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County water report

Mercer County Water Report

Drinking-water compliance, watershed health, monitoring records, and river conditions for Mercer County, Illinois.

Water grade

C

Water score

55.3

State rank

#41

of 102

Health violations

1

EPA SDWIS, 5-year lookback

Watershed impaired

46.9%

480 water bodies assessed

Monitoring sites

1

649 recent measurements

Live streamflow

65%

EDWARDS RIVER NEAR NEW BOSTON, IL

Water at a glance

Key Water Indicators for Mercer County

EPA SDWIS

Safety Grade

C

Score: 55.3 / 100

EPA SDWIS

Active Violations

1

5-year health-based lookback

EPA ATTAINS

Watershed Health

47% impaired

480 bodies assessed

USGS NWIS

Streamflow Snapshot

65% of mean

EDWARDS RIVER NEAR NEW BOSTON, IL

EPA WQP

Monitoring Sites

1

649 recent readings

Source: EPA SDWIS · Safe Drinking Water Information System

Drinking Water Compliance

Compliance grade

C

Based on EPA SDWIS compliance history.

Water score

Higher scores indicate cleaner recent compliance records.

55.3/100

Health violations

1

Health-based violations

Violations per 100K served

10.2

Population-normalized SDWIS rate

Editorial analysis

Understanding Mercer County’s Water

Drinking Water Quality Overview

EPA SDWIS

Mercer County's drinking water earned a C grade, scoring 55.3 out of 100. Over the past five years, EPA SDWIS records 1 health-based violation — a single incident worth monitoring.

Watershed Conditions

EPA ATTAINS

Under the Clean Water Act §303(d), EPA ATTAINS tracks whether waterways meet quality standards for drinking, recreation, and aquatic life (reporting cycle: 2022). A substantial 46.9% of assessed waterways are impaired (225 of 480 water bodies) across Mercer County's watersheds. The leading impairment causes are phosphorus, total and mercury. Impairment does not mean tap water is unsafe — it measures ambient waterway conditions upstream of treatment, not finished drinking water.

River & Streamflow Status

USGS NWIS

USGS NWIS gauge data (as of 2026-05-14T12:30:00.000-06:00) puts EDWARDS RIVER at 213.0 cfs — running somewhat below its historical average at 65% of mean. Streamflow is a leading indicator of drought stress, sediment load, and dilution capacity: low flows concentrate pollutants and warm water temperatures, stressing aquatic life and, in surface-water-dependent systems, the source water quality for treatment plants.

Monitoring Network

EPA WQP

EPA's Water Quality Portal (WQP) aggregates monitoring data from federal, state, and tribal agencies. Mercer County has limited coverage with 1 active monitoring site with 649 recent measurements on record. Predominant monitoring categories include inorganics, minor, metals and physical. More monitoring sites generally indicate greater scientific attention to local water conditions — and provide the baseline data that regulators use to set future impairment listings.

Editorial advisory

What the data suggests for 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 100,000 people served, this rate is high and signals significant water quality management issues.

Consumer Guidance

Tap water in Mercer County meets baseline standards but the compliance record shows room for improvement, with a Grade C rating. Mercer County's drinking-water compliance score is 55.3 out of 100. The violation rate for Mercer County is 10.2 per 100,000 people served. Residents who are immunocompromised, pregnant, or have young children may benefit from using an NSF 53-certified filter. Contacting your local utility for the current Consumer Confidence Report will confirm which specific violations were recorded and whether they have been resolved. Phosphorus, Total is the leading impairment cause in Mercer County's watershed. There is 1 active water-quality monitoring site in Mercer County. A pipeline streamflow snapshot from the EDWARDS RIVER gauge is also available on this page.

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.

Advisory text summarizes county-level public records and is not a replacement for your utility's current Consumer Confidence Report or direct local notices.

Contaminants & Resources

Key issues flagged in Mercer County's water environment

Watershed Impairment Causes (EPA ATTAINS)

  • 1

    Phosphorus (excess nutrients)

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 2

    Mercury

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 3

    Total Suspended Solids (Tss)

    Impairment cause per EPA Clean Water Act §303(d) assessment

Source: EPA ATTAINS · Reporting cycle 2022

Official EPA Resources for Mercer County

EPA How's My Waterway

Interactive watershed health data for Mercer County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for Mercer County

Consumer Confidence Reports (IL)

Annual water quality reports from utilities in Illinois

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; the percent-of-typical value compares the latest reading against that average.

Free tool

Estimate Your Water Costs

Water Cost Estimate

3

3 people  ·  ~225 gal/day

Annual Total

$558

Monthly

$47

Water Bill

$558/yr

Filter Cost

$0/yr

Safety Grade for Mercer County:C — Moderate

Some violations or watershed impairment detected.

Estimates use the national average residential water rate ($0.0068/gal, EPA/AWWA 2023) and EPA WaterSense per-person consumption baseline (75 gal/person/day). Actual bills vary by utility, usage tier, and local infrastructure fees. For informational purposes only.

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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 streamflow snapshots 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 has a pipeline snapshot of 213 cubic feet per second — 65% of the long-term mean of 327.96 cfs. Flow is within typical range for this gauge. For the latest gauge feed, 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 and 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 Evan Brooks, Data EditorUpdated Reviewed by Evan Brooks, Data Editor

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