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Illinois water profile

Nitrate in Illinois community water systems

Illinois drains its corn belt with tile, and that plumbing shields most of the state's aquifers — so its elevated systems gather where tile cannot help: shallow sand and gravel along the Mississippi bluffs and the American Bottoms, and thin glacial cover over bedrock. See the pattern and what each reporting system measured in the three years from August 9, 2023 to August 9, 2026.

Browse all 1,782 Illinois systems →

Search by the utility or system name shown on a water bill. Example: City of Quincy. Private wells are not in this inventory.

100%
of the state’s 1,788 active community water systems have a current result here
1,782
systems with a current nitrate result
12,060,936
people served by those reporting systems
3.2%
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 Illinois's systems fall across the five bands

Most systems report below 3 mg/L

Natural background
0.00–0.99 mg/L as N
1,380 systems · 9,761,135 served
Human impact likely
1.00–2.99 mg/L as N
195 systems · 1,200,544 served
Human impact clear
3.00–4.99 mg/L as N
150 systems · 772,008 served
Health-study range
5.00–9.99 mg/L as N
57 systems · 327,249 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 57 systems reporting 5 mg/L or more, 7 stayed at or above 5 across the whole three-year range; the other 50 reached it only on a high sample and otherwise ran lower.

Most recent reading Jun 2026; the typical system was last tested around Apr 2026. 1,063 systems sampled their own water; 719 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
Humboldt8.80 mg/LApr 2026a high point — runs lowerHealth-study rangeinherited from supplier361
Mattoon8.80 mg/LApr 2026a high point — runs lowerHealth-study range12 samples19,500
Moultrie County Rwd8.80 mg/LApr 2026a high point — runs lowerHealth-study rangeinherited from supplier2,600
Erie8.50 mg/LApr 2026a high point — runs lowerHealth-study range12 samples1,600
Leaf River8.36 mg/LMay 2026a high point — runs lowerHealth-study range22 samples403
Rock City8.26 mg/LJun 2026Health-study range12 samples313
Catlin8.00 mg/LMay 2026a high point — runs lowerHealth-study rangeinherited from supplier2,380
Westville8.00 mg/LMay 2026a high point — runs lowerHealth-study rangeinherited from supplier3,917
Largest systems
SystemMost recent resultBandBased onPeople served
Chicago0.37 mg/LApr 2026Natural background6 samples2,746,388
Aurora1.40 mg/LApr 2026Human impact likely3 samples183,000
Joliet0.05 mg/LJun 2026Natural background35 samples160,000
Naperville0.01 mg/LMar 2026Natural background8 samples156,406
Il American-Champaign0.05 mg/LJul 2025Natural background4 samples150,000

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

Two corridors: the Mississippi edge and shallow sands

Human impact is clear on drinking water served in 44 of Illinois’s 102 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.

IL 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.Winnebago County — 3 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveMarshall County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveLogan County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveLaSalle County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveTazewell County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveChristian County — 4 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveScott County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMacoupin County — 5 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMcDonough County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveSt. Clair County — 10 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveBond County — 4 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveOgle County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveJo Daviess County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveStephenson County — 0 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or abovePike County — 3 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveMason County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveLawrence County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveGreene County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveWarren County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveBrown County — 4 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMontgomery County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveMadison County — 13 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveShelby County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveKankakee County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveVermilion County — 1 system at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveMenard County — 1 system 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; 0 systems at 5.00 mg/L as N or aboveRock Island County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveAdams County — 10 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveLivingston County — 4 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveHenderson County — 1 system at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or aboveBoone County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWill County — 1 system at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveRandolph County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveLake County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveMacon County — 1 system at 3.00–4.99 mg/L as N; 3 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 aboveMcLean County — 0 systems at 3.00–4.99 mg/L as N; 7 systems at 5.00 mg/L as N or aboveClinton County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveHancock County — 7 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWhiteside County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveWhite County — 5 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or abovePeoria County — 7 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveColes County — 0 systems at 3.00–4.99 mg/L as N; 2 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

McLean leads this layer with 7 reporting systems, followed by Henderson, Vermilion, Macon. Two corridors: the Mississippi edge and shallow sands. Coverage note: the map places 72% of Illinois’s reporting systems. Of the rest, 169 carry a supplier’s result measured elsewhere and belong off this map; 329 sampled their own water and are missing a county in the records, so the map reads as a floor by at least that much.

Counties with the most systems at or above 3 mg/L as N
County3–4.99≥5Total ≥3
Madison13114
Adams10111
St. Clair10010
Hancock718
McLean077

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. Coverage note: the map places 72% of Illinois’s reporting systems. Of the rest, 169 carry a supplier’s result measured elsewhere and belong off this map; 329 sampled their own water and are missing a county in the records, so the map reads as a floor by at least that much.

Find a specific Illinois system

What the statewide pattern says

Tile drains protect the middle; the edges answer

Illinois' elevated systems concentrate along the Mississippi River corridor — Madison, St. Clair, Adams, Hancock, Henderson counties — and in scattered shallow sand-and-gravel and thin-cover settings, while the deep Mahomet aquifer that supplies much of east-central Illinois runs below detection.

Illinois' 154 mapped systems above the natural-background band form two visible corridors: the Mississippi River edge from Adams and Hancock through Madison and St. Clair — the American Bottoms — and a scatter of shallow-aquifer counties across the interior, led by McLean, Peoria, Winnebago and Vermilion. More than 1.1 million people are served by Illinois systems at or above 1.00 mg/L.

The state's defining mechanism is artificial drainage. Most Illinois farmland is tile-drained, and tile short-circuits infiltrating water to streams instead of letting it soak to the water table — a major reason nitrate rarely reaches deep aquifers under the corn belt (USGS Lower Illinois River Basin NAWQA). The same study drew the depth line: all 30 deep Mahomet aquifer samples ran below 0.10 mg/L, while 7 percent of shallow wells exceeded the federal limit, at 11 to 77 mg/L, every one of them less than 50 feet deep.

The deep Mahomet's protection is physical, not chemical: thick clay-rich till separates it from the surface, and age-dating near Clinton found recharge older than 1953 even in shallow permeable units above the aquifer (USGS SIR 2015-5159). Where that till is thin or the sand and gravel reaches the surface — the Mississippi bluffs, the Havana Lowlands, the Kankakee sands — the shield drops. The Illinois State Water Survey names the sources directly: nitrate from fertilizers and septic systems, tracked in shallow-aquifer studies across Kane, Lake and Will counties (ISWS Water Resource Systems).

The supported source reading is row-crop and septic nitrogen on shallow sand and gravel where tile drainage cannot intercept it — not a certified source at any individual system (Nolan and Hitt, 2006).

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 Illinois pattern does—and does not—say

Where does nitrate concentrate in Illinois community water systems?

McLean leads this layer with 7 reporting systems, followed by Henderson, Vermilion, Macon. Two corridors: the Mississippi edge and shallow sands.

Why does a farm state like Illinois have so many protected aquifers?

Tile drainage. Most Illinois cropland is artificially drained, and the tiles route nitrogen-laden water to streams rather than letting it percolate to aquifers. Combined with thick glacial till over deep aquifers like the Mahomet, most deep groundwater runs at natural background — the risk concentrates in shallow sand and gravel under 50 feet.

Should an Illinois private well be tested for nitrate?

Yes, especially shallow wells on sand and gravel along the Mississippi corridor, in the Havana Lowlands, or anywhere tile drainage is absent. USGS found 7 percent of shallow study wells over the federal limit, some at several times it — and Illinois wells can also carry natural arsenic and radium, arguing for a broad certified-lab panel.

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: 1,782 Illinois 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. USGS. Water-quality assessment of the Lower Illinois River Basin (NAWQA circular excerpt) — deep Mahomet samples all <0.10 mg/L; 7% of shallow wells >10 mg/L (11–77 mg/L, all <50 ft deep); tile drainage short-circuits recharge.
  5. USGS (2016). Application of Hydrogeology and Groundwater-Age Estimates to Assess Travel Time of Groundwater at a Landfill Site above the Mahomet Aquifer, near Clinton, Illinois. SIR 2015-5159 — tritium shows pre-1953 recharge above the Mahomet; till protects the deep aquifer.
  6. Illinois State Water Survey. Water Resource Systems research — nitrate from fertilizers and septic systems named among Illinois groundwater risks; shallow-aquifer studies in Kane, Lake and Will counties; ISWS–USGS Illinois River Basin nitrate fate-and-transport study underway.