Citation:

STANDER, E. K. AND M. BORST. Bioretention Monitoring: Designing Rain Gardens to Promote Nitrate Removal. Presented at 2008 INTERNATIONAL LOW IMPACT DEVELOPMENT CONFERENCE, SEATTLE, WA, November 16 - 19, 2008.

Impact/Purpose:

to demonstrate the success of removing nitrates

Description:

Laboratory and field-scale studies of bioretention systems have often shown these structures to have a high capacity for removal of suspended solids, heavy metals, and phosphorus. Most studies, however, failed to demonstrate the same success in removing nitrate. Typical rain garden designs use coarse-grained soils with low organic matter content and high aeration to promote stormwater infiltration and reduce retention time. While these conditions, specified in widely-cited rain garden manuals, promote infiltration and remove some water quality stressors, they are not ideal for nitrate removal. Bioretention systems remove nitrate through two primary mechanisms: denitrification and plant uptake. Plant uptake temporarily sequesters the nitrate within plant tissue during the growing season. Only denitrification, the microbial transformation of nitrate to gaseous nitrogen which is released to the atmosphere, represents permanent removal of nitrate from the system. Anaerobic and carbon-rich conditions which facilitate the denitrification process are typically provided by fine-grained soils with high clay and organic matter content. Bioretention practitioners generally avoid using these soils because they are less conducive to infiltration. Rain garden research at the EPA’s Urban Watershed Management Branch (UWMB) in Edison, NJ, focuses on modifying media design to facilitate denitrification while maintaining adequate stormwater infiltration. Sandy media amended with organic carbon-rich, unprinted newspaper will be added to rain garden mesocosms located at UWMB’s Urban Watershed Research Facility. Some mesocosms will also contain a saturated layer at depth to promote denitrification in the deeper soil profile while allowing infiltration in the upper soils. Mesocosm size will also be varied to test for the effect of media depth and volume on denitrification rates. Eventually mesocosms will be planted with either turf grass or native herbaceous plants to determine the effects of vegetation on organic matter content, denitrification rates, and rates of nitrate uptake. However, this paper focuses on the impact of media design on hydraulics; therefore, initial experiments related to hydraulics will be conducted prior to planting. Experiments use stormwater runoff collected from a system of parking lots at an adjacent community college. Water will be added to the mesocosms at high (750 L) and low (350 L) volumes over three hours and at high (25% volumetric water content) and low (10%) levels of antecedent moisture conditions. Timing and rates of infiltration and timing and volumes of effluent flows will be measured. Although this paper focuses on hydraulics, eventually this study will be expanded to include measurements of denitrification and plant uptake of nitrate using stable isotope techniques.

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