Well Water in Grant County, Wisconsin

The SWIGG study: what was tested, and what was found

The Southwest Wisconsin Groundwater and Geology (SWIGG) study was a partnership between the U.S. Geological Survey, UW-Madison, UW-Madison Division of Extension, and the health and land-conservation departments of Grant, Iowa, and Lafayette Counties. Over 2018 and 2019, the study collected 978 water samples from 816 private wells across the three counties -- a genuinely large, methodologically serious sample for a regional groundwater study, not a handful of anecdotal test results.

The headline finding: roughly a third of tested wells across the three-county region had levels of total coliform bacteria or nitrate exceeding state and federal health standards, with an earlier phase of the study finding as many as 42% of tested wells at unsafe levels for bacteria or nitrate. In one round of April sampling specifically, 27% of wells failed to meet health standards for total coliform, E. coli, or nitrate combined -- 16% tested positive for total coliform, 2% for E. coli, and 15% exceeded the nitrate-nitrogen health threshold of 10 parts per million, with the highest single sample result at 67 ppm (nearly seven times the standard) against a study-wide median of 3 ppm.

Why: agriculture, septic systems, and karst-prone bedrock together

The study attributes contamination to a combination of three factors that are all common across Grant County specifically, not just the wider region. First, agricultural land use: wells with elevated nitrate levels were statistically associated with nearby agricultural land, consistent with fertilizer and manure as nitrate sources -- relevant given Grant County's heavy dairy and row-crop base. Second, septic systems: bacterial and, in some cases, human-wastewater-marker contamination was more commonly detected in wells near private septic systems, a factor relevant to any rural parcel relying on an onsite POWTS system rather than municipal sewer. Third, and arguably the multiplier on the other two: the region's karst-prone bedrock -- fractured Ordovician-age dolomite and limestone (the Sinnipee and Prairie du Chien groups) that includes sinkholes, cracks, and other karst features -- lets both agricultural runoff and septic effluent reach groundwater faster and with less natural filtration than in areas with thicker soil cover over solid bedrock.

That combination means this isn't simply a "test your well because every well should be tested" caution (though that's always good practice everywhere) -- it's a documented, elevated regional risk specifically because Grant County combines heavy agricultural land use with fractured carbonate bedrock in a way that measurably increases the odds of contamination reaching a well.

What to actually test for and do

1. Test any existing well for total coliform bacteria, E. coli, and nitrate at minimum -- the three specific parameters the SWIGG study measured and found elevated -- through a certified lab (UW-Madison's Environmental Health Division / State Lab of Hygiene, or a private certified lab) rather than relying on a seller's past test result alone, since groundwater conditions and nearby land use can change.
2. Ask UW-Madison Division of Extension's Grant County office whether more recent or more localized SWIGG follow-up testing data exists for the specific area of the county you're considering -- the study has had multiple phases and ongoing outreach since 2018.
3. If nitrate is a concern, remember that Wisconsin's health standard (10 ppm nitrate-nitrogen) exists specifically because of health risks to infants and pregnant women (methemoglobinemia, or "blue baby syndrome") -- treat any result near or above that threshold seriously rather than as a minor water-quality nuisance.
4. For a new well on raw land, ask your well driller directly about the depth and geology at the specific site, since a well drilled below the fractured, karst-prone shallow bedrock into a deeper, better-protected aquifer zone may carry meaningfully lower contamination risk than a shallow well in the same area -- this is a site-specific engineering question, not something to assume either way.
5. If the parcel has an existing septic system, confirm its condition and setback from the well independently of the well test itself, since the study ties bacterial contamination partly to septic proximity -- see our septic-systems guide for what to check.