King County, Washington: drinking water report. King County covers Seattle and its surrounding suburbs, with a population of about 2.3 million.
King County covers Seattle and its surrounding suburbs, with a population of about 2.3 million. Seattle Public Utilities draws from two protected watersheds – the Cedar River and South Fork Tolt River – that are closed to public access, a strategy that has kept source water remarkably clean. Most suburban King County communities purchase wholesale water from Seattle or from the Cascade Water Alliance, though some operate independent groundwater systems.
Seattle's protected watershed model produces some of the cleanest municipal source water in the country. The system operates under an EPA Filtration Avoidance Determination, meaning the water is clean enough to skip conventional filtration – a distinction shared by only a few major US cities. However, PFAS has still been detected at low levels in some King County systems, particularly those drawing groundwater near Sea-Tac International Airport, where firefighting foam has been used extensively.
According to the Washington Department of Ecology, PFAS sampling near Sea-Tac has found concentrations exceeding state action levels in several monitoring wells. The airport has become one of the most significant PFAS investigation sites in the Pacific Northwest, with contamination documented in groundwater flowing toward communities south of the airport.
If your water comes from Seattle's watershed system, your baseline quality is excellent. The risk shifts for communities near Sea-Tac or those on independent groundwater systems that may not have the same source protection.
Check your water to confirm your source and see the latest monitoring data. For areas near Sea-Tac with elevated PFAS readings, an activated carbon or reverse osmosis filter provides effective household protection. Our water filter guide covers which systems work. Pull your detailed report for history and trends, and visit our Washington page for statewide data.