Natural Stormwater Treatment Saves Salmons’ Lives

Scientists spend a lot of time measuring markers of stormwater quality, and yet we often don’t know exactly how pollution levels in runoff affect fish and invertebrates. So one group of scientists, led by J.K. McIntyre of Washington State University, decided to skip right to the point in quantifying how effective biofiltration systems are at protecting aquatic ecosystems.

The researchers placed a number of juvenile salmon and two of their prey species (daphniids and mayflies) into untreated stormwater from a four-lane highway; then they placed another group into the same water once it had been treated in a biofiltration system. For the salmon and their invertebrate prey, it was a matter of life and death.

Unsurprisingly, the untreated runoff killed a great number of the three species (among the daphniid survivors, all had impaired reproduction). However, a biofiltration system—even one without plants—completely eliminated the acute toxicity of the highway runoff and avoided the reproductive problems in the daphniids.

Of course, aquatic animals aren’t usually exposed to pure highway runoff. The polluted water is usually expelled through the sewer into waterways, where it’s diluted with cleaner water (see Cleaning Up Stormwater: Understanding Pollutant Removal from Runoff).

Even so, the authors explain, the “first flush” of a storm can contain contaminant concentrations 20 times higher than average concentrations for an entire storm. It’s the average concentrations that are conventionally used in toxicity studies, so biofiltration systems are likely to be highly underappreciated. Given the drastic differences in how aquatic organisms responded to pre- and post-filtered runoff water, the authors argue that future effectiveness studies should analyze impacts at peak contaminant concentrations.

To learn more about natural stormwater treatment, read Stormwater Biofiltration That’s Also Smaller and Cheaper.