From Tire Dust to Salmon Streams: How Biochar Can Help
In March 2025, King County announced a groundbreaking development in the fight to protect coho salmon: scientists identified a soil mixture that can effectively filter out 6PPD-quinone (6PPD-Q), a toxic chemical derived from tire dust that enters waterways through stormwater runoff. This chemical, formed when the tire preservative 6PPD reacts with ozone, has been directly linked to mass die-offs of coho salmon returning to spawn in urban streams. Even trace amounts of 6PPD-Q have proven lethal, posing a serious threat to the health of Pacific Northwest ecosystems.
Biochar—a carbon-rich material produced by heating biomass in a low-oxygen environment—is best known for its role in improving soil health and sequestering carbon. In addition, its highly porous structure and surface chemistry also make it an exceptional filter for contaminants. In stormwater systems, biochar has been shown to capture heavy metals, nutrients, pesticides, and hydrocarbons. Could it also help trap emerging contaminants like 6PPD-Q?
Early research suggests the answer is yes. Biochar has been used to treat other forever chemicals like PFAS and even the removal of toxic gases leaking from chemical plants. Created by Curtis Hinman, his consulting company, and researchers at Western Washington University, this new media can be used for all sorts of stormwater filtration.
Mixing together biochar, sand, and coconut core, making up the primary layer of the high-performance bio-retention soil media (known as HPBSM) that's excellent for water filtration. Additionally, there can be a polishing layer made up of sand, iron aggregates, and activated alumina to help treat any phosphorus in the water.
“Essentially, the media knocked the 6PPQ concentrations down to essentially non-detect”
The study done in King County showed a 100% survival rate for salmon placed in water strained through the mixture, and a 100% mortality rate for water not treated by the soil mixture. These dramatic results are allowing for a positive push for further research and experimentation
Furthermore, a pilot project for the HPBSM has been launched in Whatcom County, and the results from this pilot could have a significant impact on the future of stormwater treatment. The protection of coho salmon provides ample reason to accelerate research and development of this promising innovation. While 6PPD is most toxic to Coho Salmon, its effects can also be lethal to other native salmonids like Chinook or Steelhead, and future testing is required to see if this soil mixture has the same effect for these fish as well.
At Kulshan Carbon Trust, we see biochar as part of a larger toolkit of nature-based climate solutions that fosters a circular economy across sectors—agriculture, forestry, and land stewardship—to regenerate ecosystems, sequester carbon, and build thriving communities. The recent King County study serves as a call to action: we must invest in materials and designs that promote both clean water and climate resilience. As we look to the future, this advancement not only holds potential for restoring Coho salmon populations but also contributes to broader efforts in improving water quality and ecosystem health in the region. We look forward to the future of this study and to seeing where it leads.
