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Closures of fisheries, flooding, and stream bank erosion, have led to increased use of Best Management Practices (BMPs) through regulation of stormwater. Bioretention areas have become popular among developers because they fulfill both landscape and water quality needs; however, questions on design persist. Two paired bioretention areas were studied in central North Carolina to assess removal of phosphorus and nitrogen, hydrologic retention, and the effectiveness of an internal storage (IS) zone, also termed anaerobic zone. Both pairs comprise 5% of their watersheds and are planted with trees and shrubs. Two of the cells compared, one containing IS of .45m (18 in.) and one conventional, were continuously monitored from June, 2002, to Dec, 2004, in Greensboro, NC. Groundwater recharge and ET have been quantified by comparing two conventional cells, one lined with plastic and one un-lined, constructed in Louisburg, NC.

During outflow events in Greensboro, TP concentrations leaving the IS design were significantly lower (p=0.004) than the conventional cell, whereas TN was not (p=.056). Both designs resulted in higher outflow concentrations and seasonality was noticed. NO3 concentrations were reduced by 77% in the IS design; however, TKN and NH4 concentrations were increased. Although lower outflow frequency was found for the IS design, no significant difference in outflow volume was found (p=.071). The non-agricultural fill soils at Louisburg resulted in average concentration reductions of 40-53% TP, and 25-60% TN for 10 storms. Exfiltration to groundwater was found to be significant (p=.004) despite a tight clay in-situ soil surrounding the cells in Louisburg.