Common Contaminants Found in Drinking Water

Recent research and studies of drinking water quality and contaminant levels in the U.S. have concluded that much of our tap and well water is of questionable quality and likely unsafe for human consumption. Essentially all sources of our drinking water contain some level of contamination ranging from naturally-occurring minerals to man-made chemicals and byproducts. While some contaminants are regulated by various federal, state and local agencies, most are not. And while many contaminant levels in our drinking water are not high enough to cause immediate sickness or other health effects, longer-term exposure to even low-levels of many common contaminants can cause serious health problems.

Many thousands of contaminants (some say 80,000 or so), especially man-made chemicals, such as pesticides, herbicides, pharmaceuticals and industrial and agricultural wastes, are unregulated and increasingly are found in our drinking water. For these unregulated and unmonitored contaminants, simple water filtration and perhaps other purification processes such as reverse osmosis, can provide some safety factor for those who might be vulnerable to disease and health effects from waterborne contaminants. This is especially important for infants, the frail elderly, pregnant women and their fetuses, people undergoing chemotherapy or living with HIV/AIDS, transplant patients and others with severely compromised immune systems. 

The EPA (Environmental Protection Agency) has established primary national standards for approximately 90 different types of contaminants that can be found in drinking water and secondary standards (non-enforceable guidelines) for an additional fifteen contaminants . Details on the EPA standards can be found here. We've summarized the EPA information for many common drinking water contaminants below to include general descriptions of their sources and possible health effects.

The contaminants for which maximum allowable contaminant levels have been established as primary national standards by EPA are categorized below into the following groups:
 
• Micro-organisms
• Disinfectants
• Disinfection Byproducts
• Inorganic Chemicals
• Organic Chemicals
• Radionuclides

NOTE: On March 14, 2023, EPA announced the proposed National Primary Drinking Water Regulation (NPDWR) for six PFAS including perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA), hexafluoropropylene oxide dimer acid (HFPO-DA, commonly known as GenX Chemicals), perfluorohexane sulfonic acid (PFHxS), and perfluorobutane sulfonic acid (PFBS). The proposed PFAS NPDWR does not require any actions until it is finalized. EPA anticipates finalizing the regulation by the end of 2023. EPA expects that if fully implemented, the rule will prevent thousands of deaths and reduce tens of thousands of serious PFAS-attributable illnesses.

The contaminants covered by secondary (non-enforceable guidelines) national standards are listed below as a separate category. 

Micro-organisms

E. Coli and fecal coliform bacteria, if found in drinking water, provide a warning of failure in water treatment, a break in the integrity of the water distribution system and possible contamination with pathogens. These bacteria can enter rivers through direct discharge of waste from mammals and birds, from agricultural and storm runoff, and from human sewage.

Elevated levels of these bacteria indicate the water may be contaminated with human and animal wastes, and they can cause serious, short-term health effects such as diarrhea, cramps, nausea, headaches, or other symptoms consistent with common food poisoning.

Cryptosporidium parvum is a single-celled protozoan parasite commonly found in lakes and rivers, especially when the water is contaminated with sewage and animal waste. Cryptosporidium are highly resistant to traditional disinfection practices.

Primary symptoms of Cryptosporidium infection are acute, watery, and diarrhea. Other symptoms may include nausea, vomiting and abdominal pain. People with severely weakened immune systems are prone to have severe an persistent symptoms.
  
Giardia lamblia is a single-celled protozoan parasite that colonizes and reproduces in the small intestine of humans and animals. It can be found in soil, food or water that has been contaminated with the feces from infected humans or animals.

Giardia lamblia can cause symptoms such as nausea, cramps, diarrhea, and associated headaches. 

Legionella bacteria are found naturally in the environment, usually in water. Legionella bacteria in water are a health risk if the bacteria are aerosolized (e.g., in an air conditioning system or a shower) and then inhaled.

Inhalation can result in a type of pneumonia known as Legionnaires disease.

Enteric Viruses are small viruses that live in the intestines of infected humans or animals. This group includes the polioviruses, coxsackieviruses, echoviruses, and other enteroviruses.

​Illness from viruses ranges from gastroenteritis caused by viruses such as rotavirus and norovirus (Norwalk-like virus) to meningitis caused by echovirus to myocarditis caused by Coxsackie B.

Turbidity is a measure of the cloudiness of water, and it’s used to indicate water quality and filtration effectiveness (such as whether disease-causing organisms are present). Higher turbidity levels are often associated with higher levels of disease-causing microorganisms such as viruses, parasites and some bacteria.

Disinfectants

Most public water supplies are disinfected with chemicals to eliminate disease-causing micro-organisms. Common disinfectants include chlorine, chlorine dioxide and chloramine. While disinfection is necessary, the disinfectants can react with organic materials in the water to form by-products, such as trihalomethanes and haloacetic acids, which may themselves pose health risks. So operators of water treatment facilities must regulate the amount of chemical disinfectants to provide protection from pathogens while simultaneously minimizing health risks from disinfection by-products.

Chlorine has been used by public water systems as a disinfectant for many decades.

Levels in excess of EPA standards can cause eye and nose irritation, stomach discomfort and unpleasant taste.

Chlorine Dioxide is an effective disinfectant and can be used to control taste and odor problems in drinking water.

Levels in excess of EPA standards can cause nervous system effects long-term exposure to excessive levels may cause anemia.
 
Chloramine is used as a disinfectant and is effective as a residual in water distribution systems. It’s formed when ammonia is added to water containing free chlorine.

​Levels in excess of EPA standards can cause eye and nose irritation, stomach discomfort and anemia. 

Disinfection Byproducts

When disinfectants are added to water in order to eliminate disease-causing micro-organisms, they can react with naturally-occurring organic matter in the water to form disinfectant byproducts. These byproducts, such as trihalomethanes, haloacetic acids bromate and chlorite, may themselves pose health risks.
 
Trihalomethanes are formed when chlorine and chloramine react with organic matter. They include Bromodichloromethane, Bromoform, Dibromochloromethane and Chloroform.

Long-term exposure to levels of total trihalomethanes in excess of EPA standards can cause liver, kidney, or central nervous system problems and may result in increased risk of cancer.

Haloacetic acids are formed when chlorine and chloramine react with organic matter. They include Dichloroacetic acid, Trichloroacetic acid, Chloroacetic acid, Bromoacetic acid and Dibromoacetic acid.

Long-term exposure to haloacetic acids in excess of EPA standards may result in increased risk of cancer.

Bromate is formed when bromide in the water reacts with the disinfectant, ozone.

​Long-term exposure to bromate in excess of EPA standards may result in increased risk of cancer.

Chlorite is formed when the disinfectant, chlorine dioxide, breaks down. Some infants, young children and fetuses of pregnant women can experience nervous system effects when exposed to chlorite levels in excess of EPA standards. And some people may experience anemia.

Inorganic Chemicals

A number of inorganic chemicals can be found our drinking water. These contaminants range from naturally-occurring minerals to man-made chemicals and byproducts. Maximum allowable contaminant standards have been established by EPA for the following sixteen inorganic chemicals:

Antimony is a metal found in natural deposits. The major sources of antimony in drinking water are discharge from petroleum refineries; fire retardants; ceramics; electronics; and solder.

Long-term exposure to antimony in excess of EPA standards may result in increases in blood cholesterol and decreases in blood sugar.

Arsenic is a semi-metallic element in the periodic table, and it’s odorless and tasteless. It enters drinking water supplies from natural deposits in the earth or from agricultural and industrial practices.

Long-term exposure to arsenic in excess of EPA standards may result in skin damage or circulatory system problems and may result in increased risk of cancer.

Asbestos is a fibrous mineral occurring in natural deposits. The major sources of asbestos in drinking water are decay of asbestos cement water mains; and erosion of natural deposits.

Long-term exposure to asbestos in excess of EPA standards may result in increased risk of developing benign intestinal polyps.

Barium is a lustrous metalic that exists in nature only in ores containing mixtures of elements. The major sources of barium in drinking water are discharge of drilling wastes, discharge from metal refineries, and erosion of natural deposits.

Long-term exposure to barium in excess of EPA standards may result in an increase in blood pressure.

Beryllium is a metallic element in the periodic table. Because it is an element, it does not degrade nor can it be destroyed. Compounds of beryllium are either white or colorless and do not have a particular smell. Beryllium naturally enters surface water and ground water through the weathering of rocks and soils or from industrial wastewater discharges. The major sources of beryllium are from coal and fuel oil combustion.

Long-term exposure to beryllium in excess of EPA standards may result in intestinal lesions.

Cadmium is a metal found in natural deposits such as ores containing other elements. The major sources of cadmium in drinking water are corrosion of galvanized pipes, erosion of natural deposits, discharge from metal refineries and runoff from waste batteries and paints.

Long-term exposure to cadmium in excess of EPA standards may result in kidney damage.

Chromium is an odorless and tasteless metallic element. Chromium is found naturally in rocks, plants, soil and volcanic dust, humans and animals. The most common forms of chromium that occur in natural waters in the environment are trivalent chromium (chromium-3), and hexavalent chromium (chromium-6). Chromium-3 is an essential human dietary element and occurs naturally in many vegetables, fruits, meats, grains and yeast. Chromium-3 is a nutritionally essential element in humans and is often added to vitamins as a dietary supplement. Chromium-3 has relatively low toxicity and would be a concern in drinking water only at very high levels of contamination.

Chromium-6 is more toxic and poses potential health risks. Long-term exposure to chromium-6 in excess of EPA standards may result in allergic dermatitis.

Copper is a metal found in natural deposits such as ores containing other elements. The major sources of copper in drinking water are corrosion of household plumbing systems and erosion of natural deposits. Copper can leach into water primarily from pipes, but fixtures and brass faucets  and fittings can also be a source.  The amount of copper in your water also depends on the types and amounts of minerals in the water, how long the water stays in the pipes, the amount of wear in the pipes, the water’s acidity and its temperature.

Short-term exposure to copper in excess of EPA standards may result in gastrointestinal distress, and long-term exposure may result in liver or kidney damage.

Cyanide is a carbon-nitrogen chemical unit which combines with many organic and inorganic compounds. The major source of cyanide in drinking water is discharge from industrial chemical factories.

Long-term exposure to cyanide in excess of EPA standards may result in nerve damage or thyroid problems.

Fluoride compounds are salts that form when the element, fluorine, combines with minerals in soil or rocks. Some fluoride compounds, such as sodium fluoride and fluorosilicates, dissolve easily into ground water as it moves through gaps and pore spaces between rocks. Most water supplies contain some naturally occurring fluoride. Fluoride also enters drinking water in discharge from fertilizer or aluminum factories.

Also, many communities add fluoride to their drinking water to promote dental health.  However, many researchers believe fluoride's predominant effect on tooth decay comes from topical contact with the teeth, not ingestion.

Long-term exposure to fluoride in excess of EPA standards may result in increased likelihood of bone fractures in adults, and may result in effects on bone leading to pain and tenderness. Children aged 8 years and younger exposed to excessive amounts of fluoride have an increased chance of developing pits in the tooth enamel, along with a range of cosmetic effects to teeth.

In addition, the National Research Council (NRC), with taxpayer funding, reviewed fluoride toxicology and concluded in 2006 that:

• “…it is apparent that fluorides have the ability to interfere with the functions of the brain and the body by direct and indirect means.”
• "Not only do fluorides [adversely] affect transmitter concentrations and functions but also are involved in the regulation of glucagon, prostaglandins, and a number of central nervous system peptides including vasopressin… and other hypothalamic peptides."
• "Fluorides also increase the production of free radicals in the brain through several different biological pathways. These changes have a bearing on the possibility that fluorides act to increase the risk of developing Alzheimer’s disease."

These findings and dozens more like them were published in the 507-page NRC report.

UPDATE: There are now 78 out of 87 IQ studies reporting lowered IQ from exposure to elevated levels of fluoride. Since 2017 we learned that the fluoride level of 0.7 mg/L, the "optimal" level used in U.S. and Canadian drinking water fluoridation projects, can create neurodevelopmental harm to the fetus, bottle-fed infant, and child. The fetus and bottle-fed infant were never considered in any risk assessment for water fluoridation by any regulatory agency in any fluoridating country. You can find the Fluoride Action Network's summary of these studies here.

In 2023 research out of York University found that fluoride exposure via drinking water may increase the risk of hypothyroidism in pregnant women. In a smaller subset of participants, the researchers found lower IQ scores between boys whose mothers had been diagnosed with hypothyroidism compared to boys whose mothers had normal thyroid levels.

Also, in a study published here in the Journal of the American Medical Association in May 2024, it was concluded prenatal fluoride exposure was associated with increased neurobehavioral problems in children living in the optimally fluoridated area of Los Angeles. The findings suggested there may be a need to establish recommendations for limiting fluoride exposure during the prenatal period.

And more recently, an August 2024 report from the National Toxicity Program of the U.S. Department of Health and Human Services concluded that fluoride in drinking water at twice the recommended limit is linked with lower IQ in children.  

Lead is a toxic metal found in natural deposits. The major sources of lead in drinking water are corrosion of household plumbing systems and erosion of natural deposits. Lead can leach into water primarily from pipes, but brass fixtures, faucets and fittings can also be a source. The amount of lead in your water also depends on the types and amounts of minerals in the water, how long the water stays in the pipes, the amount of wear in the pipes, the water’s acidity and its temperature.

Infants and children who drink water containing lead in excess of EPA standards can experience delays in their physical or mental development, and children can show slight deficits in attention span and learning abilities. Adults who drink this water over many years can develop kidney problems or high blood pressure.
 
Mercury is a liquid metal found in natural deposits such as ores containing other elements. The major sources of mercury in drinking water are erosion of natural deposits, discharge from refineries and factories and runoff from landfills and croplands.

Long-term exposure to mercury in excess of EPA standards may result in kidney damage.

Nitrates and Nitrites are nitrogen-oxygen chemical units which combine with various organic and inorganic compounds. The major sources of nitrates and nitrites in drinking water are runoff from fertilizer use; leaking from septic tanks, sewage; and erosion of natural deposits.

Infants below six months who are exposed to nitrates and nitrites in excess of EPA standards can become seriously ill and, if untreated, may die. Symptoms include shortness of breath and “blue baby syndrome”.

Selenium is a metal found in natural deposits such as ores containing other elements. The major sources of selenium in drinking water are discharge from petroleum and metal refineries, erosion of natural deposits and discharge from mines.

Long-term exposure to selenium in excess of EPA standards may result in hair or fingernail losses, numbness in fingers or toes, or circulation problems.

Thallium is a metal found in natural deposits such as ores containing other elements. The major sources of thallium in drinking water are leaching from ore-processing sites; and discharge from electronics, glass, and drug factories.

​Long-term exposure to thallium in excess of EPA standards may result hair loss, blood changes or kidney, intestine or liver problems.

Organic Chemicals

 number of organic chemicals can be found our drinking water. Most of these contaminants are man-made chemicals and byproducts. Maximum allowable contaminant standards have been established by EPA for the following fifty-three organic chemicals:

Acrylamide is a chemical commonly added to water during treatment of drinking water and waste water. The major source acrylamide in drinking water is as a residual when used as a coagulant aid in water treatment.

Long-term exposure to acrylamide in excess of EPA standards may result in nervous system or blood problems and increased risk of cancer.

Alachor is a common herbicide used for control of annual grasses and broadleaf weeds in crops, primarily on corn, sorghum, and soybeans. It enters water as run-off when used as a herbicide used on row crops.

Long-term exposure to alachor in excess of EPA standards may result in eye, liver, kidney or spleen problems, anemia and increased risk of cancer.

Atrazine is a widely used herbicide for control of broadleaf and grassy weeds. Effective in 1993, its uses were greatly restricted. It enters water as run-off when used as a herbicide used on row crops.

Long-term exposure to atrazine in excess of EPA standards may result in cardiovascular system or reproductive difficulties.

Benzene is a volatile, highly-flammable organic chemical formed through natural processes, such as volcanoes and forest fires. It’s also formed from industrial processes.

Long-term exposure to benzene in excess of EPA standards may result in anemia or a decrease in blood platelets and increased risk of cancer.

Benzo(a)pyrene is one of a group of compounds called polycyclic aromatic hydrocarbons. The major source of benzo(a)pyrene in drinking water is leaching from linings of water storage tanks and distribution lines.

Long-term exposure to benzo(a)pyrene in excess of EPA standards may result in reproductive difficulties and increased risk of cancer.

Carbofuran is a broad spectrum insecticide that is sprayed directly onto soil and plants just after emergence to control beetles, nematodes and rootworm. The greatest use of carbofuran is on alfalfa and rice, with turf and grapes making up most of the remainder. The major source of carbofuran in drinking water is leaching of soil fumigant used on rice and alfalfa.

Long-term exposure to carbofuran in excess of EPA standards may result in problems with blood or nervous or reproductive systems.

Carbon tetrachloride is a clear organic liquid used to make chlorofluorocarbon propellants and refrigerants, though this has been declining steadily. It has also been used as a dry cleaning agent and fire extinguisher, in making nylons and as a solvent for rubber cement, soaps, insecticides, etc. The major sources of carbon tetrochloride in drinking water are discharge from chemical plants and other industrial activities.

Long-term exposure to carbon tetrochloride in excess of EPA standards may result in liver problems and increased risk of cancer.

Chlordane is a synthetic organic chemical used in the past to kill a variety of insects. The only commercial use of chlordane products still permitted is for fire ant control in power transformers. Chlordane has been released into the environment primarily from its application as an insecticide and may persist for long periods of time in air, soil and water.

Long-term exposure to chlordane in excess of EPA standards may result in liver or nervous system problems and increased risk of cancer.

Chlorobenzene is a colorless organic liquid used in the manufacture of other organic chemicals, dyestuffs and insecticides. It is also used a solvent for adhesives, drugs, rubber, paints and dry cleaning, and as a fiber-swelling agents in textile processing. The major source of chlorobenzene in drinking water is discharge from chemical and agricultural chemical factories.

Long-term exposure to chlorobenzene in excess of EPA standards may result in problems with liver or kidneys.
 
2,4-D is a colorless, odorless powder used as a herbicide for the control of broad-leaf weeds in agriculture, and for control of woody plants along roadsides, railways, and utilities rights of way. It enters water as run-off when used as a herbicide on row crops.

Long-term exposure to 2,4-D in excess of EPA standards may result in problems with kidneys, liver, or adrenal glands.

Dalapon is a colorless liquid used as a herbicide to control grasses in a wide variety of crops, including fruit trees, beans, coffee, corn, cotton and peas. It is also registered for use in a number of non-crop applications such as lawns, drainage ditches, along railroad tracks, and in industrial areas. The major source of dalapon in drinking water is runoff when used as a herbicide on rights of way.

Long-term exposure to dalapon in excess of EPA standards may result in minor kidney changes.

1,2-Dibromo-3-chloropropane, or DBCP, is an organic liquid used primarily as an unclassified nematocide for soil fumigation of cucumbers, summer squash, cabbage, cauliflower, carrots, snap beans, okra, aster, shasta daisy, lawn grasses and ornamental shrubs. The major source of 1,2-dibromo-3-chloropropane in drinking water is runoff and leaching when used as a soil fumigant on soybeans, cotton, pineapples and orchards.

Long-term exposure to DBCP in excess of EPA standards may result in reproductive difficulties and increased risk of cancer.

ortho-Dichlorobenzene is a colorless organic liquid used as a chemical intermediate for making agricultural chemicals, primarily herbicides. Other present and past uses include solvent for waxes, gums, resins, wood preservatives, paints; insecticide for termites and borers, in making dyes and as a coolant, deodorizer or degreaser. The major source of o-dichlorobenzene in drinking water is discharge from industrial chemical factories.

Long-term exposure to o-dichlorobenzene in excess of EPA standards may result in problems with liver, kidneys or circulatory systems.
 
para-Dichlorobenzene is an organic solid used mainly as an insecticidal fumigant against clothes moths and as a deodorant for garbage and restrooms. It is also used as an insecticide and fungicide on crops, and in the manufacture of other organic chemicals and in plastics, dyes, and pharmaceuticals. The major source of p-dichlorobenzene in drinking water is discharge from industrial chemical factories.

Long-term exposure to p-dichlorobenzene in excess of EPA standards may result in anemia, damage to liver, kidneys or spleen or changes in blood.

1,2-Dichloroethane is a colorless, oily, organic liquid used in making chemicals involved in plastics, rubber and synthetic textile fibers. Other uses include as a solvent for resins and fats, photography, photocopying, cosmetics, drugs and as a fumigant for grains and orchards. The major source of 1,2-dichloroethane in drinking water is discharge from industrial chemical factories.

Long-term exposure to 1,2-dichloroethane in excess of EPA standards may result in increased risk of cancer.

1,1-Dichloroethylene is an organic liquid used in making adhesives, synthetic fibers, refrigerants, food packaging and coating resins such as the saran types. The major source of 1,1-dichloroethylene in drinking water is discharge from industrial chemical factories.

Long-term exposure to 1,1-dichloroethylene in excess of EPA standards may result in liver problems.

cis-1,2-Dichloroethylene is an odorless organic liquid that has two slightly different forms, a "cis" form and a "trans" form. Both the cis and trans forms — usually as a mixture — are used as a solvent for waxes and resins; in the extraction of rubber, as a refrigerant, in the manufacture of pharmaceuticals and artificial pearls, in the extraction of oils and fats from fish and meat and in making other organics. The major source of cis-1,2-dichloroethylene in drinking water is discharge from industrial chemical factories.

Long-term exposure to cis-1,2-dichloroethylene in excess of EPA standards may result in liver problems.

trans-1,2-Dichloroethylene is an odorless organic liquid that has two slightly different forms, a "cis" form and a "trans" form. Both the cis and trans forms — usually as a mixture — are used as a solvent for waxes and resins; in the extraction of rubber, as a refrigerant, in the manufacture of pharmaceuticals and artificial pearls, in the extraction of oils and fats from fish and meat and in making other organics. The major source of trans-1,2-dichloroethylene in drinking water is discharge from industrial chemical factories.

Long-term exposure to trans-1,2-dichloroethylene in excess of EPA standards may result in liver problems.

Dichloromethane, also known as DCM and methylene chloride, is a volatile organic and colorless liquid. The greatest use of DCM is as a paint remover. Other uses include solvent and cleaning agent in chemical manufacture, textiles, electronics, metals and plastics, pesticides industries; blowing and cleaning agent in the urethane foam industry; fumigant for strawberries and grains, and as degreener for citrus fruits, in pharmaceuticals and as an anesthetic, in extraction of caffeine, cocoa, fats, spices and beer hops and as a heat transfer agent in refrigeration products. The major source of dichloromethane in drinking water is discharge from drug and chemical companies.

Long-term exposure to dichloromethane in excess of EPA standards may result in liver problems and increased risk of cancer.

1,2-Dichloropropane is a colorless, flammable liquid used to make other organic chemicals. It is also used in making lead free gasoline, paper coating, soil fumigant for nematodes, and insecticide for stored grain. The major source of 1,2-dichloropropane in drinking water is discharge from industrial chemical factories. It may be released into the atmosphere or in wastewater during its production or use as an intermediate in chemical manufacture. There were also significant releases during its former use as a soil fumigant. It may also leach from municipal landfills.

Long-term exposure to 1,2-dichloropropane in excess of EPA standards may result in liver problems and increased risk of cancer.

Di(2-ethylhexyl) adipate is a light-colored, oily liquid used in making plastics. It is also used as a solvent, in aircraft lubricants, as a hydraulic fluid, as a plasticizer or solvent in the following cosmetics: bath oils, eye shadow, cologne, foundations, rouge, blusher, nail polish remover, moisturizers and indoor tanning preparations and in meat wrapping operations. The major source of di(2-ethylhexyl) adipate in drinking water is discharge from chemical factories.

Long-term exposure to di(2-ethylhexyl) adipate in excess of EPA standards may result in toxic effects such as weight loss, liver enlargement, or possible reproductive difficulties.

Di(2-ethylhexyl) phthalate is the most commonly used of a group of related chemicals called phthalates or phthalic acid esters. The greatest use of di(2-ethylhexyl) phthalate is as a plasticizer for polyvinylchloride (PVC) and other polymers including rubber, cellulose and styrene. A number of packaging materials and tubings used in the production of foods and beverages are polyvinylchloride contaminated with phthalic aced esters, primarily di(2-ethylhexyl) phthalate. The major source of di(2-ethylhexyl) phthalate in drinking water is discharge from rubber and chemical factories.

Long-term exposure to di(2-ethylhexyl) phthalate in excess of EPA standards may result in liver problems, reproductive difficulties and increased risk of cancer.

Dinoseb is an organic solid used as a contact herbicide for post-emergence weed control in cereals, undersown cereals, seedling lucerne and peas. Dinoseb is also used as a corn enhancer and an insecticide and miticide. The major source of dinoseb in drinking water is runoff when used as an herbicide on soybeans and vegetables.

Long-term exposure to dinoseb in excess of EPA standards may result in reproductive difficulties.

Dioxin is an organic solid and is not produced or used commercially in the United States. It is a contaminant formed in the production of some chlorinated organic compounds, including a few herbicides such as silvex. It may also be formed during combustion of a variety of chlorinated organic compounds, including a few herbicides such as silvex. It may also be formed during combustion of a variety of chlorinated organic compounds. The major sources of dioxin in drinking water are emissions from waste incineration and other combustion and discharge from chemical factories.

Long-term exposure to dioxin in excess of EPA standards may result in reproductive difficulties and increased risk of cancer.

Diquat is an organic solid used extensively in the United States as a herbicide since the late 1950s to control both crop and aquatic weeds. It is used on potatoes, as an aid in harvesting cotton, rapeseed and other oil seed crops, to wilt and dry out silage, standing hay, etc., for storage and as a plant growth regulator and sugar cane-flowering suppressant. The major source of diquat in drinking water is runoff when used as a herbicide.

Long-term exposure to diquat in excess of EPA standards may result in formation of cataracts.

Endothall is an organic solid used as a defoliant for a wide range of crops and as a herbicide for both terrestrial and aquatic weeds. It is used as a desiccant on lucerne and on potato, for the defoliation of cotton, to control aquatic weeds and as an aquatic algicide growth regulator. It has been used for sugar beets, turf, hops sucker suppression; alfalfa, clover desiccants; potato vine killers. The major source of endothall in drinking water is runoff when used as a herbicide.

Long-term exposure to endothall in excess of EPA standards may result in problems with the stomach or intestines.

Endrin is an insecticide which has been used mainly on field crops such as cotton, maize, sugarcane, rice, cereals, ornamentals, and other crops. It has also been used for grasshoppers in non-cropland and to control voles and mice in orchards. Once widely used in the United States, most uses were cancelled in 1980. The major source of endrin in drinking water is residue use of the banned insecticide.

Long-term exposure to endrin in excess of EPA standards may result in liver problems.

Epichlorohydrin is a colorless organic gas used for making glycerine and as a monomer/building block for making plastics and other polymers, some of which are used as coagulant aids in water treatment. It is also used in the paper and drug industries as an insect fumigant. The major sources epichlorohydrin in drinking water is discharge from chemical factories and as a residual when used as a coagulant aid in water treatment.

Long-term exposure to epichlorohydrin in excess of EPA standards may result in stomach problems and increased risk of cancer.

Ethylbenzene is a colorless organic liquid. The greatest use — more than 99 percent — of ethylbenzene is to make styrene, another organic liquid used as a building block for many plastics. It is also used as a solvent for coatings, and in the making of rubber and plastic wrap. The major source of ethylbenzene in drinking water is discharge from petroleum refineries.

Long-term exposure to ethylbenzene in excess of EPA standards may result in liver or kidney problems.

Ethylene dibromide, also known as EDB, and 1,2-Dibromoethane, is a colorless, heavy synthetic organic liquid used in an anti-knock gasoline mixtures, particularly aviation fuel. EDB is released during the use, storage, and transport of leaded gasoline, as well as during any spills, from its former use as a pesticide, wastewater and emissions from processes and waste waters of the chemical industries that use it. Under certain soil and climatic conditions, EDB may get into drinking water by runoff into surface water or by leaching into ground water.

Long-term exposure to ethylene dibromide in excess of EPA standards may result in problems with liver, stomach, the reproductive system or kidneys and increased risk of cancer.

Glyphosate is a non-selective herbicide used on many food and non-food crops as well as non-crop areas such as roadsides. When applied at lower rates, it serves as a plant growth regulator. The most common uses include control of broadleaf weeds and grasses in: hay/pasture, soybeans, field corn; ornamentals, lawns, turf, forest plantings, greenhouses, rights-of-way. The major source of glyphosate in drinking water is runoff when used as a herbicide.

Long-term exposure to glyphosate in excess of EPA standards may result in kidney problems or reproductive difficulties -- and recent research indicates that glyphosate may be linked to several forms of cancer.

Heptachlor is an insecticide that breaks down in the environment to form heptachlor epoxide. Most uses of heptachlor to kill termites in homes and insects on farm crops were canceled in 1978. The only permitted use of heptachlor products is for fire ant control in buried pad-mounted electric power transformers, and in underground cable television and telephone cable boxes. Heptachlor may be released directly to the soil in connection with its use. It has also been found in treated wastewater from some types of industrial facilities.

Long-term exposure to heptachlor in excess of EPA standards may result in liver damage and increased risk of cancer.

Heptachlor epoxide is formed when heptachlor breaks down in the environment. Most uses of heptachlor to kill termites in homes and insects on farm crops were canceled in 1978. The only permitted use of heptachlor products is for fire ant control in buried pad-mounted electric power transformers, and in underground cable television and telephone cable boxes. Heptachlor epoxide adsorbs strongly to soil, but is extremely resistant to biodegradation, persisting for many years in the upper soil layers. Similarly in water, heptachlor epoxide will persist, usually in sediments. It is concentrated extensively in aquatic life.

Long-term exposure to heptachlor epoxide in excess of EPA standards may result in liver damage and increased risk of cancer.

Hexachlorobenzene is a synthetic organic chemical. Currently there are no commercial uses of hexachlorobenzene in the United States, but it was widely used as a pesticide until 1965.  It also was used previously as a fungicide, to make fireworks, ammunition rubber and wood preservatives. It breaks down very slowly and still persists in the environment. Small particles stick to soil and remain in sediments in the bottoms of water bodies.

Long-term exposure to hexachlorobenzene in excess of EPA standards may result in liver or kidney problems, reproductive difficulties and increased risk of cancer.

Hexachlorocyclopentadiene is an organic liquid used as a raw material in manufacturing other chemicals, including pesticides, flame retardants, resins, dyes, pharmaceuticals, plastics, etc. It has no end uses of its own. The major source of hexachlorocyclopentadiene is discharge from chemical factories.

Long-term exposure to hexachlorocyclopentadiene in excess of EPA standards may result in kidney or stomach problems.

Lindane is an insecticide whose uses were restricted in 1983. It’s currently used primarily for treating wood-inhabiting beetles and seeds. It’s also used as a dip for fleas and lice on pets, and livestock, for soil treatment, on the foliage of fruit and nut trees, vegetables, timber and ornamentals and for wood protection. The major source of lindane in drinking water is runoff and leaching when used as an insecticide on cattle, lumber and gardens.

Long-term exposure to lindane in excess of EPA standards may result in kidney or liver problems.

Methoxychlor is an insecticide preferred to DDT for use on animals, in animal feed, and on DDT-sensitive crops such as squash, melons, etc. Since methoxychlor is more unstable than DDT, it has less residual effect. It has been used extensively in Canada for the control of biting flies, and is also effective against mosquitoes and houseflies. The major source of methoxychlor in drinking water is runoff and leaching when used as an insecticide on cattle, lumber and gardens.

Long-term exposure to methoxychlor in excess of EPA standards may result in reproductive difficulties.

Oxamyl is a synthetic organic chemical and is the chemical ingredient identified in widely used insecticides and nematicides, for example Vydate. EPA has classified most products containing oxamyl as restricted use pesticides intended for occupational use only for control of insects, mites and nematodes on field crops, fruits and ornamentals. Major sources of oxamyl in drinking water are runoff and leachate from its use as an insecticide and leakage, poor storage or improper disposal practices related to its use and manufacture.

Long-term exposure to oxamyl in excess of EPA standards may result in slight nervous system effects.

Per and polyfluoroalkyl substances – known as PFAS are a group of manufactured chemicals that have been used in industry and consumer products since the 1940s because of their useful properties. There are thousands of different PFAS, some of which have been more widely used and studied than others. One common characteristic of concern of PFAS is that many break down very slowly and can build up in people, animals, and the environment over time. Thus, these chemicals are commonly known as "forever chemicals". 

Current peer-reviewed scientific studies have shown that exposure to certain levels of PFAS may lead to: 

• Reproductive effects such as decreased fertility or increased high blood pressure in pregnant women.
• Developmental effects or delays in children, including low birth weight, accelerated puberty, bone variations, or behavioral changes.
• Increased risk of some cancers, including prostate, kidney, and testicular cancers.
• Reduced ability of the body’s immune system to fight infections, including reduced vaccine response.
• Interference with the body’s natural hormones.
• Increased cholesterol levels and/or risk of obesity.​

UPDATE: In April 2024, EPA amended the Primary Drinking Water Standards to include regulation of a long list of PFAS. These regulations become effective on June 25, 2024. A detailed description of the new PFAS rule-making can be found here. ​

Polychlorinated biphenyls (PCBs) are a group of organic chemicals that were formerly used in the United States as hydraulic fluids, plasticizers, adhesives, fire retardants, way extenders, de-dusting agents, pesticide extenders, inks, lubricants, cutting oils, in heat transfer systems, carbonless reproducing paper. The major sources of polychlorinated biphenyls in drinking water are runoff from landfills and discharge of waste chemicals.

Long-term exposure to PCBs in excess of EPA standards may result in skin changes, thymus gland problems, immune deficiencies, reproductive or nervous system difficulties and increased risk of cancer.

Pentachlorophenol is a white organic solid. The greatest use of pentachlorophenol is as a wood preservative (fungicide). Though once widely used as a herbicide, it was banned in 1987 for these and other uses, as well as for any over-the-counter sales. The major source of pentachlorophenol in drinking water is discharge from wood-preserving factories.

Long-term exposure to pentachlorophenol in excess of EPA standards may result in liver or kidney and increased risk of cancer.

Picloram is a synthetic organic chemical used in salt form as a systemic herbicide for controlling annual weeds in crops, and in combination with 2,4-D or 2,4,5-T against perennials on non-croplands for brush control. The major source of picloram in drinking water is herbicide runoff. Picloram does not adhere to soil and has been found in ground water.

Long-term exposure to picloram in excess of EPA standards may result in liver problems.

Simazine is a herbicide used as a pre-emergence herbicide for control of broad-leaved and grassy weeds on a variety of deep-rooted crops such as artichokes, asparagus, berry crops, broad beans, citrus, etc., and on non-crop areas such as farm ponds and fish hatcheries. Its major use is on corn where it is often combined with AAtrex. Other herbicides with which simazine is combined include: paraquat, on apples, peaches; Roundup or Oust for noncrop use; Surflan on Christmas trees; Dual on corn and ornamentals. The major source of simazine in drinking water is herbicide runoff.

Long-term exposure to simazine in excess of EPA standards may result in blood problems.

Styrene is an oily organic liquid. Initially, it was used primarily in the synthetic rubber industry, but it’s currently used as a building block for polymers in making plastics, resins, coatings, and paints. The major sources of styrene in drinking water are discharge from rubber and plastic factories and leaching from landfills.

Long-term exposure to styrene in excess of EPA standards may result in liver, kidney or circulatory system problems.

Tetrachloroethylene is a colorless organic liquid. The greatest use of tetrachloroethylene is in the textile industry, and as a component of aerosol dry-cleaning products. The major source of tetrachloroethylene in drinking water is discharge from factories and dry cleaners.

Long-term exposure to tetrachloroethylene in excess of EPA standards may result in liver problems and increased risk of cancer.

Toluene is an organic liquid used primarily to make benzene and urethane. The major source of toluene in drinking water is discharge from petroleum factories.

Long-term exposure to toluene in excess of EPA standards may result in nervous system, kidney or liver problems.

Toxaphene is a synthetic organic chemical and was used as an insecticide for cotton and vegetables, and on livestock and poultry. In 1982, most of its uses were banned and in 1990, all uses were banned in the United States. Major sources of toxaphene in drinking water are runoff or leaching. It is very persistent, remaining in soil for up to 14 years.

Long-term exposure to toxaphene in excess of EPA standards may result in kidney, liver or thyroid problems and increased risk of cancer.

2,4,5-TP (Silvex) is an organic herbicide. Until Silvex was banned in 1985, the greatest use of 2,4,5-TP was as a postemergence herbicide for control of woody plants and broadleaf herbaceous weeds in rice and bluegrass turf, in sugarcane, in rangeland improvement programs and on lawns. Aquatic uses included control of weeds in ditches and riverbanks, on floodways, along canals, reservoirs, streams and along southern waterways. The major sources of 2,4,5-TP in drinking water are runoff or leaching of the banned herbicide residue.

Long-term exposure to 2,4,5-TP in excess of EPA standards may result in liver problems.

1,2,4-Trichlorobenzene is an aromatic, colorless organic liquid. The greatest use of 1,2,4-trichlorobenzene is primarily as a dye carrier. It is also used to make herbicides and other organic chemicals, as a solvent, in wood preservatives and in abrasives. It was once used as a soil treatment for termite control. The major source of 1,2,4-trichlorobenzene in drinking water is discharge from textile finishing factories.

Long-term exposure to 1,2,4-trichlorobenzene in excess of EPA standards may result in changes in adrenal glands.

1,1,1-Trichloroethane is an organic liquid largely used as a solvent for removing grease from machined metal products, in textile processing and dyeing and in aerosols. The major source of 1,1,1-trichloroethane in drinking water is discharge from metal degreasing and other factories.

Long-term exposure to 1,1,1-trichloroethane in excess of EPA standards may result in liver, nervous system or circulatory system problems.

1,1,2-Trichloroethane is a volatile organic chemical used to make vinylidene chloride which is in turn used to make synthetic fibers and plastic wraps. It is also used in adhesives, production of Teflon tubing, in lacquer and coating formulations and as a solvent for fats and oils. 1,1,2-trichloroethane evaporates during its use in manufacturing other products and as a solvent. It is also released in wastewater from these uses, and in leachates and volatile emissions from landfills.

Long-term exposure to 1,1,2-trichloroethane in excess of EPA standards may result in liver, kidney or immune system problems.

Trichloroethylene is a volatile organic chemical primarily used to remove grease from fabricated metal parts and in the production of some textiles. The major source of trichloroethylene in drinking water is discharge from metal degreasing sites and other factories. Wastewater from metal finishing, paint and ink formulation, electrical components and rubber processing industries may also contain trichloroethylene.

Long-term exposure to trichloroethylene in excess of EPA standards may result in liver problems and increased risk of cancer.

Vinyl chloride is a colorless organic gas used in the manufacture of numerous products in building construction, automotive industry, electrical wire insulation and cables, piping, industrial and household equipment and medical supplies, and is depended upon heavily by the rubber, paper, and glass industries. The major sources of vinyl chloride in drinking water are leaching from polyvinyl chloride (PVC) piping; and discharge from plastics factories.

Long-term exposure to vinyl chloride in excess of EPA standards may result in increased risk of cancer.

Xylenes are any of a group of very similar organic compounds. The greatest use of xylenes is as a solvent which is much safer than benzene. Xylene mixtures are used to make phthalate plasticizers, polyester fiber, film and fabricated items and are used in gasoline as part of the BTX component (benzene-toluene-xylene). The major sources of xylenes in drinking water are discharge from petroleum factories; and discharge from chemical factories.

​Long-term exposure to xylenes in excess of EPA standards may result in damage to the nervous system.

Radionuclides

Certain rock types have naturally occurring trace amounts of "mildly radioactive" elements (radioactive elements with very long half-lives) that serve as the "parent" of other radioactive contaminants ("daughter products"). These radioactive contaminants, depending on their chemical properties, may accumulate in drinking water sources at levels of concern. Maximum allowable contaminant standards have been established by EPA for the following:
 
Alpha Particles are emitted by certain radioactive minerals as they decay. Long-term exposure to alpha radiation in excess of EPA standards may result in increased risk of cancer.
 
Beta Particles & Photons are emitted by certain radioactive minerals as they decay. Long-term exposure to beta and photon radiation in excess of EPA standards may result in increased risk of cancer.

Radium 226 and 228 (combined) are radioisotopes that emit alpha particles (radium 226) and beta particles (radium 228) as they decay. Radium 226 and 228 enter drinking water as a result of erosion of natural mineral deposits. Long-term exposure to radium 226 and 228 emissions in excess of EPA standards may result in increased risk of cancer.

Uranium is a naturally-occurring mineral and is slightly radioactive. Uranium enters drinking water as a result of erosion of natural mineral deposits. Exposure to uranium in drinking water may result in toxic effects to the kidney. Long-term exposure to alpha emitters, such as uranium, in excess of EPA standards, may result in increased risk of cancer.

"Secondary" Contaminants

In addition to the contaminants for which EPA has established national primary standards for drinking water quality, EPA also has established “secondary” standards for some fifteen contaminants. These secondary standards have been established only as guidelines to assist public water systems in managing their drinking water for aesthetic considerations, such as taste, color and odor.

There are a wide variety of problems related to secondary contaminants. These problems can be grouped into three categories: Aesthetic effects—undesirable tastes or odors; Cosmetic effects—effects which do not damage the body but are still undesirable; and Technical effects—damage to water equipment or reduced effectiveness of treatment for other contaminants.

The “secondary” contaminants and their noticeable effects when at levels in excess of the EPA guidelines are:

Aluminum -- colored water
Chloride -- salty taste
Color -- visible tint
Copper -- metallic taste; blue-green staining
Corrosivity -- metallic taste; corroded pipes/ fixtures staining
Fluoride -- tooth discoloration/ thyroid issues
Foaming agents -- frothy, cloudy; bitter taste; odor
Iron -- rusty color; sediment; metallic taste; reddish or orange staining
Manganese -- black to brown color; black staining; bitter metallic taste
Odor -- "rotten-egg", musty or chemical smell
pH -- low pH: bitter metallic taste; corrosion/high pH: slippery feel; soda taste; deposits
Silver -- skin discoloration; graying of the white part of the eye
Sulfate -- salty taste
Total Dissolved Solids (TDS) -- hardness; deposits; colored water; staining; salty taste
Zinc -- metallic taste