waterbycounty
Compiled federal source snapshot

Illinois Water Quality

Drinking water data for all 102 counties.

Avg SDWIS Compliance Index

48.9

Most Common Five-Year SDWIS Compliance Band

F

Counties with Data

102

of 102 total

County water atlas

Illinois water signals by county

A state-level 2.5D view across drinking-water compliance, watershed impairment, monitoring density, and streamflow snapshot context. Pin any county, switch layers, then use the lens controls to compare index ranges, recorded violation rates, or impaired watersheds without leaving the page.

Counties

102

Avg SDWIS index

48.9

Watersheds

102

ATTAINS counties

Monitoring

99

76 fresh USGS snapshots

State atlas layers combine EPA SDWIS health-based violations, EPA ATTAINS 303(d) impairment assessments, EPA Water Quality Portal monitoring sites, and representative USGS NWIS streamflow gauges. Streamflow values appear only for fresh pipeline snapshots, not as a continuously updated stream. County pages include the observation time and year.

Prepared by ByCounty Editorial TeamPublished by WaterByCounty

Multi-source coverage in Illinois

Beyond Drinking Water

EPA SDWIS

102/ 102

counties with drinking-water compliance data

627 health violations statewide (5yr)

EPA ATTAINS

71.0%

avg impaired across 102 counties

14,428 of 19,411 assessed bodies impaired

EPA WQP

1,319

monitoring sites across 99 counties

329,867 total readings (5yr window)

USGS NWIS

76

counties with a fresh pipeline snapshot

5 above67 below

State atlas notes

What stands out in Illinois

County water quality is not one number. The strongest read comes from comparing drinking-water compliance against watershed impairment, monitoring density, and streamflow context. Use these signals as a starting point, then open any county profile for source-level detail.

Compliance spread

Alexander County has the highest value in the published state SDWIS Compliance Index table at 86.2/100, while Pope County has the lowest at 8.3/100. That is a 77.9 point gap inside one state.

Zero health violations

15

3+ health violations

57

Watershed pressure

The atlas impairment layer points to counties where assessed water bodies are most likely to miss state quality standards. Assessment density varies, so compare the percentage with the number of assessed bodies on the county page.

Marion County90.1%Clay County88.2%Wayne County88.2%

Monitoring hubs

Cook County244 sitesLake County140 sitesMcHenry County83 sites

Lowest flow reads

Edwards County0%Marion County0%Wayne County2%

Highest fresh streamflow pipeline snapshots: Peoria County (149%, observation 2026), Shelby County (146%, observation 2026), Lake County (132%, observation 2026). These are source-data observations, not household tap-water conclusions.

All Illinois Counties

CountySDWIS Compliance Index
Alexander County86.2
Boone County86.2
Brown County86.2
Calhoun County86.2
Clay County86.2
Edwards County86.2
Gallatin County86.2
Hardin County86.2
Henderson County86.2
Jasper County86.2
Jefferson County86.2
Jersey County86.2
Marion County86.2
Moultrie County86.2
Washington County86.2
DuPage County71.9
Cook County71.7
Kendall County68.2
Lake County67.8
Kane County66.9
St. Clair County66.9
Coles County66.5
Will County66.5
Peoria County66.3
Champaign County66.1
McHenry County65.9
Franklin County65.1
Stephenson County62.9
Tazewell County62.3
Madison County61.1
Winnebago County61.1
DeKalb County60.1
Crawford County58.6
Sangamon County56.9
Adams County55.7
Ogle County55.7
Pike County55.2
Knox County54.7
Grundy County53.8
White County53.5
Monroe County53.2
Rock Island County53.0
Menard County52.0
Warren County51.8
Cass County50.7
De Witt County50.5
Macon County49.7
Wabash County48.9
Mercer County48.8
Clark County48.7
Kankakee County47.8
Logan County46.0
Vermilion County46.0
Hamilton County45.8
McLean County45.6
Schuyler County44.9
Massac County44.0
Lawrence County43.0
Greene County42.6
Whiteside County42.5
Piatt County41.2
Carroll County40.5
Ford County38.9
Jo Daviess County38.5
Clinton County38.0
Cumberland County37.5
Williamson County36.6
Saline County35.8
Livingston County35.2
Edgar County35.0
Mason County34.6
Shelby County34.6
Richland County33.4
Henry County32.7
Effingham County32.4
Morgan County31.7
Pulaski County31.2
Jackson County30.8
Perry County30.8
Lee County30.7
Marshall County30.2
Hancock County29.5
McDonough County28.1
Douglas County27.3
Iroquois County27.2
Randolph County26.1
Bureau County25.5
Fayette County24.7
Christian County24.3
Johnson County23.0
Putnam County23.0
Montgomery County21.9
Woodford County21.6
Fulton County21.3
Stark County20.5
Scott County18.6
LaSalle County18.5
Wayne County15.8
Union County15.5
Bond County14.9
Macoupin County11.8
Pope County8.3

Frequently Asked Questions

Which Illinois county has the highest SDWIS Compliance Index?
Alexander County has the highest SDWIS Compliance Index in the published Illinois cohort at 86.2/100. This WaterByCounty relative comparison is not an EPA score and does not establish current conditions for a utility or address.
What does the Five-Year SDWIS Compliance Band mean?
The band is derived only from each county aggregation's recorded health-violation rate per 100,000 people served for the stated five-year source window. It is separate from the relative SDWIS Compliance Index and is not a current determination for a specific utility or address.
What does EPA ATTAINS report across Illinois?
Across 102 published Illinois county records with EPA ATTAINS data, 14,428 of 19,411 assessed water bodies are listed as impaired. This is a watershed assessment and does not describe finished household tap water.
Which USGS streamflow records are shown across Illinois?
WaterByCounty has 76 fresh USGS pipeline snapshots in the published Illinois cohort. Each county page shows its observation timestamp and year. A pipeline snapshot is not a continuously updated reading or a tap-water safety measure.
Where can Illinois residents find current official information?
Review the applicable utility's Consumer Confidence Report, active notices, and the state drinking-water agency. WaterByCounty's county-level historical aggregations do not establish the current status of a specific utility, address, private well, or household plumbing system.
What does Water Quality Portal monitoring volume show in Illinois?
The published county records contain 329,867 results from 1,319 monitoring locations in the site's five-year aggregation. Monitoring volume does not by itself indicate better or worse water quality.

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. The WaterByCounty SDWIS Compliance Index reflects the recorded rate and is not an EPA score.

Disclaimer: This data is informational only. It is not health, legal, or professional advice. For concerns about your specific water supply, contact your local water utility.

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