Five bioretention cells were studied from 2004 through 2007 in central NorthCarolina. They were located in Charlotte (1), Graham (2), and Rocky Mount (2). Theformer three cells were located in the Piedmont and therefore were underlain by clayinfluencedsoils. The latter two cells were located in the Upper Coastal Plain, whichwere sand dominated. The cells differed by vegetation cover, media type, mediadepth, and underdrain configuration. The Charlotte cell was 1.2 m deep and filledwith a sandy-loam soil. It was drained by conventional underdrains and vegetated bytrees 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 anupturn in the underdrain to create a sump to temporarily store water. The two cellswere turf covered. In Rocky Mount, both cells were 0.9 m deep and used a sand fillmedia (98% sand). Both cells employed an upturned underdrain, but one cell wasvegetated by turf grass and the other cell was vegetated by trees and shrubs andcovered in mulch. Substantial data have been collected for the cells in Charlotte andGraham, 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 nutrientloads and concentrations, and reduce outflow volumes, as compared to inflowvolumes. Important implications of the research are a continued refinement ofbioretention performance standards, confirmation that grassed systems performcomparably well to tree and shrub systems (at least in the short term), and the role ofan upturned drainage system, particularly in a somewhat sandy in situ soil.
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