Well water in Walworth County, Wisconsin

In Walworth County, 8% of 255 private wells tested between 1990 and 2025 had arsenic above the Wisconsin enforcement standard (10 ug/L), according to the UWSP Well Water Quality Viewer.

1,692 private wells tested 2013 to 2020 in state summaries; 19 individual samples from 11 wells, 1958 to 2021. Data refreshed September 9, 2026.

What homeowner tests found

Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 2013 to 2020. 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
E. coli	2	50%
Iron	13	15%	5,757	300 ug/L
Total coliform	23	9%
Arsenic	255	8%	60.1	10 ug/L
Sodium	13	8%	32.4	20 mg/L
Nitrate	1,692	5%	73.8	10 mg/L
Lead	22	5%	138	15 ug/L
Chloride	23	0%	236	250 mg/L
Manganese	22	0%	24	300 ug/L
Copper	22	0%	0.516	1.3 mg/L

What individual samples found

Each row summarizes every public sample for that contaminant inside Walworth 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 1983 to 2021	19	74%	0%		1.35	7.89	30 ug/L EPA limit
Radium (226 + 228) 1983 to 2001	18	72%	0%		0	3	5 pCi/L EPA limit

Sampled fewer than 10 times, too few to summarize: PFOS (1), PFOA (1), Nitrate (5), Copper (1), Fluoride (1), Arsenic (1), Radon (7), Manganese (3), Sodium (2), Chloride (2).

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What this means

Among homeowners in Walworth County who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (9% of 23 wells), arsenic (8% of 255 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.

Iron exceeds the state's advisory level in a sizable share of wells; this is mostly a taste, staining or diet issue rather than a toxicity one.

What to test for in Walworth County

Volatile organic compounds (VOCs)136 leaking underground storage tank sites on record locally.
Iron15% of 13 local wells tested were above the Wisconsin nuisance level.
Total coliform9% of 23 local wells tested were above the Wisconsin limit (any detection).
Arsenic8% of 255 local wells tested were above the Wisconsin enforcement standard.
Sodium8% of 13 local wells tested were above the EPA advisory level.
Nitrate5% of 1,692 local wells tested were above the Wisconsin enforcement standard.
Lead5% of 22 local wells tested were above the Wisconsin enforcement standard.
Coliform bacteriaRecommended for every private well, every year.
RadonMedian local reading is 220 pCi/L.

Contamination sources on record in Walworth County

Wisconsin DNR records 136 leaking underground storage tank sites in Walworth County, among 508 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.

136: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
47: lined landfills and transfer facilities
325: 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
Iron: Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
Sodium: Point-of-use reverse osmosis; Distillation
Copper: Neutralizing filter to raise pH; Point-of-use reverse osmosis
Fluoride: Point-of-use reverse osmosis; Activated alumina; Distillation

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Common questions

Is there arsenic in well water in Walworth County?

Arsenic was detected in 0% of 1 public well samples in Walworth County, and 0% 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 Walworth County?

The median radon level across 7 public samples in Walworth County was 220 pCi/L, with a high of 290 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Walworth 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 Walworth County

Bloomfield5% nitrate
Bloomfield village5% nitrate
Darien13% nitrate
Delavan7% nitrate
East Troy3% nitrate
Elkhorn1% nitrate
Geneva0% nitrate
Lafayette3% nitrate
La Grange7% nitrate
Linn18% arsenic
Lyons0% nitrate
Richmond11% nitrate
Spring Prairie7% nitrate
Sugar Creek14% nitrate
Troy6% nitrate
Walworth11% nitrate
Whitewater2% nitrate

13 towns have fewer than 30 public samples and are included in the Walworth County totals above rather than shown separately: Burlington, Darien village, Delavan, East Troy village, Fontana-on-Geneva Lake village, Genoa City village, Lake Geneva, Mukwonago village, Sharon, Sharon village, Walworth village, Whitewater, Williams Bay village.

Sources

USGS Water Quality Portal: 61 samples in Walworth 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 Walworth 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: 508 regulated sites located in Walworth 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.