Well Water Nerd

Well water in Green Lake County, Wisconsin

In Green Lake County, 19% of 1,317 private wells tested between 1988 and 2025 had nitrate above the Wisconsin enforcement standard (10 mg/L), according to the UWSP Well Water Quality Viewer.

1,317 private wells tested 1988 to 2025 in state summaries; 4 individual samples from 3 wells, 1994 to 2013. Data refreshed September 9, 2026.

What homeowner tests found

Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 1988 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.

ContaminantWells testedAbove state levelMedian95th pctHighestState level
Nitrate1,31719%52.110 mg/L
Sodium7810%65.220 mg/L
Total coliform3405%
Iron765%4,111300 ug/L
Arsenic1582%52.310 ug/L
Manganese961%3,816300 ug/L
Chloride3430%186250 mg/L
Copper960%0.5281.3 mg/L
Lead960%415 ug/L
E. coli180%

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What this means

Among homeowners in Green Lake County who sent a sample to the state lab, the health-based limits most often exceeded were nitrate (19% of 1,317 wells), total coliform (5% of 340 wells), and arsenic (2% of 158 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 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 Green Lake County

  • Volatile organic compounds (VOCs)42 leaking underground storage tank sites on record locally.
  • Nitrate19% of 1,317 local wells tested were above the Wisconsin enforcement standard.
  • Sodium10% of 78 local wells tested were above the EPA advisory level.
  • Total coliform5% of 340 local wells tested were above the Wisconsin limit (any detection).
  • Iron5% of 76 local wells tested were above the Wisconsin nuisance level.
  • Arsenic2% of 158 local wells tested were above the Wisconsin enforcement standard.
  • Manganese1% of 96 local wells tested were above the Wisconsin enforcement standard.
  • Coliform bacteriaRecommended for every private well, every year.
  • RadonMedian local reading is 170 pCi/L.

Contamination sources on record in Green Lake County

Wisconsin DNR records 42 leaking underground storage tank sites in Green Lake County, among 208 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.

42
leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
26
lined landfills and transfer facilities
140
other remediation projects

Treatment that fits the local profile

Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Total coliform
Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
Sodium
Point-of-use reverse osmosis; Distillation
Iron
Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration

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Common questions

Is there arsenic in well water in Green Lake County?

Arsenic was detected in 50% of 2 public well samples in Green Lake 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 Green Lake County?

The median radon level across 3 public samples in Green Lake County was 170 pCi/L, with a high of 230 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Green Lake 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 Green Lake County

7 towns have fewer than 30 public samples and are included in the Green Lake County totals above rather than shown separately: Berlin, Green Lake, Kingston village, Markesan, Marquette village, Princeton, Seneca.

Sources

  • USGS Water Quality Portal: 13 samples in Green Lake 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 Green Lake County, up to 1,317 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: 208 regulated sites located in Green Lake 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.