Pollutant Removal in Bioretention Cells with Grass Cover

North Carolina has spent considerable effort to improve water quality in the State and its estuaries. One area of focus has been stormwater runoff quality. Bioretention is often used to treat runoff from new developments and is retrofitted in areas where development has already occurred. Typical designs have trees and shrubs planted and mulch cover, which has become the design standard. There is significant interest in bioretention with grassed cover because maintenance could be less costly and because some owners consider them to be more aesthetically pleasing and usable. Some regulators are reluctant to allow grassed systems because of a lack of research data proving that they meet current standards for pollutant removal. The goals of this study were (1) to test the performance of grassed bioretention cells in removing nitrogen, phosphorous, metals and sediment and (2) to compare the pollutant removal between 2 identical grassed cells using induced storage zones with different fill media depths. An induced storage zone is a water storage layer at the bottom of the bioretention cell created by elevating the underdrain outlet above the bottom of the cell. A field system was built in the Piedmont of North Carolina for the study. Both cells lost volume during storm events through exfiltration, which decreased the volume of outflow through the underdrains. The induced storage zone caused an increase in the exfiltration volume, in some cases preventing any outflow from the underdrains, which improved the pollutant load reduction by the cells. Nutrient load reductions for TN were approximately 70 to 80%. Phosphorus load reductions were 35 to 50%. The higher load reduction estimates were associated with the cell with a greater media depth. Fecal conform concentration removal was excellent for one cell, 97%. However, due to a limited number of storms collected (6), there was not a statistically significant finding.