Storm runoff can be toxic to aquatic life, but a new study suggests a simple and relatively inexpensive solution: Filter the water through dirt before it enters streams, rivers or the ocean.

Researchers collected runoff from a busy four-lane highway in Seattle during six storms in 2011 and 2012. They tested the toxicity of water from the first five storms and found that coho salmon fry could not survive in it, nor could the mayfly and water flea larvae they feed on.

Then they collected water from the sixth storm. The samples were also highly toxic to the insects, and all of the fish were dead within 12 hours of exposure. But in samples run through the dirt filters, the salmon and the insects flourished. In some of the filters, the researchers included plants as well. As it turned out, the soil mix alone was so effective that plants provided no additional protection. The study was published online in the journal Chemosphere.

“The reason we do this research is because in many areas, low-impact approaches will be required with new development,” said the lead author, Jennifer McIntyre, a postdoctoral researcher at Washington State University at Puyallup. “With a fully built environment, like a major city, it will be hard to do this.”