waterbycounty
EPA SDWIS

Mercer County Water Quality

Mercer County, Illinois

Water Grade

C

Water Score

55.3

Violations

1

State Rank

#41

of 102 (1 = best)

EPA SDWIS Compliance

Drinking Water Quality

Water Quality Grade

C

Based on EPA compliance history and violation data

Water Score

55.3/100

Higher = better quality

Health Violations

1

Health-based violations

Violation Rate

10.2%

Systems with violations

Water Advisory: Mercer County

Water Verdict

Mercer County receives a fair water quality assessment with a grade of C and a score of 55.3 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

Mercer County has recorded 1 health-based violation, meaning the water system experienced at least one exceedance of federal contaminant limits or treatment requirements. At 10.2 violations per 1,000 residents, this rate is high and signals significant water quality management issues.

Consumer Guidance

Tap water in Mercer 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 1 recorded health violation, 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

Mercer County has better water quality than the average county in Illinois. Its water score is 7.5 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

46.9%

225 of 480 assessed

Moderate concern

Top Impairment Causes

  • 1

    PHOSPHORUS, TOTAL

  • 2

    MERCURY

  • 3

    TOTAL SUSPENDED SOLIDS (TSS)

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

1

Active in the past 5 years

Measurements Recorded

649

649 total readings

Most Measured

  • Inorganics, Minor, Metals
  • Physical
  • Organics, Pesticide

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

213cfs

May 14, 6:30 PM UTC

vs Long-Term Average

65%

Below typical

Primary Streamgage

EDWARDS RIVER NEAR NEW BOSTON, IL

USGS site
05466500
Drainage area
445 sq mi
Long-term mean
328 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.

Improve your water quality at home

Berkey filters remove 99.9%+ of contaminants from tap water.

Shop Berkey →

Sponsored

Test your tap water

Tap Score provides professional mail-in water testing.

Get Tested →

Frequently Asked Questions

What is the water quality in Mercer County, Illinois?
Mercer County, Illinois has a drinking-water quality grade of C with a score of 55.3/100, based on EPA SDWIS compliance data. The county has 1 health-based drinking water violation over the past 5 years. Watershed health, monitoring records, and live streamflow are reported separately on this page.
Are there any water violations in Mercer County?
Mercer County has 1 health-based drinking water violation 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 Mercer County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 46.9% of Mercer County's 480 assessed water bodies are classified as impaired (225 impaired). The top reported causes are PHOSPHORUS, TOTAL, MERCURY, TOTAL SUSPENDED SOLIDS (TSS). 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 Mercer County?
EPA's Water Quality Portal records 649 measurements from 1 monitoring sites in Mercer County over the past five years. The most frequently measured characteristic groups are Inorganics, Minor, Metals, Physical, Organics, Pesticide. 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 Mercer County right now?
Mercer County's primary USGS streamgage on the EDWARDS RIVER is currently reading 213 cubic feet per second — 65% of the long-term mean of 327.96 cfs. Flow is within typical range for this gauge. For genuine real-time data, visit waterdata.usgs.gov.
How does Mercer County water compare to the Illinois average?
Mercer County's SDWIS water quality score of 55.3/100 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 Mercer County?
Based on EPA SDWIS data, Mercer County has a water quality grade of C (55.3/100). 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).
Does Mercer County have clean drinking water?
Mercer County has 1 health-based drinking water violation according to EPA records. With a water quality score of 55.3/100 and grade C, the county's drinking water has had some compliance issues but continues to be monitored. Note: drinking-water compliance speaks to the public water system, not necessarily to the watershed itself — check the Watershed Health zone for ATTAINS §303(d) data.
How does Mercer County rank for water quality in Illinois?
Mercer County ranks #41 out of 102 counties in Illinois by SDWIS water quality score (1 = best). With a score of 55.3/100, it falls in the middle 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

Wabash County

Illinois

C

55.4

Water score

Vermilion County

Illinois

C

54.5

Water score

Knox County

Illinois

C

54.4

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

Explore more data for Mercer County

Property TaxCost of LivingCrime RateIncomeHealthSchoolsDisaster RiskWeatherCounty Score