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Measure Twice, Build Once: Bench-Scale Testing to Evaluate Bioretention Media Design (Presentation)
Citation:
STANDER, E. Measure Twice, Build Once: Bench-Scale Testing to Evaluate Bioretention Media Design (Presentation). Presented at EWRI/ASCE International Low Impact Development Conference, San Francisco, CA, April 11 - 14, 2010.
Impact/Purpose:
To inform the public.
Description:
Rain garden design manuals and guidelines typically recommend using native soils or engineered media that meet specifications for low content of clay, silt, fine and very fine sands, and organic matter. These characteristics promote stormwater infiltration and sorption of heavy metals and some nutrients. However, constructed rain gardens often deviate from these recommendations for several reasons: the logistics of locating an affordable, local engineered media; variation among specifications in rain garden manuals and guidelines; and attempts to modify rain garden design to improve removal of specific stressors, such as nitrate. Planned pilot- and full-scale research at EPA’s Urban Watershed Research Facility proposed using a locally-available engineered media with a higher silt and clay content than typically recommended and adding layers of shredded newspaper as a carbon amendment to promote nitrate removal. A bench-scale experiment was conducted to test the drainage capability of the chosen media containing shredded newspaper layers. While there were differences in effluent volumes and flow rates between control and newspaper treatments, surface ponding occurred in all three treatments, suggesting that some factor other than the newspaper had an effect on drainage properties. Grain size and clay mineralogy analyses demonstrated that finer particles had migrated into the deeper soils, which could have inhibited drainage. As a result of these findings, an alternate locally-available media containing less than five percent silt and clay was obtained for the pilot- and full-scale studies. A 20-cm thick layer of double-shredded hardwood mulch was added 10 cm above the geotextile in four of the pilot-scale rain gardens as a carbon source to drive denitrification; the other four pilot-scale gardens did not receive the mulch layer and served as controls. Stormwater was collected onsite and distributed to the eight pilot-scale rain gardens. Twelve stormwater additions were completed at low and high flow rates and at low and high antecedent moisture conditions. Effluent flow rates and volumes were measured, total suspended solids concentrations and solids particle size distribution was quantified in influent and effluent samples, and time domain reflectometers continuously recorded soil volumetric water content. The performance of the two media types in terms of infiltration and drainage were qualitatively compared between the bench-scale and pilot-scale studies. The results of these studies underscore the importance of conducting bench-scale testing of bioretention media before installation in full-scale, working rain gardens, particularly when media characteristics differ from recommendations or when modified to promote stressor removal.