waterbycounty
EPA SDWIS

King County Water Quality

King County, Washington

Water Grade

A

Water Score

71.1

Violations

16

State Rank

#11

of 39 (1 = best)

EPA SDWIS Compliance

Drinking Water Quality

Water Quality Grade

A

Based on EPA compliance history and violation data

Water Score

71.1/100

Higher = better quality

Health Violations

16

Health-based violations

Violation Rate

0.5%

Systems with violations

Water Advisory: King County

Water Verdict

King County receives a good water quality assessment with a grade of A and a score of 71.1 out of 100. While the water supply is generally safe, occasional monitoring gaps or minor contaminant detections may occur.

Violation Context

King County has recorded 16 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 0.5 violations per 1,000 residents, this rate is relatively low compared to many U.S. counties.

Consumer Guidance

Tap water in King 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 16 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

King County has better water quality than the average county in Washington. Its water score is 11.8 points higher than the state average, indicating stronger water system performance relative to neighboring counties.

Past 5 years

Water Quality Monitoring

Monitoring Sites

253

Active in the past 5 years

Measurements Recorded

40K

39,669 total readings

Most Measured

  • Physical
  • Organics, Pesticide
  • Not Assigned

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

3,860cfs

May 14, 6:15 PM UTC

vs Long-Term Average

104%

Near typical

Primary Streamgage

SNOQUALMIE RIVER NEAR CARNATION, WA

USGS site
12149000
Drainage area
603 sq mi
Long-term mean
3,724 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 King County, Washington?
King County, Washington has a drinking-water quality grade of A with a score of 71.1/100, based on EPA SDWIS compliance data. The county has 16 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 King County?
King County has 16 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 much water-quality monitoring happens in King County?
EPA's Water Quality Portal records 39,669 measurements from 253 monitoring sites in King County over the past five years. The most frequently measured characteristic groups are Physical, Organics, Pesticide, Not Assigned. 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 King County right now?
King County's primary USGS streamgage on the SNOQUALMIE RIVER is currently reading 3,860 cubic feet per second — 104% of the long-term mean of 3,724.48 cfs. Flow is within typical range for this gauge. For genuine real-time data, visit waterdata.usgs.gov.
How does King County water compare to the Washington average?
King County's SDWIS water quality score of 71.1/100 is higher than the Washington state average of 59.3. The average water quality grade across Washington is D, based on data from 39 counties with available SDWIS data.
Is tap water safe to drink in King County?
Based on EPA SDWIS data, King County has a water quality grade of A (71.1/100). 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 King County have so many water violations?
King County has 16 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 King County rank for water quality in Washington?
King County ranks #11 out of 39 counties in Washington by SDWIS water quality score (1 = best). With a score of 71.1/100, 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

Clark County

Washington

B

69.5

Water score

Chelan County

Washington

B

68.0

Water score

Kitsap County

Washington

B

67.5

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.

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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