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What it means: DBCP is a halogenated aliphatic soil fumigant used to control nematodes on crops including pineapple, grapes, and citrus. Its use was banned in the United States in 1979 after it was linked to male sterility in factory workers. DBCP is highly mobile in soil and extremely persistent in groundwater, and remains a significant contaminant in agricultural regions such as California's Central Valley.

Health effects: DBCP is classified by EPA as a probable human carcinogen (Group B2). It causes male reproductive toxicity — including reduced sperm count and sterility — at relatively low exposure levels. Chronic exposure is also associated with kidney damage. Men of reproductive age and developing fetuses are most at risk.

Limit: 0.0002 mg/L (0.2 ppb)

What it means: 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a chlorinated aromatic compound formed as a byproduct of industrial processes including waste incineration, herbicide manufacturing, and paper bleaching. It is not intentionally produced. TCDD is highly persistent in the environment and bioaccumulates in fatty tissue; drinking water is generally a minor exposure pathway compared to food.

Health effects: TCDD is classified as a known human carcinogen (Group A) by EPA. Even at extremely low concentrations it is associated with increased cancer risk, particularly soft tissue sarcoma and lymphoma. It also causes chloracne, liver damage, immune suppression, and reproductive and developmental toxicity. The MCL is set at a vanishingly low level reflecting its extreme potency.

Limit: 0.00000003 mg/L (0.03 ppt)

What it means: Silvex (2,4,5-trichlorophenoxypropionic acid) was a chlorophenoxy herbicide used to control woody plants and broadleaf weeds on cropland, orchards, and roadsides. All uses were canceled in the United States in 1985 due to contamination with dioxin (TCDD) during manufacture. Legacy residues persist in agricultural soils and can leach into groundwater.

Health effects: Chronic exposure to Silvex is associated with liver damage. EPA classifies it as a possible human carcinogen (Group C). The dioxin contamination historically associated with its production added to concern about its carcinogenicity, though the MCL applies to Silvex itself. Long-term exposure above the MCL is the primary concern.

Limit: 0.05 mg/L (50 ppb)

What it means: 2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most widely used herbicides in the world, applied to lawns, turf, pastures, and crops to control broadleaf weeds. It remains in active use in the United States. 2,4-D is water-soluble and can enter drinking water through agricultural and residential runoff and leaching into groundwater.

Health effects: Chronic exposure to 2,4-D is associated with kidney, liver, and adrenal gland damage in animal studies. IARC classifies it as possibly carcinogenic to humans (Group 2B); EPA's classification is not likely to be carcinogenic under current guidelines. Individuals with kidney or liver conditions may face greater risk from long-term exposure.

Limit: 0.07 mg/L (70 ppb)

What it means: Atrazine is one of the most widely used herbicides in the United States, applied primarily to corn and sorghum fields. It is chemically stable and highly mobile in soil, making it one of the most frequently detected pesticides in US groundwater and surface water. Agricultural runoff after spring planting is the primary pathway into drinking water sources.

Health effects: Chronic exposure to atrazine is associated with endocrine disruption — particularly effects on reproductive hormones — and cardiovascular and adrenal effects in animal studies. EPA classifies atrazine as not likely to be a human carcinogen under current guidelines, though its endocrine-disrupting properties remain under research scrutiny. Fetuses and infants may be most sensitive to hormonal effects.

Limit: 0.003 mg/L (3 ppb)

What it means: Benzo(a)pyrene (BaP) is a polycyclic aromatic hydrocarbon (PAH) formed during incomplete combustion of organic material. It enters drinking water primarily through leaching from coal tar linings in older water storage tanks and distribution pipes, as well as from contaminated soil and industrial runoff. It is one of the most studied PAHs due to its potent carcinogenicity.

Health effects: Benzo(a)pyrene is classified by EPA as a probable human carcinogen (Group B2) and is a well-established animal carcinogen. Long-term exposure is associated with increased cancer risk, particularly for reproductive organ cancers. It is also linked to immune system effects and developmental toxicity in animal studies.

Limit: 0.0002 mg/L (0.2 ppb)

What it means: Carbofuran is a systemic carbamate insecticide and nematicide used on crops including corn, rice, and soybeans. EPA regulatory action beginning in 2006 led to cancellation of most food uses by 2009 due to acute toxicity to birds and other wildlife. Carbofuran is water-soluble and mobile in soil and has been detected in groundwater near agricultural areas.

Health effects: Carbofuran is one of the most acutely toxic carbamate pesticides. It inhibits acetylcholinesterase, causing cholinergic toxicity at the nervous system. Short-term exposure above the MCL can cause nausea, headache, blurred vision, and in high doses respiratory failure. Children are most vulnerable to acute effects. EPA has not classified carbofuran as a human carcinogen.

Limit: 0.04 mg/L (40 ppb)

What it means: Chlordane is a broad-spectrum organochlorine insecticide used extensively for termite control and on crops including corn and citrus. All uses were banned in the United States in 1988. Chlordane is extremely persistent in soil and can leach slowly into groundwater over decades. Legacy contamination is most prevalent in areas with intensive historical use.

Health effects: Chronic chlordane exposure is associated with liver damage and increased cancer risk. EPA classifies it as a probable human carcinogen (Group B2). Animal studies show liver tumors at elevated doses. It bioaccumulates in fatty tissue and can be passed to infants through breast milk. Children and immunocompromised individuals are most vulnerable.

Limit: 0.002 mg/L (2 ppb)

What it means: Dalapon is an organochlorine herbicide used to control grassy weeds in sugarcane, corn, and other crops, as well as along roadsides and waterways. It remains in agricultural use in the United States. It reaches water primarily through agricultural runoff and leaching.

Health effects: Long-term exposure to dalapon is associated with liver and kidney effects in animal studies. EPA has not classified it as a human carcinogen based on available data. The general population is the primary concern for chronic low-level exposure through drinking water.

Limit: 0.2 mg/L (200 ppb)

What it means: Di(2-ethylhexyl) adipate (DEHA) is a synthetic plasticizer used to make PVC and other plastics flexible. It enters drinking water through discharge from chemical factories and leaching from PVC plastic materials including some water distribution components. It is one of the less commonly detected regulated organic contaminants in US public water systems.

Health effects: Chronic exposure to DEHA is associated with reproductive effects and decreased body weight in animal studies. EPA classifies it as a possible human carcinogen (Group C). Long-term exposure above the MCL is the primary concern; the general population is at risk, with potential for greater sensitivity in developing fetuses and reproductive-age adults.

Limit: 0.4 mg/L (400 ppb)

What it means: Di(2-ethylhexyl) phthalate (DEHP) is the most widely used plasticizer in PVC products including pipes, flooring, medical tubing, and food packaging. It enters drinking water through discharge from rubber and chemical factories and leaching from PVC water distribution materials. It is one of the more commonly detected phthalates in US drinking water.

Health effects: Chronic exposure to DEHP is associated with reproductive and developmental toxicity — it acts as an endocrine disruptor and can interfere with hormone signaling. EPA classifies it as a probable human carcinogen (Group B2) based on animal liver tumor data. Developing fetuses, infants, and reproductive-age adults are most susceptible to its hormonal effects.

Limit: 0.006 mg/L (6 ppb)

What it means: Dinoseb is a dinitrophenol herbicide and pesticide formerly used on crops including soybeans, cereals, and vegetables. Its US pesticide registrations were voluntarily canceled in 1986 after it was found to cause severe reproductive and developmental toxicity. It persists in soil and can leach into groundwater from historically treated agricultural fields.

Health effects: Dinoseb uncouples oxidative phosphorylation, disrupting cellular energy production. Chronic exposure causes reproductive toxicity: impaired sperm production in males and developmental harm, including birth defects, in developing fetuses. EPA has not established a formal carcinogenicity classification. Reproductive-age adults and pregnant women face the greatest risk.

Limit: 0.007 mg/L (7 ppb)

What it means: Diquat is a broad-spectrum contact herbicide used to control aquatic weeds, algae, and terrestrial vegetation. It remains in active use in the United States, applied directly to water bodies and irrigation canals. Diquat binds tightly to soil but can enter surface water directly during aquatic applications.

Health effects: Chronic exposure to diquat is associated with cataracts and kidney damage in animal studies. Acute high-level exposure causes gastrointestinal distress. EPA has not classified diquat as a human carcinogen. Individuals with kidney conditions may be at elevated risk from chronic exposure.

Limit: 0.02 mg/L (20 ppb)

What it means: Endothall is an aquatic and terrestrial herbicide used to control algae, weeds, and grass in irrigation canals, ponds, and lakes. It is still in use in the United States and is applied directly to water bodies for aquatic vegetation management. It degrades relatively quickly in aquatic environments but can appear in source water shortly after application.

Health effects: Short-term exposure above the MCL can cause gastrointestinal irritation and stomach cramps. Chronic exposure is associated with liver, kidney, and gastrointestinal damage in animal studies. EPA has not classified endothall as a human carcinogen. The general population is at risk from water system violations.

Limit: 0.1 mg/L (100 ppb)

What it means: Ethylene dibromide (EDB) is a halogenated organic compound used as a soil and grain fumigant and as a lead scavenger in leaded gasoline. Its use as a pesticide was banned in the United States in 1984 following discovery of widespread groundwater contamination. EDB is highly persistent in groundwater and continues to be detected at legacy agricultural and fuel storage sites.

Health effects: EDB is classified by EPA as a probable human carcinogen (Group B2). It is acutely toxic at high doses, causing liver, kidney, and reproductive damage. Chronic low-level exposure is associated with increased cancer risk. Men of reproductive age face particular risk due to EDB's well-documented testicular toxicity.

Limit: 0.00005 mg/L (0.05 ppb)

What it means: Glyphosate is the most widely used herbicide in the United States, applied to crops, lawns, and roadsides. It is the active ingredient in Roundup and related products. It enters water through agricultural and urban runoff.

Health effects: Long-term exposure above the MCL is associated with kidney and reproductive damage in animal studies. EPA's current classification does not designate glyphosate as a human carcinogen, though IARC classifies it as probably carcinogenic (Group 2A). This classification remains a subject of active scientific and regulatory debate.

Limit: 0.7 mg/L (700 ppb)

What it means: Hexachlorobenzene (HCB) is a chlorinated aromatic compound formerly used as a fungicide on grain seeds and as an industrial chemical. All pesticide uses were canceled in the United States by 1984. It is also produced as a byproduct in the manufacture of certain chlorinated solvents and pesticides. HCB is highly persistent and bioaccumulates in fatty tissue and sediment.

Health effects: Chronic exposure to hexachlorobenzene is associated with liver damage, skin lesions, and neurological effects. EPA classifies it as a probable human carcinogen (Group B2). It can be transferred to infants through breast milk. Reproductive and developmental toxicity have been observed in animal studies.

Limit: 0.001 mg/L (1 ppb)

What it means: Hexachlorocyclopentadiene (HCCP) is a chlorinated cyclic compound used as a chemical intermediate in the manufacture of pesticides, flame retardants, and resins. It is not used directly as a pesticide. It enters drinking water through discharge from chemical manufacturing facilities. It is reactive and degrades relatively quickly in the environment.

Health effects: Chronic exposure to hexachlorocyclopentadiene is associated with kidney and stomach damage in animal studies. EPA has not classified it as a human carcinogen based on available data. Short-term high-level exposure may cause irritation of mucous membranes. Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.05 mg/L (50 ppb)

What it means: The Interim Enhanced Surface Water Treatment Rule (IESWTR), finalized in 1998, strengthened protections for systems serving 10,000 or more people by tightening turbidity limits and requiring Cryptosporidium removal. It was an intermediate step between the original SWTR and the Long-Term Enhanced SWTRs.

Health effects: Violations indicate failure to meet treatment technique requirements that reduce exposure to Cryptosporidium, Giardia, and other microbial pathogens. Cryptosporidium poses the greatest risk to immunocompromised individuals, infants, and the elderly, and can cause severe or fatal illness in vulnerable populations.

Limit: Treatment technique — no numeric MCL; compliance means meeting turbidity limits (0.3 NTU in 95% of monthly samples) and Cryptosporidium removal benchmarks

What it means: Nitrate (NO₃) is the most common chemical contaminant in US groundwater, entering from fertilizers, septic systems, and animal waste. It is odorless and tasteless — the only way to know if it's present is to test.

Health effects: Methemoglobinemia in infants under 6 months: gut bacteria convert nitrate to nitrite, which blocks oxygen transport in blood. Pregnant women should also take precautions. Chronic adult exposure at elevated levels may increase cancer risk.

Limit: 10 mg/L as nitrogen (NO₃-N)

What it means: Oxamyl is a carbamate insecticide and nematicide sold under the trade name Vydate, used on potatoes, apples, citrus, and other crops. It remains in agricultural use in the United States. Oxamyl is water-soluble and mobile in soil, making it prone to leaching into groundwater and entering surface water through runoff.

Health effects: Short-term exposure above the MCL can cause nervous system effects including headache, blurred vision, muscle weakness, and nausea, consistent with acetylcholinesterase inhibition by carbamate compounds. Chronic effects include nervous system damage. Children are most susceptible to acute neurological effects. EPA has not classified oxamyl as a human carcinogen.

Limit: 0.2 mg/L (200 ppb)

What it means: Pentachlorophenol (PCP) is a chlorinated phenol used as a wood preservative, primarily for utility poles, railroad ties, and fence posts. It remains in limited industrial use in the United States. PCP leaches from treated wood into surrounding soil and groundwater, and can enter surface water through runoff from treated wood storage sites.

Health effects: Chronic exposure to pentachlorophenol is associated with liver and kidney damage and immune system effects. EPA classifies it as a probable human carcinogen (Group B2). Acute high-level exposure can cause fever, profuse sweating, and cardiovascular collapse. Workers and communities near wood treatment facilities face the greatest exposure risk.

Limit: 0.001 mg/L (1 ppb)

What it means: Picloram is a systemic pyridine herbicide used to control broadleaf weeds and woody plants on rangeland, pasture, rights-of-way, and forestry sites. It remains in active use in the United States. Picloram is highly water-soluble and mobile in soil, persistent in the environment, and prone to leaching into groundwater and entering surface water through agricultural runoff.

Health effects: Chronic exposure to picloram is associated with liver damage in animal studies. EPA classifies it as a possible human carcinogen (Group C). The general population faces chronic risk from prolonged exposure above the MCL, particularly in agricultural regions where it is applied to rangeland and rights-of-way.

Limit: 0.5 mg/L (500 ppb)

What it means: Simazine is a triazine herbicide used to control broadleaf weeds and grasses in orchards, vineyards, turf, and some crops. It remains in use in the United States and is chemically related to atrazine. Simazine is mobile in soil and readily leaches into groundwater, particularly in sandy soils.

Health effects: Chronic exposure to simazine is associated with potential cancer risk; EPA classifies it as a possible human carcinogen (Group C) based on animal studies. Blood and nervous system effects have also been observed in animal studies. Long-term exposure at levels above the MCL is the primary concern for public water system violations.

Limit: 0.004 mg/L (4 ppb)

What it means: All systems using surface water (rivers, lakes, reservoirs) or groundwater under surface influence (GWUDI) must filter and disinfect. Treatment technique violations mean the system failed to maintain adequate disinfectant residual, exceeded turbidity limits, or lacked required permits. Snowmelt and storm events can raise turbidity and pathogen loads in surface water.

Health effects: Without proper filtration/disinfection: Giardia lamblia (severe diarrhea, cramping), Cryptosporidium (life-threatening for immunocompromised individuals), viruses (hepatitis A, norovirus). Turbidity itself is not a health risk but can shield pathogens from disinfection.

Limit: Treatment technique: turbidity ≤0.3 NTU in 95% of monthly samples; detectable disinfectant residual in 95% of distribution samples

What it means: TTHMs are four compounds — chloroform, bromodichloromethane, dibromochloromethane, bromoform — formed when chlorine reacts with organic matter in water. They are among the most common disinfection byproducts and form more readily in surface water systems with high organic loads.

Health effects: Long-term exposure is associated with increased bladder cancer risk. Some studies link high TTHM exposure to adverse reproductive outcomes. Bromodichloromethane is classified as a possible human carcinogen.

Limit: 0.080 mg/L (80 ppb) locational running annual average (LRAA)

What it means: HAA5 are five haloacetic acids formed when chlorine disinfectants react with naturally occurring organic matter in source water. Higher levels occur with more organic-rich source water or when water sits longer in distribution pipes. The violation indicates the locational running annual average exceeded the MCL.

Health effects: Dichloroacetic acid is a probable human carcinogen (Group B2). Long-term exposure to trichloroacetic acid may also increase cancer risk. Animal studies show liver, kidney, and reproductive effects at high doses.

Limit: 0.060 mg/L (60 ppb) locational running annual average (LRAA)

What it means: Toxaphene was a complex mixture of chlorinated camphenes used extensively as an insecticide on cotton and other crops in the southern United States. It was banned in 1990. Toxaphene is persistent in soil and sediment and can enter surface water through runoff from historically treated agricultural land, particularly in the South.

Health effects: Chronic toxaphene exposure is associated with liver and kidney damage and effects on the immune and nervous systems. EPA classifies it as a probable human carcinogen (Group B2). Children and those with compromised liver or kidney function face greater risk from chronic exposure.

Limit: 0.003 mg/L (3 ppb)