Illinois Water Quality Deep Dive — 2,276 Systems Analyzed

Executive Summary

Illinois operates 2,276 public water systems monitored through state and federal testing programs, serving communities across 1,608 ZIP codes. Our analysis of 40,969 individual test results from EPA, state laboratory data, and Consumer Confidence Reports reveals 278 instances where contaminant levels exceeded federal or state Maximum Contaminant Levels (MCLs) — an overall exceedance rate of 0.68%.

The state's primary water quality challenges center on Chicago's massive lead service line inventory and naturally occurring radium in downstate groundwater. Geographic risk patterns across Illinois reflect widespread lead service lines in Chicago and older industrial cities, naturally occurring radium in deep aquifers, PFAS contamination from manufacturing and military sites, and agricultural nitrate in central Illinois.

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 Illinois's water supply. The following analysis covers both regulated contaminants with federal MCLs and state-specific standards — Illinois follows federal MCLs but passed landmark lead service line replacement legislation requiring full replacement; IEPA has been aggressive on radium enforcement.

Top Contaminants by MCL Exceedance Rate

PFAS ("Forever Chemicals") in Illinois

PFAS monitoring in Illinois covers 14,851 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 Illinois report.

State vs. Federal Standards

Illinois follows federal MCLs but passed landmark lead service line replacement legislation requiring full replacement; IEPA has been aggressive on radium enforcement.

This regulatory landscape creates a two-tier compliance reality. A water system in Illinois 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.

ELGIN, IL leads with 7 exceedances in our dataset. AQUA ILLINOIS-UNIVERSITY PARK follows with 6 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 Illinois's 2,276 monitored systems, the top 10 worst offenders account for 46 of the state's 278 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 2% of enforcement actions in Illinois are formal (court orders, consent decrees, administrative penalties). The remaining 98% 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.

551 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. Chicago's lead service line replacement program is one of the largest municipal infrastructure projects in the nation; radium treatment upgrades continue in dozens of downstate communities.

Areas with Most Health Violations

Geographic Risk Patterns

Water quality risk in Illinois is not evenly distributed. Widespread lead service lines in Chicago and older industrial cities, naturally occurring radium in deep aquifers, PFAS contamination from manufacturing and military sites, and agricultural nitrate in central Illinois 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 Illinois:

High-severity findings:

• ZIP 61103 (severity 8/10): PFAS cluster: 6 adjacent ZIPs near Rockford, IL all exceed limits — View full report
• ZIP 61111 (severity 8/10): PFAS cluster: 6 adjacent ZIPs near Loves Park, IL all exceed limits — View full report
• ZIP 61115 (severity 8/10): PFAS cluster: 6 adjacent ZIPs near Machesney Park, IL 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 Illinois, Chicago has the most lead service lines of any US city — an estimated 400,000+ — while downstate communities face aging groundwater systems with radium exceedances. 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 Illinois, 1,242 ZIP codes have elevated or high lead pipe risk based on housing age, and 727 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.

• 921 ZIP codes in Illinois have FEMA flood insurance claims on record
• 47,168 total flood insurance claims filed historically
• $535.9 million in total flood damage payouts

The average flood insurance claim payout in Illinois is $11,361. 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 1,479 ZIP codes in Illinois, documenting 921 self-reported violations and 531 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

Chicago's lead service line replacement program is one of the largest municipal infrastructure projects in the nation; radium treatment upgrades continue in dozens of downstate communities.

Three major regulatory forces are reshaping water quality across Illinois 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 Illinois's 2,276 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: Illinois follows federal MCLs but passed landmark lead service line replacement legislation requiring full replacement; IEPA has been aggressive on radium enforcement. 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 Illinois Residents Should Do

Based on our analysis of 40,969 test results and 1,608 ZIP codes, here are specific actions for Illinois 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 Illinois (Lead, PFAS, Radium), 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 — IL — Lead & Copper 90th Percentile (EPA ECHO LCR); IL — PFAS Monitoring (UCMR5 National Dataset) (40,969 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 1,479 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 2026-06-04.

Related Reports

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

Highest-Risk ZIP Codes in Illinois

• 60931 Donovan Water Report — Lead: 96 ppb
• 60946 Kempton Water Report — Lead: 29.9 ppb
• 61378 West Brooklyn Water Report — Lead: 25.3 ppb
• 61412 Alexis Water Report — Lead: 22.5 ppb
• 61484 Vermont Water Report — Lead: 15 ppb