Well Water in Iowa County, Wisconsin

A county-specific dataset, separate from the regional SWIGG study

Iowa County is one of three counties (with Grant and Lafayette) covered by the 2018-2019 Southwest Wisconsin Groundwater and Geology (SWIGG) study, which tested 816 private wells across the three-county region and found roughly a third exceeding health standards for bacteria or nitrate. That study's full methodology and figures are covered on our Grant County well-water page, since SWIGG reports its results as a three-county aggregate rather than broken out county by county -- there is no confirmed Iowa-County-only breakdown of the SWIGG numbers specifically.

Separately, and worth knowing on its own terms, is a larger, longer-running, county-specific well-testing dataset: search-indexed sources report that 1,318 Iowa County wells have been tested, with 13% exceeding the state's nitrate-nitrogen health standard, 26% testing positive for total coliform bacteria, and 40% showing detectable levels of atrazine, a widely-used corn and soybean herbicide. This figure traces to a regional karst-education source (driftlessprairies.org) and likely draws on UW-Extension Iowa County's own long-running groundwater testing and outreach program, though this research pass could not confirm the exact testing years or access the likely primary document directly (see the methodology note below).

Why atrazine specifically is worth flagging

Atrazine is one of the most widely used herbicides in U.S. corn production and is applied across much of Iowa County's row-crop acreage. It's also mobile enough in groundwater that Wisconsin's Department of Agriculture, Trade and Consumer Protection (DATCP) maintains a statewide Atrazine Prohibition Area program under Wisconsin Administrative Code ATCP 30, banning atrazine use in specific townships where groundwater monitoring has already documented contamination -- a regulatory response that exists because atrazine detection in well water is a genuine, recurring finding in parts of southern and southwestern Wisconsin's agricultural areas, not a one-off result. This research pass could not confirm whether any specific Iowa County township is currently designated an Atrazine Prohibition Area; check DATCP's interactive mapping tool directly for a specific parcel's township.

Unlike bacteria, which is typically an immediate health signal tied to septic or surface-water intrusion, atrazine in well water is a longer-term exposure concern tied to agricultural land-use history in the surrounding area -- meaning a clean recent bacteria test doesn't tell you anything about atrazine risk, and vice versa. Both need to be tested for separately.

Karst bedrock is the common thread behind all three contaminants

Iowa County sits within Wisconsin's unglaciated Driftless Area, where bedrock is often only 0-50 feet below the soil surface and fractured by karst features (sinkholes, cracks, and underground channels). That shallow, fractured bedrock is what lets surface contaminants -- whether bacteria from septic systems and manure, nitrate from fertilizer, or atrazine from herbicide application -- reach the groundwater aquifer quickly and with little natural filtering, which is the same underlying driver behind both the SWIGG study's regional findings and Iowa County's own testing data.

What to actually test for and do

1. Test any existing well for total coliform bacteria, nitrate-nitrogen, and atrazine specifically -- not just the standard bacteria/nitrate pair -- through a certified lab (the Iowa County Health Department is a DATCP-certified bacteria testing lab; the State Laboratory of Hygiene or a private certified lab can test for atrazine and other pesticides).
2. Ask the Iowa County Health Department ((608) 930-9870) about eligibility for the free WellTAP testing program if there is a pregnant person, a child under 12, or an immunocompromised household member and testing cost is a barrier.
3. Check Wisconsin DATCP's Atrazine Prohibition Area interactive map for the specific township a parcel sits in, since atrazine use is legally banned in townships where past groundwater monitoring has already found contamination -- a designation is itself a signal about local groundwater vulnerability even apart from any specific well's test result.
4. Ask UW-Madison Division of Extension's Iowa County office whether more recent SWIGG follow-up data or additional county-specific well-testing results exist for the specific area of the county you're considering, since testing programs and results are updated periodically.
5. For a new well on raw land, ask the well driller about depth and geology at the specific site -- a well completed below the shallow, fractured karst zone into a deeper, better-protected aquifer may carry meaningfully lower contamination risk than a shallow well nearby, a site-specific engineering question rather than something to assume either way.