stormwater gutterRemember the good old days when you could self-righteously point to spewing smoke stacks and foul outfalls as the big polluters? These days, when it comes to Puget Sound’s water pollution, we’ve met the enemy and he is us.

For the majority of contaminants sullying the Sound, they’re getting there via stormwater. Stormwater is the rain that streams from roads, parking lots, roofs, highways, and some landscapes washing toxic chemicals along with it.

Stormwater flowing from the developed landscape is the No. 1 source for lead and petroleum products getting into the Sound. Runoff contributes 35 times more mercury to Puget Sound than industrial polluters, according to estimates from a Department of Ecology study. A follow-up report estimates that stormwater is dumping 134 metric tons of copper into the Sound each year—that’s the weight of more than 53 million pennies (copper can prove deadly to salmon by deadening their sense of smell, which is used for finding food and mates and avoiding predators).

How can a little rain wreak so much havoc?!

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  • It turns out that the wet Northwest produces tidal waves of stormwater each year. There aren’t great numbers available for the total amount (Ecology’s trying to better answer this question in upcoming studies), but you can get a sense of the volume just by looking at what comes from a single house.

    A good-sized rainstorm falling on a house with a 1,200-square-foot roof could send about 10 bathtubs full of water out of the home’s downspouts. And that’s just one storm, at one house. Over the course of an average year in Seattle, approximately 400 bathtubs worth of water are gushing off that single roof.

    And the stormwater situation will only get more challenging as the Puget Sound likely continues its rapid pace of sprawling development. That’s because when you take farmland, grassland, or forests and turn them into houses and roads, you can expect to double or even triple the amount of runoff racing off the landscape (again see Ecology’s phase 2 study). That argues for more compact, high-density development that leaves more of the natural landscape untouched so that it can sponge up the rain where it falls.

    So how does a drop of rain make its way to the Sound? In an older city such as Seattle or Tacoma or Everett, the rain runs from roads and roofs into gutters and ditches, which empty into creeks and streams, and they lead directly to the sea. That’s a largely straight shot to the Sound, bypassing any kind of treatment to remove toxic chemicals and sediment, or to slow the flow to reduce erosion.

    In some cases, the stormwater is piped into the sewer system, in which case it gets treated along with the waste from your toilets and sinks (unless there’s so much rain that the system overflows and untreated sewage and runoff get dumped into the Sound through a Combined Sewer Overflow, or CSO).

    In newer developments or redevelopment, the stormwater might be channeled into ponds that hold the water and allow it to soak more slowly into the ground, or it goes into underground “vaults”—perhaps under a big parking lot—that also catch and hold some of the water allowing dirt and other particles of pollution to settle out. The trouble with these systems is that while they can provide some level of treatment, it only works if the dirty basins are regularly cleaned out and the sediment disposed of, which is a costly undertaking and may occur infrequently.

    What’s better is something called “Low-Impact Development” where buildings and landscapes are designed to trap most of the rainfall on site rather than allowing it to runoff. LID projects—which can be applied to new construction or retrofitting developed areas—use strategies including porous driveways and sidewalks, rain gardens, and rain barrels. Stay tuned for more on LID in a later post.

    Rain gutter photo courtesy of Flickr user jc.westbrookunder a Creative Commons license.