Seattle’s tap water is safe to drink, meeting or exceeding all water quality standards set by both the federal government and Washington state. Seattle Public Utilities (SPU) manages the system, supplying water to over 1.6 million people in the greater metropolitan area. This high standard of purity begins with the protected nature of the source waters and continues through a multi-barrier treatment process.
Where Seattle’s Water Comes From
Seattle’s drinking water originates from two primary sources located high in the Cascade Mountains: the Cedar River Watershed and the Tolt River Watershed. These two watersheds provide a pristine water supply that requires less intensive treatment than sources used by many other major cities. The Cedar River Watershed is the larger source, supplying approximately 60 to 70 percent of the water, while the Tolt River Watershed provides the remaining 30 to 40 percent.
The City of Seattle owns and manages nearly all of the land within the Cedar River Municipal Watershed, and a large portion of the Tolt River Watershed, restricting public access. This watershed protection plan prohibits activities like agriculture, industry, and unsupervised recreation. This minimizes the opportunity for contaminants to enter the raw water supply, as the water is collected as rain and snowmelt before flowing into the reservoir system.
The Treatment and Purification Process
Seattle Public Utilities employs a multi-step treatment process for both the Cedar and Tolt supplies, though the exact steps vary due to differing source water characteristics. The Tolt Water Treatment Facility utilizes conventional filtration to remove suspended particles, followed by disinfection. Due to its exceptional purity, the Cedar River supply is one of the few large surface water systems in the U.S. that does not require filtration under a Limited Alternative to Filtration status granted by regulators.
Both treatment streams use advanced disinfection techniques to inactivate any remaining pathogens. Primary disinfection involves ozonation, which uses ozone gas to destroy microorganisms and also improves the water’s taste and odor. The Cedar supply adds an extra layer of protection using ultraviolet (UV) light disinfection, which is highly effective against chlorine-resistant parasites like Cryptosporidium.
Final disinfection for both supplies involves the addition of free chlorine to ensure a residual disinfectant remains active as the water travels through the distribution pipes. The process also includes corrosion control. Lime is added to the water to adjust the pH and alkalinity, making the water less corrosive to the metal in the pipes and household plumbing.
Addressing Specific Contaminant Concerns
The primary concern for residents is not the quality of the water leaving the treatment plants, but potential contamination from aging plumbing once it enters the localized distribution system. Lead and copper are the most common contaminants, which leach into the water from older service lines, lead solder, and brass fixtures inside homes. The water itself does not contain lead when it leaves SPU’s facility.
SPU’s corrosion control program, achieved through pH adjustment, creates a protective mineral coating on the interior of pipes. This is highly effective at preventing lead and copper from dissolving into the water.
Residents in older homes can take steps to reduce risk. Running the cold water tap for two minutes after it has been standing in the pipes for several hours, such as first thing in the morning, flushes out any accumulated metals. Water used for drinking or cooking should always be taken from the cold water tap, as lead dissolves more quickly in hot water.
Regarding emerging contaminants like Per- and Polyfluoroalkyl Substances (PFAS), Seattle Public Utilities conducts testing for over 200 compounds, well beyond federal requirements. Recent testing has found no detectable levels of PFAS in the main Cedar and Tolt surface water supplies. Trace amounts of PFAS have been detected in a few emergency backup wells, but these wells are rarely used and have been kept offline to protect the water supply.
Regulatory Oversight and Quality Reports
The safety of Seattle’s drinking water is overseen by two regulatory bodies: the U.S. Environmental Protection Agency (EPA) and the Washington State Department of Health (DOH). SPU must comply with the standards set by these agencies, which includes a monitoring and testing schedule. The utility performs thousands of tests annually across the source water, treatment plants, and distribution system for a wide range of microbial, organic, and inorganic contaminants.
To keep the public informed, the EPA requires all public water systems to publish an annual Consumer Confidence Report (CCR). Seattle Public Utilities makes this report available to all customers, detailing the previous year’s water quality results and comparing them against regulatory limits. The CCR provides data on compounds detected, the levels found, and confirmation that SPU has maintained full compliance with health-based standards.