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.
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.
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).
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Treating arsenic in Wisconsin
Arsenic is not removed by carbon pitchers or faucet filters. Reverse osmosis is the treatment that brings it below the federal limit at your tap.
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