Well water in Wisconsin
In Wisconsin, 10% of 11,282 private wells tested between 1990 and 2025 had total coliform above the Wisconsin limit (any detection), according to the UWSP Well Water Quality Viewer.
121,661 private wells tested 1994 to 2025 in state summaries; 605 individual samples from 330 wells, 1935 to 2024. Data refreshed September 9, 2026.
What homeowner tests found
Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 1994 to 2025. Shares are against the level the state applies to each contaminant, named in the last column; several are stricter than EPA. County and state figures combine the town summaries, weighted by wells tested; medians are not combined.
| Contaminant | Wells tested | Above state level | Median | 95th pct | Highest | State level | |
|---|---|---|---|---|---|---|---|
| Iron | 6,371 | 18% | 61,640 | 300 ug/L | |||
| Sodium | 6,432 | 11% | 1,022 | 20 mg/L | |||
| Total coliform | 11,282 | 10% | |||||
| Nitrate | 121,661 | 8% | 129 | 10 mg/L | |||
| Arsenic | 22,114 | 4% | 6,300 | 10 ug/L | |||
| E. coli | 1,091 | 4% | |||||
| Manganese | 7,925 | 4% | 9,964 | 300 ug/L | |||
| Lead | 7,937 | 3% | 2,529 | 15 ug/L | |||
| Copper | 7,890 | 1% | 15.9 | 1.3 mg/L | |||
| Chloride | 15,485 | <1% | 3,010 | 250 mg/L |
What individual samples found
Each row summarizes every public sample for that contaminant inside Wisconsin. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Samples | Detected | Above limit | Median | Highest | Limit | |
|---|---|---|---|---|---|---|---|
| PFOA 2019 to 2021 | 10 | 40% | 30% | 0 | 9.6 | 4 ng/L EPA limit | |
| PFOS 2019 to 2021 | 10 | 30% | 20% | 0 | 16 | 4 ng/L EPA limit | |
| Nitrate 1935 to 2018 | 194 | 78% | 11% | 0.595 | 44 | 10 mg/L EPA limit | |
| Lead 1956 to 2023 | 89 | 39% | 3% | 0 | 200 | 15 ug/L EPA limit | |
| Radium (226 + 228) 1983 to 2018 | 350 | 71% | 2% | 0.09 | 31 | 5 pCi/L EPA limit | |
| Arsenic 1973 to 2023 | 90 | 68% | 1% | 0.22 | 16.5 | 10 ug/L EPA limit | |
| Uranium 1954 to 2024 | 605 | 80% | <1% | 0.457 | 41.9 | 30 ug/L EPA limit | |
| Fluoride 1945 to 2014 | 94 | 82% | 0% | 0.11 | 2.8 | 4 mg/L EPA limit | |
| Copper 1955 to 2023 | 88 | 50% | 0% | 0 | 0.23 | 1.3 mg/L EPA limit | |
| Radon 1982 to 2014 | 199 | 100% | no limit | 390 | 26,000 | 2,000 pCi/L health guideline | |
| Chloride 1935 to 2021 | 97 | 98% | no limit | 4 | 480 | 250 mg/L nuisance level | |
| Sodium 1952 to 2017 | 96 | 100% | no limit | 2.48 | 130 | 20 mg/L health guideline | |
| Manganese 1935 to 2017 | 95 | 84% | no limit | 30 | 1,400 | 300 ug/L health guideline | |
| Iron 1945 to 2022 | 94 | 85% | no limit | 100 | 18,000 | 300 ug/L nuisance level |
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What this means
Among homeowners in Wisconsin who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (10% of 11,282 wells), nitrate (8% of 121,661 wells), and arsenic (4% of 22,114 wells). These are ordinary wells tested by their owners, so the shares are the best available picture of a typical well here, with one caveat: people test more often when they suspect a problem.
Iron and sodium exceed the state's advisory level in a sizable share of wells; these are mostly a taste, staining or diet issue rather than a toxicity one.
What to test for in Wisconsin
- Volatile organic compounds (VOCs)10,362 leaking underground storage tank sites on record locally.
- PFOA30% of 10 local samples were above the limit.
- PFOS20% of 10 local samples were above the limit.
- Iron18% of 6,371 local wells tested were above the Wisconsin nuisance level.
- Sodium11% of 6,432 local wells tested were above the EPA advisory level.
- Total coliform10% of 11,282 local wells tested were above the Wisconsin limit (any detection).
- E. coli4% of 1,091 local wells tested were above the Wisconsin limit (any detection).
- Manganese4% of 7,925 local wells tested were above the Wisconsin enforcement standard.
- Radium (226 + 228)2% of 350 local samples were above the limit.
Contamination sources on record in Wisconsin
Wisconsin DNR records 10,362 leaking underground storage tank sites in Wisconsin, among 34,512 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 10,362
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 3,016
- lined landfills and transfer facilities
- 21,134
- other remediation projects
Treatment that fits the local profile
- Total coliform
- Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Iron
- Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
- Sodium
- Point-of-use reverse osmosis; Distillation
- PFOA
- Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
- PFOS
- Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
Common questions
Is there arsenic in well water in Wisconsin?
Arsenic was detected in 68% of 90 public well samples in Wisconsin, and 1% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in Wisconsin?
The median radon level across 199 public samples in Wisconsin was 390 pCi/L, with a high of 26,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Wisconsin?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Counties in Wisconsin
- Adams10% total coliform
- Ashland4% arsenic
- Barron9% nitrate
- Bayfield<1% nitrate
- Brown3% arsenic
- Buffalo9% nitrate
- Burnett14% total coliform
- Calumet14% nitrate
- Chippewa12% nitrate
- Clark9% total coliform
- Columbia18% nitrate
- Crawford12% total coliform
- Dane18% nitrate
- Dodge12% total coliform
- Door11% total coliform
- Douglas9% total coliform
- Dunn15% nitrate
- Eau Claire5% nitrate
- Florence14% arsenic
- Fond du Lac12% total coliform
- Forest4% arsenic
- Grant6% nitrate
- Green14% nitrate
- Green Lake19% nitrate
- Iowa14% total coliform
- Iron0% arsenic
- Jackson5% nitrate
- Jefferson17% total coliform
- Juneau9% nitrate
- Kenosha5% arsenic
- Kewaunee11% total coliform
- La Crosse12% nitrate
- Lafayette26% total coliform
- Langlade6% total coliform
- Lincoln11% total coliform
- Manitowoc10% total coliform
- Marathon9% total coliform
- Marinette12% arsenic
- Marquette11% total coliform
- Menominee10% arsenic
- Milwaukee<1% nitrate
- Monroe9% nitrate
- Oconto13% total coliform
- Oneida8% lead
- Outagamie19% arsenic
- Ozaukee11% arsenic
- Pepin20% total coliform
- Pierce18% total coliform
- Polk15% total coliform
- Portage21% nitrate
- Price12% total coliform
- Racine6% arsenic
- Richland17% total coliform
- Rock23% nitrate
- Rusk2% nitrate
- Sauk13% nitrate
- Sawyer6% total coliform
- Shawano12% arsenic
- Sheboygan10% total coliform
- St. Croix11% nitrate
- Taylor9% total coliform
- Trempealeau21% nitrate
- Vernon12% total coliform
- Vilas8% total coliform
- Walworth8% arsenic
- Washburn11% total coliform
- Washington16% total coliform
- Waukesha4% arsenic
- Waupaca12% total coliform
- Waushara11% nitrate
- Winnebago19% arsenic
- Wood9% total coliform
Sources
- USGS Water Quality Portal: 2,111 samples in Wisconsin. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- Wisconsin Well Water Quality Viewer samples: summaries for 10 contaminants in Wisconsin, up to 121,661 wells each. Private well samples compiled by the UWSP Center for Watershed Science and Education from the DNR Groundwater Retrieval Network, DATCP surveys, the UWSP Water and Environmental Analysis Lab and county health programs, published in the Wisconsin Well Water Quality Viewer with Wisconsin DHS. Samples are located to the town, village or city they fall in; one result per well and parameter (the latest) so repeat samples do not double count; wells without a state well number are keyed by location. Shares are against Wisconsin NR 140 enforcement standards (arsenic 10 ug/L, nitrate 10 mg/L as N, manganese 300 ug/L, lead 15 ug/L, copper 1.3 mg/L) and the iron and chloride secondary levels. Many samples were submitted by homeowners with a reason to test, so shares can run above a random survey. Retrieved September 9, 2026.
- Wisconsin DNR regulated sites: 34,512 regulated sites located in Wisconsin. Open and closed remediation activities (BRRTS, including leaking underground storage tanks and spills) and landfill and waste sites from the Wisconsin Department of Natural Resources. Retrieved September 9, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.