Sept. 10, 2007 — Paved areas pollute. They harbor exhaust-spewing cars, absorb and radiate heat, and collect contaminants that are eventually washed into the ground through rainwater runoff.
But a new kind of engineered soil could curb pavement pollution. Made with natural and locally available materials, the aggregate can filter storm water as well as provide a better soil bed for trees, which offer shade, scrub the air of emissions, reduce ambient temperatures, and intercept rainfall.
"Paved surfaces account for up for 20 to 40 percent of a city's surface," said Greg McPherson, director for the U.S. Forest Service's Center for Urban Forest Research, which is located on the campus of the University of California, Davis. "How do we green up these impervious surfaces?"
McPherson and colleague Qingfu Xiao, a research water scientist at UC-Davis, are working together on improving water quality as part of a larger project to minimize runoff from pavement. The work involves Cornell University and Virginia Tech.
The answer, they say, lies with trees. A 40- or 50- foot maple or oak can soak up about 50 gallons of rain in its canopy alone, preventing the water from becoming run-off. Plus trees provide shade and breathe in carbon dioxide and other air pollution.
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Unfortunately, conventional soils and methods for tree-planting reduce their effectiveness. When constructing a parking lot or sidewalk, two or three feet of the topsoil is scraped off, the remainder is compacted and filled in with layers of rock and then asphalt.
Although the compacted soil keeps out moisture and provides good support for the pavement, it works against a healthy environment. Trees, which are typically planted in soil islands throughout the lot, cannot stretch their roots into the dirt, nor likely reach their full canopy potential.
