Water Quality and Hydrologic Benefits of Five Bioretention Cells in North Carolina

Five bioretention cells were studied from 2004 through 2007 in central North Carolina. They were located in Charlotte (1), Graham (2), and Rocky Mount (2). The former three cells were located in the Piedmont and therefore were underlain by clay-influenced soils. The latter two cells were located in the Upper Coastal Plain, which were sand dominated. The cells differed by vegetation cover, media type, media depth, and underdrain configuration. The Charlotte cell was 1.2 m deep and filled with a sandy-loam soil. It was drained by conventional underdrains and vegetated by trees and shrubs and covered by mulch. The Graham cells were 0.6 and 0.9 m deep, employed a proprietary soil comprised primarily of expanded slate fines, and had an upturn in the underdrain to create a sump to temporarily store water. The two cells were turf covered. In Rocky Mount, both cells were 0.9 m deep and used a sand fill media (98% sand). Both cells employed an upturned underdrain, but one cell was vegetated by turf grass and the other cell was vegetated by trees and shrubs and covered in mulch. Substantial data have been collected for the cells in Charlotte and Graham, while data collection in Rocky Mount is on-going. To date, all five cells, despite their many different design elements, have been shown to reduce nutrient loads and concentrations, and reduce outflow volumes, as compared to inflow volumes. Important implications of the research are a continued refinement of bioretention performance standards, confirmation that grassed systems perform comparably well to tree and shrub systems (at least in the short term), and the role of an upturned drainage system, particularly in a somewhat sandy in situ soil.