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Well water testing in Marathon County, Wisconsin

What official data says about private wells in Marathon County, what to test for, and the state-certified labs nearby.

People on private wells (2015)45,126

About 33% of the county's 135,868 residents. Rank 5 of 72 in Wisconsin.

Arsenic above EPA’s limit (USGS estimate)about 2,687 people

95% range 2,147 to 3,378 private-well users (2010 base year). Rank 3 of 72 in Wisconsin.

EPA radon zone (indoor air)Zone 1

Highest potential (average indoor radon may be greater than 4 pCi/L). EPA says the map should not be used to decide whether an individual home needs testing.

Certified labs in this county2

Labs with an address in this county, from the official state lists on this site, matched by ZIP code.

What to test for in Marathon County

Private wells are not regulated like public water: EPA advises owners to test their well every year for total coliform bacteria, nitrates, total dissolved solids and pH. Your state's own schedule:

  • Bacteria once a year and when the taste, color or smell changes. Nitrate once a year and before the well is used by someone who is or may become pregnant. Arsenic once every five years for most well users; every year in Outagamie, Winnebago or Brown county or if arsenic was found before. source

Arsenic. USGS estimates that about 2,687 people using private wells in Marathon County had arsenic above 10 µg/L, EPA's limit for public water (2010 base year; 95% range 2,147 to 3,378). USGS describes this kind of estimate as useful "for broad screening of the population at risk", so it says nothing certain about any one well. Arsenic has no taste or smell: test for it by name.

Radon. EPA puts Marathon County in Zone 1: highest potential (average indoor radon may be greater than 4 pCi/L). That map is about radon in indoor air, and EPA says it should not be used to decide whether an individual home needs testing. If radon is found in your air and you have a well, EPA suggests testing the water too: see radon in well water.

For anything specific to your home (farms nearby, old plumbing, a new baby, a smell or stain), use the which-tests checklist. When the results arrive, check your numbers against EPA's limits.

Estimated private-well users with arsenic above 10 µg/L: Marathon County and Wisconsin's highest countiespeople (2010 base year)
Show the numbers
ItemValueNote
Waukesha County4,188 people (2010 base year)
Winnebago County2,991 people (2010 base year)
Marathon County (this county)2,687 people (2010 base year)
Kenosha County2,460 people (2010 base year)
Racine County2,416 people (2010 base year)
Sheboygan County2,247 people (2010 base year)

USGS model estimate for broad screening, not a measurement of any well. Only a test tells you about your own well. Source: USGS, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter (Ayotte, Medalie and Qi, 2017), accessed 2026-11-01.

PFAS in public water near Marathon County (public supply, not your well)

This is public water data, not your well. A private well draws its own groundwater, which can have more or less PFAS than the public supply nearby. Only a test of your well tells you.

Under EPA’s fifth Unregulated Contaminant Monitoring Rule (UCMR 5), public water systems sampled their water for 29 PFAS between Jan 3, 2023 and Jun 17, 2026. 5 public systems that serve ZIP codes mainly in Marathon County reported results. 3 of them had at least one sample with PFOA or PFOS at or above 4.0 ng/L, and 4 had at least one of the 29 PFAS measured at or above its reporting level.

Public water systems with the highest single PFOA or PFOS result
Public water systemSystem IDHighest PFOA (ng/L)Highest PFOS (ng/L)
Weston Municipal UtilitiesWI73701635.913
Rib Mountain Sanitary DistWI73706605.29.4
Schofield WaterworksWI73701624.7

4.0 ng/L (parts per trillion) is both EPA’s UCMR 5 reporting level for PFOA and PFOS and the number in EPA’s 2024 limit for public water. That limit is judged on a running annual average, so one sample at that level is not a violation. EPA proposed changes to its PFAS rules in May 2026: see PFAS in well water for the current status and who should test a well. To test your own well, find a lab certified for PFAS.

Method: a system counts for this county when it reports serving a ZIP code whose largest land part is in this county, so a system serving several counties counts in each. Source: U.S. EPA, UCMR 5 occurrence data (by state), file dated Oct 15, 2026; downloaded Nov 1, 2026.

Testing programs and help in Wisconsin

  • The Wisconsin PFAS Private Well Testing Program offers a free PFAS kit and analysis to eligible well owners. Apply through the DNR page. source

Buying a home with a well in Marathon County

Loan test (FHA, VA, USDA) in Marathon County, WI

  • VA. Test results are valid for 90 days from the date the local health authority certifies them, unless the local authority says otherwise. VA’s chapter names no list of contaminants: the water must meet the local health authority’s requirements, or EPA guidelines where it has none. Your lender may ask for specific tests.
  • FHA. A well test is required in the situations FHA’s handbook lists, among others, and the report may not be more than 180 days old at disbursement.
  • USDA. Guaranteed loans need a water quality test for a private well, done by the local health authority or a state-certified lab, and no more than 180 days old at closing.

Here:

  • Wisconsin State Laboratory of Hygiene, prices effective July 1, 2026: HUD/FHA-VA loan package, $146 (checked 1 Nov 2026) source
  • U.s. Water, LLC prints a loan or home-sale package: Real Estate Testing Package $79 Arsenic, Coliform, Nitrate. Popular with Real estate agents, home buyers, home sellers. source

Lenders can ask for more (extra tests, a particular lab, who takes the sample): ask your lender before you book. Federal rules as read on Nov 3, 2026 source 1 source 2 source 3. Full comparison: VA, FHA and USDA well water test requirements. Loan test kits sold by mail, with price and days to results: kits compared.

State-certified labs in Marathon County

Marathon County Public Health Lab

1000 Lakeview Dr, Wausau, WIPublic samples: ask the labColiform / E. coli

U.s. Water, LLC

6905 Venture Cir, Weston, WIHomeowner samples: yes (lab’s website)Coliform / E. coli

Testing by mail from Marathon County. Wisconsin State Laboratory of Hygiene, prices effective July 1, 2026 publishes these prices: Bacteria (total coliform / E. coli) $35; Nitrate $35; Arsenic $38 (checked 1 Nov 2026) source. All Wisconsin state-lab prices and how to get a kit.

Labs on the Wisconsin list whose own websites say they take homeowner samples: Agsource Cooperative Services (Marshfield), Ashland County Health & Human Services (Ashland), Badger Laboratories, Inc. (Green Bay), Badger Laboratories, Inc. (Stevens Point), Badger Laboratories, Inc. (Neenah), Chippewa County (Chippewa Fls), Clark County Health Department (Neillsville), Commercial Testing Laboratory Inc (Colfax) and 24 more. Use Check my ZIP to sort labs by distance.

See all Wisconsin labs in the finder and filter by the test you need and by labs that take homeowner samples.

Sources and method Private-well population is the USGS 2015 estimate of people on domestic self-supplied water. The arsenic figure is a USGS model estimate on a 2010 population base, not a count of tested wells. Labs are placed in a county by the ZIP code on the state list. These figures come from different USGS products, years and methods, so they can disagree with each other (for example, the arsenic estimate and the well population use different base years). Read them as context for the county, not as facts about any one well. Checked 1 Nov 2026. General information, not medical advice.
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