waterbycounty
EPA SDWIS

Mercer County Water Quality

Mercer County, New Jersey

Water Grade

B

Water Score

62.3

Violations

15

State Rank

#8

of 21 (1 = best)

EPA SDWIS Compliance

Drinking Water Quality

Water Quality Grade

B

Based on EPA compliance history and violation data

Water Score

62.3/100

Higher = better quality

Health Violations

15

Health-based violations

Violation Rate

5.0%

Systems with violations

Water Advisory: Mercer County

Water Verdict

Mercer County receives a fair water quality assessment with a grade of B and a score of 62.3 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 15 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 5.0 violations per 1,000 residents, this rate is moderate and suggests recurring water quality challenges.

Consumer Guidance

Tap water in Mercer County is generally safe to drink based on available data. Residents should still review their utility's annual Consumer Confidence Report for transparency on detected contaminants. 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

Mercer County has better water quality than the average county in New Jersey. Its water score is 6.6 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

92.3%

12 of 13 assessed

High concern

Top Impairment Causes

  • 1

    PCBS IN FISH TISSUE

  • 2

    MERCURY IN FISH TISSUE

  • 3

    DDT IN FISH TISSUE

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

158

Active in the past 5 years

Measurements Recorded

42K

42,283 total readings

Most Measured

  • Physical
  • Organics, Pesticide
  • 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

11.0Kcfs

May 14, 6:15 PM UTC

vs Long-Term Average

91%

Near typical

Primary Streamgage

Delaware River at Trenton NJ

USGS site
01463500
Drainage area
6,780 sq mi
Long-term mean
12.1K 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 Mercer County, New Jersey?
Mercer County, New Jersey has a drinking-water quality grade of B with a score of 62.3/100, 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 Mercer County?
Mercer 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 Mercer County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 92.3% of Mercer County's 13 assessed water bodies are classified as impaired (12 impaired). The top reported causes are PCBS IN FISH TISSUE, MERCURY IN FISH TISSUE, DDT IN FISH TISSUE. 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 42,283 measurements from 158 monitoring sites in Mercer County over the past five years. The most frequently measured characteristic groups are Physical, Organics, Pesticide, 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 Mercer County right now?
Mercer County's primary USGS streamgage on the Delaware River is currently reading 11,000 cubic feet per second — 91% of the long-term mean of 12,071.75 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 New Jersey average?
Mercer County's SDWIS water quality score of 62.3/100 is higher than the New Jersey state average of 55.7. The average water quality grade across New Jersey is D, based on data from 21 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 B (62.3/100). This indicates good to excellent water quality with strong SDWIS compliance. 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 Mercer County have so many water violations?
Mercer 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 Mercer County rank for water quality in New Jersey?
Mercer County ranks #8 out of 21 counties in New Jersey by SDWIS water quality score (1 = best). With a score of 62.3/100, 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

Middlesex County

New Jersey

C

61.7

Water score

Ocean County

New Jersey

B

63.0

Water score

Burlington County

New Jersey

C

60.6

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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