Rain water that beats down on paved highways or parking lots can't penetrate into the soil and ultimtaely has to find someplace else to go. On its way, this stormwater runoff collects pollutants such as oil, dirt, lawn fertilizers and other chemicals – dangerous toxins that researchers have linked to the decline of adult coho salmon populations in urban streams along the U.S. West Coast. But there may be some good news, according to a new study. 

Researchers from the National Oceanic and Atmospheric Administration recently discovered that an inexpensive runoff filitration system involving a simple column of sand and soil can completely prevent the toxic effects of contaminated stormwater on fish. Their study was recently published in the Journal of Applied Ecology. 

"Untreated urban runoff is very bad for salmon health," Julann Spromberg, a research scientist at NOAA Fisheries' Northwest Fisheries Science Center in Seattle, said in a news release. "Our goal with this research is to find practical and inexpensive ways to improve water quality. The salmon are telling us if they work."

For their study, scientists examined the impacts of urban stormwater on salmon living in and around Puget Sound in Washington. In this area, more than half of the coho salmon returning to stormwater-dominated streams every year die before they can spawn, according to the release. In fact, coho living in California, Oregon and southwestern Washington are listed under the Endangered Species Act. If stormwater continues to kill significant numbers of salmon before they can spawn, they could be pushed closer to extinction, researchers reported in their study.

However, there is good news. The sand and soil filtration columns are similar to "rain gardens." These "green" stormwater systems could reverse the population declines if they become integrated into future development and redevelopment projects. 

"If we can incorporate clean water design strategies into future growth, as some transportation projects are already doing, wild salmon might have a chance," Nat Scholz, manager of the Ecotoxicology Program at the NWSFC in Seattle and a coauthor of the study, said in a statement. "They can't take the kinds of losses we've documented in urban streams."

To test this, researchers exposed adult coho to different combinations of polluted and clean water, in order to better understand what the fish can withstand. These combinations allowed researchers to simulate runoff from a busy urban highway in Seattle, and all fish exposed to this degree of toxic stormwater died within 24 hours.

However, after researchers filtered the water through a three-foot-high soil column containing layers of gravel, sand, compost and bark, all the exposed coho survived as well as they did in clean water. This suggests that filitration columns ultimately reduced toxic heavy metals by 58 percent and polyaromatic hydrocarbons, which are byproducts of gasoline combustion, by 94 percent, according to their study. 

"What impressed me most was the effectiveness of the treatment," Jen McIntyre, co-author and researcher at the stormwater program at WSU's Puyallup Research and Extension Center, said in a release. "It's remarkable that we could take runoff that killed all of the adult coho in less than 24 hours -- sometimes less than four hours -- and render it non-toxic, even after putting several storms worth of water through the same soil mixture."

While previous studies concluded that household or agricultural pollutants such as pharmaceuticals or pesticides were not to blame for coho salmon population declines, in the recent study researchers discovered the most devastating runoff may include toxins from exhaust, leaking oil and dust from brakes and tires as they wear. However, additional testing and analysis is required to confirm this. 

"The recurring coho spawner deaths have been a high-profile mystery for many years, and we're now much closer to the cause," Scholz added in the release. "Although we haven't identified a smoking gun, our study shows that toxic stormwater is killing coho, and that the problem can be addressed."

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