Rain gardens blossom as a storm-water management tool
New tweaks help make a low-tech method for reducing urban runoff work even better.
When it rains, it pours—and that’s the problem for scientists and engineers working in the field of storm-water management. But a study posted today on ES&T’s Research ASAP website (10.1021/es051644f) confirms the effectiveness of rain gardens, an increasingly popular—and aesthetically pleasing—technique for reducing the flow of common pollutants after storms in cities and suburbs. Authors Michael Dietz and John Clausen of the University of Connecticut also show how a simple design modification can enhance rain gardens’ ability to treat the polluted rainwater flowing off roofs. The authors hope that their results will help contribute to the growing acceptance of rain gardens by developers and homeowners.
A rain garden, in its simplest form, is a shallow depression in the earth, landscaped with permeable soil and hardy grasses, shrubs, or trees and covered by a thin layer of mulch. Rain gardens collect the storm-water runoff from impervious surfaces, such as roofs and driveways, and allow it to percolate into the ground instead of draining into sewers or waterways. In previous studies, rain gardens—which are also known as bioretention facilities—have been shown to provide the additional benefit of improving water quality by trapping pollutants in mulch and plantings and promoting the conversion of some compounds to less harmful forms.
The storm-water runoff from a house roof or a parking lot may not seem like a major threat to water quality. “People think of pollution as something coming from a factory or a sewage treatment plant, dumping directly into a river,” says Dietz. But runoff carries the signature of the urbanized areas through which it flows, including nitrogen and phosphorus compounds from atmospheric deposition and fertilizers, as well as metals, oils, and other particulate matter. The volume and velocity of runoff from a heavy rain can also overwhelm urban sewer systems. According to the U.S. EPA, more than half of the rainwater that falls on a typical city block, one with 75% or more impervious cover, will leave as runoff.
Of the pollutants commonly found in storm-water runoff, Dietz and Clausen focused on nitrates, which previous laboratory studies had shown to be poorly retained by bioretention facilities. To encourage the bacteria in the soil to convert the nitrates into nitrogen gas via the denitrification process, the authors modified the design of their garden to allow water to pool in the bottom. Saturation by water is expected to reduce oxygen levels in the soil, Dietz explains, creating anaerobic conditions favorable for denitrification. The simple modification produced an 18% decrease in total nitrogen levels compared with the control garden.
Allen Davis of the University of Maryland believes that the results show the potential for engineering rain gardens to target specific pollutants. “The two interesting points in the paper: One, they are trying to focus on nutrients and especially nitrogen,” Davis says. “A corollary to that is that working with saturation gives us an engineering tool that we can play with.”
Rain gardens, and similar techniques that promote infiltration of storm water into the ground, have become increasingly popular in the U.S. in the past two decades, after a 1987 amendment to the Clean Water Act led EPA to establish storm-water discharge standards for industries, new construction sites, and municipal sewer systems. In 1990, Prince George’s County, Md., became the first local government to promote rain gardens as a storm-water management tool, and numerous states, counties, and municipalities across the country soon followed.
“We are depending more and more on rain gardens to help us achieve our storm-water management goals,” says Roger Bannerman of the Wisconsin Department of Natural Resources. Bannerman emphasizes that the success of storm-water management programs is dependent on the quality of information available to practitioners. “[This] paper is very timely—it’s the kind of stuff we need to know.”
Researchers concur that there is much more to learn about bioretention systems and that special care must be taken in treating extremely polluted water. But for the relatively clean runoff from rooftops like the one used in the study, rain gardens are extremely effective at reducing the burden on sewer systems and receiving waters. “We found that 99% or so of the flow over the course of 2 years was retained by the garden,” Dietz says. “Along with that 99% flow goes 99% of that pollutant load.” —LIZZ THRALL
