State-Wide Arsenic Data

About the Drinking Water Data

Tables on this page:

Table 1 below provides the number of Community Water Systems by annual mean arsenic concentration category. A total of 959 systems were active as of April 2010; the nine systems with no monitoring data from 1999-2009 were most likely not yet active.

The maximum contaminant level (MCL) for arsenic was lowered from 50 micrograms per liter (µg/L) to 10 µg/L in January 2006. This is reflected in the fewer number of systems with annual arsenic concentrations in the 10-<20 µg/L concentration category or higher over time. The number of systems in actual violation of the >10 µg/L MCL standard is not equivalent to the number of systems listed in the table with concentrations ≥10 µg/L, mainly because the MCL is based on a running annual average at each entry point to the distribution system rather than a system-wide calendar year average. Even so, a number of systems in Minnesota are still working to achieve compliance with the arsenic standard. According to the MDH Drinking Water Protection Program, approximately 40 community water systems had arsenic levels >10 µg/L when the maximum MCL was modified in January of 2006. By the end of 2009, 10 community water systems, including six municipal systems, still exceeded that level. The systems in exceedance are working with MDH to come into compliance and are also communicating regularly with their residents about the situation.

Annual mean arsenic concentrations in Minnesota Community Water Systems (CWS), 1999-2009

Year Cumulative Number of Community Water Systems Sampled, 1999-2009 Number of Community Water Systems by Concentration Category (µg/L)
<5 5 to <10 10 to <20 20 to <30 30+
1999 381 (40%)  361 10 6 2 2
2000 658 (69%) 609 30 11 3 5
2001 833 (87%) 758 43 19 8 5
2002 848 (88%) 769 52 15 6 6
2003 858 (90%) 770 56 21 7 4
2004 881 (92%) 790 59 21 8 3
2005 901 (94%) 812 60 18 9 2
2006 917 (96%) 829 65 17 4 2
2007 926 (97%) 844 63 14 5 0
2008 941 (98%) 863 60 12 1 5
2009 950 (99%) 871 57 16 2 4

*µg/L means micrograms per liter of water.


Table 2 below shows the number of Community Water Systems by annual maximum concentration category during years 1999-2009.  A Community Water System may have a number of different wells or 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.

Annual maximum arsenic concentrations in Minnesota Community Water Systems (CWS), 1999-2009

Year Cumulative Number of Community Water Systems Sampled, 1999-2009 Number of Community Water Systems by Concentration Category (µg/L)
<5 5 to <10 10 to <20 20 to <30 30+
1999 381 (40%) 354 14 9 2 2
2000 658 (69%) 595 41 14 3 5
2001 833 (87%) 742 57 19 10 5
2002 848 (88%) 758 56 18 6 10
2003 858 (90%) 761 57 24 8 8
2004 881 (92%) 779 61 26 9 6
2005 901 (94%) 802 60 24 9 6
2006 917 (96%) 814 68 25 6 4
2007 926 (97%) 835 63 20 5 3
2008 941 (98%) 858 61 13 4 5
2009 950 (99%) 866 55 18 4 7

*µg/L means micrograms per liter of water.


Table 3 below shows the number of people served by community water systems within each arsenic mean concentration category. A total of 4,234,479 people were estimated to be served by community water systems in Minnesota as of April 2010. Arsenic monitoring data are available for over 99% of these people. The table shows that 99.8% of people served by CWS in 2009 received drinking water with arsenic concentrations less than 10 µg/L. Even so, over 10,000 people were still estimated to have received drinking water with an annual average of 10 µg/L or higher. Note that the estimates of number of people served by each system are rough calculations periodically updated by water purveyors. Estimates are not available for years past.

Number of people served by Community Water Systems by mean arsenic concentration, Minnesota 2009

Year Cumulative Number of Community Water Systems Sampled, 1999-2009 Number of People Served by Community Water Systems by Concentration Category (µg/L)
<5 5 to <10 10 to <20 20 to <30 30+
2009 4,227,942 (99.1%) 4,189,670 (98.9%) 28,018 (0.7%) 8,635 (0.2%) 566 (0.01%) 1,053 (0.03%)

*µg/L means micrograms per liter of water.