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City water profile

Mercer Island water quality

Compare local ZIP estimates, evidence quality, associated public systems, and compliance history for Mercer Island, Washington.

Median indexed hardness

61PPM

One indexed ZIP

Range 61.3–61.3 PPM

State comparison
43 PPM below
State hardness rank
#100 of 133
Matched utilities
1
Active health records
0

City summary

What the indexed records say

Mercer Island has 1 published ZIP profile across King County. The indexed median is 61 PPM, compared with 104 PPM across Washington.

Among the 133 Washingtoncities with an indexed median, Mercer Island ranks #100from highest to lowest. Across all 189 state ZIP profiles, available estimates run from 20.6to 238 PPM.

Because only one ZIP is represented, the city median and range are the same local estimate. It should not be read as multiple independent samples across the community. At least one estimate is marked low confidence, so observation distance and date deserve extra attention.

Hardness distribution

How local ZIP estimates compare

Soft0 ZIPs
Moderately hard1 ZIP
Hard0 ZIPs
Very hard0 ZIPs

1 of 1 ZIP profiles have a hardness estimate. Bars count ZIP profiles, not households or laboratory samples.

Evidence quality

Distance, dates, and confidence

Median nearest distance
25.4 miles
Profiles with evidence
1/1

Confidence labels

High: 0Moderate: 0Low: 1

Observation window: Jan 13, 2016 to Mar 20, 2026.

ZIP directory

Water reports in Mercer Island

Open a ZIP for site-level provenance and measurements.

ZIP code

98040

Moderately Hard

61.3 PPM

Nearest site
25.4 mi
Confidence
Low

View full ZIP evidence →

Public water systems

Utilities associated with this city

Mercer Island City Of

PWSID WA5353640

Surface water
System population served
31,057
Last reported
Jun 19, 2026

A municipal system, in-state city, and served county all agree with this Census ZCTA.

Compliance history

What EPA records show

No active health-based violation appears among the associated systems in the represented EPA reporting quarter. This does not mean every contaminant was tested at every tap.

No violation records are included in the current lookback window.

Practical next steps

Move from index to evidence

  1. 1. Open your ZIP. Check distance, dates, confidence, and the associated utility.
  2. 2. Read the current CCR. Confirm source water, detections, limits, ranges, and notices.
  3. 3. Test the tap when needed. Confirm health concerns or treatment sizing with the right method.

Questions people ask

About water in Mercer Island

Is the water hard?

The indexed city median is 61 PPM. Open the ZIP report to verify the distance and confidence behind that estimate.

Is the tap water safe?

Compliance history is utility-wide and cannot guarantee conditions at every tap. Check current notices and test when address-specific evidence is needed.

Do I need treatment?

Choose treatment only after confirming the problem at the property. A softener targets hardness; contaminant treatment requires a matching, verified performance claim.