As the saying goes, when it rains, it pours. Now, as developed surfaces cover more and more land in the U.S., often when it rains, it floods—or sweeps contaminants into our drinking water supplies.

According to Rooftops to Rivers, a report released by the National Resources Defense Council this week, an estimated 10 trillion gallons a year of untreated stormwater runs off roofs, roads, parking lots, and other paved surfaces, often through the sewage systems, ultimately ending up in our waterways and rivers. According to the NRDC, “Stormwater runoff from the built environment remains one of the great challenges of modern water pollution control, as this source of contamination is a principal contributor to water quality impairment of water bodies nationwide.” Not only do these polluted stormwater flows get into our drinking supply, they also damage ecosystems, degrade riverbanks and flood fragile habitats.

Breaking up impervious surfaces, like this stormwater catch at street corner in Tucson, Arizona, helps rainwater absorb directly into the ground. Areas like this also help beautify urban communities, providing an area for vegetation to grow and break up the streetscape. (photo by Megan Kimble)

Part of the problem in effective stormwater management is that as cities pave new roads and build new rooftops, the ground itself absorbs less water. From 1982 to 2007, land under development—land covered with roads, rooftops, or parking lots—increased by 56 percent. These surfaces are impervious to water absorption, which increases storm runoff, as well as the risk of floods and water contamination.

It’s the difference between pouring a glass over water on tile or carpet. After a storm, water flows rapidly across these slick surfaces in quick bursts, accumulating debris and pollutants, like fertilizers, bacteria, animal waste, metals, and oils, which end up in local aquifers and degrade overall water quality. Contrastingly, on undeveloped land—land covered with dirt or vegetation—only about 10 percent of a storm’s total rainfall becomes runoff (the rest is absorbed into the ground or evaporated).

But cities across the U.S. are pitching in to stop runoff pollution. The NRDC report details the work of 14 cities that are working to develop green infrastructure, which helps ease runoff by capturing rainwater before it hits the ground running. Developments such as green roofs, street trees, increased green space, rain barrels, rain gardens, and permeable pavement ease runoff pollution by storing rainwater for future use or helping it filter back into the ground.

Green infrastructure projects attempt to make cities more like carpet than tile. Check out individual NRDC case studies to see how each of the 14 cities fared on the “Emerald City Scale.” Philadelphia got a perfect 6, with Milwaukee, New York, Portland, Syracuse, NY, and Washington, D.C. ranking just below it.

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