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Bioretention Design to Improve Nitrogen Removal
Citation:
STANDER, E., S. Gilchrist, M. BORST, T. P. OCONNOR, AND A. A. Rowe. Bioretention Design to Improve Nitrogen Removal . Presented at Philadelphia Low Impact Development Symposium: Greening the Urban Environment, Philadelphia, PA, September 25 - 28, 2011.
Impact/Purpose:
To inform the public.
Description:
Bioretention has been shown to effectively remove a variety of stormwater stressors, including oil/grease, heavy metals, phosphorus, and ammonium. However, reported nitrate and total nitrogen removal performance is highly variable. The media typically used in bioretention installation is coarse-grained with low organic matter content, which facilitates high infiltration rates but fails to provide the anaerobic conditions and carbon availability necessary to promote nitrate removal by denitrification. EPA's research at the Urban Watershed Research Facility investigates the effects of media carbon amendments, introduced internal storage zones, plant type, and media volume on nitrogen removal. Initial bench-scale tests informed media and carbon amendment choices. A locally-available, sandy media with low organic matter content was added to eight experimental, pilot-scale rain gardens above a shallow pea gravel drainage layer. The media was separated from the pea gravel with a nonwoven geotextile. Double-shredded hardwood wood chips were chosen as a carbon amendment and added as a 20-cm layer 10 cm above the geotextile in four of the eight pilot-scale rain gardens; the other four did not receive the mulch layer. Four rain gardens were constructed with an elevated outlet pipe to create an internal storage zone; the other four drain freely. Pilot-scale rain gardens were constructed in tanks of two sizes to test the effects of media volume. After initial hydrologic tests. four rain gardens will be planted with turfgrass and the other four with native herbaceous plants. Hydrologic tests will be repeated following plant establishment. The four design variables were assigned to the eight pilot-scale rain gardens in a partial factorial design.
