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You are here:   Home → Natural Environment → Drinking Water Quality
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Indicator: Drinking Water Quality

Data and Data Discussion provided by Sustainable Seattle

Figure 2: Lead and Copper Monitoring Results, 2007, Seattle Public Utilities

Figure 1: Drinking Water Quality 2007 Results, Seattle Public Utilities

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Upstream Indicators
  •   303d Listed Watercourses --- Although approximately 50% of watercourses included on the 1998 Washington State 303(d) list have been de-listed, the total number of listed water segments has increased in the past decade. The most significant increase in listings has occurred with regards to temperature.   (view data graph)
  •   Freshwater Habitat Index --- Overall freshwater quality in King County is below standards except for lakes. These findings are based on 2007 monitoring results for rivers, streams, groundwater, and lakes. Of the total sampled stream sites in King County in 2006-07, 45 percent were considered moderate to high water quality, and 55 percent were rated to be of high concern. This was a decrease in the number of sites considered moderate to high water quality from previous years.   (view data graph)
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Downstream Indicators

There are currently no published Downstream Indicators.

Related Goals

Sustainability Snapshot:

The quality our drinking water not only is critical for public health but also provides an excellent indicator of environmental protection and sustainable land use practices in our watersheds. Clean drinking water requires constant vigilance to protect the source from bacteria, other contaminants and metals — all the way from the reservoirs to the taps in people’s homes. Seattle's water, mostly, comes from rain and snowpack in the 103,839-acre Cedar River and Tolt River watersheds. These areas are access-controlled; they have no industry and no residences, resulting in a source of exceptionally clean water. Other cities and places in King County use different watershed and also groundwater sources.

Sustainability Trend:

Seattle Public Utilities (SPU) regularly tests its water for 179 compounds, including metals, chemical contaminants and disease-causing organisms. From 2001 through 2008, SPU-supplied water easily met all state and federal drinking water standards. No compounds were detected above allowable levels.

Data Discussion

The Indicator Defined

Results of regular monitoring of drinking water quality by Seattle Public Utilities.

Data Interpretation/Evaluation

To ensure that tap water is safe to drink, the Environmental Protection Agency and the Washington State Department of Health prescribe regulations that limit the amount of certain contaminants in water provided by public water systems.

Seattle has some of the lowest levels of water contaminants of any major city in North America.  In Seattle’s surface water supplies, potential sources of contamination include:

  • Microbial contaminants, such as viruses, bacteria, and protozoa from wildlife.  
  • Inorganic contaminants, such as salts and metals, which are naturally occurring.
  • Organic contaminants, which result from chlorine combining with the naturally occurring organic matter.

Some people may be more vulnerable to contaminants in drinking water than the general population.

The results of monitoring in 2007 are shown in Figures 1 and 2.  These results are for parameters regulated by the federal and state agencies.  In 2007 and 2008, none of the monitored compounds in SPU’s testing were found to have exceeded allowable levels.

In 2004, SPU completed an agreement with the Washington State Department of Health related to the reduction of lead levels in residential homes.  Although there is no detectable lead in our source water, tests show there are sometimes elevated levels of lead and copper in some samples, primarily because of corrosion of household plumbing systems. These results show that it is very important that homeowners, business owners and others be aware of their type of plumbing, and how the plumbing affects their drinking water quality.  The majority of homes have some risk of lead contamination in water that sits in pipes for longer than two hours. Where you live, when your plumbing was installed and what type of plumbing you have, all play a part in determining your potential exposure level. To have your home tested, contact a certified lab near your area.

Data Source and Limitations

Data compiled by Sustainable Seattle from Seattle Public Utilities reports for water quality.   

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Additional Resources

  • Historical Monthly Precipitation and Water Use
    Most of the time we have plenty of water available to meet demand. During the summer, however, water use increases from 50 to 250 percent because of irrigation of lawns, golf courses, and parks. This graph compares precipitation levels to average drinking water use.