Engineered Soil Greens Up Parking Lots

By Tracy Staedter
Washington, DC (September 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.
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.
The hard soil also promotes contaminated runoff and other pollution. McPherson and Xiao’s solution involves a soil mixture that is 70 percent stone and 30 percent clay loam. The stone is a very porous, lightweight lava rock found locally in the Sierra Nevada range.
In lab tests, Xiao found that the so-called “Davis soil” filtered out at least 50 percent of nitrogen and phosphorous from run off water and removed about 75 percent of heavy metals. They also showed that if the soil were used to plant a tree in a hole that was 4-foot deep by 4-foot square, it could collect and filter 150 gallons of water in addition to the 50 gallons a tree crown can naturally capture and store.
The Davis soil could be used for more than planting parking lot trees. It could also be deposited under sidewalks, and even as a small soil reservoir alongside paved areas. The soil reservoir could catch rainwater runoff and filter it before it seeps into the subsoil and replenishes the ground water.
“The beauty of that is that you are much more closely mimicking thesnatural system,” said Susan Day, research assistant professor of urban forestry at Virginia Tech in Blacksburg, VA, and lead researcher on the project called, Development of a Green Infrastructure Technology that Links Trees and Engineered Soil to Minimize Runoff from Pavement. “The tree is a functioning part of the storm water system management instead of just an add-on,” she said.
The vision, said McPherson is for landscape architects and engineers to eventually think of the soil and the trees as mini reservoir system designed to filter a quantifiable amount of water and air. “We will have more success in arguing to have adequate space for trees, for the soil volume that they need to grow to fullfill their potential and improve the quality of life in our cities,” he said.
For the full article, visit Discovery News.
Related Resources:
Center for Urban Forest Research
UC Davis
Trees and Engineered Soil Research
Parking Spaces Outnumber Drivers 3 To 1
UConn Stormwater Nonpoint Education for Municipal Officials