The role of ecologists in designing rain gardens: Enhancing nitrate removal performance

Background/Question/Methods Rain gardens are vegetated depressions designed to receive stormwater runoff from roads, roofs, and parking lots. Stormwater infiltration through rain gardens’ sandy soils is intended to have both water quantity and quality benefits, through stream peak flow reduction and pollutant transformations. Studies have demonstrated high removal rates of phosphorus and heavy metals through adsorption. Nitrate removal, however, is usually low. Terrestrial denitrification experts, often trained as terrestrial biogeochemists or microbial ecologists, are needed to engage in rain garden research to enhance rain garden design in order to improve ecological function and facilitate monitoring efforts. Rain garden research at the US Environmental Protection Agency explores the use of shredded, unprinted newspaper as a media carbon amendment and deep zones of saturation to facilitate denitrification by providing labile carbon and anoxic conditions in experimental rain garden mesocosms. A bench-scale test was performed to determine the drainage capability of a locally-available, engineered, sandy media containing shredded newspaper layers. Stormwater was introduced at high and low flow rates to bins containing zero, one, and two layers of shredded newspaper at varying depths. Effluent volumes and flow rates were compared among newspaper treatments. The media was analyzed for grain size, clay mineralogy, and soil characteristics.
Results/Conclusions

There were some differences in volumes and flow rates between the newspaper and control treatments. However, because surface ponding occurred in all three treatments during the high flow test, it is likely that some other factor besides the newspaper had an effect on drainage properties. The engineered media had a high percentage of fines (9% silt, 10% clay) compared to most rain garden applications. Particle size analysis suggested the migration of smaller particles into deeper soils. This could have negatively affected drainage, particularly because one of the minerals contained in the clay, illite, may become gel-like when saturated. Engineered media for rain garden applications should be chosen to avoid drainage problems related to too much clay, too great a range in particle sizes, or clay mineralogy containing clays that swell or otherwise impede drainage. The results of the bench-scale study will be used to inform the design of the pilot-scale research, which will include measurements of nitrate removal via denitrification. An important goal of EPA’s research in urban design includes specifically designing green infrastructure projects to be monitored, as current designs are typically not conducive to field stormwater monitoring efforts.