Rain Garden Research at EPA’s Urban Watershed Research Facility

1. 60 cm freeboard media 6 cm shredded newspaper 2.5 cm media shredded newspaper 2.5 cm 9.5 cm media 3.75-cm diameter perforated drainage pipe wrapped in geotextile Rain Garden Research at EPA’s Urban Watershed Research Facility Emilie K. Stander, Michael Borst, Thomas P. O’Connor, and Amy A. Rowe U.S. Environmental Protection Agency, Office of Research and Development, Water Supply and Water Resources Division, Urban Watershed Management Branch2890 Woodbridge Ave, MS-104, Edison, NJ 08837 Emilie K. Stander lstander.emilie@epa.govl 732-906-6898 Introduction Experimental Design of Bench-Scale Test Results Effluent volumes were significantly larger in the control treatment compared to both one-layer and two-layer newspaper treatments during the low flow test (F2,15 = 11.3, p < 0.01), suggesting that the shredded newspaper retained the added stormwater (Figure 6). There were no significant differences among treatments during the high flow test. Rain gardens are vegetated depressions designed to capture and infiltrate stormwater runoff from impervious surfaces such as roofs, parking lots, and roads. The potential benefits compared to traditional curb and gutter drainage systems include peak flow attenuation in receiving waters, increased groundwater recharge, stream baseflow maintenance, and filtration and transformation of environmental stressors, such as heavy metals and nutrients, commonly found in runoff. These structures are quickly gaining popularity as green infrastructure components of low impact development planning. Background Few studies have quantified the ability of rain gardens to effectively manage chemical stressors. These studies indicate that the gardens function well in removing heavy metals and phosphorus, but the results for nitrogen have been mixed (Davis 2007). A nitrogen species of particular interest is nitrate, which is known to contribute to algal blooms and hypoxia in receiving waters. Current rain garden design, which typically includes rapidly draining sandy soils with low levels of organic matter, is not conducive to nitrate removal through denitrification. Compost is often mixed in with the sandy media to increase adsorption of heavy metals and phosphorus and increase organic matter content, but without the proper conditions for nitrate removal, compost may be a nitrate source in rain garden effluent. Alternative carbon sources should be considered. EPA’s rain garden research in Edison, NJ, studies enhancements of rain garden design to promote increased nitrate removal through denitrification and plant uptake while maintaining adequate drainage and high levels of heavy metal and phosphorus removal. A bench-scale test (Figures 3-5) of the pilot-scale rain garden mesocosm study (Figures 1 and 2) was conducted in 90 x 60 x 60 cm polyethylene tanks to test for possible impacts of shredded newspaper layers on drainage. The tanks were filled with a locally-available sand-clay media used for baseball infields in New Jersey. The media is 81% sand, 9% silt, and 10% clay. Stormwater collected from an adjacent community college parking lot was mixed and applied to the media surface at low and high flow rates using a spray bar. Slotted pipes, wrapped in geotextile to prevent clogging, at the tank bottoms allowed for effluent drainage to collection containers. Water level sensors in the collection containers measured effluent depth, used as a surrogate for effluent flow rate, at six-minute intervals. Newspaper was chosen as a carbon amendment based on preliminary work by Kim et al. (2003) demonstrating high nitrate removal in bioretention columns amended with newspaper. Tanks were constructed with zero (as a control), one, or two layers of shredded, unprinted newspaper to determine the effect of the newspaper on effluent volumes and flow rates. Figure 6. This graph shows effluent volumes for each treatment during the low flow test. Effluent flow rates were significantly larger in the control treatment compared to both newspaper treatments during the low flow test (F2,16 = 9.0, p < 0.01) (Figure 7a). However, during the high flow test, all three treatments displayed significant ponding (Figure 5), and there were no significant differences among treatments (Figure 7b). These results suggest that something common to all three treatments was a factor in slowing drainage rates, possibly related to the migration of small clay particles in the media (Figure 8). 5 cm 34.5 cm Experimental Design of Pilot-Scale Study Figure 7. Effluent flow rates are shown by treatment for the low flow (a) and high flow tests (b). Eight pilot-scale rain garden mesocosms will be tested in 2.1-m and 2.5-m diameter, conical-bottom HDPE tanks at EPA’s Urban Watershed Research Facility (Figures 1 and 2). Figure 8. The coefficient of uniformity at 3-cm soil depth increments increased in the bottom half of the media, suggesting that smaller clay particles may have migrated from the top half to the bottom half of the media. Figure 3. This schematic shows the bench-scale design for a tank containing two newspaper layers. In the one-layer treatment the bottom layer of shredded newspaper was eliminated. Future Directions Figure 1. Elevated rain gardens facilitate collection and analysis of water that drains from the gardens. Figure 2. Experimentally-added stormwater drains through a gravel layer and out a bottom drain to a collection tank. Stormwater Additions in Pilot-Scale Mesocosms Stage 1: a series of twelve 3-hour stormwater additions will be carried out during two months to test hydraulic properties (infiltration/exfiltration rates, volumes, and timing; soil water content using TDR; standing water depth) Stage 2: chemical analysis of effluent (heavy metals and nutrients) Stage 3: quantification of denitrification rates and plant uptake The experimental design consists of four treatments, listed below, assigned to eight mesocosms using a partial factorial design: • high / low volume of organic matter (layers of shredded newspaper) • presence / absence of a saturated zone at depth • high / low hydraulic loading (different sized tanks) • turf grass / native, herbaceous vegetation References Notice The U.S. Environmental Protection Agency, through its Office of Research and Development, funded and managed, or partially funded and collaborated in, the research described herein. It has been subjected to the Agency’s peer and administrative review and has been approved for external publication. Any opinions expressed are those of the author (s) and do not necessarily reflect the views of the Agency, therefore, no official endorsement should be inferred. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. Davis, A. P. 2007. Field performance of bioretention: water quality. Environmental Engineering Science 24(8): 1048-1064.Kim, H., E. A. Seagren, and A. P. Davis. 2003. Engineered bioretention for removal of nitrate from stormwater runoff. Water Environmental Research 75(4): 355-367. Figure 4. This photograph illustrates the experimental setup for the bench-scale study. Figure 5. All three treatments demonstrated significant surface ponding of the added stormwater during the high flow test. This was an unexpected result.