Diamond Plastics

What it means: 1,1,1-Trichloroethane (methyl chloroform) was a widely used industrial solvent for metal cleaning, electronic component degreasing, and aerosol propellants. Production was phased out under the Montreal Protocol due to ozone depletion; US manufacture essentially ended by 1996. Legacy groundwater contamination persists at former industrial sites and Superfund locations across the country.

Health effects: Chronic exposure to 1,1,1-trichloroethane affects the central nervous system (dizziness, incoordination) and can cause liver and cardiovascular effects at elevated doses. EPA has not classified it as a human carcinogen. It degrades in anoxic groundwater to 1,1-dichloroethylene and eventually vinyl chloride, which are more acutely toxic breakdown products.

Limit: 0.2 mg/L (200 ppb)

What it means: 1,1,2-Trichloroethane is a chlorinated solvent used as a chemical intermediate and industrial cleaning agent. It enters drinking water through industrial discharge and improper solvent disposal. It is less commonly detected than its isomer 1,1,1-trichloroethane but persists in groundwater and has been found at industrial contamination sites.

Health effects: Chronic exposure to 1,1,2-trichloroethane is associated with liver and kidney damage. EPA classifies it as a probable human carcinogen (Group B2) based on animal tumor data. Long-term exposure above the MCL is the primary concern; the general population near contaminated groundwater sources faces the greatest risk.

Limit: 0.005 mg/L (5 ppb)

What it means: 1,1-Dichloroethylene (1,1-DCE) is a chlorinated solvent used in the production of polyvinylidene chloride (PVDC) plastic wraps and coatings. It also occurs as a breakdown product of 1,1,1-trichloroethane and trichloroethylene in contaminated groundwater. Industrial discharge and solvent disposal are the primary pathways into drinking water sources.

Health effects: Chronic exposure to 1,1-dichloroethylene is associated with liver and kidney damage. EPA classifies it as a possible human carcinogen (Group C) based on animal data. Acute high-level exposure can cause central nervous system depression. Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.007 mg/L (7 ppb)

What it means: 1,2,4-Trichlorobenzene is an industrial solvent and chemical intermediate used in dye manufacturing, herbicide production, and as a dielectric fluid. It enters drinking water through discharge from industrial facilities and leaking underground storage tanks. It is moderately persistent in groundwater and has been detected in groundwater near industrial sites.

Health effects: Chronic exposure to 1,2,4-trichlorobenzene is associated with changes in adrenal gland function and liver and kidney effects in animal studies. EPA classifies it as a possible human carcinogen (Group C). Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.07 mg/L (70 ppb)

What it means: 1,2-Dichloroethane (ethylene dichloride) is a chlorinated solvent used as a chemical intermediate in vinyl chloride production and in other manufacturing processes. It is one of the highest-volume chlorinated compounds produced in the US. It enters drinking water through industrial discharge, leaking underground storage tanks, and improper disposal. It is moderately persistent in groundwater.

Health effects: Chronic exposure to 1,2-dichloroethane is associated with liver and kidney damage and neurological effects. EPA classifies it as a probable human carcinogen (Group B2) based on animal evidence of gastrointestinal and mammary gland tumors. Long-term exposure above the MCL is the primary concern; communities near chemical manufacturing facilities and Superfund sites face the greatest risk.

Limit: 0.005 mg/L (5 ppb)

What it means: 1,2-Dichloropropane (propylene dichloride) is a chlorinated solvent used as a chemical intermediate and industrial cleaning agent. It was historically used in fumigant mixtures for soil treatment but agricultural uses were canceled. It enters drinking water through industrial discharge, improper disposal, and leaching from contaminated soil. It is moderately persistent in groundwater.

Health effects: Chronic exposure to 1,2-dichloropropane is associated with liver and kidney damage. EPA classifies it as a probable human carcinogen (Group B2) based on animal evidence of liver and mammary gland tumors. Long-term exposure above the MCL is the primary concern; communities near Superfund sites and former agricultural fumigation areas face elevated exposure risk.

Limit: 0.005 mg/L (5 ppb)

What it means: Arsenic is a naturally occurring metalloid found in bedrock and soil. It dissolves into groundwater from natural deposits and can also come from mining, smelting and past pesticide use. EPA lowered the MCL from 50 ppb to 10 ppb in 2001.

Health effects: Long-term arsenic exposure increases risk of bladder, lung, and skin cancers. It is also associated with cardiovascular disease, type 2 diabetes, and developmental effects in children. Arsenic is classified as a Group A human carcinogen by the EPA.

Limit: 0.010 mg/L (10 ppb), set in 2001 and enforceable since 2006; the previous MCL was 0.050 mg/L (50 ppb)

What it means: Barium is a naturally occurring alkaline earth metal found in mineral deposits, particularly barite and witherite. It enters drinking water through dissolution of natural deposits and, to a lesser extent, from industrial discharge — metal refining, coal combustion, and oil and gas drilling operations. Groundwater drawn from barium-bearing rock can carry detectable levels.

Health effects: Short-term exposure above the MCL can cause increases in blood pressure. Chronic exposure is associated with cardiovascular effects, kidney damage, and gastrointestinal disturbance. The general population is at risk; those with pre-existing hypertension or kidney disease face greater concern.

Limit: 2 mg/L (2,000 ppb)

What it means: Benzene is a fundamental aromatic hydrocarbon and component of crude oil, gasoline, and many industrial chemicals. It enters drinking water primarily through leaking underground fuel storage tanks, industrial discharge, and contaminated runoff. It is one of the most commonly detected VOCs at Superfund sites and underground storage tank cleanup sites across the United States.

Health effects: Benzene is classified by EPA as a known human carcinogen (Group A). Long-term exposure causes leukemia, particularly acute myeloid leukemia (AML), and other blood disorders including aplastic anemia. There is no established safe threshold. Children and individuals with compromised immune systems are most vulnerable to hematological effects.

Limit: 0.005 mg/L (5 ppb)

What it means: Beryllium is a rare alkaline earth metal that enters drinking water through discharge from metal refineries, coal burning, and electrical and aerospace manufacturing industries. Natural erosion of beryllium-containing minerals can also contribute. It is among the less commonly detected regulated metals in US drinking water systems.

Health effects: Chronic oral exposure to beryllium is associated with intestinal lesions and bone damage. EPA classifies beryllium as a probable human carcinogen (Group B2) based on animal evidence and limited human data. Occupational inhalation exposure causes a distinct lung disease (berylliosis), but the oral route primarily targets the GI tract and bone.

Limit: 0.004 mg/L (4 ppb)

What it means: Cadmium is a heavy metal that enters drinking water primarily through corrosion of galvanized pipes and solder, industrial discharge from metal refining and battery manufacturing, and leaching from natural mineral deposits. It is also a byproduct of zinc production. Levels in water are typically low but can accumulate in plumbing infrastructure over time.

Health effects: Cadmium accumulates in the kidneys and can cause kidney damage with long-term exposure. It is classified by the EPA as a probable human carcinogen (Group B1). Bone demineralization and reproductive effects have also been observed at elevated exposure levels.

Limit: 0.005 mg/L (5 ppb)

What it means: Carbon tetrachloride is a chlorinated solvent formerly used extensively as a cleaning and degreasing agent, fire extinguisher propellant, and grain fumigant. Its production was phased out under the Montreal Protocol. Legacy groundwater contamination is widespread at former industrial facilities, Superfund sites, and agricultural fumigation areas.

Health effects: Carbon tetrachloride is classified by EPA as a probable human carcinogen (Group B2). Chronic exposure causes liver and kidney damage and is associated with liver cancer in animal studies. Acute high-dose exposure causes severe hepatotoxicity — nausea, vomiting, jaundice, and acute liver failure. Communities near contaminated groundwater plumes and historical industrial sites face the greatest long-term risk.

Limit: 0.005 mg/L (5 ppb)

What it means: Chlorobenzene is an aromatic solvent and chemical intermediate used in the production of DDT, nitrochlorobenzenes, and other industrial chemicals. It enters drinking water through discharge from chemical manufacturing facilities and leaching from contaminated industrial sites. It is moderately volatile and moderately persistent in groundwater.

Health effects: Chronic exposure to chlorobenzene is associated with liver and kidney effects in animal studies. EPA has not classified it as a human carcinogen based on available data. Short-term exposure at high levels can cause nervous system depression. Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.1 mg/L (100 ppb)

What it means: Chromium occurs naturally in rocks, soil, and volcanic dust and enters drinking water through natural leaching from mineral deposits and industrial discharge from electroplating, leather tanning, and textile manufacturing. Hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the two main forms; both are regulated together under this MCL.

Health effects: Hexavalent chromium is associated with increased cancer risk through ingestion in animal studies and is classified by EPA as a probable human carcinogen (Group B1) for inhalation, with growing evidence for oral carcinogenicity. Chronic exposure to total chromium causes liver and kidney damage, allergic dermatitis, and gastrointestinal irritation. Individuals with pre-existing liver or kidney conditions face greater risk.

Limit: 0.1 mg/L (100 ppb) — total chromium

What it means: Cyanide in drinking water originates primarily from industrial discharges — metal plating, steel manufacturing, and certain mining operations — and from leaching of road salt formulations that contain ferrocyanide compounds. It can also enter from improper disposal of industrial waste. Natural background levels are negligible. It is measured as free cyanide.

Health effects: Acute high-dose cyanide exposure interferes with cellular oxygen use and can cause rapid deterioration of the central nervous system, including headache, dizziness, and in severe cases, loss of consciousness. Chronic exposure at lower levels is associated with thyroid dysfunction and nerve damage. Most drinking water violations pose chronic rather than acute risk.

Limit: 0.2 mg/L (200 ppb)

What it means: Dichloromethane (DCM), also called methylene chloride, is a widely used industrial solvent employed in paint stripping, pharmaceutical manufacturing, and metal degreasing. It enters drinking water through industrial discharge and improper disposal. It is volatile and evaporates quickly from open water but can persist in groundwater.

Health effects: Chronic exposure to dichloromethane is associated with liver and kidney damage and neurological effects. EPA classifies it as a probable human carcinogen (Group B2); it is metabolized in the body to carbon monoxide and has been linked to cardiovascular stress. Workers in industrial settings and communities near manufacturing sites face the greatest risk.

Limit: 0.005 mg/L (5 ppb)

What it means: Ethylbenzene is an aromatic hydrocarbon and BTEX compound used as a solvent and in the production of styrene. It is a natural component of petroleum and coal tar and enters drinking water through leaking underground fuel storage tanks, petroleum spills, and industrial discharge. It is moderately volatile and degrades relatively quickly under aerobic conditions.

Health effects: Chronic exposure to ethylbenzene is associated with liver and kidney effects. EPA classifies it as a possible human carcinogen (Group D — inadequate evidence) based on limited animal data. Short-term high-level exposure can cause eye and throat irritation and dizziness. Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.7 mg/L (700 ppb)

What it means: Inorganic mercury enters drinking water through erosion of natural deposits, discharge from refineries and factories, and runoff from landfills. Coal combustion deposits mercury in watersheds via atmospheric deposition, making it a diffuse source for surface water systems. The MCL applies to inorganic mercury; methylmercury bioaccumulation in fish is a separate concern governed by FDA.

Health effects: Chronic exposure to inorganic mercury in water primarily damages the kidneys. Neurological effects — tremors, memory loss, cognitive impairment — are more associated with organic mercury (methylmercury) than with the inorganic form regulated in drinking water. Developing fetuses and young children are most vulnerable.

Limit: 0.002 mg/L (2 ppb)

What it means: Nickel is a naturally occurring metal that leaches from rock and soil into groundwater and surface water. Industrial sources include metal plating, battery manufacturing, and mining operations. Nickel can also leach from faucets, fittings, and stainless steel plumbing components, particularly in low-pH water. It was added to the NPDWR framework as part of the Phase II rule.

Health effects: Chronic oral exposure to nickel in drinking water is associated with cardiovascular and kidney effects. Nickel compounds are classified as known human carcinogens (Group A) by IARC for inhalation; the oral carcinogenicity evidence is less established. Individuals with nickel sensitivity may also experience dermatological reactions.

Limit: 0.1 mg/L (100 ppb)

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: Nitrite (NO₂) enters water from fertilizer runoff, septic systems, and natural mineral deposits. Unlike nitrate, nitrite acts very quickly in the body — it converts hemoglobin to methemoglobin, which cannot carry oxygen. Nitrite can also form in distribution systems through microbial nitrification when chloramine disinfection breaks down.

Health effects: Methemoglobinemia ('blue baby syndrome') in infants under 6 months: reduced oxygen delivery to tissues can be life-threatening. Pregnant women and people with glucose-6-phosphate dehydrogenase (G6PD) deficiency are also at elevated risk. Adults are generally less susceptible due to more effective methemoglobin reduction.

Limit: 1 mg/L as nitrogen (NO₂-N)

What it means: Selenium is a naturally occurring metalloid present in certain soils and rock formations. It enters drinking water through weathering of seleniferous soils and rock, agricultural irrigation return flows, and discharge from petroleum refineries and metal smelters. Selenium is an essential trace nutrient at low concentrations but is toxic at levels above the MCL.

Health effects: Chronic exposure above the MCL causes selenosis — characterized by hair and nail loss, peripheral neuropathy, fatigue, and gastrointestinal disturbance. Liver damage can occur with prolonged high intake. The margin between nutritional requirement and toxic dose is narrow, making the MCL an important threshold.

Limit: 0.05 mg/L (50 ppb)

What it means: Styrene is an aromatic hydrocarbon monomer used in the manufacture of polystyrene plastics, synthetic rubber, and resins. It enters drinking water through industrial discharge, leaching from plastic linings in water storage tanks, and contaminated runoff from manufacturing sites. It is moderately volatile and biodegrades under aerobic conditions.

Health effects: Chronic exposure to styrene is associated with liver, kidney, and nervous system effects. EPA classifies it as a possible human carcinogen (Group C) based on limited animal evidence. Short-term exposure can cause irritation of the eyes, nose, and throat. Long-term exposure above the MCL is the primary concern for public water system violations.

Limit: 0.1 mg/L (100 ppb)

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: Tetrachloroethylene (PCE), also called perchloroethylene, is a chlorinated solvent used extensively in dry cleaning and metal degreasing. It is one of the most frequently detected VOCs in US groundwater, found near dry cleaning facilities, industrial sites, and military bases. PCE is dense and sinks through soil into aquifers, where it persists for decades.

Health effects: PCE is classified by EPA as a probable human carcinogen (Group B2), with evidence linking it to bladder and non-Hodgkin lymphoma risk. Chronic exposure is also associated with liver and kidney damage and neurological effects. Communities near dry cleaning operations and contaminated military installations face the greatest long-term exposure risk.

Limit: 0.005 mg/L (5 ppb)

What it means: Thallium is a rare heavy metal that enters drinking water through leaching from ore-processing operations, discharge from electronics and pharmaceutical manufacturing, and natural mineral weathering. It is one of the least common contaminants detected in US public water systems but has a very low MCL due to its high toxicity.

Health effects: Chronic thallium exposure causes hair loss, changes in blood chemistry, kidney damage, intestinal problems, and liver effects. It can also impair the nervous system. EPA has classified thallium as a possible human carcinogen (Group C). The very low MCL reflects its acute toxicity even at small doses.

Limit: 0.002 mg/L (2 ppb)

What it means: Toluene is an aromatic hydrocarbon solvent found in gasoline, paints, adhesives, and chemical manufacturing. It is a BTEX compound (benzene, toluene, ethylbenzene, xylene) and enters drinking water through leaking underground fuel storage tanks, petroleum spills, and industrial discharge. It is highly volatile but can persist in groundwater at contamination sites.

Health effects: Short-term exposure above the MCL can cause nervous system effects including headache, dizziness, and impaired coordination. Chronic exposure is associated with kidney and liver damage and neurological effects. EPA has not classified toluene as a human carcinogen. Developing fetuses may be particularly sensitive to its neurotoxic effects.

Limit: 1 mg/L (1,000 ppb)

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: Trichloroethylene (TCE) is a chlorinated solvent widely used for metal degreasing and as a chemical intermediate. It is one of the most common groundwater contaminants in the United States, found at hundreds of Superfund sites. TCE enters water through industrial discharge, improper solvent disposal, and leaking underground storage tanks. It is persistent in anoxic groundwater.

Health effects: TCE is classified by EPA as a known human carcinogen (Group A), associated primarily with kidney cancer and non-Hodgkin lymphoma. It is also linked to liver cancer and cervical cancer. Prenatal exposure is associated with cardiac defects. Workers at industrial facilities and residents near contaminated groundwater plumes face the greatest risk.

Limit: 0.005 mg/L (5 ppb)

What it means: Vinyl chloride is an industrial chemical used to manufacture polyvinyl chloride (PVC) plastic and related products. It enters drinking water primarily as a breakdown product of trichloroethylene and tetrachloroethylene in contaminated groundwater, and through discharge from PVC pipe manufacturing facilities. It is highly volatile but can persist in anoxic groundwater.

Health effects: Vinyl chloride is classified by EPA as a known human carcinogen (Group A). It is associated with a rare liver cancer called hepatic angiosarcoma, as well as liver and brain cancers. There is no safe threshold — any exposure carries some cancer risk. Workers in PVC manufacturing and communities near contaminated groundwater sites face the greatest risk.

Limit: 0.002 mg/L (2 ppb)

What it means: Xylenes are a group of three isomeric aromatic hydrocarbons (ortho-, meta-, and para-xylene) used as solvents and chemical intermediates in the production of plastics, synthetic fibers, and paints. They enter drinking water primarily through leaking underground fuel storage tanks and industrial spills, as they are components of gasoline and jet fuel.

Health effects: Short-term exposure above the MCL can cause nervous system effects including headache, dizziness, and impaired coordination. Chronic exposure is associated with liver and kidney damage. EPA has not classified total xylenes as human carcinogens. The general population is at risk; children may be more sensitive to neurological effects.

Limit: 10 mg/L (10,000 ppb)

What it means: cis-1,2-Dichloroethylene is a chlorinated solvent that occurs primarily as a breakdown product of trichloroethylene (TCE) and tetrachloroethylene (PCE) in contaminated groundwater. It is rarely used directly as an industrial chemical. Its presence in water typically indicates an upstream TCE or PCE contamination source undergoing natural attenuation.

Health effects: Chronic exposure to cis-1,2-dichloroethylene is associated with liver, nervous system, and immune system effects in animal studies. EPA has not classified it as a human carcinogen based on available data. Its detection in water often signals the presence of more acutely toxic parent compounds such as TCE or vinyl chloride.

Limit: 0.07 mg/L (70 ppb)

What it means: o-Dichlorobenzene (1,2-dichlorobenzene) is a chlorinated aromatic solvent used as a chemical intermediate, insecticide, and degreaser. It enters drinking water through industrial discharge and leaking underground storage infrastructure. It is moderately persistent in groundwater and has low but detectable volatility.

Health effects: Chronic exposure to o-dichlorobenzene is associated with liver, kidney, and blood effects in animal studies. EPA has not classified it as a human carcinogen based on current data. Long-term exposure above the MCL is the primary concern; the general population is at risk from contaminated groundwater sources.

Limit: 0.6 mg/L (600 ppb)

What it means: p-Dichlorobenzene (1,4-dichlorobenzene) is a chlorinated aromatic compound used as a moth repellent, toilet deodorizer, and chemical intermediate. It is one of the more commonly detected VOCs in US drinking water due to its widespread consumer and industrial use. It enters water through industrial discharge, landfill leachate, and volatilization from consumer products into indoor air that contacts water.

Health effects: Chronic exposure to p-dichlorobenzene is associated with liver and kidney damage. EPA classifies it as a possible human carcinogen (Group C) based on animal tumor data. Long-term exposure above the MCL is the primary concern. Individuals with pre-existing liver or kidney conditions may be at greater risk.

Limit: 0.075 mg/L (75 ppb)

What it means: trans-1,2-Dichloroethylene (trans-1,2-DCE) forms primarily as a breakdown product of trichloroethylene (TCE), tetrachloroethylene (PCE), and other chlorinated solvents in contaminated groundwater during natural attenuation. Like its cis isomer, it is rarely used directly as an industrial chemical; its presence signals upstream solvent contamination.

Health effects: Chronic exposure to trans-1,2-dichloroethylene is associated with central nervous system effects in animal studies. EPA has not classified it as a human carcinogen based on available data. Its primary public health significance is as a marker of more acutely toxic parent compounds — TCE, PCE, or vinyl chloride — that may also be present in the same groundwater.

Limit: 0.1 mg/L (100 ppb)