City of Sammamish Water Hardness & Quality Report (2026)

Sammamish Plateau Water and Sewer Water Company serves approximately 54,780 residents in the Sammamish area of King County, Washington, primarily the Sammamish Plateau. All drinking water is sourced from groundwater via five wells — three in the Evans Creek Valley and two on the plateau. No surface water is used. Treatment is minimal: filtration is applied to Wells 3 and 4 for arsenic, and to Well 3 for hydrogen sulfide odor removal. No disinfection chemicals are added, and the water is not fluoridated following a voter decision decades ago. Annual Consumer Confidence Reports are available via spwater.org.

The supply originates within the Sammamish Plateau's local groundwater recharge areas, part of the broader Sammamish Lake watershed influenced by the Cascade foothills. Water infiltrates through glacial till and outwash plains into the Vashon Aquifer, a shallow unconfined system of Pleistocene-age sands and gravels formed during the Fraser Glaciation. Dissolved minerals from glacial deposits over the Seattle Uplift's sedimentary bedrock impart a moderately mineralised character, with variations between valley and plateau wells due to differing recharge paths and till compositions.

At moderately hard levels, scale buildup occurs in water heaters, dishwashers, and washing machines, reducing efficiency and lifespan; faucets and fixtures may develop spots. Boilers and coffee makers require more frequent descaling. Maintenance tips include regular vinegar flushes, installing drain screens, and using scale-inhibiting filters. A water softener is recommended for households with aesthetic concerns or heavy appliance use. Water quality meets federal and state standards, with pH ranging from 7.2 in Evans Creek Valley wells to 7.4 on the plateau; arsenic and hydrogen sulfide are addressed via filtration where needed.