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That Plume You See is Sediment, Not Sewage

The power of imagery was on full display in a March 12 front page Seattle Times aerial photograph of West Point Treatment Plant and its adjacent waters. The photo purported to show a plume of sewage and stormwater that had discharged from the plant’s emergency outfall before the bypass gate was closed early in the morning of Feb. 16. The accompanying story began:  “A mighty river of brown, raw sewage and stormwater makes a plume offshore at Discovery Park, plainly seen from the air…It’s from the West Point Treatment plant, gushing untreated wastewater into Puget Sound.”


Looking out to where the emergency bypass outfall discharges, about 500 feet offshore from Discovery Park’s North Beach

Unfortunately, that was untrue and the power of the photo is in how it was wrongly described. At King County’s request, the Times clarified its reporting on March 20 to reflect that what its readers were actually seeing is sediment. The sediment plume regularly builds up around West Point in the shallow beach area, south of the lighthouse.


“West Point is a pronounced geographic feature that interrupts natural sediment movement along the shoreline,” says Bruce Nairn, King County’s oceanographer. “On an ebb tide, flows move north.”

This particular Feb. 16 sediment plume is corroborated by field scientists, water quality samples, historical photos, wind patterns, currents, and tidal movements.

WestPointOct2015It is true there was an emergency wastewater bypass that ended about seven hours before the Times’ drone took the photograph. However, a discharge from the combined stormwater and sewer system is about 85- 90 percent stormwater during heavy rainstorms, and would not look like the plume in the Times photograph. The discharge would appear much clearer and less concentrated, and especially seven hours after the bypass.

At the same time King County was pinpointing the source of the plume, we were contacted by Rob Casey of Salmon Bay Paddle. Rob diagrammed the photo with tidal and wind movements to help debunk the Times description.

RobCasey.salmonbaypaddle.westpoint.waterDiagrammed photo courtesy of Rob Casey, Salmon Bay Paddle

“In my experience, discolored water like what you see in the photograph is typically sediment from a high tide and big ebb current pushing north around West Point,” said Casey.

Nairn points out that the plume on the south side of the point would be unlikely to contain sewage.

“There was an ebb tide from about 6 a.m. until 2 p.m.,” says Nairn. “The wastewater would travel north, not around the point to the south.”

Casey said that in his experience the sediment plume is common and that the eroding feeder bluffs at Discovery Park’s South Beach are a typical sediment source after a high tide.

The Duwamish River and Owl Creek, which feeds the engineered wetland at the North Beach, can also contribute to significant sedimentation after rainstorms.


Commonly know as the “duck pond”, the engineered wetland along the North Beach trail is fed by Owl Creek, and overflows through an outlet to Puget Sound.

box of jugsBen Budka, Field Science Supervisor at the King County Environmental Laboratory, said water samples taken near the plant on Feb. 16 show some elevated fecal coliform levels measured by colony forming units (CFU), including a high of 770 CFU per 100 milliliters, but nothing approaching what would happen if the plume shown in the photo was actually sewage.

“If the Seattle Times image was actually sewage we would have seen fecal coliform bacteria counts in the hundreds of thousands or millions of CFU per 100 milliliters,” Budka said. “…And if this was sewage there would be a very strong odor evident along the nearshore and in any of the sample bottles we collected.  This was not the case.”

King County doesn’t dispute that an emergency bypass occurred, and we are providing updates and water quality monitoring information as restoration of West Point Treatment Plant continues. However, we feel strongly about setting the record straight that the photograph was misleading by identifying the sediment plume as sewage.

Meanwhile, crews working seven days a week remain on track for restoring full wastewater treatment function at the West Point Treatment Plant by April 30. Wastewater treatment is continuing at the plant, including screening, some solids settling and disinfection. No emergency bypasses of highly diluted stormwater and wastewater into Puget Sound have occurred since Feb. 16 at the plant, and all beaches have been open since Feb. 21.West Point Beach