Bioretention Performance in the Upper Coastal Plain of North Carolina

Bioretention is a principal low impact development (LID) practice. A study underway in North Carolina is examining the performance of bioretention cells with respect to hydrology and water quality under varying vegetative covers and media depths in the sandy in-situ soils of the upper coastal plain. Two types of cells are being monitored in both Rocky Mount and Nashville. In Rocky Mount, the cells were designed with 0.9 m deep media depth and a 0.6 m deep internal water storage layer (IWS). The IWS is intended to create anaerobic conditions to increase the rate of nitrate and total nitrogen reduction. Current data shows the IWS having a strong effect on flow reduction. Out of 37 events that were monitored since September 2007, outflow was generated in the grassed cell and mulch/shrub cell two and one time, respectively. Grab samples have been collected from the underdrains after an event to quantify treatment. Preliminary data from ten events show concentration reductions of total nitrogen, total phosphorus, and total suspended solids of 80, 72, and 92%, respectively. In Nashville, two sets of cells were designed without IWS and with media depths of 0.6 and 0.9 m. Preliminary data shows the deeper media depths meet a LID hydrology goal of volume reduction more frequently. Data collected from these four sets of cells will be used to compare performance of bioretention in the sandy in-situ soils of the upper coastal plain to those sited in clayey soils from past studies in the Piedmont region.