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Study reducing toxicity of stormwater to salmon

Untreated road runoff is known to carry contaminants that are toxic to salmon. Bioretention is one of the most commonly applied and adaptable management practices in Low Impact Development (LID). A specific mixture of soil — called a bioretention soil mix — is recommended for use in stormwater projects like rain gardens and retention walls. 

Bioretention toxicity study

This study evaluated how effective the recommended bioretention soil mixture in our Stormwater Management Manual for Western Washington is at treating contaminants in stormwater. A research team found that the default mixture protects adult and embryonic Coho salmon from toxins in untreated stormwater. 

Study questions

  • Does the default bioretention soil mix reduce the following pollutants?
    • Polycyclic Aromatic Hydrocarbons (PAH)
    • Metal
    • Fecal coliform
    • Nutrient pollution from highway runoff
  • Does the default bioretention soil mix protect adult and embryonic Coho salmon from toxins?

Study findings

This Stormwater Action Monitoring (SAM) Effectiveness study funded ongoing work to evaluate how well bioretention soils to protect Coho salmon from toxins in urban stormwater. 

Recommended soil mixture protects salmon

The study showed that the default 60:40 bioretention soil mix we recommend in the Stormwater Management Manual for Western Washington is capable of filtering toxins to protect adult and embryonic Coho salmon.

Study leads

U.S. Fish and Wildlife partnered with Suquamish Tribe and Washington State University-Puyallup to specifically test the default mix (60 percent sand and 40 percent compost) for toxicity treatment.

What does this study tell us?

This new information helps inform stormwater managers across Washington as they adaptively manage stormwater runoff in Washington's rapidly developing watersheds. 

The study found that the recommended soil mix is effective at:

  • Protecting adult and embryonic Coho salmon
  • Filtering toxins out of stormwater
  • Reducing concentrations of:
    • Metals
    • PAHs 
    • Fecal coliform

Our stormwater management manual will continue to specify the 60:40 mix as the standard for bioretention. 

Study and more information