Arsenic in Community Water Systems

For Minnesota's Community Water Systems:

Some have arsenic levels above the MCL

Very few Community Water Systems (CWS) have arsenic levels above the Maximum Contaminant Level (MCL), but there are systems with arsenic at levels of concern.

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 the type of arsenic, the length and level of exposure, and individual susceptibility - which can depend on things like age and genetics. 

How much arsenic is too much?

The MCL for arsenic in CWSs is 10 micrograms per liter (µg/L). All systems must test for arsenic and other contaminants. If a system's arsenic levels are above the MCL, MDH will help the system reduce arsenic.


Mean arsenic levels in Minnesota Community Water Systems

Percent of tested systems, by mean arsenic concentration
The MCL is 10 µg/L. Concentrations are the mean annual levels for community water systems in Minnesota.
The MCL is 10 µg/L. Concentrations are the mean annual levels for community water systems in Minnesota.

The percent of systems in violation of the MCL is not equal to the percent of systems shown in the chart because whether a system is in violation is based on a running annual average at each entry point to the system, rather than a system-wide calendar year average. A number of systems in Minnesota are still working to bring their arsenic levels below the MCL.


Maximum arsenic levels of Minnesota Community Water Systems

Percent of tested systems, by maximum arsenic concentration
The MCL is 10 µg/L. Concentrations are the maximum annual levels for community water systems in Minnesota.
The MCL is 10 µg/L. Concentrations are the maximum annual levels for community water systems in Minnesota.

The chart above shows the percent of tested systems by maximum concentration category. A system may have a number of different entry points to the distribution system with different levels of arsenic. In this case, the maximum value reflects the highest concentration in any single sample from any entry point, prior to any blending of water. The level of arsenic in blended water that ultimately reaches the consumer's tap may be lower because of dilution from other entry points with lower levels of arsenic.


Most Minnesotans drink water with arsenic below the MCL

Compared to the total population of Minnesotans served by community water systems, the number of people served by systems with arsenic levels of concern in Minnesota is low. Note that the estimates of number of people served by each system are rough calculations periodically updated by water system managers. 


Population served

People served, by mean arsenic annual concentration of their Community Water Systems, 2015
Arsenic Systems Population served Percent
0 - 5 896 4,311,156 93.5%
5+ - 10 57 29,249 6.0%
10+ - 20 4 521 0.4%
20+ - 30 1 430 0.1%
30+ 0 0 0.0%

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 it can be found in water throughout Minnesota.

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.

What can be done about arsenic in drinking water?

Customers of community water systems can find out the level of arsenic in their drinking water by reading the Consumer Confidence Report (sometimes called the Water Quality Report) issued each year by their water utility. Those wishing to take extra precautions may install a home water treatment unit (point of use device). Distillation, reverse osmosis, columns of anion exchange, and columns of absorptive media may be used to reduce arsenic levels in drinking water.

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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