Arsenic in Private Wells: Facts & Figures

Arsenic can be harmful to your health

Long-term exposure to arsenic, even at low levels, is associated with adverse health effects, including cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate. Other possible health effects include cardiovascular disease, developmental and reproductive effects, diabetes, and skin changes. The risks from exposure to arsenic depend on many factors, including the type of arsenic, the duration and level of exposure, and individual susceptibility, which can depend on things like age and genetics. 

Most arsenic in the 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.

How much arsenic is too much?

The Maximum Contaminant Level (MCL) for arsenic in community water systems is 10 micrograms per liter (μg/L). While community water systems must keep arsenic levels below the MCL, private wells are not required to meet this standard. It is up to each well owner to decide whether to take steps to reduce levels of arsenic in their water. The Minnesota Department of Health (MDH) recommends that people don't use water with more than 10 μg/L of arsenic for drinking or cooking.

Exposures to levels of arsenic below 10 μg/L may also increase risks for cancer depending on the amount and duration of arsenic exposure over time, and individual susceptibility. For questions or additional information about the health risks from arsenic in private well water, contact the MDH Well Management Program

Some MN 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 2013 show that on average, 10.4 percent of these wells have arsenic levels above 10 μg/L. A few have levels as high as 300 μg/L.

Private wells in Minnesota, by arsenic concentration

Arsenic concentrations:

Data are from tests of newly-constructed wells.

Geology shows where arsenic levels are likely to 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).

Testing 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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