Mercer Island water-supply pipeline update: design 60% complete; construction expected 2026–27

Mercer Island staff told the City Council on Oct. 21 that the city’s water supply pipeline project is at roughly 60% design and is on track to finish design in February 2026, with water-main construction anticipated to begin in 2026 and run through mid-2027.

The project would install about 5,000 linear feet of new 24-inch ductile iron pipeline along a new alignment on Southeast 36th and Gallagher Hill, replace approximately 1,500 feet of asbestos-cement (AC) main on Gallagher Hill, and provide a new city-owned supply connection that would change how Shorewood receives potable water. “We are using a design–construction alternative process with a contractor at the table early,” project staff said; Scarcella Construction is participating on the design team, staff added.

Staff said the water portion of the project currently carries an estimated cost of $12.5 million to $13.5 million; an additional suite of roadway, pedestrian and bicycle improvements along the same corridor will follow the water work and remain under design. The state legislature approved roughly $1.2 million in capital-budget funding toward the pipeline, staff said.

Timing and sequencing were central to the council discussion. Staff said they plan to prioritize construction during summer months where feasible to limit impacts to school traffic and to coordinate workloads with King County and other nearby projects. Design completion is targeted for Feb. 2026; staff estimated water-main construction could occur Feb. 2026–April 2027, with roadway improvements phased afterward and a target of substantial completion by Dec. 2027.

Shorewood’s service will be reconfigured to take water from Mercer Island’s distribution system through two new city meter points; Shorewood would remain responsible for its internal distribution and customer metering past the new city meters, staff said. Project modeling indicates Mercer Island can supply Shorewood without adverse effects on the city distribution system, according to staff.

Staff also discussed next steps: finalize construction sequencing, coordinate traffic and school impacts, continue contract negotiations with Seattle Public Utilities (SPU) on asset transfers and return to council with a Q1 2026 update. City staff said they are also negotiating terms and timing for when water service would be shifted from SPU to the city.

Public comment during the meeting raised a separate but related infrastructure concern: a resident, Eric Radman, asked the council to inspect and consider protections for the reservoir site’s diesel generator and fuel supply because trees near the equipment could fall on the hardware. Staff responded that Parks and Right-of-Way staff will perform tree-risk assessments and that staff will follow up with the Department of Health about any additional protective requirements for the generator and fuel tank.

Why it matters: staff described the pipeline as a “generational” investment that will increase local control and resiliency by replacing a vulnerable SPU supply reach, removing segments of aging AC pipe and creating a direct city-owned supply connection. The work will require permits, interagency coordination and substantial construction impacts for a corridor that serves schools and residential neighborhoods.

Less-critical details: staff said final cost estimates will be refined as design advances and that funding and contract terms related to asset transfer (SPU to city) are ongoing negotiation items that will be presented to council next year.