Soil Mixture Confirmed To Completely Remove Coho-killing 6PPD-q From Street Runoff

King County researchers say they’ve confirmed that a soil mix formula is 100 percent effective at removing a coho-killing chemical from road runoff.

Scientists at the county’s Water and Land Resources Division and stormwater researcher Curtis Hinman found that running stormwater through a mix of roughly 70 percent sand, 20 percent coconut fiber and 10 percent biochar, with a small proportion of engineered sand, iron aggregate and aluminum thrown in, filtered out 6PPD-quinone in a laboratory setting, rendering treated water safe for the salmon.

They now plan to try it in the wild on Whatcom Creek by the end of 2025.

“It’s not every day that we get to say with certainty that we can solve such a complicated problem, but I feel hopeful about this one,” said WLRD’s Josh Latterell in a press release announcing the findings. “This issue isn’t unique to King County, but King County is uniquely positioned to do something about it and our researchers are building a whole toolkit of strategies that will make cleaner, safer water a reality.”

According to a video produced alongside the release, the sand allows untreated runoff to soak into the treatment media, the coconut fiber and biochar help catch pollutants, while the remaining materials trap “elusive” contaminants such as heavy metals.

King County’s results built on initial work by the cities of Tacoma and Redmond, Kitsap and King Counties, Herrera Environmental and other researchers, and confirm that the mix they stumbled onto works to remove 6PPD-q, a chemical linked to a tire preservative and how it breaks down when exposed to ozone.

It’s the latest news from a problem first spotted on West Seattle’s Longfellow Creek but germane anywhere high-traffic roads intersect salmon streams. When adult coho encounter runoff laden with 6PPD-q, they die, regardless of whether they’ve spawned or not. Steelhead and Chinook are also affected, though to lower degrees than coho.

As Portland scientists and municipal managers also rally to the issue, King County and state agencies are now identifying roadways where the toxin is highly concentrated and drumming up a priority list for creating new or bolstering already in place treatment facilities. The county is also updating its stormwater management protocols for developers to OK the use of the soil mix while building bioretention swales outside urban growth areas.

“The urgency of the 6PPDQ problem is leading to a lot of research, coordination, resources, and general societal investment,” said Chelsea Mitchell, a King County WLRD senior ecotoxicologist, in that press release. “I think if we keep up this momentum and investment, we can tackle the problem with aggressive source control and stormwater treatment. Plus, the treatments that are effective for 6PPD-q also mitigate other toxic stormwater contaminants, so we can address many of these problems at the same time.”