Water Quality Monitoring Results

The results of monitoring in 2023 are shown in the tables below. These results are for parameters regulated by the federal and state agencies.

View general information about our water quality reports, including archived results from previous years. For other water quality information, please see our extensive water quality web pages or call (206) 615-0827. We can also send you a list of the more than 200 compounds for which we tested but did not find in our surface water supplies, including unregulated contaminants.

Water quality monitoring data can be difficult to interpret. To make all the information fit, we used many acronyms that are defined below the table. In Seattle, if you live south of Green Lake, your water probably comes from the Cedar. Areas north of Green Lake usually receive Tolt water. Each source can provide water to other areas in Seattle if needed.

2023 Monitoring Results

** SPU is required to monitor your drinking water for specific contaminants on a regular basis. Results of regular monitoring are an indicator of whether or not your drinking water meets health standards. During October 2023, a bromate sample was not analyzed for the Tolt supply, and therefore SPU cannot be sure of the quality of your drinking water during that time. However, based on historical data and results since October 2023, Tolt bromate levels are generally ND.

Lead and Copper Monitoring Results

+ The concentration of a contaminant which, if exceeded, triggers treatment or other requirements which a water system must follow.
++ 90th Percentile: i.e., 90 percent of the samples were less than the values shown.

Lead is an important topic when it comes to the safety of your drinking water. If present, elevated levels of lead can cause serious health problems, especially for pregnant women and young children.

Sources of Lead

There is no detectable lead in our source water, but tests show there are sometimes elevated levels of lead and copper in some home tap samples, primarily because of corrosion of household plumbing systems. In Washington state, lead in drinking water is primarily from materials and components associated with service lines and home plumbing. 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. Learn more about water quality and lead.

Learn About Your Plumbing

While there are no known lead service lines in Seattle’s water distribution system, there are a small number of homes and buildings that may have short lead connections. In addition, individual homes and businesses may have other plumbing components that could corrode and introduce contaminants into the water. SPU is responsible for providing high-quality drinking water but cannot control the variety of materials used in plumbing components. SPU treats the water to minimize the tendency for lead to enter the water through corrosion, and results show that we have been very successful at this.

Lower Your Risk, Don’t Let it Sit

The risk of lead contamination in water increases when water sits in pipes for longer than six hours. If you are concerned about lead and your water has been sitting for several hours, you can minimize the potential for lead exposure by flushing your tap for 30 seconds to two minutes before using water for drinking or cooking. You can use the flushed water for washing dishes, watering plants, or general cleaning.

Lead Testing is Available

If you are concerned about lead in your water, you may wish to have your water tested. Information on lead in drinking water, testing methods, and steps you can take to minimize exposure is available from the Safe Drinking Water Hotline at 800-426-4791 or at epa.gov/safewater/lead. Customers enrolled in the City of Seattle's Utility Discount Program can access free testing by calling SPU's Water Quality Lab at 206-615-0827.

Finally, remember that drinking water is only a minor contributor to overall exposure to lead. Other sources, including paint, soil, and food, also contribute.

Definitions

MCLG: Maximum Contaminant Level Goal - The level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety.

MCL: Maximum Contaminant Level - The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to the MCLGs as feasible using the best available treatment technology.

MRDL: Maximum Residual Disinfectant Level - The highest level of a disinfectant allowed in drinking water. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.

MRDLG: Maximum Residual Disinfectant Level Goal - The level of a drinking water disinfectant below which there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control microbial contaminants.

TT: Treatment Technique - A required process intended to reduce the level of a contaminant in drinking water.

NTU: Nephelometric Turbidity Unit - Turbidity is a measure of how clear the water looks. The turbidity MCL that applied to the Cedar supply in 2023 was 5 NTU, and for the Tolt it was 0.3 NTU for at least 95 percent of the samples in a month. 100 percent of the samples from the Tolt in 2023 were below 0.3 NTU.

NA: Not Applicable

ND: Not Detected

ppm: 1 part per million = 1 mg/L = 1 milligram per liter

ppb: 1 part per billion = 1 ug/L = 1 microgram per liter

1 ppm =1000 ppb