Wisconsin Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Wisconsin. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Wisconsin groundwater. USGS modeling estimates about 72,668 Wisconsin residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Waukesha County (~4,188), Winnebago County (~2,991), Marathon County (~2,687). For nitrate, about 0.8% of Wisconsin's ~1,520,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
0.8%
of the private-well population is modeled above the 10 mg/L nitrate limit
~12,500
people on wells estimated above the limit
~1,520,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Wisconsin has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across Wisconsin, an estimated ~72,668 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Waukesha County (~4,188 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Waukesha County | ~4,188 | 3,423–5,179 |
| Winnebago County | ~2,991 | 2,306–3,920 |
| Marathon County | ~2,687 | 2,147–3,378 |
| Kenosha County | ~2,460 | 2,042–2,966 |
| Racine County | ~2,416 | 2,017–2,904 |
| Sheboygan County | ~2,247 | 1,894–2,684 |
| Dane County | ~2,038 | 1,664–2,507 |
| Outagamie County | ~1,977 | 1,643–2,396 |
| Oconto County | ~1,896 | 1,538–2,341 |
| Walworth County | ~1,823 | 1,460–2,284 |
| Washington County | ~1,809 | 1,525–2,144 |
| Oneida County | ~1,797 | 1,387–2,340 |
| Fond du Lac County | ~1,724 | 1,396–2,148 |
| Jefferson County | ~1,613 | 1,320–1,990 |
| Manitowoc County | ~1,581 | 1,365–1,839 |
| Dodge County | ~1,574 | 1,308–1,907 |
| Waupaca County | ~1,565 | 1,252–1,954 |
| Chippewa County | ~1,553 | 1,256–1,926 |
| Brown County | ~1,509 | 1,267–1,802 |
| St. Croix County | ~1,435 | 1,154–1,798 |
| Wood County | ~1,422 | 1,109–1,838 |
| Clark County | ~1,312 | 1,056–1,627 |
| Vilas County | ~1,265 | 964–1,663 |
| Shawano County | ~1,254 | 1,029–1,536 |
| Marinette County | ~1,214 | 950–1,559 |
| Kewaunee County | ~1,105 | 942–1,296 |
| Douglas County | ~1,087 | 892–1,330 |
| Ozaukee County | ~1,060 | 911–1,239 |
| Lincoln County | ~1,051 | 845–1,310 |
| Polk County | ~1,029 | 834–1,272 |
| Barron County | ~1,009 | 831–1,228 |
| Portage County | ~1,004 | 756–1,341 |
| Columbia County | ~991 | 797–1,236 |
| Taylor County | ~925 | 772–1,111 |
| Waushara County | ~868 | 666–1,125 |
| Marquette County | ~845 | 671–1,061 |
| Rock County | ~799 | 652–977 |
| Burnett County | ~787 | 620–1,001 |
| Dunn County | ~711 | 565–900 |
| Sawyer County | ~706 | 561–892 |
| Juneau County | ~704 | 515–957 |
| Adams County | ~687 | 499–953 |
| Eau Claire County | ~685 | 537–870 |
| Langlade County | ~634 | 529–764 |
| Price County | ~594 | 485–729 |
| Sauk County | ~563 | 456–702 |
| Washburn County | ~560 | 443–708 |
| Monroe County | ~538 | 418–692 |
| Bayfield County | ~532 | 434–655 |
| La Crosse County | ~524 | 383–714 |
| Green Lake County | ~489 | 382–627 |
| Rusk County | ~475 | 393–578 |
| Calumet County | ~456 | 347–606 |
| Pierce County | ~432 | 334–569 |
| Forest County | ~348 | 283–430 |
| Vernon County | ~309 | 245–397 |
| Green County | ~286 | 229–360 |
| Grant County | ~280 | 220–359 |
| Jackson County | ~259 | 203–329 |
| Ashland County | ~241 | 203–287 |
| Door County | ~220 | 179–273 |
| Trempealeau County | ~196 | 156–248 |
| Richland County | ~182 | 147–227 |
| Iron County | ~176 | 140–221 |
| Milwaukee County | ~176 | 149–211 |
| Buffalo County | ~161 | 124–216 |
| Florence County | ~156 | 124–197 |
| Crawford County | ~145 | 115–186 |
| Iowa County | ~138 | 111–172 |
3 additional Wisconsin counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 1 has measured USGS well samples: Menominee.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 63 Wisconsin counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Winnebago County | 40% | 3 mg/L | 5 wells |
| Marquette County | 27% | 3.3 mg/L | 11 wells |
| Waushara County | 19% | 2 mg/L | 16 wells |
| Waupaca County | 13% | 1.9 mg/L | 15 wells |
| Portage County | 8% | 4.3 mg/L | 38 wells |
| Fond du Lac County | 8% | 1.7 mg/L | 12 wells |
| Shawano County | 7% | 0.2 mg/L | 15 wells |
| Sheboygan County | 6% | 0 mg/L | 36 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Pierce County | 23% | 5 µg/L | 13 wells |
| Monroe County | 20% | 150 µg/L | 5 wells |
| Forest County | 17% | 6 µg/L | 6 wells |
| Shawano County | 13% | 40 µg/L | 15 wells |
| Sheboygan County | 10% | 28 µg/L | 20 wells |
| Fond du Lac County | 8% | 68 µg/L | 12 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
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Wisconsin well water: FAQ
Is there arsenic in well water in Wisconsin?
Yes. USGS modeling estimates about 72,668 people in Wisconsin use private wells with arsenic above 10 µg/L, across 72 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Wisconsin counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Waukesha County (~4,188), Winnebago County (~2,991), Marathon County (~2,687). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Wisconsin?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Wisconsin well water?
USGS models about 0.8% of Wisconsin's private-well population (~12,500 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Wisconsin?
Acidic well water is uncommon in Wisconsin, but it does occur. In USGS lab results from 1,399 wells across 63 Wisconsin counties, 6% had pH below 6.5. The most acidic sampled counties are Chippewa County (50% of 14 wells), Vilas County (44% of 48 wells), Eau Claire County (40% of 15 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Wisconsin well water — what should I do?
In USGS lab results from 244 wells across 22 Wisconsin counties, 4% were above the EPA's 300 µg/L health advisory for manganese and 22% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Pierce County (23% of 13 wells), Monroe County (20% of 5 wells), Forest County (17% of 6 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.