Seattle.gov Home Page
Seattle.gov This Department

Water Sources and Contaminants

  • Masonry Pool in the Cedar River Watershed.
More information about your water.

The Details About Our Water Sources and Their Potential Contaminants

Our region’s water comes from two major sources: the Cedar River (60 percent) and the South Fork Tolt River (40 percent). These surface water sources begin in the Cascade Mountains and have very large protected watersheds. The system also uses wells, located in Burien, to meet peak summer demand. The wells provided less than 0.2 percent of the supply in 2012.

As water travels over the surface of the land or through the ground, it dissolves naturally occurring minerals and, in some cases, radioactive material, and can pick up substances resulting from the presence of animals or from human activity.

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

All drinking water, including bottled water, contains at least small amounts of some contaminants, the presence of which does not necessarily indicate a health risk. More information about contaminants and potential health effects can be obtained by calling the Environmental Protection Agency’s Safe Drinking Water Hotline at (800) 426-4791.

Washington’s Source Water Assessment Program is conducted by the Department of Health (DOH) Office of Drinking Water. According to DOH, all surface waters in Washington are given a susceptibility rating of “high,” regardless of whether contaminants have been detected or whether there are any sources of contaminants in the watershed. The Seattle wells have been given a susceptibility rating of “low” because of the type of aquifer, depth of well, and lack of contaminant detection. Information on the source water assessments is available from the DOH website.

Since both watersheds are publicly owned, Seattle Public Utilities is able to prohibit agricultural, industrial, and recreational activities in the watersheds. Even so, there is always some potential for natural sources of contamination.

In Seattle’s surface water supplies, the 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; and
  • 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. Immuno-compromised persons such as persons with cancer undergoing chemotherapy, persons who have undergone organ transplants, people with HIV/AIDS or other immune system disorders, some elderly, and infants can be particularly at risk from infections. These people should seek advice about drinking water from their health care providers. Environmental Protection Agency/Centers for Disease Control guidelines on appropriate means to lessen the risk of infection by cryptosporidium and other microbial contaminants are available from the Safe Drinking Water Hotline at (800) 426-4791.