Well water in Oconto County, Wisconsin

In Oconto County, 13% of 104 private wells tested between 2000 and 2025 had total coliform above the Wisconsin limit (any detection), according to the UWSP Well Water Quality Viewer.

2,029 private wells tested 2003 to 2025 in state summaries; 11 individual samples from 8 wells, 1935 to 2024. Data refreshed September 9, 2026.

What homeowner tests found

Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 2003 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
Iron	23	13%	1,110	300 ug/L
Total coliform	104	13%
Arsenic	434	7%	129	10 ug/L
Lead	35	6%	77	15 ug/L
Sodium	23	4%	32	20 mg/L
Nitrate	2,029	1%	129	10 mg/L
Chloride	99	1%	284	250 mg/L
Copper	35	0%	0.288	1.3 mg/L
Manganese	35	0%	84	300 ug/L
E. coli	12	0%

What individual samples found

Each row summarizes every public sample for that contaminant inside Oconto 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 2002 to 2024	11	100%	0%		0.769	4.75	30 ug/L EPA limit

Sampled fewer than 10 times, too few to summarize: Radium (226 + 228) (8), Arsenic (2), Copper (2), Fluoride (1), Radon (4), Manganese (1), Chloride (1), Sodium (1).

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What this means

Among homeowners in Oconto County who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (13% of 104 wells), arsenic (7% of 434 wells), and lead (6% of 35 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 Oconto County

Volatile organic compounds (VOCs)54 leaking underground storage tank sites on record locally.
Iron13% of 23 local wells tested were above the Wisconsin nuisance level.
Total coliform13% of 104 local wells tested were above the Wisconsin limit (any detection).
Arsenic7% of 434 local wells tested were above the Wisconsin enforcement standard.
Lead6% of 35 local wells tested were above the Wisconsin enforcement standard.
Sodium4% of 23 local wells tested were above the EPA advisory level.
Nitrate1% of 2,029 local wells tested were above the Wisconsin enforcement standard.
Chloride1% of 99 local wells tested were above the Wisconsin nuisance level.
Coliform bacteriaRecommended for every private well, every year.

Contamination sources on record in Oconto County

Wisconsin DNR records 54 leaking underground storage tank sites in Oconto County, among 235 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.

54: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
52: lined landfills and transfer facilities
129: 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
Lead: Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching
Iron: Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
Fluoride: Point-of-use reverse osmosis; Activated alumina; Distillation
Manganese: Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis

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Common questions

Is there arsenic in well water in Oconto County?

Arsenic was detected in 100% of 2 public well samples in Oconto 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 Oconto County?

The median radon level across 4 public samples in Oconto County was 430 pCi/L, with a high of 600 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

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

Abrams0% arsenic
Brazeau2% nitrate
Breed2% nitrate
Chase0% nitrate
Doty3% nitrate
Gillett7% nitrate
Lakewood2% nitrate
Little River0% nitrate
Little Suamico1% arsenic
Morgan34% arsenic
Mountain4% nitrate
Oconto0% nitrate
Oconto Falls0% nitrate
Pensaukee0% nitrate
Riverview1% nitrate
Spruce1% nitrate
Stiles0% nitrate
Townsend<1% nitrate
Underhill2% nitrate

10 towns have fewer than 30 public samples and are included in the Oconto County totals above rather than shown separately: Bagley, Gillett, How, Lena, Lena village, Maple Valley, Oconto, Oconto Falls, Pulaski village, Suring village.

Sources

USGS Water Quality Portal: 31 samples in Oconto 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 Oconto County, up to 2,029 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: 235 regulated sites located in Oconto 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.