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Mercer Island obtains its water from the Seattle Public Utilities (SPU). The City of Mercer Island purchases and distributes most of the water consumed on the Island under a new long-term contract with SPU that guarantees an adequate supply through the year 2062. In 1997, the City assumed the Mercer Crest Water Association that for many years had been an independent purveyor of SPU. It served a largely residential base with customers residing in the neighborhoods south of the Shorewood Apartments, and east and west of the Mercer Island High School campus areas of the Island. The Mercer Crest system was intertied and consolidated into the City utility during 1998-99. One small independent water association, Shorewood, remains as a direct service customer of SPU. The City is one of 21 wholesale customers (Cascade Water Alliance and 20 neighboring cities and water districts) of SPU.

The bulk of the Island’s water supply originates in the Cedar River watershed and is delivered through the Cedar Eastside supply line to Mercer Island’s 30-inch supply line. Mercer Island also is served periodically through the South Fork of the Tolt River supply system.

Water is distributed by the City through 115 miles of mains (4-, 6-, and 8-inch) and transmission lines (10- to 30-inch) constructed, operated and maintained by the City. The City’s distribution system also includes two 4-million-gallon storage reservoirs, two pump stations, and 86 pressure-reducing valve stations.

Minimizing supply interruptions during disasters is a longstanding priority in both planning efforts and the City’s capital improvement program. The City completed an Emergency Supply Line project in 1998-99. In 2001 following the Nisqually Earthquake, SPU strengthened sections of the 16-inch pipeline.

The year before the earthquake, the City completed extensive seismic improvements to its two storage reservoirs. As a result, neither was damaged in the earthquake. The improvements were funded through a hazard mitigation grant from the Federal Emergency Management Agency.

The City also constructed an emergency well, which was designed and permitted to provide 5 gallons per day for each person on the Island for a period of 7 to 90 days.

In 2014, the City took significant action to ensure high water quality standards after two boil water advisory alerts, including additional expanded collection of water quality samples, injection of additional chlorine, research into potential equipment upgrades and improvements, and a thorough review of the City’s cross-contamination program, including the best means of overseeing the registration of certification of backflow prevention devices.

In 2013, the City’s total number of water customers was 7,376.