Impacts of Media Depth on Effluent Water Quality and Hydrologic Performance of Undersized Bioretention Cells

Abstract

Fill media and excavation volume are the main costs in constructing bioretention cells, but the importance and impact of media depth in these systems is relatively unknown. Two sets of loamy-sand-filled bioretention cells of two media depths (0.6 m and 0.9 m), located in Nashville, North Carolina, were monitored from March 2008 to March 2009 to examine the impact of media depth on their performance with respect to hydrology and water quality. Construction and design errors resulted in the surface storage volume being undersized for the design event (2.5 cm). The actual surface storage volume was only 28% and 35% of the design volume for the 0.6-m and 0.9-m media depth cells, respectively. Overflow (bypass) occurred at least three times more frequently than intended. The exfiltration volume was much higher in the deeper media cells, presumably because of greater storage volume in the media and more exposure to side walls. Evapotranspiration (ET) plus exfiltration accounted for 42% of the inflow runoff in the 0.9-m media cells, while ET and exfiltration accounted for only 31% of the inflow runoff in the 0.6-m media cells. With the increase in exfiltration, the deeper media depth met a previously defined low-impact development (LID) hydrology goal of volume reduction more frequently than the shallower media system (44% of events compared to 21%). Larger outflow reduction consequently increased the reduction in pollutant loads. Estimated annual pollutant load reduction for total nitrogen, total phosphorus, and total suspended solids were 21, 10, and 71% for the 0.6-m media cells and 19, 44, and 82% for the 0.9-m media cells, respectively. Overall, nitrogen reduction was poor owing to suspected export of nitrate from the fertilizer use, and phosphorus removal was hampered because of irreducible concentrations in the inflow. Pollutant reduction was limited because the cells were undersized as a result of construction and design errors.