With new facility, Mountlake Terrace to treat chemical that kills coho

MOUNTLAKE TERRACE — In 1999, as a Seattle Public Utilities employee, Laura Reed noticed something was killing coho salmon before they could spawn.

She was in charge of monitoring creek restoration projects in the city. Along the shores of Longfellow Creek in West Seattle, she and a coworker found scores of dead fish, while conducting spawning surveys.

Reed tipped off the National Oceanic and Atmospheric Administration, though no one had any concrete theories about the cause.

“It was thought that that the primary problem with stormwater was scour and volume — not water quality,” she said. “It wasn’t clear that there was a connection between fish health and stormwater at that point.”

Now, as Mountlake Terrace’s stormwater program manager, Reed is leading an effort to address the pollutant that killed those salmon 25 years ago: 6PPD, a chemical that helps tires last longer.

When 6PPD reacts with ozone in the air, it becomes 6PPD-quinone. When it rains, the chemical often flows off roads and into local waterways. In 2021, researchers found coho salmon die after only a few hours of exposure — devastating a critical resource for local tribes and food staple for orcas, sea lions and seals. Scientists are still determining why coho salmon, versus other species, are more vulnerable to 6PPD.

Last month, the city of Mountlake Terrace received about $560,000 from the state Department of Ecology to design an underground stormwater treatment facility. The project, expected to cost about $2 million total, would clean heavily polluted runoff from a 34-acre stretch of the city that feeds into Hall Creek and eventually Lake Ballinger.

Contractors would reroute stormwater in the drainage basin through a pipe and into an underground stormwater vault, Reed said. The vault would contain a special soil mixture that could filter out 6PPD and other pollutants before the stormwater returns to Hall Creek.

Rerouting stormwater runoff underground may also reduce flooding in the streets along the stream. Heavy winter rains can turn intersections into lakes.

The section of Hall Creek expected to benefit from the planned stormwater treatment facility happens to run through the sole industrial zone in the city.

“We have to take care of it,” said Mountlake Terrace spokesperson Rikki Fruichantie. “We’re going to continue to develop. As (the city) develops, we have to preserve green spaces — not just for recreation but also habitat.”

Along Hall Creek, 220th Street SW attracts heavy traffic — over 20,000 cars every day.

“You can imagine the amount of tire dust that accumulates,” Reed said.

Further south, Lake Ballinger is also a popular swimming destination. The Lake Ballinger watershed, just east of Highway 99, is the city’s highest priority for environmental work.

Historically, the lake was considered the dirtiest in the Puget Sound region. But restoration efforts have improved conditions.

Just last month, the city and the U.S. Army Corps of Engineers partnered to complete a $7 million restoration project at Ballinger Park.

Contractors removed more than 16 acres of invasive vegetation — putting native plants there instead. Workers also improved in-stream fish habitat and shifted one of Hall Creek’s channels to reflect a more natural meander.

Treating stormwater runoff from 220th Street SW, as well as nearby commercial and residential neighborhoods, will ideally protect both fish and locals who use the watershed, Reed said.

She hopes construction for the project can begin in three years.

“I’ll be very happy to see this project come to pass,” Reed said, “to feel like we’re doing as much as we can for this area.”

Ta’Leah Van Sistine: 425-339-3460; taleah.vansistine@heraldnet.com; Twitter: @TaLeahRoseV