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Last Edited: 8/11/10

Drinking Water Contaminants: Arsenic

For more information on arsenic including where it comes from, how you can be exposed to it, and the type of health effects it can have, view Environmental Protection Agency's (EPA) fact sheet on arsenic (PDF).  

View data on arsenic levels in drinking water for California.


What is arsenic and where does it come from?

Arsenic is a toxic element that occurs naturally in the environment.  It can be found in soil, rocks, and minerals.  Arsenic cannot be destroyed; it can only change its form. In the natural environment, arsenic combines with oxygen, chlorine, and sulfur to form inorganic arsenic compounds. Inorganic arsenic naturally occurs in the earth’s crust and soil in a wide range of concentrations.  Arsenic absorbed or ingested by animals and plants combines with carbon and hydrogen to form organic arsenic compounds. 

Arsenic is also a by-product of some agricultural and industrial activities.  Inorganic arsenic compounds are used as preservatives, mainly in wood. Some organic arsenic compounds are used as pesticides, primarily on cotton fields and in orchards.   

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How does arsenic get in our water?

Arsenic may enter the air, water, and land from wind-blown dust.  It can enter drinking water through the ground or as run-off into surface water sources.  Many common arsenic compounds can dissolve in water. Most of the arsenic in water will ultimately end up in soil or sediment. 

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What are the health effects of arsenic?

Arsenic has been identified as a human carcinogen by the International Agency for Research on Cancer.  Exposure to arsenic at a concentration of hundreds of micrograms per liter (µg/L) in the drinking water has been associated with lung, bladder, liver, and skin cancers in Taiwan, Chile, Argentina, Mexico, Bangladesh, and India.  Other adverse health effects of arsenic include nausea, cardiovascular disease, developmental and reproductive effects, diabetes, and skin keratosis and hyperpigmentation.

Some people who drink water containing arsenic in excess of EPA’s standard over many years could experience skin damage or problems with their circulatory system, and may have an increased risk of getting cancer.

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How much arsenic is allowed in drinking water?

Since 1977, the maximum concentration level (MCL) of arsenic allowed in drinking water was 50 micrograms (or 0.05 milligrams) per liter.  In 2001, US EPA reduced the MCL to 10 micrograms (or 0.010 milligrams) per liter (µg/L).  Water systems were required to comply with the new rule by 2006.  The US EPA has also set limits on the amount of arsenic that industrial sources can release into the environment and has restricted or stopped many of the uses of arsenic in pesticides. 

Compliance with the arsenic MCL is based on samples taken by water systems over a 9-year period (2002-2010).  The frequency of sampling depends on the water sources and ranges from yearly to once in 9 years.  If a sampling point exceeds 10 µg/L, that point is then sampled quarterly, and the running annual average is used to determine compliance.  MCL violations before and after the change in this rule are not comparable, which means violations prior to 2006 can not be compared to the ones after January 2006.

For more information on monitoring requirements, see the EPA's guide to the standardized monitoring framework (PDF).  See Drinking Water Monitoring for more information on how water samples are collected.

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How do I know if there is arsenic in my drinking water?

Overall, based on the current understanding of the health effects of arsenic, the potential for adverse health effects from drinking water exposure to arsenic in the United States is generally very low for most community water systems.

If your water comes from a municipal or privately-owned water company that meets the definition of a community water system, they are already testing for arsenic in your water.

If you have your own household water supply, you are responsible for maintaining and testing it. Contact your local health department to find out whether arsenic is a contaminant of concern in your area.  If it is, you may consider having your water tested by a certified lab and installing a home treatment device that can remove arsenic.

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