2024 Test Results — Key Findings

• Overall Compliance: SPU’s 2024 Water Quality Report confirms Seattle’s tap water met or exceeded all EPA and Washington State drinking water standards across every regulated parameter — consistent with the utility’s long track record of full compliance.
• Lead and Copper: SPU’s source water (Cedar and Tolt) contains no detectable lead or copper. The 90th percentile lead level remains well below the EPA action level of 15 ppb. In homes with older plumbing, lead can leach from internal fixtures — SPU advises flushing taps for 30 seconds to 2 minutes after extended non-use periods as a precaution.
• Testing Scope: Seattle Public Utilities conducts over 20,000 water quality tests annually, including comprehensive monitoring at both treatment facilities and throughout the 1,800-mile distribution system.
• 2024 Minor Monitoring Note: A minor administrative monitoring violation was recorded at the Cedar Treatment Facility on 21 June 2024 when UV equipment briefly failed to log data for one of seven operating UV units. UV treatment continued without interruption throughout the event and there were no public health implications. Repairs and improved system programming were completed promptly.

PFAS Testing — Consistently Clean

• Main Supplies — Zero Detections: PFAS testing of the Cedar and Tolt finished waters in 2023 (conducted as part of EPA’s UCMR 5 programme, the most comprehensive PFAS monitoring cycle to date) found no detections across all 29 PFAS compounds tested. Prior rounds in 2015, 2018, and 2022 also returned zero detections for the primary supplies.
• Emergency Wells — Trace Detection in One Well Only: SPU maintains three emergency wells that have not been used operationally since 2015. Testing in 2018 and 2022 found trace PFAS in the Blvd Park well at a maximum of 5.7 ppt for PFOA — above the EPA’s current 4 ppt MCL (enforceable from 2031) but well below previous advisory levels. The other two wells were clean. These wells do not supply drinking water under normal operations.
• Washington State PFAS Monitoring: State legislation required all public water systems to complete PFAS testing by December 2025. Districts purchasing water from SPU confirmed no PFAS detections in SPU-supplied water through 2024 testing rounds.

Protected Watershed Sources

• Cedar River Watershed: The primary source (approximately 72% of supply) includes over 90,000 acres of protected forestland managed for water quality, wildlife habitat, and public recreation. Areas south of Green Lake are primarily served by Cedar water.
• South Fork Tolt River: The secondary source (approximately 28% of supply) features a pristine mountain watershed with strict development restrictions and habitat protection. Areas north of Green Lake typically receive Tolt water, though the sources can serve either area as needed.
• Natural Filtration: Both watersheds naturally filter water through old-growth forests, streams, and natural ecosystems, providing exceptionally pure source water that requires minimal treatment before reaching taps.

Treatment Philosophy

• Cedar Water Treatment Facility: Uses ozonation, UV disinfection, and fine filtration to stabilise seasonal water quality variations driven by rainfall and runoff. Chlorine is added post-treatment at around 1.5 mg/L leaving the plant, averaging approximately 0.8 mg/L through the distribution system.
• Disinfection Byproducts: EWG’s independent review identifies trace TTHMs and HAA5s that exceed EWG’s more stringent health-based benchmarks, though all levels remain below federal MCLs. These byproducts rise slightly during autumn and winter when higher rainfall increases watershed organic content. SPU manages this through optimised ozonation and filtration adjustments.
• Fluoride: Seattle has fluoridated its drinking water since 1970 at the CDC-recommended level of 0.7 ppm to promote dental health.

Climate Resilience and Sustainability

• Drought Preparedness: Despite abundant Pacific Northwest precipitation, SPU maintains active drought contingency plans including reservoir management, emergency well standby capacity, and tiered conservation measures.
• Infrastructure Resilience: Ongoing seismic upgrades and system redundancy programmes ensure reliable water delivery during emergencies including major earthquakes — a significant planning priority in the Puget Sound region.
• Forest Climate Adaptation: Watershed forest management practices are being adapted to address increasing wildfire risk, changing snowpack patterns, and shifting precipitation across the Cascades.

Environmental Leadership

Seattle Public Utilities demonstrates national leadership in watershed-based drinking water management, with comprehensive forest stewardship, salmon habitat restoration, and recreational access programmes. The utility’s source-protection approach — maintaining pristine watersheds rather than relying on heavy treatment — delivers exceptional water quality with minimal environmental impact. Seattle’s system serves as a model for sustainable urban water supply. For the broader Pacific Northwest water quality picture, see our Washington State water quality page, or check for any active boil water notices in your area.