Arsenic in Drinking Water in Illinois
What residents of Illinois need to know about arsenic in drinking water — including natural geological sources, private well risk, which utilities have documented violations, and how to remove arsenic from tap water.
Source: EPA SDWIS, Illinois Environmental Protection Agency, USGS · Last reviewed: 2025-01-01
Quick Answer
Is arsenic in drinking water a concern in Illinois?
Yes. Illinois has documented arsenic in groundwater, particularly in the glacial aquifer systems that underlie much of northern and central Illinois. The Mahomet Aquifer — a major buried glacial aquifer in central Illinois — has documented naturally elevated arsenic in some areas. Illinois is one of the Midwestern states with documented arsenic occurrence in private wells from glacial sediment geology.
Where does arsenic come from in Illinois's water?
Glacial aquifer geology is the primary arsenic source in Illinois. Glacial till and sand-and-gravel aquifers can contain arsenic from mineral weathering under reducing geochemical conditions. The Mahomet Aquifer in central Illinois and glacial aquifers in northern Illinois have documented arsenic occurrence. Private wells and small water systems drawing from these aquifers face elevated risk.
What should Illinois residents know?
Private well owners in central Illinois (McLean, DeWitt, Champaign, Ford, and Iroquois counties near the Mahomet Aquifer) and in northern Illinois counties with glacial aquifer systems should test for arsenic. Illinois EPA monitors public water systems, but private wells — used by about 1.5 million Illinois residents — are unregulated.
Key Facts
| EPA MCL | 10 µg/L (10 ppb) |
| MCLG | Zero |
| Primary source | Glacial aquifer geology — reducing geochemical conditions mobilize arsenic from mineral sediments |
| Mahomet Aquifer | Major buried glacial aquifer in central Illinois — documented arsenic in some areas |
| State regulator | Illinois Environmental Protection Agency (IEPA) |
| Private well concern | ~1.5 million Illinois residents use private wells — unregulated for arsenic |
| Health effects | Bladder, lung, skin cancer; cardiovascular; diabetes risk |
| Effective treatment | Reverse osmosis or activated alumina |
Why Arsenic Matters in Illinois
Illinois's arsenic occurrence is tied to its glacial geology — the thick glacial deposits that blanket most of the state contain minerals that can release arsenic under reducing (low-oxygen) groundwater conditions. The Mahomet Aquifer, which supplies water to communities across east-central Illinois, has documented arsenic in some areas. A significant number of rural Illinois residents use private wells in glacial aquifers, and arsenic testing is not required for these systems. Illinois EPA has studied arsenic occurrence and provides guidance, but private well owners must test and treat independently.
Illinois Arsenic Program
Illinois EPA monitors arsenic under federal SDWIS. Illinois's glacial aquifer geology presents moderate natural arsenic in some areas, particularly in the Illinois River basin. Arsenic is not a primary concern for most large Illinois utilities drawing from Lake Michigan, but private well owners and small system customers in agricultural areas should test.
Largest Illinois Water Utilities
No arsenic violations on record in EPA SDWIS for Illinois utilities in our database. Browse the largest utilities to review their full water quality record.
How Does Arsenic Get Into Drinking Water?
Arsenic in drinking water is almost always naturally occurring — it leaches from arsenic-bearing rocks and minerals into groundwater over time. New England granite, Southwest volcanic geology, and Upper Midwest glacial aquifers are the primary high-risk formations. It has no taste, odor, or color.
Full arsenic overview — geology maps, health effects, all 50 statesWho Should Pay Closest Attention
Private well owners in McLean, DeWitt, Champaign, Ford, Iroquois, and surrounding central Illinois counties near the Mahomet Aquifer, and northern Illinois counties with glacial sand-and-gravel aquifers, face the highest documented risk.
Private well owners near mining districts or agricultural areas
Residents in states with documented volcanic or geothermal geology
Long-term consumers of water from small groundwater systems
Households in homes built before 1960 with older well casings
Residents whose well water has never been tested for arsenic
Anyone living in a state where bedrock wells are common
How to Check Your Situation in Illinois
- 1
Identify your water source. If you use a public utility, use the ZIP lookup on this page to find your system and check its compliance record.
- 2
If on public water, review your utility's most recent Consumer Confidence Report (CCR) for arsenic monitoring data. The MCL is 10 ppb — your report should show recent test results.
- 3
If on a private well, order an arsenic test from a Illinois Environmental Protection Agency-certified laboratory. A basic arsenic test costs $15–$40. The state agency website maintains a certified lab list.
- 4
Test your well at the tap — not just at the wellhead. The entire water distribution system within your home can affect water quality.
- 5
If your test shows arsenic above 5 ppb, install certified treatment immediately. If above 10 ppb, do not use the water for drinking or cooking until treatment is installed.
- 6
Retest after installing treatment to confirm it is working as certified. Replace filter media on the manufacturer's schedule — an exhausted filter may not perform as rated.
Treatment Options for Arsenic
Boiling does not remove arsenic — it concentrates it. Standard activated carbon filters (Brita, etc.) do not effectively remove arsenic. Certified treatment is required.
NSF/ANSI Standard 58 — Reverse Osmosis
RO systems certified to NSF/ANSI 58 remove 85–95% of arsenic. Under-sink installation. Most effective for removing multiple contaminants simultaneously. Replace membranes and pre-filters on schedule.
Activated Alumina Filters
Activated alumina is specifically designed for arsenic and fluoride removal. Point-of-use or whole-house options available. Must be certified by NSF International or WQA for arsenic reduction. Requires periodic media regeneration or replacement.
Iron/Manganese Oxidation Filters
Effective for arsenic in iron-rich well water, which is common in the Midwest and New England. Oxidation converts dissolved iron and arsenic to a form that can be filtered out. Best when arsenic is co-occurring with high iron levels.
What does NOT work for arsenic
Standard activated carbon filters (Brita, refrigerator filters, most pitcher filters) do NOT effectively remove arsenic. Boiling concentrates arsenic. Water softeners do not remove arsenic. Only use products with NSF certification specifically for arsenic reduction.
Take Action Now
If you use a private well in Illinois, test for arsenic — especially if you are in a region with granite, volcanic, or sedimentary geology. A basic arsenic test costs $15–$40 at a state-certified lab.
Public water users: check your utility's Consumer Confidence Report (CCR) for arsenic results. The EPA MCL is 10 ppb — any detection warrants attention.
If arsenic is detected above 10 ppb (or even below it, given MCLG is zero), install an NSF/ANSI 58-certified reverse osmosis system for drinking and cooking water.
Standard carbon filters do NOT remove arsenic — do not rely on a Brita or refrigerator filter for arsenic protection.
Frequently Asked Questions
Related Pages
Arsenic — National Overview
All U.S. utilities with arsenic records
Illinois State Overview
All utilities and water quality data
Illinois Well Water Guide
Testing, risks & certified labs for private wells
Reverse Osmosis Guide
Removes 85–95% of arsenic
Best Arsenic Filter Guide
What actually works (carbon doesn't)
PFAS in Drinking Water
The 'forever chemical' contamination overview
All Contaminants
Complete reference library
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
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Illinois Environmental Protection Agency ↗