Evaluation of Bioretention Hydrology and Pollutant Removal in the Upper Coastal Plain of North Carolina

摘要

Bioretention is a principal low impact development practice. A study underway in North Carolina is examining the performance of bioretention cells with respect to hydrology and water quality under varying vegetative cover, media depths, and drainage configurations 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. Only two out of 32 events that were monitored since September2007 produced outflow (total storm sizes - 49.3 mm and 83.3 mm). Grab samples have been taken from the underdrains after an event to quantify treatment. Preliminary data from six events show concentration reductions of total nitrogen, total phosphorus, and total suspended solids of 65.7, 17.3, and 94.0%, respectively. In June 2008, the IWS will be reduced to 0.3 m. In Nashville, two sets of cells were designed with 0.6 and 0.9 m deep media depths and without IWS. Monitoring in Nashville started in April2008. 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.