Arsenic in Private Wells: Facts & Figures

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Some groundwater has arsenic levels of health concern

Starting in 2008, all new private wells in Minnesota must be tested for arsenic. Data from 2008 to 2015 show that on average 10.7 percent of these wells have arsenic levels above 10 μg/L.

Arsenic (As) concentration in MN private wells

Data are from newly-constructed wells, Aug 2008 - Dec 2015.
Data are from newly-constructed wells, Aug 2008 - Dec 2015.

How much arsenic is too much?

The Maximum Contaminant Level (MCL) for arsenic in community water systems is 10 μg/L. Community water systems must keep arsenic levels below the MCL, but private well owners must decide whether to test and reduce arsenic levels in their water. MDH recommends that people don't drink or cook with water if arsenic levels are above 10 μg/L. Even exposure to arsenic levels below this can increase cancer risk. For  more about health risks from arsenic, contact the MDH Well Management Program

Geology shows where arsenic may be high

Arsenic can occur in groundwater almost anywhere in Minnesota. High levels are most likely to occur from the Twin Cities and west, to the border with the Dakotas. However, arsenic levels can vary from one well to the next, even if they are close to each other. If you or your family drinks from a private well, it's best to test your well for arsenic. See tips at private well testing

Most arsenic in Minnesota is from geologic materials left by glaciers thousands of years ago. The most recent glaciation in Minnesota occurred about 14,000 years ago, and left a clay-rich material called the Des Moines lobe till. Wells within the area once covered by this glacier are more likely to have high levels of arsenic. 

Regional geology and arsenic levels

Regional geology and arsenic levels
Note: Adapted with permission from Erickson and Barnes (2005).

Arsenic can be harmful to your health

Long-term exposure to arsenic, even at low levels, is associated with increased risks for certain cancers and other diseases and conditions. The level of risk depends on many factors, including the type of arsenic, the length and level of exposure, and individual susceptivility - which can depend on things like age and genetics. 

Most arsenic in water and soil is from natural sources

Arsenic occurs naturally in soil and certain kinds of rock. Arsenic can dissolve into groundwater, the main source of drinking water for much of Minnesota. It has no smell or taste, so water must be tested in a laboratory to see if it has arsenic. Groundwater in the west-central and northwestern parts of Minnesota tends to have higher levels of arsenic, though arsenic can be found in groundwater throughout the State.

Some arsenic in the environment comes from human activity. It was once an ingredient in some pesticides and was used as a wood preservative for things like wood foundations, decks, and children's play structures.

Test your well for arsenic

The MDH Well Management Program protects both public health and groundwater by assuring the proper construction of new wells and borings, and the proper sealing of unused wells and borings.  In addition, they provide information to the public about testing private wells. For more information about arsenic in well water, see Arsenic in Minnesota's Well Water. For more information on how to reduce high levels of arsenic, see Reducing Arsenic in Drinking Water.

For well owners, the only way to know if a well contains arsenic is to test the water. Water testing services are available from state and county health agencies and private laboratories. To ensure that a laboratory is certified by MDH to test drinking water for arsenic, see How to Search for Accredited Laboratories.

Also see the U.S. Environmental Protection Agency's Arsenic in Your Drinking Water: Just the Facts for Consumers for additional information.

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