Selection of Vegetation Covers for Runoff Mitigation Practices.

EPA National Pollutant Discharge Elimination System Stormwater Program (NPDES II) requires on-site treatment of stormwater in urbanized areas (i.e. municipal separate storm sewer systems - MS4s, and construction and industrial activities). Treatment goals include attenuating runoff peak flow and reducing runoff volume, and runoff composition (e.g. heavy metals, sediments, excess nutrients, etc.). Soil-based permitted practices, e.g. bioinfiltration (rain gardens) and intermittent pocket wetlands, are the best performing, cost effective, and hence, highly recommended best management practices by the permitting and regulatory agencies. These systems are located in low-lying areas, downhill from hardscape runoff contributing areas. Current selection of plant material is made mostly for their ornamental value rather than their contribution to the overall system performance in meeting the above goals. We compared 20 different grasses for their performances in a bioinfiltration-type setting. Seven-gallon buckets, filled according to bioinfiltration specifications (i.e. 5-cm pea gravel layer underneath a permeable geotextile fabric, overlain by 30 cm of bioinfiltration soil mix), were planted, each with different plant material and flooded with synthetic urban runoff on a weekly interval (after plant establishment) during the 2009 growing season (Jun-Aug). Leachate volume and composition were monitored and analyzed for metals, nutrients, and fecal bacteria. Composition and water uptake mass balances were calculated for each plant material in order to compare vegetation performances. Marked differences were observed between plant material for their ability to remove water and nutrients. The study demonstrates that performance-based selection of vegetation could markedly improve overall runoff mitigation systems performances.