Bioretention/Bioinfiltration Performance in the Mid-Atlantic

Researchers in Maryland, North Carolina, and Pennsylvania have been conducting parallel bioretention/bioinfiltration research since fall 2007. Various designs have been tested including those that rely on underdrains, have internal water storage (IWS) layers, or are underdrain-free. The cells provide a range of watershed practice size ratios and employ a variety of land covers. Researchers are pooling water quantity and quality data to help create new design standards. Initial results from the shared data will be presented, with specific attention to bioretention design parameters that control flow modification and water quality improvement. Two bioretention cells of varying vegetative cover are being monitored in Rocky Mount, NC. This site is located in the upper coastal plain with sandy in-situ soils. These cells were designed with a 0.9 m media depth and a 0.6 m deep internal water storage (IWS) layer. Another bioretention cell is being monitored in Silver Spring, MD. It was constructed with a 0.9 m media depth and a 0.3 m pooling depth. Finally, there are two bioinfiltration cells being monitored in Villanova, PA. The first is the "Traffic Island" bioinfiltration cell, which has been monitored since 2003. The bowl is only designed for 1.2 cm over the impervious surface, yet overflow rarely occurs for events less than 5.1 cm. The site had groundwater wells installed in 2007 and is the subject of an ongoing study on the groundwater effects. The second site has only been monitored for approximately six months. Cumulatively, the four bioretention cells extensively examined have dramatically reduced outflow volumes, completely assimilating all events less than 1.2 cm — and in some designs much greater events. By aggregating the data, runoff reduction by bioretention can be profound.