Natural Remedy
Researchers enhance the pollution treatment performance of stormwater bioretention systems

Challenge
Much of the pollution in streams, rivers, lakes, estuaries, and coastal waters comes from stormwater runoff that has washed across parking lots, roadways, buildings, and agricultural fields. The runoff carries pollutants such as heavy metals, nitrogen, oil, and fecal matter, and deposits them directly into surface waters. Contaminated runoff poses one of the greatest threats to water quality nationwide, and that risk is expected to grow as more land—which serves as a natural filter for polluted runoff—is developed.

To address the problem, many municipalities are turning to low impact development (LID), a practice that leverages green space to minimize runoff. Wider adoption, however, LID approaches must be proven effective, affordable to operate and maintain, and adaptable to sites with limited space.

Response
With support from CICEET, investigators from the University of Maryland refined the process of bioretention, one of the more common LID approaches to managing stormwater runoff. Bioretention relies on a combination of soil, mulch, grasses, shrubs, and small trees to control runoff and treat the pollution it contains. Until recently, quantifiable data on the performance of bioretention systems at different scales has been limited. Also, design specifications have been vague and not correlated to performance results.

To avoid the installation of larger and deeper bioretention facilities when not necessary, these researchers worked to determine which soil mixes are most effective at capturing a range of pollutants, including heavy metals, suspended solids, and nutrients. The goal was to get optimal water quality treatment in the smallest possible footprint.

To determine which filter media deliver the highest pollutant reduction and removal rates, they conducted laboratory studies in which runoff was directed into three-foot deep bioretention columns that contained different combinations of sand, mulch, and soil. Researchers then studied these systems to determine which materials were most effective at treating specific contaminants. Controlled field studies were also used to support laboratory results.

Impact
Bioretention has proven to be particularly effective during smaller storm events, and for removing heavy metals, oils, and solids from stormwater runoff. For many pollutants, shallow depth and sandy media are adequate for efficient pollutant removal. The incorporation of organic matter in the media promotes nitrogen and phosphorus removal.

Based on this study, researchers developed two profiles of bioretention filter media. With the appropriate modifications, the design for this media can be adapted for use in stormwater systems in different regions in the United States. Researchers have partnered with Prince George’s County, Maryland—which developed and installed its first bioretention facility nearly twenty years ago—and with other state and local governments.

Learn More
Dr. Allen Davis
Maryland Water Resources Research Center
University of Maryland
T: 301.405.1958
E: apdavis@eng.umd.edu

Related Projects
Field Demonstration of Wood Filter Technology for Stormwater Treatment [Project Brief]

UNH Stormwater Center [Web Site]

Engineering Bioretention for Treatment of Stormwater Runoff [Final Report]

Sorptive Clarification as an Environmental Technology to Passively Treat Stormwater from Elevated Transportation Infrastructure [Progress Report]

Development of a Decision Support Model for Compliance with the Clean Water Act National Pollutant Discharge Elimination System (NPDES) Stormwater Program [Final Report]