Water is delivered to Mercer Island by Seattle Public Utilities via 16" and 24" water supply lines. The water is stored in two 4-million gallon reservoirs for domestic use and fire protection. Water is distributed through 4" to 30" waterlines and is delivered to customers through approximately 7,300 water meters. Water quality testing and monitoring is performed as required by the Environmental Protection Agency Safe Water Drinking Act.
Cross-Connections & Backflow Prevention Devices
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Water Utility Billing
Visit the Water Utility Services page here.
2014 Boil-Water Incident
In late September, routine water quality testing conducted by the City revealed the presence of E. coli and Total Coliform bacteria at several different locations in the City’s water delivery system over two days. Working in conjunction with the state Department of Health, the City determined that it should issue a Boil-Water Advisory on Saturday September 27 until Monday Sept 29. This applied to all residents and forced the closure of all food service establishments due to Public Health – Seattle & King County regulations. Despite initial corrective measures, contamination was detected again two days later, and the City had to reinstate the Boil-Water Advisory from Thursday October 1, until Wednesday October 8. No cause of the transient contamination was ever found, and no cases of confirmed E. coli illness were linked to the water system.
The state Department of Health cited five milestones allowing the end of the Advisory: six sequential days of clear test results; completion of an Investigative Action Plan; maintained boosted chlorine residuals in the water reservoirs; demonstrated and monitored elevated chlorine residuals across the distribution system; and development of a post-rain event inspection protocol for high-risk underground valve vaults identified in field surveys.
Click here for a selection of water quality data mapping from the incident.
To identify further potential operating improvements to its water delivery system, the City contracted for expert review with water system specialists at Confluence Engineering Group. With the support of the City Council and the City’s Utility Board, staff are already in the process of implementing many of the recommendations. The Draft Technical Summary of their findings on the incident is available here.
Based on its experience with emergency communications during the incident, the City has implemented a number of improvements to notification systems, Email software, and other mass-outreach technologies. A “Dine Local” campaign was launched to encourage support of restaurants impacted financially by the closure orders. The City also helped facilitate access to low-interest emergency loans from the federal Small Business Administration.
Read the system update letter sent to all residents in March 2015.
Watersheds: The Source of Our Water
The region’s fresh water supply water comes primarily from two sources: the Cedar River Watershed and the Tolt River Watershed, both located in eastern King County. The watersheds are large, uninhabited land areas that gather and store rain and snowmelt. Year-round, 26 cities and water districts rely on a limited supply of stored water from these two sources to meet most of the daily needs of business, government, institutions and 1.3 million people in our region. In addition to providing clean, clear, reliable drinking water, the watersheds also provide habitat for fish and wildlife.
Water Conservation Tips: How to be Waterwise
Water conservation is important for our region, our environment, future generations and your pocketbook. Click here to visit our conservation pages and learn how to be waterwise.
Just how much water does Mercer Island use anyway?
Historically, Mercer Island has been one of our region's biggest water users during the summer "demand" season. A variety of conservation efforts funded through the Water Utility are targeting this seasonal peak in consumption. Click here to see how much water the Island uses each month.
Water Quality Report
The City of Mercer Island is pleased to inform residents that compliance with all state and federal drinking water laws remains exemplary. We are committed to delivering the best quality drinking water. To that end, we remain vigilant in meeting the challenges of water system protection, water conservation and community education. To view the 2014 report, please click here.
[Note: annual reports issue by July of the following year; the 2015 report will be ready in July 2016]
To view the "UCMR3" report, which provides additional detailed water quality testing information, click here.
Water Standard Details
Click here to view the most current version of the Water Standard Details.
Click here to view a list of local laboratories to contact for residential water testing.
Water System Plan
The purpose of the Water System Plan (WSP) is to examine the existing system and its operation, analyze system hydraulics, study current and projected future conditions by computer modeling and develop a plan to meet the City’s water system needs to the year 2028 and beyond. The drafted 2008 WSP has been reviewed and commented on by the Department of Health (DOH) and the Seattle Public Utilities (SPU). Responses to both agencies were addressed in response letters provided by the City. Revisions to various subjects and areas of the draft 2008 WSP were made in accordance to the agencies' review comments. The final 2008 WSP was adopted by the City Council on July 7, 2008. A copy of the 2008 Water System Plan is available at City Hall.
Central Puget Sound Water Suppliers Forum
Click here to learn about the Forum's purpose, members and most commonly asked questions.
The City developed the emergency well to supply Island residents with water for an extended period of time without significant assistance from outside during an emergency; the well water requires treatment before it is suitable for consumption. Click here for more information.