Using Trees to Curb Pollution

North Oakland's Urban Releaf leads a revitalization of neglected blocks while studying benefits of trees in storm-water management.

In a few years’ time, 31st Street between Martin Luther King Jr. Way and Market Street will be completely transformed. Instead of a sea of concrete and neglected curbside planting strips, its sidewalks will feature an interconnected series of street-level swales. Instead of a nearly treeless horizon punctuated by power lines and street signs, it will enjoy a growing canopy of young trees. And instead of funneling toxics directly into the nearby Ettie Street Pump Station and out to the bay, the two residential West Oakland blocks will actively remove pollutants from the watershed.

The 31st Street National Demonstration Project is not your average neighborhood clean-up effort. Supported by UC Davis scientists and developed with the assistance of city, state, and national agencies, the innovative project spearheaded by Oakland nonprofit Urban Releaf — which since 1999 has planted more than 15,000 trees throughout the city — is a multifaceted endeavor that takes urban greening to the next level. Through an on-site experiment designed by UC Davis water scientist Qingfu Xiao, it’ll even help quantify the long-recognized but rarely measured ability of trees to filter pollutants from storm-water runoff.

The targeted blocks, located in West Oakland’s Hoover neighborhood, pose both a challenge and a unique opportunity. The immediate area is an epicenter of crime, poverty, and toxicity; asthma rates and water and air pollution are well above average, and the median annual household income hovers near $20,000. Urban Releaf director Kemba Shakur, who launched her organization twelve years ago only a few blocks away, said she was motivated by issues of environmental justice to transform the street and eventually hold it up as a model for others like it.

“Poor areas and areas of color are beset by environmental degradation, and are the last areas to be remedied,” she said. “We’ve been working on that block for a while, but now we can showcase it as a national project, because it has not been done in any other poor community in the nation. It’s really a new science on the benefits of trees.”

Using an experimental framework developed by Xiao, Oakland civil engineering firm Hyphae Design Laboratory has led the way in planning the project and obtaining permits. At the heart of their efforts is a newly devised structural soil that has never been used in street-tree planters, Xiao said. It consists of lava rock and clay loam soil and was engineered with two purposes in mind: first, to provide for maximum water retention and filtration; and second, to allow tree roots considerably more space to grow than they have in typical soil. “What we’re doing is creating a model here,” said Brent Bucknam of Hyphae, “to see how it performs versus plopping a tree into native soil.”

Tree wells will be cut from the existing concrete to a width of four feet and a length as large as is allowable between driveways, then excavated at least three feet deep to make room for the new soil. Finally, cuts will be made into the concrete curbs themselves at the location of each new swale. These will allow rainwater running down the gutters to flow directly into the tree wells, instead of disappearing into the storm drain at the end of the block. The water will filter down through the engineered soil and eventually be sucked back up through the tree, which will consume phosphates, nitrogen, and other pollutants picked up from the air and street surfaces. The end result is a self-watering landscape that reduces the level of water pollution flowing from the city into the bay.

By March 2012, both blocks should be outfitted with flow-through swales and planted with trees. But the research component kicks off sooner. Measurements of water flow into the street’s storm drains have already been taken to establish a baseline level. Within a few weeks, permits and design work should be complete for the new test site between West and Market streets that will house Xiao’s experiment.

The 50- to 100-foot swale comprising four to six individual planters will feature a pump at each end, one designed to push water into the system and one designed to pull it back out. Between them, a flow meter and soil moisture sensors will assess the amount of water being diverted through the swale. Another series of pumps automated to pull water samples from the swale on a regular, timed basis will assist in measuring the quality of the water as it passes through the system. The test site should be complete by next summer and stay in operation for two years; ultimately, the data it provides could help spur further investment in similar efforts on a much larger scale.

The project’s budget of $181,000 comes from the California Department of Water Resources, the West Oakland Project Area Committee, the City of Oakland Redevelopment Agency, and others. Using additional funds from the American Reinvestment and Recovery Act, Urban Releaf will hire local at-risk youth to assist in community outreach and tree planting.

In the end, the 31st Street National Demonstration Project will incorporate scientific research, social justice, and bioremediation into a single entity on the front lines of urban environmental degradation.

Xiao, who has been working with Urban Releaf since 2004, is so jazzed by the project that he has opted to contribute some of his free time. “I just think it’s the right thing to do.”


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