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Wisconsin water profile

Nitrate in Wisconsin community water systems

Wisconsin's higher nitrate concentrates in three vulnerable landscapes: the northeastern dolomite karst, the Central Sands, and the Driftless southwest. See where the pattern shows in public systems and what each reporting system measured in the three years from August 9, 2023 to August 9, 2026.

Browse all 1,001 Wisconsin systems →

Search by the utility or system name shown on a water bill. Example: Madison Water Utility. Private wells are not in this inventory.

98%
of the state’s 1,022 active community water systems have a current result here
1,001
systems with a current nitrate result
4,036,213
people served by those reporting systems
8.1%
of reporting systems at or above 5 mg/L
How to read these numbers Each system is placed into a band by its most recent reported result within a rolling three-year window. The bands describe how strongly human activity, read from nitrate, has marked a water source; they are not a safety verdict. Figures describe system reporting, not nitrate measured at an individual tap.

How Wisconsin's systems fall across the five bands

Most systems report below 3 mg/L

Natural background
0.00–0.99 mg/L as N
632 systems · 2,593,806 served
Human impact likely
1.00–2.99 mg/L as N
184 systems · 808,448 served
Human impact clear
3.00–4.99 mg/L as N
104 systems · 296,108 served
Health-study range
5.00–9.99 mg/L as N
81 systems · 337,851 served
Above the federal limit
10.00 mg/L and above
0 systems · 0 served

Each system appears in one band only, placed by its most recent reported result in the current three-year window. Counts sit beside people served so a small number of larger systems cannot hide behind a row of tiny ones. Of the 81 systems reporting 5 mg/L or more, 24 stayed at or above 5 across the whole three-year range; the other 57 reached it only on a high sample and otherwise ran lower.

Most recent reading Jul 2026; the typical system was last tested around Feb 2026. 964 systems sampled their own water; 37 systems carry a supplier’s result.

The named systems

Every reading here belongs to a utility with a name

Highest recent readings
SystemMost recent resultBandBased onPeople served
Fifield Sanitary District9.75 mg/LMay 2026a high point — runs lowerHealth-study range14 samples300
Westwind Estates9.70 mg/LMar 2026Health-study range11 samples160
Country Cove Court Llc9.30 mg/LApr 2026Health-study range10 samples150
Country Road Estates9.15 mg/LJun 2026Health-study range14 samples120
Maizefield Mhc (Manufactured Home Community)8.33 mg/LApr 2026a high point — runs lowerHealth-study range12 samples145
Monticello Waterworks8.10 mg/LApr 2026a high point — runs lowerHealth-study range18 samples1,217
Thorp Waterworks7.90 mg/LJun 2026a high point — runs lowerHealth-study range62 samples1,636
Amherst Waterworks7.87 mg/LMar 2026a high point — runs lowerHealth-study range31 samples1,119
Largest systems
SystemMost recent resultBandBased onPeople served
Milwaukee Waterworks0.36 mg/LMay 2026Natural background3 samples626,000
Madison Water Utility2.71 mg/LJun 2026Human impact likely68 samples272,000
Green Bay Waterworks0.22 mg/LMay 2025Natural background2 samples107,369
Racine Waterworks0.34 mg/LFeb 2026Natural background3 samples105,100
Kenosha Water Utility0.50 mg/LJan 2025Natural background2 samples99,218

Each result shows its date and what it rests on, so a lone sample is never taken for a settled measurement. A single high sample is not a complete account of what a household drinks.

Where higher nitrate concentrates

Higher results gather in the vulnerable landscapes

Human impact is clear on drinking water served in 50 of Wisconsin’s 72 counties. Each is shaded by how many of its systems reported 3.00 mg/L as N or above. Shading counts systems, not severity — a county with many small utilities can outrank one holding a single larger system that reads higher. The table beside the map splits the two bands so that difference stays visible.

WI counties shaded by number of community water systems reporting nitrate at or above 3.00 mg/L as NCounties are shaded by how many community water systems reported nitrate at or above 3.00 mg/L as N in the three years from August 9, 2023 to August 9, 2026. The same counts appear in the table below this map.Marinette County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWaushara County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWaupaca County — 6 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveManitowoc County — 2 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveLincoln County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveTaylor County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveWaukesha County — 3 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveMarathon County — 8 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveClark County — 4 systems at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or abovePortage County — 3 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveWood County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveGreen Lake County — 0 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveRichland County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveRock County — 1 system at 3.00–4.99 mg/L as N; 6 systems at 5.00 mg/L as N or aboveKenosha County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveMarquette County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or abovePrice County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveLanglade County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveChippewa County — 3 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveDodge County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or abovePierce County — 1 system at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or abovePolk County — 4 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSt. Croix County — 3 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveColumbia County — 7 systems at 3.00–4.99 mg/L as N; 8 systems at 5.00 mg/L as N or abovePepin County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveCalumet County — 0 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveBarron County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveCrawford County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveDane County — 10 systems at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or aboveGrant County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveLa Crosse County — 0 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveEau Claire County — 3 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveJackson County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveDoor County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveGreen County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveVilas County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveShawano County — 4 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveDunn County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveTrempealeau County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveFond du Lac County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWashington County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveWalworth County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveAdams County — 1 system at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveSauk County — 2 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveJefferson County — 4 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveMonroe County — 1 system at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveForest County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveJuneau County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveIowa County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSheboygan County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or above
none1–23–56–1112–2526+Number of drinking water systems reporting ≥3.00 mg/L NO₃–N per county as of August 9, 2026

Columbia leads this layer with 8 reporting systems, followed by Rock, Clark, Dane. Higher results gather in the vulnerable landscapes.

Counties with the most systems at or above 3 mg/L as N
County3–4.99≥5Total ≥3
Columbia7815
Dane10515
Marathon8311
Clark459
Chippewa347

Counts use each system's most recent reported result in the current three-year window. A system appears in only one of the two bands.

Find a specific Wisconsin system

What the statewide pattern says

Three vulnerable geologies

Wisconsin's higher nitrate is concentrated in three geologies — northeastern dolomite karst, the irrigated Central Sands, and Driftless fractured carbonate — and in the dolomite country the water often carries bacteria along with the nitrate.

Three Wisconsin geologies do most of the work: the fractured Silurian dolomite along Lake Michigan, the sandy Central Sands, and the Driftless karst of the southwest — the same creviced-limestone belt that crosses into northeast Iowa (Nolan and Hitt, 2006).

In the northeastern dolomite, the deciding factor is how shallow the bedrock sits. Where thin soil covers fractured rock, manure and fertilizer nitrogen reach the aquifer with little to stop them. The landmark Kewaunee County study sampled 624 private wells and found roughly a quarter to a third contaminated with nitrate or bacteria — with depth-to-bedrock and nearby agricultural land, not well construction or septic systems, the dominant risk factors (Borchardt and others, 2021). A companion microbial source-tracking study in the same aquifer traced contamination to both cattle and human sources (Environmental Health Perspectives, 2021).

The Central Sands are the opposite texture with the same result: coarse, oxygen-rich sand under irrigated vegetables lets nitrate move down and persist, because the low-oxygen conditions that would break it down are absent (Dubrovsky and others, 2010). The supported source reading is agricultural — manure-heavy in the dairy karst, fertilizer-heavy in the irrigated sands — not a certified source at any individual system.

One caution about the public-system map: in the northeastern dolomite counties, much of the population drinks from private wells, so few public systems exist there to count. The map's footprint — south-central and central counties — understates the karst problem precisely where the private-well literature says it is worst.

The general science — how nitrate moves, why some ground is vulnerable, and why the legacy lingers — is laid out with its sources on Where nitrate comes from.

Questions this page answers

What the Wisconsin pattern does—and does not—say

Where does nitrate concentrate in Wisconsin community water systems?

Columbia leads this layer with 8 reporting systems, followed by Rock, Clark, Dane. Higher results gather in the vulnerable landscapes.

Why does the map show little in the northeast karst counties?

Because that region drinks from private wells, not public systems, so there are few public systems to count. The private-well literature — a quarter to a third of Kewaunee County wells contaminated — shows the karst problem is real; the public-system map simply cannot see it.

Can this page say the source is agriculture?

At the zone level, yes — carefully: manure-heavy in the dairy karst, fertilizer-heavy in the irrigated Central Sands. Microbial source tracking in Kewaunee County found both cattle and human signatures, so manure, fertilizer, and septic cannot be fully separated at each system.

Does a high public-system result mean a nearby private well is high?

No. It is a local sentinel, not a well-specific result. It shows that nitrogen and a transport pathway have reached groundwater in the area; a certified-lab test is still the only way to know a specific private well.

Sources and provenance

What this page rests on

  1. NitrateSmart inventory: 1,001 Wisconsin community water systems with a current nitrate result in the three years from August 9, 2023 to August 9, 2026. Figures count people served by reporting systems, not measured tap exposure.
  2. Nolan BT, Hitt KJ (2006). Vulnerability of Shallow Groundwater and Drinking-Water Wells to Nitrate in the United States. Environmental Science & Technology 40(24):7834–7840.
  3. Dubrovsky NM, and others (2010). Nutrients in the Nation's Streams and Groundwater, 1992–2004. USGS Circular 1350.
  4. Borchardt MA, and others (2021). Sources and risk factors for nitrate and microbial contamination of private household wells in the fractured dolomite aquifer of northeastern Wisconsin. Environmental Health Perspectives 129(6):067004.
  5. Borchardt MA, and others (2021). Farm to faucet? Agricultural waste and private well contamination in Kewaunee County, Wisconsin. Environmental Health Perspectives 129(11) — microbial source tracking found cattle and human signatures.