Well water in Fond du Lac County, Wisconsin

In Fond du Lac County, 12% of 219 private wells tested between 2001 and 2025 had total coliform above the Wisconsin limit (any detection), according to the UWSP Well Water Quality Viewer.

1,692 private wells tested 2002 to 2025 in state summaries; 13 individual samples from 11 wells, 1955 to 2015. Data refreshed September 9, 2026.

What homeowner tests found

Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 2002 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	Highest	State level
Sodium	122	34%	461	20 mg/L
Iron	118	34%	6,612	300 ug/L
Total coliform	219	12%
Arsenic	333	8%	91	10 ug/L
Nitrate	1,692	5%	27.9	10 mg/L
Lead	174	3%	125	15 ug/L
Chloride	218	2%	492	250 mg/L
Copper	174	<1%	1.54	1.3 mg/L
Manganese	174	<1%	443	300 ug/L
E. coli	26	0%

What individual samples found

Each row summarizes every public sample for that contaminant inside Fond du Lac County. 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
Uranium 1958 to 2015	13	77%	0%		0.768	5.5	30 ug/L EPA limit

Sampled fewer than 10 times, too few to summarize: Radium (226 + 228) (6), Nitrate (5), Fluoride (2), Copper (1), Radon (5), Manganese (3), Iron (3).

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What this means

Among homeowners in Fond du Lac County who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (12% of 219 wells), arsenic (8% of 333 wells), and nitrate (5% of 1,692 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.

Sodium and iron 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 Fond du Lac County

Volatile organic compounds (VOCs)188 leaking underground storage tank sites on record locally.
Sodium34% of 122 local wells tested were above the EPA advisory level.
Iron34% of 118 local wells tested were above the Wisconsin nuisance level.
Total coliform12% of 219 local wells tested were above the Wisconsin limit (any detection).
Arsenic8% of 333 local wells tested were above the Wisconsin enforcement standard.
Nitrate5% of 1,692 local wells tested were above the Wisconsin enforcement standard.
Lead3% of 174 local wells tested were above the Wisconsin enforcement standard.
Chloride2% of 218 local wells tested were above the Wisconsin nuisance level.
Copper<1% of 174 local wells tested were above the Wisconsin enforcement standard.

Contamination sources on record in Fond du Lac County

Wisconsin DNR records 188 leaking underground storage tank sites in Fond du Lac County, among 672 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.

188: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
61: lined landfills and transfer facilities
423: 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
Arsenic: Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Nitrate: Point-of-use reverse osmosis; Anion exchange; Distillation
Sodium: Point-of-use reverse osmosis; Distillation
Iron: Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
Fluoride: Point-of-use reverse osmosis; Activated alumina; Distillation

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Common questions

How high is radon in well water in Fond du Lac County?

The median radon level across 5 public samples in Fond du Lac County was 620 pCi/L, with a high of 850 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Fond du Lac County?

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.

Towns in Fond du Lac County

Alto8% nitrate
Ashford4% nitrate
Auburn1% nitrate
Byron25% nitrate
Calumet3% nitrate
Eden11% nitrate
Eldorado0% nitrate
Empire8% nitrate
Fond du Lac2% nitrate
Forest13% nitrate
Friendship0% nitrate
Lamartine0% nitrate
Marshfield0% nitrate
Metomen11% nitrate
Oakfield6% nitrate
Osceola0% nitrate
Ripon8% nitrate
Rosendale0% nitrate
Rosendale village2% nitrate
Springvale2% nitrate
Taycheedah9% arsenic
Waupun3% nitrate

12 towns have fewer than 30 public samples and are included in the Fond du Lac County totals above rather than shown separately: Brandon village, Campbellsport village, Eden village, Fairwater village, Fond du Lac, Kewaskum village, Mount Calvary village, North Fond du Lac village, Oakfield village, Ripon, St. Cloud village, Waupun.

Sources

USGS Water Quality Portal: 38 samples in Fond du Lac County. 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 Fond du Lac County, up to 1,692 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: 672 regulated sites located in Fond du Lac County. 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.