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EPA SDWIS

St. Clair County Water Quality

St. Clair County, Illinois

Water Grade

C

Water Score

61.0

Violations

15

State Rank

#30

of 102 (1 = best)

EPA SDWIS Compliance

Drinking Water Quality

Water Quality Grade

C

Based on EPA compliance history and violation data

Water Score

61/100

Higher = better quality

Health Violations

15

Health-based violations

Violation Rate

5.8%

Systems with violations

Water Advisory: St. Clair County

Water Verdict

St. Clair County receives a fair water quality assessment with a grade of C and a score of 61.0 out of 100. The water supply meets baseline federal standards, but there may be periods of elevated contaminant levels or infrastructure concerns worth monitoring.

Violation Context

St. Clair County has recorded 15 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 5.8 violations per 1,000 residents, this rate is moderate and suggests recurring water quality challenges.

Consumer Guidance

Tap water in St. Clair County meets baseline standards, but residents who are immunocompromised or have young children may want to use an NSF-certified water filter as a precaution. With 15 recorded health violations, staying informed about utility communications and boil-water notices is especially important. For long-term peace of mind, request your utility's latest Consumer Confidence Report and consider independent water testing if you have specific health concerns.

Regional Context

St. Clair County has better water quality than the average county in Illinois. Its water score is 13.2 points higher than the state average, indicating stronger water system performance relative to neighboring counties.

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

29.2%

90 of 308 assessed

Some impairment

Top Impairment Causes

  • 1

    PHOSPHORUS, TOTAL

  • 2

    ALTERATION IN STREAM-SIDE OR LITTORAL VEGETATIVE COVERS

  • 3

    DISSOLVED OXYGEN

Source: EPA ATTAINS · Reporting cycle 2022

Impairment is determined under the Clean Water Act §303(d): a water body is impaired when it fails to meet state-defined quality standards for designated uses (drinking, recreation, aquatic life). Assessment coverage varies by state — counties without assessed water bodies are not shown.

Past 5 years

Water Quality Monitoring

Monitoring Sites

19

Active in the past 5 years

Measurements Recorded

2.6K

2,590 total readings

Most Measured

  • Inorganics, Minor, Metals
  • Physical
  • Nutrient

Categories measured most frequently

Data from the EPA Water Quality Portal (WQP), aggregating monitoring records from federal, state, and tribal sources. Each measurement represents a single sample analyzed for a specific characteristic (e.g., E. coli, pH, dissolved oxygen, nitrogen).

Live USGS Streamgage

River & Stream Conditions

Current Discharge

8,360cfs

May 14, 6:00 PM UTC

vs Long-Term Average

154%

Well above typical

Primary Streamgage

KASKASKIA RIVER AT NEW ATHENS, IL

USGS site
05595000
Drainage area
5,189 sq mi
Long-term mean
5,418 cfs

One representative streamgage (the one with the largest drainage area in the county). Many counties have multiple gauges — this view summarizes the primary one. The long-term mean is the full-record annual average; "% of typical" compares the latest reading against that average.

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Frequently Asked Questions

What is the water quality in St. Clair County, Illinois?
St. Clair County, Illinois has a drinking-water quality grade of C with a score of 61.0/100, based on EPA SDWIS compliance data. The county has 15 health-based drinking water violations over the past 5 years. Watershed health, monitoring records, and live streamflow are reported separately on this page.
Are there any water violations in St. Clair County?
St. Clair County has 15 health-based drinking water violations recorded by the EPA over the past 5 years. Health-based violations indicate instances where contaminant levels exceeded EPA Maximum Contaminant Levels (MCLs). Violations may have been resolved — check with your local water utility for current status.
How healthy are the watersheds in St. Clair County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 29.2% of St. Clair County's 308 assessed water bodies are classified as impaired (90 impaired). The top reported causes are PHOSPHORUS, TOTAL, ALTERATION IN STREAM-SIDE OR LITTORAL VEGETATIVE COVERS, DISSOLVED OXYGEN. Impairment means the water body fails to meet state quality standards for at least one designated use — drinking water source, recreation, aquatic life, or fish consumption. Note: watershed impairment doesn't always translate to tap-water issues; treatment plants can remove most regulated contaminants.
How much water-quality monitoring happens in St. Clair County?
EPA's Water Quality Portal records 2,590 measurements from 19 monitoring sites in St. Clair County over the past five years. The most frequently measured characteristic groups are Inorganics, Minor, Metals, Physical, Nutrient. Each measurement is a single sample analyzed for one characteristic (E. coli, pH, dissolved oxygen, etc.). High monitoring density means more scientific evidence behind any reported signal — it does not by itself indicate water quality.
What's happening with rivers in St. Clair County right now?
St. Clair County's primary USGS streamgage on the KASKASKIA RIVER is currently reading 8,360 cubic feet per second — 154% of the long-term mean of 5,417.67 cfs. This is well above typical — often a signal of recent precipitation or storm runoff. For genuine real-time data, visit waterdata.usgs.gov.
How does St. Clair County water compare to the Illinois average?
St. Clair County's SDWIS water quality score of 61.0/100 is higher than the Illinois state average of 47.8. The average water quality grade across Illinois is D, based on data from 102 counties with available SDWIS data.
Is tap water safe to drink in St. Clair County?
Based on EPA SDWIS data, St. Clair County has a water quality grade of C (61.0/100). This indicates moderate compliance. Some violations have been recorded but overall standards are maintained. The grade speaks to the public water system, not the watershed — for watershed-level concerns, see the Watershed Health zone. For the most up-to-date information, contact your local water utility or review your Consumer Confidence Report (CCR).
Why does St. Clair County have so many water violations?
St. Clair County has 15 health-based drinking water violations on record from the EPA SDWIS database. A higher violation count can result from aging infrastructure, underfunded water utilities, agricultural runoff contamination, or industrial pollution. Counties with more water systems may also see more violations simply due to scale. Residents concerned about water quality should consider independent water testing and home filtration systems.
How does St. Clair County rank for water quality in Illinois?
St. Clair County ranks #30 out of 102 counties in Illinois by SDWIS water quality score (1 = best). With a score of 61.0/100, it falls in the top third of counties statewide. The ranking reflects EPA SDWIS compliance only — not watershed impairment, monitoring density, or streamflow, which are tracked separately on this page.

Counties with Similar Water Quality

Ogle County

Illinois

C

60.8

Water score

Pike County

Illinois

C

60.4

Water score

Tazewell County

Illinois

B

62.6

Water score

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. Scores reflect compliance history and health-based violation counts.

Watershed health and impaired-waterway data from the EPA ATTAINS Clean Water Act §303(d) assessments — state-reported, EPA-finalized.

Water-quality monitoring counts from the EPA Water Quality Portal (WQP) — federated USGS, EPA, and state agency sampling records over a rolling 5-year window.

Live streamflow from the USGS National Water Information System (NWIS) — continuous discharge measurements from the largest-drainage gauge in each county, compared against the full-record long-term annual mean.

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.

By Logan Johnson, Founder & Data EditorUpdated Reviewed by Logan Johnson, Founder & Data Editor

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