Ohop Creek Stormwater Management Pilot Project

Can compost prevent toxic roadway runoff from reaching salmon streams?

Project Overview

Tire dust and other chemicals in roadway runoff are deadly to salmon. Tire dust is especially harmful for coho salmon, causing high rates of mortality even at low levels. To protect coho spawning habitat in a sensitive restoration area along Highway 7, LLTK, the Nisqually Indian Tribe, and partners are testing a compost-based biofiltration system to collect and clean stormwater before it reaches Ohop Creek.

View the technical report completed for the pilot project’s initial round of data collection and testing.

Aerial photo of the side of Highway 7 crossing Ohop Valley, a forested wetland with hills in the distance. A bright green container with a pipe running from the roadway is a stormwater treatment system.
Pilot stormwater system in the Ohop Valley.

The Problem

For decades, coho salmon in urban streams across the West Coast have been dying from exposure to stormwater runoff – water that washes from roads, carrying with it a wide variety of pollutants. In 2020, researchers at Washington State University Puyallup and University of Washington Tacoma determined that a chemical called 6PPD-quinone was responsible for the harmful effects. 6PPD-quinone is formed when particles of tire dust react with ozone from the atmosphere as they wear off onto roads. When it rains, this new chemical then washes into stormwater and can end up in streams. It represents a huge threat to coho survival: between 20% and 90% of coho die within hours of exposure to contaminated stormwater. Recent studies found that it is harmful to Chinook and steelhead as well.

Identifying 6PPD-quinone was a critical first step to finally addressing coho mortality from urban stormwater, but there is still an urgent need for effective and adaptable solutions to treat roadway runoff before it reaches salmon streams. While policymakers and scientists are working with the tire industry to find safe alternatives, 6PPD-quinone will be present in stormwater for years to come. In the Nisqually Watershed in South Puget Sound, some of the most important coho habitat occurs in Ohop Creek, a major habitat restoration site crossed by an increasingly busy state highway. As Puget Sound’s population grows, so does the traffic along Highway 7 (the main road to and from Mount Rainier National Park). Current traffic deposits around 12 pounds of tire dust on the road crossing Ohop Creek each year, washing potentially lethal amounts of 6PPD-quinone into the creek.

Our Solution

The Nisqually Indian Tribe has teamed up with Long Live the Kings, Cedar Grove, Herrera, and other partners to install and test a biofiltration system to filterstormwater and protect the salmon of Ohop Creek. Biofiltration is a method that uses living material – such as plants, mulch, and compost – to remove pollution from contaminated water. Rain gardens and bioswales are examples of biofiltration systems which capture runoff and allow it to slowly filter through vegetation and layers of mulch and soil, removing pollutants. Because Ohop Creek is a sensitive habitat restoration site, constructing a large traditional bioswale below ground wasn’t a good option. Instead, our partners at Cedar Grove developed a containerized, mobile biofiltration unit to treat runoff. The unit collects stormwater from over 13,000 square feet of roadway in Ohop Valley and filters it through layers of compost-based filtration media to remove toxic compounds. Because compost can add excess nutrients to the water, this system includes a final, external polishing layer to remove phosphorous before the water is discharged to the wetlands near the creek.

During the first year of this pilot project, LLTK sampled water moving through this system during three storm events in the spring of 2022. (To qualify for this study, a storm must meet rainfall and duration requirements set by the Washington Department of Ecology to ensure consistent data.) The samples were sent to a lab for chemical analysis, comparing the levels of contaminants in the runoff before it entered the system, at the midpoint, and when it was discharged. While this first round of data is small, the results are promising: the system appears to be effective in removing over 90% of the 6PPD-quinone from untreated stormwater, reducing it to levels safe for coho salmon.

Despite the small sample size, preliminary results suggest this stormwater management system is a promising technology for the removal of 6PPD-quinone—with removal efficiencies that far exceed those seen for the other parameters tested. Based on the results of the water quality data, this stormwater biofiltration system has the potential to significantly reduce toxic stormwater runoff, and with a few adjustments, excess nutrients. In addition, the toxicology results support that the biofiltration treatment system shows promise as a solution for treating large quantities of roadway runoff before they enter receiving waters. View the project’s technical report for a more in-depth analysis.

The biofiltration system is currently on site and operational, collecting and filtering stormwater before it reaches salmon habitat. Long Live the Kings is seeking funding to conduct more sampling in future years. Our goal is to collect enough data (from at least 15 qualifying storms) so that this system can be approved for use in projects across Washington to protect critical streams for salmon recovery.

Download Project Factsheet

A person in a rain jacket leans over sampling equipment (large plastic barrels with buttons on top) in a stormwater treatment unit. A gutter runs along the side of Highway 7 to collect stormwater for treatment. A stormwater treatment system in a large square green container with a tarp-covered external section, with bright green discharge pipes extending to the ground. Three people, two in bright high-visibility gear, work on assembling a stormwater treatment unit.

Project Impact


Square feet of roadway runoff captured


of 6PPDQ removed from treated water


storm events needed for data collection

Project Partners

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Project Contact

Keith Estes

Project Manager (he/him/his)