Public water system · WA5353640
ActiveMercer Island City of
At a glance
City of Mercer Island (PWSID WA5353640) is an active community water system in King County, Washington. It reports serving 31,057 people through 9,807 service connections. Its primary water source is purchased surface water, and EPA lists it as owned by a local government. EPA's records list 4 violations for this system; 1 is health-based, and the most recent began in December 1992.
Dataset updated . Source: EPA SDWIS/ECHO public records.
Population served
31,057
Service connections
9,807
Primary source
SWP
Purchased surface water
System type
CWS
Community water system
System record
How does City of Mercer Island compare with other water systems in King County?
By EPA's size categories, City of Mercer Island is a large system (10,001 to 100,000 people). King County has 214 active public water systems. Its population served is about twice the county average of 15,523 people per system, and accounts for 0.9% of the 3,321,960 people reported by systems filed under the county. Across Washington, the 4,705 active systems average 2,172 people each. Nationally, the average active community water system reports serving 6,703 people, so City of Mercer Island is about 4.6 times that figure.
What does “community water system” mean?
A community water system serves at least 15 connections used by year-round residents, or regularly serves at least 25 year-round residents. Homes, apartments and mobile-home parks are served by these. Washington has 2,392 active community water systems, 51% of the state's 4,705 active systems. Purchased surface water means the system buys surface water from another public water system rather than drawing its own. 130 Washington systems (2.8%) list purchased surface water as their primary source.
Who owns and regulates City of Mercer Island?
EPA lists City of Mercer Island as owned by a local government. The WA state drinking water program is its primacy agency, the regulator with primary enforcement responsibility for the system under the Safe Drinking Water Act. 58 of the 214 active systems in King County (27%) are also owned by a local government.
Does City of Mercer Island have drinking water violations?
EPA's records list 4 violations for this system; 1 is health-based, and the most recent began in December 1992. The violations listed here began between January 1992 and December 1992. By type: 3 monitoring and reporting and 1 contaminant level above the legal limit. The contaminant or rule involved most often is Total coliform bacteria (TCR) (4 of them). EPA lists 4 as archived. Archived means the violation was not marked resolved but is more than five years past the end of the noncompliance period, so it no longer counts toward current compliance status.
| Began | Violation | Contaminant or rule | Health-based | Status |
|---|---|---|---|---|
| 1992-12-01 | Monitoring and reportingMonitoring, Routine Minor (TCR) · code 24 | Total coliform bacteria (TCR)code 3100 | No | Archived |
| 1992-08-01 | Monitoring and reportingMonitoring, Routine Minor (TCR) · code 24 | Total coliform bacteria (TCR)code 3100 | No | Archived |
| 1992-05-01 | Monitoring and reportingMonitoring, Repeat Major (TCR) · code 25 | Total coliform bacteria (TCR)code 3100 | No | Archived |
| 1992-01-01 | Contaminant level above the legal limitMaximum Contaminant Level Violation, Monthly (TCR) · code 22 | Total coliform bacteria (TCR)code 3100 | Yes | Archived |
All 4 violations in EPA's records, newest first.Labels and codes come from EPA's SDWIS reference tables. A monitoring or reporting violation means a required test or report was missed or late; it does not by itself mean a contaminant was found. "Health-based" marks violations of a legal limit or required treatment.
City of Mercer Island: frequently asked questions
Who runs City of Mercer Island?
City of Mercer Island (PWSID WA5353640) is a community water system owned by a local government. It is regulated by the WA state drinking water program.
How many people does City of Mercer Island serve?
City of Mercer Island reports serving 31,057 people through 9,807 service connections in King County, Washington.
Where does City of Mercer Island get its water?
EPA SDWIS lists this system's primary water source as purchased surface water. Purchased surface water means the system buys surface water from another public water system rather than drawing its own.
Does City of Mercer Island have any drinking water violations?
EPA's records list 4 violations for this system; 1 is health-based, and the most recent began in December 1992. A monitoring or reporting violation means a required test or report was missed or late; it does not by itself mean a contaminant was found.
Is City of Mercer Island's water safe to drink?
AquaCensus shows EPA compliance records, not current water test results, so it cannot say whether the water is safe at a given moment. EPA's records list 4 violations for this system; 1 is health-based, and the most recent began in December 1992. For current results, consult the system's annual water quality report, EPA ECHO, or the WA state drinking water program.
Where can I find City of Mercer Island's water quality report?
As a community water system, City of Mercer Island must deliver an annual water quality report, called a Consumer Confidence Report, to its customers by July 1 each year, covering the previous calendar year. The system can provide a copy on request.
Other water systems in King County
| Water system | County | Population served | Type | Source |
|---|---|---|---|---|
| Seattle Public Utilities | King | 1,161,961 | Community | Surface water |
| Bellevue City of | King | 321,349 | Community | Purchased surface water |
| Kent Water Department | King | 166,421 | Community | Purchased surface water |
| Redmond Water System City of | King | 163,335 | Community | Purchased surface water |
| Lakehaven Water and Sewer District | King | 148,809 | Community | Purchased surface water |
| Renton City of | King | 108,091 | Community | Groundwater |
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About this data
Source: U.S. EPA Safe Drinking Water Information System (SDWIS) and SDWA violation records, via EPA ECHO, as published October 1, 2026 — AquaCensus refreshes it quarterly. See the methodology for how records are matched, cleaned and scored.
Cite as: AquaCensus, “Mercer Island City of, PWSID WA5353640, Washington”, https://aquacensus.com/water-systems/washington/mercer-island-city-of-wa5353640 (data as of October 1, 2026).