Performance of a Bioretention Area and a Level Spreader-Grass Filter Strip at Two Highway Sites in North Carolina

The pollutant removal efficiency of a bioretention area and a level spreader-grass filter strip implemented at North Carolina highway facilities was assessed. The assessment consisted of monitoring inflow, outflow, and on-site rainfall for at least 13 storm events. Monitoring included continuous discharge measurement and collecting and analyzing flow-proportional samples for each event. All samples were analyzed for solids, turbidity, and nitrogen and phosphorus forms and selected samples were analyzed for metals. The level spreader-grass filter strip had the best overall efficiency with load reduction efficiencies in all pollutants ranging from 24 to 83% and the highest reduction for total suspended solids (TSS). Much of the efficiency of this best management practice can be attributed to the 49% reduction in runoff volume from inflow to outflow. Pollutant reduction efficiencies for the bioretention area ranged from -254 to 76% with the highest reduction for TSS. The lowest or large negative efficiency was for nitrate+nitrite nitrogen (NO2+3-N). The increase in NO2+3-N likely resulted from a combination of nitrogen additions within the cell and conversion of other forms of nitrogen to NO2+3-N. Statistical analyses suggested that all of the mass reductions for the grass filter strip and many of those for the bioretention area were significant.