Illinois Water Quality

Alexander County has the highest SDWIS water quality score in Illinois at 86.0/100 (Grade: A). Note: this ranking reflects drinking-water compliance only — watershed health, monitoring density, and streamflow are tracked separately on each county page.

Union County has among the lowest SDWIS water quality scores in Illinois at 9.7/100. See the individual county page for detailed violation history, watershed assessments, monitoring records, and streamflow data.

Across the 102 Illinois counties with EPA ATTAINS §303(d) assessments, an average of 38.0% of assessed water bodies are classified as impaired — 14,428 of 35,477 reported assessments. Impairment is a Clean Water Act designation that a water body fails to meet state quality standards for one or more designated uses.

Of the 77 Illinois counties with an active USGS streamgage, 19 are currently flowing above their long-term mean and 53 are flowing below. Above-typical can indicate recent storm runoff; below-typical can indicate drought stress on source water. See each county page for the specific gauge and reading.

Illinois has an average SDWIS water quality score of 47.8/100 across counties with reporting. Individual county scores vary — check your specific county's page for compliance, watershed health, monitoring records, and streamflow snapshots.

EPA SDWIS tracks violations for regulated contaminants like lead, nitrates, bacteria, disinfection byproducts, and others. EPA ATTAINS captures broader watershed impairments including mercury, E. coli, sediment, nutrients, and PCBs. The Water Quality Portal aggregates monitoring records from federal, state, and tribal sources. See individual county pages for source-specific detail.

Each one measures a different layer of water. EPA SDWIS tracks drinking-water compliance — whether your public water system met federal Safe Drinking Water Act standards. EPA ATTAINS records §303(d) assessments — what share of a county's rivers, lakes, and streams fail state quality standards under the Clean Water Act. EPA WQP aggregates monitoring records — how many samples have been taken and what's being measured. USGS NWIS provides streamflow snapshots — how much water was flowing through the county's primary streamgage when the pipeline last ran. SDWIS speaks to your tap; the other three speak to source water and the watershed.

An 'impaired' designation under Clean Water Act §303(d) means the state has determined the water body fails to meet its designated-use quality standards — drinking water source, recreation, aquatic life, or fish consumption — for one or more pollutants. Top causes nationally include mercury, E. coli (and other fecal indicator bacteria), nutrients, sediment, and PCBs. Impairment is a structural signal about the watershed, not necessarily about what comes out of your tap (treatment plants can remove or reduce contaminants before delivery).