Paved paradise? Impervious surfaces affect a region's hydrology, ecosystems—even its climate

Science News, Sept 4, 2004 by Sid Perkins

Parking lots not only make up a large proportion of an urban area's impervious surfaces but also provide many pollutants to urban runoff. Motor oil and other fluids that leak from parked vehicles find their way into city streams. New research suggests that significant quantities of suspected carcinogens known as polycyclic aromatic hydrocarbons (PAHs) can leach from asphalt-based and coal tar-based sealants used on paved lots.

The sealants first came under scrutiny when chemical analyses of sediments collected in 2001 and 2002 from creeks near parking lots in Austin, Texas detected unusually high concentrations of PAHs, says Peter C. Van Metre of the U.S. Geological Survey in Austin. In 2003, he and his colleagues tested the sealants' impact by running distilled water over 5-by-10-meter areas of sealed and unsealed pavement in local parking lots. The scientists sprayed 100 liters of water over each pavement patch and analyzed the chemicals in dirt swept up by the faux runoff.

Previous research by other scientists detected adverse ecological effects for aquatic organisms when the PAH concentration in sediments reached about 23 milligrams/kilogram, says Van Metre. Material washed from even the unsealed parking lots showed higher concentrations of these contaminants, his team found.

On average, each kilogram of particles from unsealed concrete and asphalt lots contained 54 mg of PAHs. Those substances could have come from vehicle emissions, the smoke and soot from power plants, or material left behind by tires, says Van Metre.

Particles filtered from the runoff from asphalt-sealed parking lots contained 620 mg of PAHs per kilogram, and those from lots treated with coal tar-based sealants held 3.5 grams of PAHs per kilogram of sediment.

ECOLOGICAL HAVOC A recent census of the aquatic larvae of mayflies, stoneflies, and caddisflies--key links in a stream's rood chain--showed that the number of species within the three groups dropped in a typical stream as the percentage of urban land in a watershed increased. Those studies, conducted in and around the metropolitan areas of Boston, Chicago, Philadelphia, and Cincinnati, were reported by USGS scientists earlier this year.

The amount of impervious surface that begins to cause ecological damage in streams varies in different areas. In some estuarine ecosystems in South Carolina, water levels fluctuate up to 2 m between high tide and low tide, says Dana Beach, executive director of the South Carolina Coastal Conservation League in Charleston. That constant flushing action masks to some degree the detrimental effects of impervious surfaces. An aquatic ecosystem there doesn't begin to degrade until impervious surfaces make up 15 percent of its watershed area, Beach has found.

In contrast, studies showed parts of Alaska to be unusually sensitive to damage from pavement. There, the populations of some aquatic invertebrates start to decline when the proportion of impervious surface in a watershed reaches 5 percent, says Steven A. Frenzel, a hydrologist with USGS in Anchorage.