Editor's Note: Expedition Blue Planet, led by Jacques Cousteau's granddaughter Alexandra Cousteau, is traveling 14,500 miles of road over 138 days to investigate and film some of North America's most pressing water-use and management stories. Expedition members will file dispatchs from the field for Scientific American until the expedition concludes on November 12 in Washington, D.C. This is their fourth blog post.

Right from the start of our expedition, stormwater runoff has held center stage when it comes to water quality issues. It's a constant refrain: Tell us about the water quality issues that this watershed faces? Answer: Stormwater runoff.

More than three quarters of North Americans live in urban areas, and by 2030, 60 percent of the world's population is expected to live in them. Not only do these concrete jungles alter our watersheds and hydrological cycles, but they also shunt the runoff from impervious pavement and rooftops, into the water body at hand, whether it's a lake, creek, river or coastline.

"People think storm water is pristine," says Lou Di Gironimo, general manager for Toronto Water. "Well, it is when it comes out of the clouds but not when it hits an urban surface." Despite the fact that Toronto's stormwater does not meet the criteria for discharging it untreated, the bulk of it is collected in storm drains and funneled straight into the city's creeks, rivers and lakes, like in so many urban centers worldwide.

According to the Environmental Protection Agency, some 80,000 miles of streams and rivers are impaired by urbanization in the U.S. And the amount of impervious surface superimposed on the country's watersheds ranges between 12.5 percent and 30 percent—high enough to degrade aquatic habitats.

About 13 percent of U.S. rivers, 18 percent of lakes and 32 percent of estuaries are made unsafe for swimming or fishing thanks to stormwater, making it a leading source of water pollution. Just as an example, 75 percent of the toxic chemicals that arrive in Seattle's Puget Sound are carried by stormwater that slicks paved roads, rooftops, yards and other urban features.

"We all pollute the environment in one way or another, and the rainwater, all it does is it picks it up, mixes it all together, puts it in a pipe and we're having to deal with it somewhere," explains Di Gironimo.

Urban streams suffer from increased phosphorus concentrations due to the ubiquitous application of lawn and garden fertilizers. Urban areas also increase the nitrogen concentration in rivers for hundreds of miles. Then there are the metals, of which lead, zinc, chromium, copper, manganese, nickel and cadmium are the most common in runoff.

Brake linings, tires and engines contain nickel, chromium, lead, zinc, lead, manganese and copper, among others—all of which accumulate on parking lots and roads. 

(One study found that the copper levels in stormwater runoff have the potential to increase mortality in salmon by way of compromising their sensory abilities and increasing the odds that they'll get plucked off by predators.)

Then there are pesticides, PCBs and the petroleum-hydrocarbons that drip off our cars. Drop by drop, oil stain by oil stain, it all adds up. By one estimate, the Los Angeles River alone contributes 1 percent of the annual world petroleum hydrocarbon input into the ocean.

The EPA is expected to enact stormwater regulations come 2012 but in the meantime, there are small steps that many urbanites are making already. In natural systems, rainwater never really traveled that far from where it fell. The soil and plants would soak it up.

Indeed, the best ways to deal with urban stormwater is to go back to that principle: use the rainwater where it falls.

"Capture that rainwater, put it in a rain barrel, put it in your garden and use that water," urges Di Gironimo.

"If you let it fall and hit your driveway or the road, it is now polluted and we're gong to have to deal with it somehow."

Photo: Mud Creek's path in Toronto near Yonge Street and Eglington. Courtesy of Flickr.