tissue. Long-term exposure increases the risk of bone cancer, leukemia, and other cancers.
How it enters water: Natural geological erosion from radium-bearing rock formations into groundwater.
Uranium
Uranium occurs naturally in groundwater in many parts of the US, particularly in the West and Great Plains.
EPA MCL: 30 ppb
Health effects: Unlike most radioactive contaminants where radiation is the primary concern, uranium’s chemical toxicity to the kidneys is the main health risk at drinking water concentrations. It can cause kidney damage and may contribute to cancer at high exposures.
How it enters water: Natural geological sources; uranium mining operations.
Organic Chemicals
Organic chemical contaminants include industrial solvents, pesticides, and petroleum products. Many are known or probable carcinogens.
Benzene
Benzene is a volatile organic compound (VOC) and component of gasoline. It evaporates readily, so exposure from drinking water also occurs through inhalation during showering.
EPA MCL: 5 ppb
Health effects: Benzene is a known human carcinogen. Long-term exposure causes aplastic anemia and leukemia, particularly acute myeloid leukemia (AML).
How it enters water: Underground storage tank leaks, industrial discharge, and gasoline spills.
Atrazine
Atrazine is one of the most widely used herbicides in the US, applied primarily to corn fields. It is frequently detected in surface water and groundwater in agricultural regions.
EPA MCL: 3 ppb
Health effects: Atrazine is an endocrine disruptor. Animal studies show reproductive effects, and some human studies link atrazine exposure to birth defects and menstrual irregularities. The EPA classifies it as “not likely” to be carcinogenic, but it remains controversial.
How it enters water: Agricultural runoff following spring herbicide application.
Trichloroethylene (TCE)
TCE is an industrial solvent used in metal degreasing. It’s one of the most common groundwater contaminants at Superfund sites.
EPA MCL: 5 ppb
Health effects: TCE is a known human carcinogen associated with kidney cancer, non-Hodgkin lymphoma, and liver cancer. It also causes neurological effects and may cause liver and immune system damage at lower exposures.
How it enters water: Industrial discharge, improper disposal, and contaminated groundwater from Superfund sites.
Tetrachloroethylene (PCE)
PCE, also called perchloroethylene or “perc,” is used in dry cleaning and metal degreasing. Like TCE, it persists in groundwater.
EPA MCL: 5 ppb
Health effects: Probable human carcinogen associated with bladder cancer. Also causes neurological effects (dizziness, headaches) and liver damage.
How it enters water: Dry cleaning facility discharge and improper disposal of industrial solvents.
Vinyl Chloride
Vinyl chloride is used to make PVC plastic. It can also form in groundwater when TCE or PCE partially degrades under certain conditions.
EPA MCL: 2 ppb
Health effects: Vinyl chloride is a known human carcinogen, particularly associated with a rare liver cancer called hepatic angiosarcoma. It is also a probable cause of liver cancer more broadly.
How it enters water: Industrial discharge, leaching from PVC pipes (particularly older ones), and degradation of other chlorinated solvents.
Disinfection Byproducts
A central challenge in water treatment is that the chemicals used to kill pathogens can react with naturally occurring organic matter to form potentially harmful compounds.
Total Trihalomethanes (TTHMs)
Trihalomethanes — including chloroform, bromodichloromethane, dibromochloromethane, and bromoform — form when chlorine reacts with humic acids and other natural organic matter.
EPA MCL: 80 ppb (as a running annual average)
Health effects: Long-term exposure to elevated TTHM levels is associated with increased risk of bladder cancer. Some studies also suggest associations with adverse reproductive outcomes.
Haloacetic Acids (HAA5)
HAA5 encompasses five haloacetic acids (mono-, di-, and trichloroacetic acids, and mono- and dibromoacetic acids) that form through the same chlorination process as trihalomethanes.
EPA MCL: 60 ppb
Health effects: Similar to TTHMs — long-term exposure is linked to increased cancer risk, particularly bladder cancer.
Note: Utilities that draw from surface water with high organic content (e.g., reservoirs with algae blooms) tend to have higher DBP levels. Switching from chlorine to chloramines for disinfection reduces TTHMs but can increase other byproducts.
Emerging Contaminants: PFAS
Per- and polyfluoroalkyl substances (PFAS) represent a newer category of concern. These synthetic chemicals were used in industrial and consumer products for decades and are now widespread in water supplies.
PFOA and PFOS
PFOA (perfluorooctanoic acid) and PFOS (perfluorooctane sulfonate) are the two most studied PFAS compounds. In April 2024, the EPA finalized the first-ever federal MCLs for PFAS in drinking water.
EPA MCL (2024 rule): 4 parts per trillion (ppt) each for PFOA and PFOS individually
Health effects: PFOA and PFOS are associated with kidney and testicular cancer, thyroid disease, immune system suppression (including reduced vaccine effectiveness), high cholesterol, and reproductive issues. They are classified as possible-to-likely human carcinogens.
Why “forever chemicals”: PFAS contain chains of carbon-fluorine bonds — among the strongest in chemistry. They do not break down naturally in the environment or in the human body, where they bioaccumulate. The name “forever chemicals” reflects this persistence.
How they enter water: Firefighting foam (aqueous film-forming foam, or AFFF) used at military bases and airports is a major source of groundwater contamination. Manufacturing facilities, non-stick cookware production, waterproof fabric treatment, and food packaging also contribute.
Compliance timeline: Water utilities have until 2029 to comply with the 2024 PFAS MCL rule, meaning many systems are still in the process of assessment and treatment upgrades.
Understanding EPA Limits
EPA MCLs are set considering both health effects and the feasibility of treatment at scale. An MCL is not always the same as a “safe” level — for carcinogens especially, there may be no truly safe threshold. The EPA typically sets MCLs at levels that keep lifetime cancer risk below 1 in 10,000 or 1 in 1,000,000, balancing risk against treatment costs.
For non-regulated contaminants like chromium-6 and many PFAS compounds, public water systems are not required to test or report results — which means the absence of data doesn’t mean the absence of contamination.
To check your water system’s reported contaminant levels, review your utility’s annual Consumer Confidence Report (CCR), which must be delivered to customers by July 1 each year. You can also look up your water system’s data through EPA’s ECHO database or state drinking water databases.