Drinking Water Contaminants


Arsenic occurs naturally in the earth's crust. Most arsenic in drinking water comes from natural rock formations. As water flows through these formations, it can dissolve arsenic and carry it into underground aquifers, streams, or rivers that may become drinking water supplies. Arsenic can also come from human activities, such as mining or smelting arsenic-containing ores. Learn more about arsenic in drinking water

View Arsenic Detections in Washington Public Water Supplies

Questions and Answers - Arsenic in Drinking Water (PDF)


Chromium, including hexavalent chromium, is an odorless and tasteless metallic element that occurs naturally in rocks, animals, plants, and soils. Learn more about chromium in drinking water


Coliform bacteria are organisms that are present in the environment and in the feces of all warm-blooded animals and humans. Coliform bacteria will not likely cause illness. However, their presence in drinking water indicates that disease-causing organisms (pathogens) could be in the water system. Learn more about coliform bacteria in drinking water


Copper is a mineral and natural component of soils. It is an essential nutrient for humans and plants. Industrial pollution, domestic wastewater, mining wastewater, and weathering of copper-bearing rocks are major sources of copper in surface and ground waters. Discharges of copper into sewer systems from some residential areas have also been found. In Washington, most copper in drinking water comes from corrosion of household plumbing. Learn more about copper in drinking water


Cryptosporidium is a one-celled parasite that can cause a gastrointestinal illness called cryptosporidiosis. It occurs in the feces of infected animals or humans. It is environmentally resistant and may survive outside the body for long periods of time. To become infected, a person must consume contaminated food or water, including from streams or rivers. Learn more about Cryptosporidium

Disinfectant Byproducts

What are disinfection byproducts and how are they formed? Chlorine is added to drinking water to kill or inactivate harmful organisms that cause various diseases. This process is called disinfection. However, chlorine is a very active substance. It reacts with naturally occurring substances to form compounds known as disinfection byproducts (DBPs). The most common DBPs formed when chlorine is used are trihalomethanes (THMs), and haloacetic acids (HAAs). Learn more about disinfectants and disinfection byproducts

Question and Answers - Disinfection Byproducts (PDF)


Lead in drinking water usually comes from water distribution lines or household plumbing rather than lakes, wells or streams. Lead from other sources, such as ingesting old-paint chips or dust, can add to the effects of lead in water. Lead poisoning is a particular problem because there may be no unique signs or symptoms associated with lead exposure. Early symptoms of lead poisoning may include loss of appetite, fatigue, irritability, anemia and, sometimes, abdominal pain. Because of the general nature of symptoms at this stage, lead poisoning is not often suspected. Learn more about lead in drinking water


Manganese is a naturally occurring mineral found in rocks, groundwater, and surface water. Small amounts of manganese are essential nutrients for humans. Your body needs some manganese to stay healthy, but too much can be harmful, especially to infants. Manganese in your water can also stain your laundry and create a brownish-black or black stain on your toilet, shower, bathtub, or sink. Manganese can make your water look, smell, or taste bad.

Manganese in Drinking Water: Recommendations for Public Water System Managers 331-741 (PDF 682KB)

Manganese in Drinking Water: What Customers Should Know 331-740 (PDF 564KB)


Nitrate is a chemical found in most fertilizers, manure, and liquid waste discharged from septic tanks. Natural bacteria in soil can convert nitrogen into nitrate. Rain or irrigation water can carry nitrate down through the soil into groundwater. Your drinking water may contain nitrate if your well draws from this groundwater. Nitrate is an acute contaminant. That means one exposure can affect a person's health. Learn more about nitrate in drinking water

Per- and Polyfluoroalkyl Substances (PFAS)

Per- and Polyfluoroalkyl Substances (PFAS) are a family of similar chemicals manufactured since the 1950s. PFAS chemicals have special properties that have made them useful for grease-proof and water-proof coatings such as coated papers used in food packaging, Scotchgard® used on textiles or Gore-Tex® used in waterproof fabrics. PFAS were also used to make durable non-stick surfaces such as Teflon® pans and medical devices. Learn more about PFAS