Well water in Clark County, Wisconsin
In Clark County, 9% of 153 private wells tested between 1999 and 2025 had total coliform above the Wisconsin limit (any detection), according to the UWSP Well Water Quality Viewer.
1,135 private wells tested 2001 to 2024 in state summaries; 1 individual samples from 1 wells, 1963 to 1980. Data refreshed September 9, 2026.
What homeowner tests found
Wisconsin Well Water Quality Viewer samples: private wells tested and reported to the state, 2001 to 2024. 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.
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What this means
Among homeowners in Clark County who sent a sample to the state lab, the health-based limits most often exceeded were total coliform (9% of 153 wells), nitrate (6% of 1,135 wells), and copper (6% of 71 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 Clark County
- Volatile organic compounds (VOCs)64 leaking underground storage tank sites on record locally.
- Iron32% of 50 local wells tested were above the Wisconsin nuisance level.
- Total coliform9% of 153 local wells tested were above the Wisconsin limit (any detection).
- Sodium6% of 48 local wells tested were above the EPA advisory level.
- Nitrate6% of 1,135 local wells tested were above the Wisconsin enforcement standard.
- Copper6% of 71 local wells tested were above the Wisconsin enforcement standard.
- Lead5% of 74 local wells tested were above the Wisconsin enforcement standard.
- Manganese3% of 71 local wells tested were above the Wisconsin enforcement standard.
- Arsenic<1% of 151 local wells tested were above the Wisconsin enforcement standard.
Contamination sources on record in Clark County
Wisconsin DNR records 64 leaking underground storage tank sites in Clark County, among 278 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.
- 64
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 57
- lined landfills and transfer facilities
- 157
- 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
- Copper
- Neutralizing filter to raise pH; Point-of-use reverse osmosis
- 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
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Common questions
How often should I test a private well in Clark 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 Clark County
- Colby5% nitrate
- Dewhurst0% nitrate
- Eaton5% nitrate
- Fremont0% nitrate
- Hixon12% nitrate
- Hoard3% nitrate
- Lynn6% nitrate
- Mentor3% nitrate
- Pine Valley10% nitrate
- Warner12% nitrate
- Weston16% nitrate
- Withee3% nitrate
- York18% nitrate
33 towns have fewer than 30 public samples and are included in the Clark County totals above rather than shown separately: Abbotsford, Beaver, Butler, Colby, Curtiss village, Dorchester village, Foster, Grant, Granton village, Green Grove, Greenwood, Hendren, Hewett, Levis, Longwood, Loyal, Loyal, Mayville, Mead, Neillsville, Owen, Reseburg, Seif, Sherman, Sherwood, Stanley, Thorp, Thorp, Unity, Unity village, Washburn, Withee village, Worden.
Sources
- USGS Water Quality Portal: 4 samples in Clark 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 Clark County, up to 1,135 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: 278 regulated sites located in Clark 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.