Wisconsin Water Quality Deep Dive - 3,516 Systems Analyzed

Executive Summary

Wisconsin operates 3,516 public water systems monitored through state and federal testing programs, serving communities across 974 ZIP codes. Our analysis of 26,356 individual test results from EPA, state laboratory data, and Consumer Confidence Reports reveals 384 instances where contaminant levels exceeded federal or state Maximum Contaminant Levels (MCLs) - an overall exceedance rate of 1.46%.

The state's primary water quality challenges center on PFAS contamination, naturally occurring radium, and lead infrastructure in Milwaukee. Geographic risk patterns across Wisconsin reflect PFAS contamination from Tyco/Marinette manufacturing - one of the largest PFAS sites in the Midwest; naturally occurring radium in sandstone aquifers; nitrate from dairy farming; lead service lines in Milwaukee and older cities.

This report is not a summary - it is a ground-level examination of what the data actually shows. Every number comes from EPA SDWIS enforcement records, state laboratory testing programs, Consumer Confidence Reports filed by utilities, FEMA flood insurance claims, and Census Bureau housing stock data. Where the data tells a clear story, we state it plainly. Where it is ambiguous or incomplete, we note that too.

Contaminant Analysis

State laboratory testing and EPA monitoring data reveal the scope of contamination across Wisconsin's water supply. The following analysis covers both regulated contaminants with federal MCLs and state-specific standards - Wisconsin set PFAS standards among the most comprehensive in the nation; DNR has been aggressive on radium enforcement and lead service line replacement requirements.

Top Contaminants by MCL Exceedance Rate

How to read the copper row. EPA's federal reporting schema defines the CU90 code as a Copper Action Level Exceedance: the record exists only where a system's 90th-percentile copper result was at or above the 1.3 mg/L action level. Every copper record in this dataset is therefore an exceedance, which is why no rate is shown - there is no denominator of clean copper results to compute one from. For the same reason, a system with no copper record has not been shown to be clean: non-exceedance copper results are optional to report for systems serving 3,300 people or fewer. We do not publish a copper level because the same schema transmits no verified unit of measure for this code.

Copper appears here differently from every other contaminant in the table. EPA's federal database records a copper 90th-percentile result under the CU90 code specifically when it exceeds the 1.3 mg/L action level, so Wisconsin's 145 copper records across 88 systems are 145 action-level exceedances, not a sample of routine copper testing. Copper leaches from the same plumbing that leaches lead - household pipes, solder and fixtures - and the corrosion control that keeps one in check is what keeps the other in check. Read the count, not a rate: there is no reported population of clean copper results to divide by, and a system missing from this line has not been shown to be clean, only to have reported nothing.

PFAS ("Forever Chemicals") in Wisconsin

PFAS monitoring in Wisconsin covers 8,808 individual tests across multiple PFAS compounds. Current testing shows no exceedances of the 2024 EPA PFAS MCLs, though monitoring is ongoing and detection does not require exceedance to pose health concerns.

For detailed PFAS data by ZIP code, see the PFAS in Wisconsin report.

State vs. Federal Standards

Wisconsin set PFAS standards among the most comprehensive in the nation; DNR has been aggressive on radium enforcement and lead service line replacement requirements.

This regulatory landscape creates a two-tier compliance reality. A water system in Wisconsin may appear "in compliance" on federal reports while actually exceeding stricter state limits. For residents reading their annual Consumer Confidence Report, this distinction matters enormously - the report may reference federal standards while the state is enforcing tighter ones.

The gap between state and federal standards also affects how violations are counted. Our dataset captures both tiers, which is why the exceedance counts above may differ from EPA-only reporting. When we say a system "exceeds the MCL," we mean the applicable limit - federal or state, whichever is stricter.

Worst Water Systems by Violations

The following systems had the highest number of MCL exceedances in our dataset. A critical caveat: exceedance count alone does not mean a system is currently unsafe. Many exceedances are resolved through treatment adjustments, blending, or switching water sources. However, patterns of repeated violations across multiple contaminants or multiple years indicate systemic issues - underfunding, aging treatment infrastructure, or management failures - that are unlikely to resolve without intervention.

PWSID WI6270392 leads with 8 exceedances in our dataset. PWSID WI2670394 follows with 5 exceedances.

If you receive water from any of these systems, we recommend checking your specific ZIP code report for the most current violation status and filtration recommendations. Exceedance data tells you what has happened - your ZIP report tells you what to do about it.

Of Wisconsin's 3,516 monitored systems, the top 10 worst offenders account for 47 of the state's 384 total exceedances. This concentration pattern is common - a small number of chronically non-compliant systems drive a disproportionate share of violations statewide.

Enforcement & Compliance

EPA and state enforcement actions tell the story of how violations translate (or fail to translate) into accountability. The enforcement pipeline works in stages: a violation is detected, an informal action (like a warning letter) may be issued, and if non-compliance persists, formal enforcement - consent orders, administrative orders, or court actions - follows. The ratio between informal and formal actions reveals how aggressively a state pursues compliance.

Only 3% of enforcement actions in Wisconsin are formal (court orders, consent decrees, administrative penalties). The remaining 97% are informal - warning letters, compliance schedules, and technical assistance. This ratio matters: informal actions carry no legal penalty and rely on voluntary compliance. When systems repeatedly violate MCLs without facing formal enforcement, the deterrent effect weakens.

324 violations remain officially unresolved across the state. Each unresolved violation represents a system where contamination was detected, documented, and - as of our latest data - not yet remediated to the satisfaction of regulators. Marinette/Peshtigo PFAS contamination from Tyco drove statewide PFAS regulations; Milwaukee's lead service line replacement program is underway; radium treatment upgrades continue in dozens of communities.

Areas with Most Health Violations

Geographic Risk Patterns

Water quality risk in Wisconsin is not evenly distributed. PFAS contamination from Tyco/Marinette manufacturing - one of the largest PFAS sites in the Midwest; naturally occurring radium in sandstone aquifers; nitrate from dairy farming; lead service lines in Milwaukee and older cities create distinct regional patterns that are visible in the data.

Understanding where water quality problems concentrate is as important as understanding what contaminants are present. A statewide average conceals enormous ZIP-to-ZIP variation - two communities 20 miles apart may have completely different risk profiles based on their water source, treatment infrastructure, and local geology.

Data Anomalies & Notable Findings

Our automated anomaly detection system flagged 8+ patterns worth investigation in Wisconsin:

High-severity findings:

• ZIP 53080 (severity 8/10): PFAS cluster: 3 adjacent ZIPs near Saukville, WI all exceed limits - View full report
• ZIP 53186 (severity 8/10): PFAS cluster: 3 adjacent ZIPs near Waukesha, WI all exceed limits - View full report
• ZIP 54701 (severity 8/10): PFAS cluster: 3 adjacent ZIPs near Eau Claire, WI all exceed limits - View full report

Lead Exposure & Infrastructure Age

Lead contamination in drinking water is almost never caused by the water source itself - it leaches from lead service lines, lead solder in copper pipes, and brass fixtures as water sits in contact with these materials. This means lead risk is fundamentally an infrastructure problem, and infrastructure age is the single strongest predictor.

In Wisconsin, Milwaukee has one of the highest concentrations of lead service lines in the nation; rural systems in dairy country face nitrate challenges; radium is a persistent problem in deep well communities. The federal Lead and Copper Rule (LCR) requires utilities to test a sample of high-risk homes and report the 90th percentile lead level - meaning 90% of samples must be below the 15 ppb action level. But this sampling methodology has long been criticized: utilities often avoid the worst homes, and the action level itself is not a health-based standard (the EPA has stated there is no safe level of lead exposure).

Across Wisconsin, 747 ZIP codes have elevated or high lead pipe risk based on housing age, and 302 have elevated electrical system risk. These infrastructure age indicators are derived from Census Bureau American Community Survey data on housing stock vintage.

The connection between housing age and water contamination risk is well-documented: homes built before 1986 (when the federal ban on lead solder took effect) are significantly more likely to have lead in their plumbing. Homes built before 1950 face even higher risk, as lead service lines were standard construction practice in many parts of the country during that era.

Highest Lead Exposure Risk ZIP Codes

Flood Risk & Water Infrastructure

Flooding directly threatens water quality through multiple mechanisms: overwhelmed treatment plants release partially treated water, floodwaters can infiltrate well heads and contaminate groundwater sources, damaged distribution lines create entry points for bacteria and sediment, and power outages disable treatment systems entirely. In the aftermath of major flood events, boil-water advisories become common - but many residents in affected areas may not receive timely notification.

• 500 ZIP codes in Wisconsin have FEMA flood insurance claims on record
• 9,372 total flood insurance claims filed historically
• $136.1 million in total flood damage payouts

The average flood insurance claim payout in Wisconsin is $14,518. While flood damage is typically associated with structural property damage, the water quality implications are often overlooked. Communities with repeated flooding face compounding infrastructure degradation - each event weakens pipes, treatment facilities, and distribution systems that may not be fully restored before the next event.

Consumer Confidence Reports (CCRs)

Water utilities are required to publish annual Consumer Confidence Reports. We have parsed CCR data for 278 ZIP codes in Wisconsin, documenting 32 self-reported violations and 111 systems with detectable lead levels.

CCR data is self-reported by utilities and may undercount actual contamination events. Cross-referencing CCR data with EPA SDWIS violation records provides a more complete picture - which is exactly what ZipCheckup reports do for every ZIP code.

Trend Analysis & Regulatory Outlook

Marinette/Peshtigo PFAS contamination from Tyco drove statewide PFAS regulations; Milwaukee's lead service line replacement program is underway; radium treatment upgrades continue in dozens of communities.

Three major regulatory forces are reshaping water quality across Wisconsin and the country:

Lead and Copper Rule Improvements (LCRI): The 2024 LCRI represents the most significant update to lead regulation since the original 1991 rule. It requires all water systems to complete a lead service line inventory, lower the action level trigger from 15 ppb to 10 ppb, and replace all lead service lines within 10 years. For Wisconsin's 3,516 systems, this means billions in infrastructure investment - and a fundamental reshaping of the lead risk landscape we document above.

PFAS National Primary Drinking Water Regulation (2024): For the first time, EPA set enforceable MCLs for six PFAS compounds - PFOA and PFOS at 4 ppt, and four others at various levels. Systems nationwide are still in the initial monitoring phase, which is why our PFAS data captures detections that may not yet have triggered formal violations. Treatment to remove PFAS (primarily granular activated carbon or reverse osmosis) is expensive, and many small systems will struggle to comply within the 3–5 year implementation timeline.

State-level action: Wisconsin set PFAS standards among the most comprehensive in the nation; DNR has been aggressive on radium enforcement and lead service line replacement requirements. As federal regulation catches up to state standards in some areas, the patchwork of requirements creates an uneven compliance landscape that makes cross-state comparisons complex but ZIP-level analysis essential.

What Wisconsin Residents Should Do

Based on our analysis of 26,356 test results and 974 ZIP codes, here are specific actions for Wisconsin residents:

1. Check your ZIP code report - enter your ZIP at ZipCheckup.com to see contaminant data, violation history, and risk scores specific to your address
2. Request your utility's CCR - if your ZIP is not in our CCR database, request the latest Consumer Confidence Report directly from your water utility
3. Test your water independently - home water testing kits ($30–$150) can detect lead, bacteria, and common contaminants. Lab testing ($100–$400) provides more comprehensive results
4. Consider filtration - for the contaminants most prevalent in Wisconsin (PFAS, Lead, Nitrate), reverse osmosis or NSF-certified carbon filters provide the most effective protection
5. Check for lead service lines - if your home was built before 1986, contact your utility to determine if you have a lead service line. Many utilities now offer free inspections
6. Prepare for flood events - if you're in a flood-prone area, keep bottled water reserves and know how to shut off your water main. After any flood, do not use tap water until your utility confirms safety

Methodology & Data Sources

This analysis combines multiple data sources:

• EPA SDWIS - Safe Drinking Water Information System violation and enforcement records
• State laboratory data - WI — Lead & Copper 90th Percentile (EPA ECHO LCR); WI — PFAS Monitoring (UCMR5 National Dataset); WI — SDWIS Health-Based Violations (EPA Envirofacts); WI — Community Water Systems (EPA SDWIS) (26,356 records)
• EPA ECHO - Enforcement and Compliance History Online, including PFAS detections and enforcement actions
• Consumer Confidence Reports - parsed and cross-referenced with EPA data for 278 ZIP codes
• FEMA NFIP - National Flood Insurance Program claims data
• Census ACS - Housing age and demographic data for infrastructure risk modeling
• Lead exposure modeling - ZipCheckup's proprietary lead risk score combining housing age, water test results, and service line data

All data is updated regularly. This report reflects data available as of 2026-07-19.

Related Reports

• Wisconsin State Overview - ZIP rankings, county breakdown, and safety scores
• PFAS in Wisconsin - detailed contaminant breakdown
• Lead in Wisconsin - detailed contaminant breakdown
• Nitrate in Wisconsin - detailed contaminant breakdown
• Radium in Wisconsin - detailed contaminant breakdown
• Water Safety Rankings by State - compare Wisconsin to other states

Highest-Risk ZIP Codes in Wisconsin

• 53969 Wyocena Water Report - Lead: 16.8 ppb
• 53039 Juneau Water Report - Lead: 19 ppb
• 53094 Watertown Water Report - Lead: 15 ppb
• 53594 Waterloo Water Report - Lead: 18 ppb
• 53806 Cassville Water Report - Lead: 15.1 ppb

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  author = {{ZipCheckup}},
  title  = {{Wisconsin Water Quality Deep Dive - 3,516 Systems Analyzed}},
  year   = {2026},
  url    = {https://zipcheckup.com/states/wisconsin/deep-dive/}
}