Paving paradise: the peril of impervious surfaces

Environmental Health Perspectives, July, 2005 by Lance Frazer

Paved surfaces are quite possibly the most ubiquitous structures created by humans. In the United States alone, pavements and other impervious surfaces cover more than 43,000 square miles--an area nearly the size of Ohio--according to research published in the 15 June 2004 issue of Eos, the newsletter of the American Geophysical Union. Bruce Ferguson, director of the University of Georgia School of Environmental Design and author of the 2005 book Porous Pavements, says that a quarter of a million U.S. acres are either paved or repaved every year. Impervious surfaces can be concrete or asphalt, they can be roofs or parking lots, but they all have at least one thing in common--water runs off of them, not through them. And with that runoff comes a host of problems.

Globally, it is a little more difficult to judge the square mileage of impervious surfaces. "We can extrapolate from the United States to a degree," says Ferguson, "but there are too many variables to judge accurately." The United States has a lot of automobiles, and compared to many other countries, Americans tend to build more (and wider) roads, more (and bigger) parking lots, more (and more expansive) shopping centers, and larger houses (with accompanying larger roofs). He says, "The United States might be on a par with Europe, but we'd be very different from India, for example, or any country where large numbers of the populace live in smaller, scattered villages, mostly without paved roads, parking lots, and the like."

According to the non-profit Center for Watershed Protection, as much as 65% of the total impervious cover over America's landscape consists of streets, parking lots, and driveways--what center staff refer to as "habitat for cars." Says Roger Bannerman, a researcher with the State of Wisconsin Department of Natural Resources: "You see some truly insane things in this country. I've seen subdivisions with streets that are thirty to forty feet wide. That's as wide as a two-lane highway. Most developers are going back to a twenty-five- to twenty-eight-foot width, but you can still see these huge streets."

Upon these automotive habitats fall a variety of substances, and thereby hangs the rest of the tale. Impervious surfaces collect particulate matter from the atmosphere, nitrogen oxides from car exhaust, rubber particles from tires, debris from brake systems, phosphates from residential and agricultural fertilizers, and dozens of other pollutants. "On a parking lot, for example, we have demonstrated buildups of hydrocarbons, bacterial contamination, metals from wearing brake linings, and spilled antifreeze," says Ferguson.

On a road of open-graded aggregate (stone), much of that material would seep down into the pavement and soil, and the community of microorganisms living there would begin a rapid breakdown process. But pollutants can't penetrate an impervious surface, and the rapid flow of rainwater off of impervious surfaces means these pollutants end up in the water. "So then," says Ferguson, "not only do you have too much water, all moving too fast, you have polluted water that kills fish and makes water unfit for drinking or recreation."

When Water Has Nowhere to Go

Areas across the country are being impacted by the growth in coverage by impervious surfaces. In Maryland, for example, when watershed imperviousness exceeds 25%, only hardier reptiles and amphibians can thrive, while more pollution-sensitive species are eliminated, according to a 1999 Maryland Department of Natural Resources report titled From the Mountains to the Sea. Watershed imperviousness exceeding 15% results in streams that are impossible to rate "good," states the report, and even 2% imperviousness can affect pollution-sensitive brook trout.

The 1.1-million-acre Chesapeake Bay watershed, one of the most diverse and delicate ecosystems in the world, is now being impacted by the 400,000 acres of impervious surfaces in Maryland. The Great Lakes, the streams and rivers of the Pacific Northwest, the Everglades of Florida--all are being impacted in one or more ways by runoff from streets, parking lots, and rooftops.

Bannerman has spent the last 30 years studying stream flow and the effect of urbanization on watersheds, including the depletion of groundwater reserves. "Not allowing the rainfall to infiltrate back into the aquifer is a very serious issue," he says. "If that happens, you lose the base flow [the portion of water derived from underground sources] for streams, and you lose the wetlands fed by springs. It's a complete disruption of the hydrologic cycle."

Bannerman cites the example of Lake Wingra, a 1.3-square-kilometer lake in Madison. "A hundred years ago," he says, "this lake was fed by around thirty-five separate springs. But today, because the lake is now almost entirely surrounded by urban areas, there are only four streams feeding the lake. Local organizations have gotten active in trying to restore the lake's water quality, but it's not the same lake it was a hundred years ago." Lake Wingra now suffers from algal blooms caused by overfertilization, beach closures due to bacterial contamination, turbidity, and drying of surrounding wetlands.