Sorting Out Stormwater In Puget Sound

Mini Cooper driving through stormwater runoff

Stormwater picks up contaminants from vehicles. Photo: Daniel Parks (CC BY-NC 2.0)

Toxic Stormwater Runoff Killing Coho Salmon

Coho salmon are dying in large numbers as they return to watersheds across Puget Sound, and research concludes that toxic stormwater runoff is the likely cause.As researchers attempt to discover which chemicals in the runoff are contributing to the deaths, an even more fundamental question arises: What, exactly, is stormwater?

Though toxic stormwater may be Puget Sound’s best-known pollutant, it is also, paradoxically, the least understood. Eric Wagner’s excellent article, “What is Killing the Coho?” in the Salish Sea Currents’ Encyclopedia of Puget Sound explores the issues and considers some of the proposed solutions. Finding answers and crafting workable strategies to prevent further deaths in this essential species will be as complex as the chemical makeup of the water itself. But a broad coalition of scientists comprising groups from Washington State University (WSU), NOAA Fisheries, the U.S. Fish and Wildlife Service and the University of Washington are working together to solve the longstanding mystery.

The Puget Sound region receives up to 40 inches of precipitation annually, most of it as rain. Before the I-5 corridor became the congested urban expanse it is today, much of that rain seeped into the soil or collected on leaves and grass and then evaporated back into the atmosphere; less than 1 percent was left to trickle into the Sound as surface runoff.

As human activity altered the Puget Sound drainage basin, so too did the fate of the rains change. To paraphrase Joni Mitchell, they paved Paradise and put up a parking lot — and streets, and roads, highways, building roofs and on and on until it now presents more than 350,000 acres of impervious surfaces with between 20 and 30 percent of precipitation turning into surface runoff. This translates into more than 370 billion gallons of stormwater per year pouring into Puget Sound.

Washington Department of Ecology found that at least 33 pollutants have a 50 percent or greater detection frequency in stormwater, meaning that they are found in at least half of samples. The list includes almost everything from fecal coliform to polycyclic aromatic hydrocarbons, or PAHs, which are known carcinogens. On top of those pollutants, 16 others are found in at least 20 percent of samples, along with hundreds of other chemicals as well.

All these pollutants and toxins can have profoundly negative effects on Puget Sound’s biota, such as aquatic insects and, especially, salmon runs, several of which are federally listed as threatened.

Nathaniel Scholz, a biologist with NOAA, and colleagues from several government agencies showed in 2011 that between 60 to 100 percent of coho salmon returning to some lowland urban streams in Puget Sound die before spawning. More recent work found that juvenile and adult coho salmon die within hours of exposure to untreated runoff from the 520 bridge between Seattle and the east side of Lake Washington. In a new paper in Ecological Applications, biologists from NOAA and the Washington Department of Fish and Wildlife found that across 40 percent of the coho’s Puget Sound range, fish returning to spawn are being especially hard hit in urban areas, primarily due to stormwater.