Water Quality

The Cascade Mountains supply our drinking water. Two very large, protected watersheds, the Cedar River Watershed and the South Fork Tolt River Watershed, supply almost all of Seattle’s metropolitan area with drinking water. We purchase all of our water from Seattle. Most of our water comes from the Tolt River Watershed, but occasionally we receive water from the Cedar River Watershed. In 2018, all of Woodinville’s supply came from the Tolt.

How The Water Is Treated

Because of our pristine source water, we provide high quality drinking water with very little additional treatment. Seattle Public Utilities protects, collects and treats our region’s water supply, which is among the nation’s finest. The data shows that Seattle’s water has some of the lowest levels of contaminants of any major city in North America. For every EPA-monitored substance, our water quality easily beats the standards. The water is tested up to 50 times a day for bacteria, metals, and chemical contaminants both before and after it goes into the treatment facility. The finished water is sent through pipes to our reservoirs. Since January 2001 our water has been filtered and ozonated at the Tolt Treatment Facility.

The plant is designed to treat up to 120 million gallons of water per day. The full treatment process for the Tolt supply is described below.

1. Ozonation: Adding ozone gas, a powerful disinfectant which destroys Cryptosporidium, Giardia, bacteria, and viruses and aids the filtration process.

2. Coagulation and Flocculation:Gently stirring the water with small doses of chemicals to bring small particles together to form large particles that can be more easily filtered.

3. Filtration: Removing the suspended particles by passing the water through a 6-foot deep bed of anthracite (crushed coal).

4. Chlorination: Adding a secondary disinfectant to provide continued protection in the distribution system against microbial contamination.

5. Fluoridation: Adding fluoride, which helps prevent tooth decay. Seattle added this to comply with a public vote in November 1968.