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Nitrate in Wisconsin well water, by county

By the Tested Water teamLab data from the USGS and EPA Water Quality Portal, 2000 to 2025Updated November 8, 2026

Nitrate is widespread, over the limit in 15% or more of tested wells, in 16 of the 61 Wisconsin counties with 10 or more tested wells. 20 more look high, but those results likely overstate it. A county's share tells you how likely a problem is nearby. Only a test of your own well tells you what's in it.

Find a nitrate test See every county

How Wisconsin compares with the rest of the country

A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit. The public lab data put it at 32% of tested wells, but those wells were picked by research and monitoring programs, not at random. Across all tested wells in the US it's about 1 in 12. Wisconsin is about average, so the county you live in tells you more than the state does.

Why it matters: It's dangerous for babies under 6 months, because it keeps their blood from carrying enough oxygen.

Nitrate soaks down into groundwater from fertilizer, manure and septic systems.

Nitrate in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA limit of 10 mg/L. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.

CountyWells testedWells over the limitLevel
Iowa County 49
65%* 32 of 49
 Widespread
Adams County 114
63%* 72 of 114
 Widespread
Rock County 99
56%* 55 of 99
 Widespread
Dunn County 77
55%* 42 of 77
 Widespread
Waushara County 113
50%* 57 of 113
 Widespread
Pepin County 26
50%* 13 of 26
 Widespread
Calumet County 53
49%* 26 of 53
 Widespread
Portage County 174
49%* 85 of 174
 Widespread
Sauk County 99
47%* 47 of 99
 Widespread
Green Lake County 78
46%* 36 of 78
 Widespread
Juneau County 78
44%* 34 of 78
 Widespread
Dane County 138
43%* 59 of 138
 Widespread
Columbia County 114
40%* 46 of 114
 Widespread
Trempealeau County 62
40%* 25 of 62
 Widespread
Green County 59
37%* 22 of 59
 Widespread
Lafayette County 36
36%* 13 of 36
 Widespread
Langlade County 37
35%* 13 of 37
 Widespread
Richland County 20
35%* 7 of 20
 Widespread
Grant County 49
35%* 17 of 49
 Widespread
Chippewa County 48
33%* 16 of 48
 Widespread
Dodge County 64
33% 21 of 64
 Widespread
St. Croix County 34
32% 11 of 34
 Widespread
Marathon County 50
32% 16 of 50
 Widespread
Marquette County 37
30% 11 of 37
 Widespread
Waupaca County 64
30% 19 of 64
 Widespread
La Crosse County 14
29% 4 of 14
 Widespread
Winnebago County 18
28% 5 of 18
 Widespread
Jackson County 29
28% 8 of 29
 Widespread
Barron County 56
25% 14 of 56
 Widespread
Pierce County 44
23% 10 of 44
 Widespread
Clark County 19
21% 4 of 19
 Widespread
Buffalo County 16
19% 3 of 16
 Widespread
Jefferson County 38
18% 7 of 38
 Widespread
Monroe County 46
17% 8 of 46
 Widespread
Brown County 72
17% 12 of 72
 Widespread
Walworth County 63
16% 10 of 63
 Widespread
Eau Claire County 34
15% 5 of 34
 Worth checking
Manitowoc County 41
15% 6 of 41
 Worth checking
Kewaunee County 14
14% 2 of 14
 Worth checking
Fond du Lac County 32
12% 4 of 32
 Worth checking
Wood County 33
12% 4 of 33
 Worth checking
Washington County 25
12% 3 of 25
 Worth checking
Vernon County 17
12% 2 of 17
 Worth checking
Shawano County 30
10% 3 of 30
 Worth checking
Crawford County 11
9% 1 of 11
 Worth checking
Marinette County 26
8% 2 of 26
 Worth checking
Burnett County 14
7% 1 of 14
 Worth checking
Oconto County 29
7% 2 of 29
 Worth checking
Outagamie County 20
5% 1 of 20
 Worth checking
Oneida County 21
5% 1 of 21
 Rare
Door County 67
4% 3 of 67
 Rare
Polk County 45
4% 2 of 45
 Rare
Waukesha County 28
4% 1 of 28
 Rare
Vilas County 27
0% 0 of 27
 Rare
Forest County 26
0% 0 of 26
 Rare
Price County 21
0% 0 of 21
 Rare
Washburn County 17
0% 0 of 17
 Rare
Bayfield County 16
0% 0 of 16
 Rare
Racine County 13
0% 0 of 13
 Rare
Ashland County 11
0% 0 of 11
 Rare
Lincoln County 10
0% 0 of 10
 Rare

* Likely overstated. These shares come from a narrow set of wells, so they probably run high for the county as a whole:

  • Iowa County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Adams County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Rock County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Dunn County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Waushara County: That's more than 6 times what a 2016 random sample of private wells found across Wisconsin (about 1 in 12), so this likely overstates it.
  • Pepin County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Calumet County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Portage County: That's more than 5 times what a 2016 random sample of private wells found across Wisconsin (about 1 in 12), so this likely overstates it.
  • Sauk County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Green Lake County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Juneau County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Dane County: That's more than 5 times what a 2016 random sample of private wells found across Wisconsin (about 1 in 12), so this likely overstates it.
  • Columbia County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Trempealeau County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Green County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Lafayette County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Langlade County: That's more than 4 times what a 2016 random sample of private wells found across Wisconsin (about 1 in 12), so this likely overstates it.
  • Richland County: All of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Grant County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.
  • Chippewa County: Almost all of these tests come from Wisconsin's agriculture department, so this likely overstates it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit.

10 more Wisconsin counties have 1 to 9 tested wells for nitrate, too few to rate.

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Where to get your well tested for nitrate in Wisconsin

1

Your county health department

Wisconsin doesn't run a statewide free testing program that we know of. Some county health departments test wells or sell kits, so call yours first.

Wisconsin's private well page
3

A mail-in kit

Fill the bottles at your kitchen tap and mail them back. We mark the kits that include nitrate.

Compare kits

Keep reading about wells in Wisconsin

Questions people ask about nitrate in Wisconsin

Is there nitrate in Wisconsin well water?

Nitrate is widespread, over the limit in 15% or more of tested wells, in 16 of the 61 Wisconsin counties with 10 or more tested wells. 20 more look high, but those results likely overstate it. A 2016 random sample of private wells found about 1 in 12 Wisconsin wells over the limit. The public lab data put it at 32% of tested wells, but those wells were picked by research and monitoring programs, not at random. Across all tested wells in the US it's about 1 in 12. Wisconsin is about average, so the county you live in tells you more than the state does.

Where can I get my well tested for nitrate in Wisconsin?

Use a lab certified by Wisconsin for drinking water, or a mail-in kit that includes nitrate from a certified lab. Some county health departments test wells too, so call yours first.

See Wisconsin's list of certified labs, or compare mail-in kits.

Where these numbers come from

  • Lab tests: Water Quality Portal (USGS and EPA), well samples since 2000. A well counts as over if any of its tests was over the EPA limit of 10 mg/L. Wells tested for studies and monitoring, not a random sample of homes.
  • Likely overstated: our own check of who tested the wells, when and where. How we check.
  • State programs and links: Department of Natural Resources (DNR).