Exxon Mobil Oil Refinery

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: 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: 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: This entry tracks violations of the Maximum Residual Disinfectant Level (MRDL) for chlorine — meaning too much disinfectant residual in the distribution system, not too little. Chlorine is added to water to kill pathogens, but excess residual can react with organic matter to form disinfection byproducts.

Health effects: Acute high-dose exposure to chlorinated water can cause eye and nose irritation, stomach discomfort, and nausea. Chronic exposure above the MRDL is primarily a concern because elevated chlorine accelerates formation of trihalomethanes and haloacetic acids, which are associated with increased cancer risk over time.

Limit: MRDL: 4.0 mg/L as an annual average

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: 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: The 2013 RTCR (effective 2016) replaced the original TCR. It triggers automatic E. coli testing whenever total coliform is found. A single E. coli-positive sample is an acute MCL violation. It also requires systems exceeding 5% total coliform-positive samples to conduct a formal assessment and correct any deficiencies, helping prevent contamination before it escalates.

Health effects: Total coliform bacteria indicate that the disinfection barrier has been breached or fecal contamination is entering the system. E. coli specifically indicates fecal contamination and the possible presence of disease-causing pathogens (norovirus, Salmonella, Shigella, Campylobacter). Illness can be severe in elderly, young children, and immunocompromised individuals.

Limit: Zero E. coli; ≤5% of monthly samples total coliform-positive

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: 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: 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: 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)