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New Study Suggests Rain Gardens Can Save Salmon

A rain garden in Seattle at 76th and Winona, installed with support from the city's Neighborhoods Projects Fund. Scientists are just now proving that common soil mixes used in green infrastructure can be highly effective in protecting wildlife.

The lethal effects of urban runoff that kills some salmon and their prey can be reversed by filtering the water through a common soil mix, according to new research by state and federal scientists.

When it rains or people wash their cars, the water that runs over pavement picks up toxic chemicals such as oils, heavy metals and residue from car emissions. This can go straight into our waterways.

So-called Green infrastructure - things like rain gardens and green roofs - uses soils and other natural materials to slow down and filter this urban runoff. But does it work?

“Nobody has ever looked at whether the runoff that comes out of those systems is going to be healthy for aquatic animals,” said Jenifer McIntyre, an eco-toxicologist at Washington State University’s Research and Extension center in Puyallup.

Together with researchers from NOAA's Northwest Fisheries Science Center and the US Fish and Wildlife Service, she used a common bio-retention method to see how it affected juvenile coho salmon and some of the bugs they eat.

First, they immersed the fish in untreated runoff from a busy urban highway, the 520 bridge. The effects were almost immediate.

"We were surprised to find it killed them within 12 hours of exposure," she said.  

“On the flip side, that same water, when it had gone through the soil system, was not only not toxic as far as causing any obvious health problems." McIntyre said, "they were completely normal looking in the runoff that had been treated.”   

The soil mix they used was 60 percent sand, with smaller amounts of compost,  shredded bark and particulate material left over from water treatment mixed in. 

McIntyre said they are still testing how long the soil system will remain effective and whether plants growing in it change the situation.

Co-author Nat Scholz, the manager of the Ecotoxicology Program at NOAA Fisheries, said it is one of the first papers on the topic, but the findings are striking. 

"Relatively simple and inexpensive treatment technologies are proving to be very effective, in terms of protecting the health of salmon and stream invertebrates," Scholz said. 

The research is published in the journal Chemosphere.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to