Seattle Public Utilities reports results separately for Cedar River and South Fork Tolt water. The two supplies follow different treatment paths. To read the figures, keep the source name, sampling place and period together. That lets you distinguish an annual summary from a measurement taken at one site.
The 2025 Drinking Water Quality Report says areas south of Green Lake probably receive Cedar water and those north usually receive Tolt water. Either source can supply other areas when needed. Ask SPU about a particular address if its source matters; the neighbourhood description leaves room for operating changes.
How the treatment paths differ
Cedar's protected source has an exemption from filtration requirements. It still receives treatment: ozone and ultraviolet light to disinfect the water, followed by acidity adjustment (pH adjustment) and chlorination. SPU explains that chlorine remains in the water to help protect it during distribution. Its Cedar facility guide describes the sequence.
Tolt receives ozone, water conditioning and filtration through beds of grains that trap particles, called granular media. Acidity (pH) and alkalinity adjustments and chlorination follow. Alkalinity describes the water's ability to resist changes in acidity; SPU uses these adjustments to reduce pipe corrosion. SPU identifies the disinfectant in its distribution system as free chlorine, not chloramines. Chloramines are compounds containing chlorine and ammonia, as the CDC's disinfection guide explains. The Tolt facility guide covers this route. These process accounts explain the treatment without measuring the chlorine at your own tap.
Read an annual figure as an annual figure
For both Cedar and Tolt, the 2025 finished-water tables give a chlorine average of 1.0 ppm and a range of 0 to 1.8 ppm. The page defines ppm as milligrams per litre for these measurements.
Although these values match, retain the source labels when saving them. An average and range from a year's monitoring cannot be presented as today's measurement in a hotel or home.
Compare the sampling points too
The May 2026 quarterly sheet shows why location matters. It records samples collected on 19 and 20 May 2026 and labels the data non-regulatory and informational.
Free chlorine was 1.61 mg/L at the Cedar plant outlet and 1.08 mg/L at its named distribution site. The corresponding Tolt results were 1.58 mg/L and 0.63 mg/L. The sheet names the Cedar sites CLT-5 and M-1A, and the Tolt sites TPT-3 and B-2. These individual results are not averages for the quarter or measurements for every residence.
A plant outlet and a distribution sampling point sit at different stages of the system. Cedar has more than one result on the same sheet, so the source name alone cannot identify a measurement. Those figures neither predict an untested address's concentration nor establish a problem there.
Finally, check whether a row describes raw or finished water. Raw-water results concern water before treatment; finished-water results concern the treated supply. Use each table's definitions and footnotes, and keep an individual building concern separate from the system summaries. Our Seattle tap-water guide provides broader traveller context; the original reports supply the dated measurements.
