Well water in Wisconsin

1in10

Testing recommended

private wells tested between 1990 and 2025 in Wisconsin had total coliform above the Wisconsin limit (any detection), according to the UWSP Well Water Quality Viewer.

The strongest signal is PFOA: 30% of 10 local samples were above the limit, too few samples to carry the headline but first on the test list below.

121,661 private wells tested 1994 to 2025 in state summaries; 51,489 test results across 14 contaminants from at least 3,880 wells, 1875 to 2026; nitrate has the most at 10,364. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 1,598,065 people in Wisconsin get their drinking water from a private well, per USGS 2015 county water use.

Separately, the USGS national arsenic model estimates that 5% of the 1,598,065 people on private wells in Wisconsin draw water above 10 ug/L (the model's range is 4% to 6%), from geology and well depth rather than local tests.

What homeowner tests found

Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 1994 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	Above state level	Wells tested	Highest	State level
Iron	18%	6,371	61,640	300 ug/L
Sodium	11%	6,432	1,022	20 mg/L
Total coliform	10%	11,282
Nitrate	8%	121,661	129	10 mg/L
Arsenic	4%	22,114	6,300	10 ug/L
E. coli	4%	1,091
Manganese	4%	7,925	9,964	300 ug/L
Lead	3%	7,937	2,529	15 ug/L
Copper	1%	7,890	15.9	1.3 mg/L
Chloride	<1%	15,485	3,010	250 mg/L

What individual samples found

Each row summarizes every public sample for that contaminant inside Wisconsin. 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	Above limit	Samples	Detected	Median	Highest	Limit
PFOA 2019 to 2021	30%	10	40%	0	9.6	4 ng/L EPA limit
PFOS 2019 to 2021	20%	10	30%	0	16	4 ng/L EPA limit
Nitrate 1934 to 2026	13%	10,364	79%	0.27	208	10 mg/L EPA limit
Arsenic 1973 to 2024	2%	1,190	70%	0.38	250	10 ug/L EPA limit
Radium (226 + 228) 1983 to 2018	2%	350	71%	0.09	31	5 pCi/L EPA limit
Lead 1955 to 2024	2%	1,806	42%	0	16,200	15 ug/L EPA limit
Uranium 1954 to 2024	<1%	605	80%	0.457	41.9	30 ug/L EPA limit
Copper 1954 to 2024	<1%	1,718	44%	0	2.4	1.3 mg/L EPA limit
Fluoride 1944 to 2024	<1%	6,057	82%	0.1	7.5	4 mg/L EPA limit
Iron 1875 to 2024	no limit	7,871	83%	120	483,000	300 ug/L nuisance level
Chloride 1875 to 2026	no limit	7,868	98%	4	9,940	250 mg/L nuisance level
Manganese 1920 to 2024	no limit	7,116	78%	20	12,000	300 ug/L health guideline
Sodium 1875 to 2024	no limit	6,325	100%	2.9	2,490	20 mg/L health guideline
Radon 1982 to 2014	no limit	199	100%	390	26,000	2,000 pCi/L health guideline

Over time

Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.

Contaminant	before 2000	2000s	2010s	2020s
Nitrate	11%9,307 samples	24%446 samples	23%472 samples	27%139 samples
Fluoride	<1%5,566 samples	0%202 samples	0%234 samples	0%55 samples
Lead	2%1,361 samples	0%170 samples	0%201 samples	3%74 samples
Copper	<1%1,273 samples	0%171 samples	0%200 samples	0%74 samples
Arsenic	2%741 samples	2%174 samples	<1%201 samples	9%74 samples
Uranium	0%189 samples	0%162 samples	<1%199 samples	0%55 samples

Arsenic above the limit in Wisconsin rose from <1% of 201 samples in the 2010s to 9% of 74 in the 2020s.

Nitrate, fluoride, lead, copper, uranium have held within a few points between the two most recent periods with enough samples.

What this means

Among homeowners in Wisconsin who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (10% of 11,282 wells), nitrate (8% of 121,661 wells), and arsenic (4% of 22,114 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 and sodium 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 Wisconsin

1
PFOA
30% of 10 local samples were above the limit.
2
PFOS
20% of 10 local samples were above the limit.
3
Nitrate
13% of 10,364 local samples were above the limit.
4
Total coliform
10% of 11,282 local wells tested were above the Wisconsin limit (any detection).
5
Arsenic
4% of 22,114 local wells tested were above the Wisconsin enforcement standard.
6
E. coli
4% of 1,091 local wells tested were above the Wisconsin limit (any detection).
7
Lead
3% of 7,937 local wells tested were above the Wisconsin enforcement standard.
8
Radium (226 + 228)
2% of 350 local samples were above the limit.
9
Volatile organic compounds (VOCs)
10,362 leaking underground storage tank sites on record locally.

Test your Wisconsin well for PFOA, PFOS and nitrate

Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.

Order the Tap Score well water test kit, from $209 Already tested? Upload the report

Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.

Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.

Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.

Where to get a well tested in Wisconsin: certified labs, state programs and prices.

Already have results back? Get your lab report read against the Wisconsin numbers, $29. Every line explained, ranked by what matters, with the fix for each.

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Send me the Wisconsin report

The numbers above, which tests to order and where, and the treatment that fits the local profile, in one email.

The rock under Wisconsin

Not a principal aquifer

The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.

Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the state centroid. A single well can sit in a different unit, especially near a boundary.

Official help in Wisconsin

Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.

Wells, Wisconsin DNR
Laboratory Certification, Wisconsin DNR

Contamination sources on record in Wisconsin

Wisconsin DNR records 10,362 leaking underground storage tank sites in Wisconsin, among 34,512 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.

10,362: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
3,016: lined landfills and transfer facilities
21,134: 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

Nitrate

Point-of-use reverse osmosis; Anion exchange; Distillation

Iron

Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration

Sodium

Point-of-use reverse osmosis; Distillation

PFOA

Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange

PFOS

Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange

Systems we would look at first

Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.

For total coliform

Aquasana Rhino Well Water with UV

whole-house, at the point of entry, about $2,871

Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.

For nitrate

Under-sink reverse osmosis system

under-sink or countertop, about $300

Look for NSF/ANSI 58 certification listing the contaminant you need removed.

For nitrate

Aquasana SmartFlow reverse osmosis

under-sink or countertop, about $225

Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.

For iron

Iron and manganese oxidizing filter

whole-house, at the point of entry, about $1,800

Air-injection or greensand media; fixes staining at the source.

For PFOA

Aquasana Claryum 3-Stage under-sink filter

under-sink or countertop, about $175

Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.

For PFOA

Whole-house PFAS carbon system

whole-house, at the point of entry, about $3,000

Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.

For PFOA

Aquasana HealthShield whole-house filter

whole-house, at the point of entry, about $2,099

Whole-house carbon system the maker certifies to reduce PFAS (PFOA and PFOS) and VOCs at every tap; a UV option adds bacteria protection.

Common questions

Is there arsenic in well water in Wisconsin?

Arsenic was detected in 70% of 1,190 public well samples in Wisconsin, and 2% 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 Wisconsin?

The median radon level across 199 public samples in Wisconsin was 390 pCi/L, with a high of 26,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Wisconsin?

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.

Counties in Wisconsin

Want the numbers themselves? Download the Wisconsin dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 51,489 samples in Wisconsin. 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 Wisconsin, up to 121,661 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.
USGS national arsenic model (Ayotte 2017): estimated share of 1,598,065 people on private wells in Wisconsin above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
Wisconsin DNR regulated sites: 34,512 regulated sites located in Wisconsin. 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.