Nitrate in Wisconsin well water
In Wisconsin, 8% of 121,661 private wells tested between 1984 and 2026 had nitrate above the Wisconsin enforcement standard (10 mg/L), according to the UWSP Well Water Quality Viewer.
Nitrate reaches wells from fertilizer, septic systems and manure. Above the limit it can cause methemoglobinemia (blue baby syndrome) in infants under six months, and long-term exposure is associated with some cancers. Shallow and dug wells near farmland or dense septic systems are most at risk.
Check your nitrate number
Type the value from your lab report. It is compared to the Wisconsin enforcement standard and the EPA limit, the same way the full interpretation does it.
Getting it fixed
Book a free in-home water test with a licensed local company, or see the treatment guide for what removes nitrate.
By county, from homeowner tests
Share of private wells above the state guidance level (10 mg/L) in tests homeowners sent to the state lab, 1988 to 2025.
| County | Wells tested | Above state level | Highest | |
|---|---|---|---|---|
| Rock | 2,220 | 23% | 64.4 mg/L | |
| Portage | 3,842 | 21% | 76 mg/L | |
| Trempealeau | 894 | 21% | 48.2 mg/L | |
| Green Lake | 1,317 | 19% | 52.1 mg/L | |
| Pepin | 390 | 19% | 39.5 mg/L | |
| Columbia | 2,862 | 18% | 48.1 mg/L | |
| Dane | 7,299 | 18% | 127 mg/L | |
| Dunn | 1,265 | 15% | 45.4 mg/L | |
| Green | 2,533 | 14% | 69.9 mg/L | |
| Calumet | 824 | 14% | 38.9 mg/L | |
| Sauk | 2,708 | 13% | 94.4 mg/L | |
| La Crosse | 1,208 | 12% | 39.2 mg/L | |
| Pierce | 1,818 | 12% | 37.8 mg/L | |
| Chippewa | 2,684 | 12% | 34.9 mg/L | |
| St. Croix | 3,738 | 11% | 40.4 mg/L |
Towns with the highest share above the state level
- La Prairie, Rock County76% of 67 wells
- Caledonia, Trempealeau County69% of 68 wells
- Pine Grove, Portage County62% of 61 wells
- Leeds, Columbia County62% of 90 wells
- Kingston, Green Lake County60% of 149 wells
- Plainfield, Waushara County53% of 66 wells
- Richfield, Adams County53% of 34 wells
- Trempealeau, Trempealeau County48% of 145 wells
- Janesville, Rock County46% of 50 wells
- Arlington, Columbia County46% of 74 wells
- Plover, Portage County45% of 205 wells
- Buena Vista, Portage County45% of 150 wells
- Cottage Grove village, Dane County44% of 45 wells
- Plymouth, Rock County42% of 45 wells
- Scott, Columbia County41% of 94 wells
- Spring Green, Sauk County40% of 226 wells
- Buena Vista, Richland County39% of 128 wells
- Hampden, Columbia County39% of 57 wells
- Holland, La Crosse County39% of 236 wells
- Almond, Portage County38% of 126 wells
- Turtle, Rock County38% of 90 wells
- Harmony, Rock County37% of 117 wells
- Lincoln, Adams County36% of 66 wells
- Bradford, Rock County36% of 36 wells
- DeForest village, Dane County36% of 36 wells
By county, from individual samples
Public monitoring samples against the EPA limit (10 mg/L); counties with at least 30 samples.
| County | Samples | Detected | Above EPA limit | Median | Highest |
|---|---|---|---|---|---|
| Portage | 32 | 91% | 31% | 4.3 | 44 mg/L |
Sources
- Wisconsin Well Water Quality Viewer samples: 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.
- Individual samples from the USGS Water Quality Portal and state monitoring, summarized per geography. nitrate counts and shares are listed on each county and town page with their sources.