privatewelldata / States / Wisconsin / Nitrate
Nitrate in well water in Wisconsin
4% of 3,666 sampled wells in Wisconsin had nitrate above the EPA limit of 10 mg/L (as nitrogen) (latest result per well, 70 counties, samples 1934–2023). The highest county share was Marathon County: 31% of 13 wells. Data updated 2026-10-18.
of sampled wells above 10 mg/L (as nitrogen)
wells with a result
counties with data
counties where 1 in 4+ wells were above
Nitrate in wells most often comes from fertilizer, manure and septic systems, so shallow wells near fields or septic fields are more exposed, and levels can change with the seasons. Only a lab test shows it. Federal limits apply to public water systems; a private well is the owner's responsibility to test (EPA).
% of sampled wells above the EPA limit (latest result per well; counties with 10+ wells). Source: USGS/EPA Water Quality Portal, 2026-10-18 · privatewelldata.com
Nitrate by county in Wisconsin
| County | Above 10 mg/L (as nitrogen) | Median | 90th pct. | Years |
|---|---|---|---|---|
| Marathon County | 31% of 13 | 2.5 mg/L as N | 12.628 | 1964–2014 |
| Green Lake County | 20% of 10 | 5.026 mg/L as N | 12 | 1960–2013 |
| Pepin County | 20% of 10 | 1.495 mg/L as N | 17.01 | 1965–1992 |
| Portage County | 19% of 142 | 4.245 mg/L as N | 17 | 1951–2019 |
| Marquette County | 16% of 19 | 2.372 mg/L as N | 20.3 | 1973–2015 |
| Waupaca County | 15% of 20 | 1.62 mg/L as N | 10.8 | 1958–2014 |
| Waushara County | 14% of 29 | 1.983 mg/L as N | 11.227 | 1963–2015 |
| Crawford County | 12% of 33 | 1.83 mg/L as N | 9.3 | 1962–1983 |
| Rock County | 12% of 58 | 3.276 mg/L as N | 10.165 | 1935–1993 |
| Iowa County | 11% of 36 | 2.007 mg/L as N | 10 | 1935–1991 |
| Monroe County | 11% of 19 | 0.72 mg/L as N | 9.3 | 1962–1991 |
| Dunn County | 9% of 43 | 0.93 mg/L as N | 7.229 | 1966–2014 |
| Winnebago County | 8% of 12 | 0.016 mg/L as N | 8.57 | 1965–2015 |
| Price County | 8% of 13 | 0.15 mg/L as N | 1.3 | 1961–2004 |
| Dane County | 8% of 131 | 0.64 mg/L as N | 6.8 | 1935–2022 |
| Shawano County | 8% of 92 | 1.3 mg/L as N | 7.5 | 1976–2015 |
| Green County | 7% of 27 | 0.44 mg/L as N | 6.6 | 1935–1991 |
| Juneau County | 7% of 15 | 0.81 mg/L as N | 3 | 1964–1983 |
| Lafayette County | 6% of 32 | 0.41 mg/L as N | 4.4 | 1935–1991 |
| Grant County | 6% of 50 | 0.41 mg/L as N | 4.2 | 1935–2014 |
| La Crosse County | 6% of 51 | 2.3 mg/L as N | 8.81 | 1935–2014 |
| Barron County | 6% of 53 | 1.83 mg/L as N | 6.325 | 1961–1980 |
| Columbia County | 5% of 115 | 1.807 mg/L as N | 9.262 | 1961–2015 |
| Fond du Lac County | 4% of 45 | 0.09 mg/L as N | 2.01 | 1955–2015 |
| St. Croix County | 4% of 113 | 2.2 mg/L as N | 5.873 | 1934–2018 |
| Sheboygan County | 4% of 69 | 0.181 mg/L as N | 4.1 | 1963–2013 |
| Forest County | 4% of 100 | 0.29 mg/L as N | 2.12 | 1965–2022 |
| Adams County | 4% of 27 | 2.6 mg/L as N | 7.5 | 1964–2023 |
| Calumet County | 4% of 27 | 0.294 mg/L as N | 2.8 | 1956–1993 |
| Pierce County | 3% of 29 | 2.711 mg/L as N | 7.906 | 1962–2018 |
| Chippewa County | 3% of 31 | 2.2 mg/L as N | 6.099 | 1961–2013 |
| Wood County | 3% of 165 | 0.023 mg/L as N | 4.5 | 1964–1986 |
| Sauk County | 3% of 33 | 1.626 mg/L as N | 6.8 | 1954–2019 |
| Vilas County | 3% of 120 | 0.01 mg/L as N | 0.76 | 1964–2014 |
| Dodge County | 2% of 108 | 0.055 mg/L as N | 3.4 | 1961–1991 |
| Manitowoc County | 2% of 54 | 0.09 mg/L as N | 2.6 | 1961–1993 |
| Menominee County | 1% of 70 | 0 mg/L as N | 1.5 | 1982–2003 |
| Waukesha County | 1% of 195 | 0.203 mg/L as N | 3.5 | 1957–2021 |
| Brown County | 1% of 103 | 0.02 mg/L as N | 0.26 | 1947–2015 |
| Walworth County | 1% of 109 | 0.14 mg/L as N | 1.92 | 1954–2021 |
| Washington County | 1% of 123 | 0.02 mg/L as N | 3.163 | 1961–2013 |
| Douglas County | 0% of 94 | 0.043 mg/L as N | 0.32 | 1949–1986 |
| Jefferson County | 0% of 80 | 0.326 mg/L as N | 2.711 | 1947–2016 |
| Ashland County | 0% of 79 | 0 mg/L as N | 0.251 | 1961–2004 |
| Milwaukee County | 0% of 78 | 0.205 mg/L as N | 0.61 | 1947–1995 |
| Bayfield County | 0% of 77 | 0.07 mg/L as N | 0.24 | 1935–2014 |
| Ozaukee County | 0% of 66 | 0.015 mg/L as N | 0.565 | 1951–2013 |
| Langlade County | 0% of 64 | 0.23 mg/L as N | 5.806 | 1970–2014 |
| Burnett County | 0% of 58 | 0.055 mg/L as N | 2.485 | 1964–1986 |
| Polk County | 0% of 55 | 0.994 mg/L as N | 2.711 | 1935–2018 |
| Door County | 0% of 38 | 0.4 mg/L as N | 2.711 | 1966–1980 |
| Racine County | 0% of 38 | 0.021 mg/L as N | 1.75 | 1963–2021 |
| Vernon County | 0% of 36 | 0.77 mg/L as N | 5 | 1935–1981 |
| Marinette County | 0% of 34 | 0 mg/L as N | 0.95 | 1960–2015 |
| Eau Claire County | 0% of 31 | 0.61 mg/L as N | 3.2 | 1963–2014 |
| Clark County | 0% of 29 | 2.711 mg/L as N | 4.744 | 1966–1980 |
| Kenosha County | 0% of 29 | 0 mg/L as N | 0.113 | 1957–2001 |
| Trempealeau County | 0% of 28 | 0 mg/L as N | 1.2 | 1935–2014 |
| Lincoln County | 0% of 22 | 0.305 mg/L as N | 2.372 | 1966–2013 |
| Sawyer County | 0% of 22 | 0.026 mg/L as N | 0.52 | 1965–1982 |
| Rusk County | 0% of 21 | 0.09 mg/L as N | 1.807 | 1962–2013 |
| Washburn County | 0% of 21 | 0.113 mg/L as N | 2.01 | 1966–2010 |
| Oconto County | 0% of 20 | 0 mg/L as N | 0.615 | 1935–2014 |
| Florence County | 0% of 18 | 0.026 mg/L as N | 3.614 | 1966–2003 |
| Kewaunee County | 0% of 16 | 0.113 mg/L as N | 0.41 | 1954–1972 |
| Richland County | 0% of 16 | 0.42 mg/L as N | 3.6 | 1962–1991 |
| Jackson County | 0% of 14 | 0.306 mg/L as N | 0.93 | 1935–1983 |
| Oneida County | 0% of 14 | 0.035 mg/L as N | 0.113 | 1970–1977 |
| Buffalo County | 0% of 12 | 0.011 mg/L as N | 1.3 | 1935–1975 |
| Outagamie County | 0% of 12 | 0.497 mg/L as N | 4.83 | 1935–2015 |
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Download this table (CSV) · public-domain source data: Water Quality Portal (USGS, EPA, NWQMC). How we calculate these numbers.
How Wisconsin compares
Share of sampled wells above the EPA nitrate limit in each state we cover (states with 100+ wells).
| # | State | Above 10 mg/L (as nitrogen) |
|---|---|---|
| 1 | California | 10% of 16,344 |
| 2 | Pennsylvania | 8% of 6,478 |
| 3 | Minnesota | 6% of 7,259 |
| 4 | North Carolina | 5% of 1,964 |
| 5 | New York | 5% of 6,236 |
| 6 | Massachusetts | 4% of 2,556 |
| 7 | New Jersey | 4% of 3,915 |
| 8 | Wisconsin | 4% of 3,666 |
| 9 | Virginia | 3% of 1,394 |
| 10 | Maine | 2% of 726 |
| 11 | Michigan | 2% of 1,970 |
| 12 | Ohio | 1% of 1,239 |
| 13 | New Hampshire | 0% of 1,685 |
Where to test: official lab list for Wisconsin
About this data
- Each well counts once, using its latest nitrate result in the Water Quality Portal (siteType = Well), downloaded 2026-10-18.
- Results reported as "not detected" count as below the limit. Results far outside the plausible range are withheld for review, not shown.
- Sampled wells include monitoring and research wells, not only household wells, and are not a random sample, so the share is a guide to local risk, not a rate for all homes.
- The limit is the EPA maximum contaminant level for public water systems (EPA).
Questions
Is nitrate common in Wisconsin well water?
In our data, 141 of 3,666 sampled wells in Wisconsin (4%) had a latest nitrate result above the EPA limit of 10 mg/L (as nitrogen), across 70 counties. That ranks Wisconsin 8 of 13 states we cover for this share.
Which Wisconsin counties have the most wells above the nitrate limit?
Among counties with at least 10 sampled wells: Marathon County (31%, 4 of 13 wells); Green Lake County (20%, 2 of 10 wells); Pepin County (20%, 2 of 10 wells).
Does this tell me whether my well is safe?
No. These are past results from wells sampled by USGS, EPA and state programs, including monitoring wells, and levels differ from well to well. The only way to know your water is to test your own well at a certified lab.
How do I test my well for nitrate in Wisconsin?
Order a nitrate test from a lab on Wisconsin's official certified-lab list (linked on this page) and ask for its bottles and sampling instructions. Our test helper suggests what else to test for at the same time.
What should I do if my nitrate result is above 10 mg/L (as nitrogen)?
Retest to confirm, use another water source for drinking and cooking while you sort it out, and look at certified treatment options. Our guide walks through each step with sources.