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STORMWATER RUNOFF QUALITY AND QUANTITY FROM TRADITIONAL AND LOW IMPACT DEVELOPMENT WATERSHEDS Mean concentrations of nitrogen in runoff from all watersheds in this study (Tables 3 and 4) were on the low end of the broad ranges reported in a literature review by Makepeace et al. (1995). Nutrients — Phosphorus Traditional Watershed. The mean concentration of TP in runoff from the traditional watershed signifi- cantly decreased (p < 0.001) by 79% in the postcon- struction period when compared with values predicted by the calibration equation (Table 3). Previ- ous research reported that TP was associated with suspended solids in urban stormwater (Mallin et al., 2002). In this study, TP concentrations in runoff from the traditional watershed were also associated with TSS concentrations during the postconstruction per- iod (r = 0.698; data not shown). TP concentrations likely decreased during the postconstruction period because the TSS concentrations decreased, although higher flows could also have diluted TP concentra- tions. Mean TP concentrations (Table 3) were between the concentrations reported by Dietz and Clausen (2004) in Connecticut and those in the NURP report (USEPA, 1983b). Despite the decrease in stormwater TP concentra- tions, the mass export of TP increased by 24 times in the postconstruction period because the flow increased (Table 3). In a Wisconsin study, feeder streets (low traffic residential streets), lawn areas, and residential driveways were important sources of TP (Bannerman et al., 1993). Together, these sources contributed more than 75% of the total load in resi- dential areas. Low Impact Development Watershed. In the LID watershed, TP concentrations significantly increased (p < 0.001) by more than ten times during the postconstruction period (Table 4). Mass export of TP also significantly increased (p = 0.01) by more than three times in the postconstruction period (Table 4). Spikes in the TP concentrations and exports in the postconstruction period occurred mostly during the growing season and autumn leaf fall. This timing suggested that increases in TP were likely due to fertilization of the grassed swales as well as leaching from autumn leaves. Fertilizer appli- cations from residential houses contribute to nutri- ents in stormwater runoff (Myers et al., 1985), and leaves have been reported to leach soluble phospho- rus in stormwater (Cowen and Lee, 1973). Interestingly, TP concentrations in runoff from the LID watershed were greater than from the tradi- tional watershed during the postconstruction period based on a paired t -test (t = — 2.97, p = 0.004). Since the primary conveyance of stormwater in the LID watershed was from the grassed swales, perhaps they alone may have contributed to the higher TP concen- trations. Parking lot runoff in another study reported higher TP concentrations from lots with grassed swales compared with lots with no grassed swales (Rushton, 2001). However, TP export from traditional watershed runoff was still greater than export from the LID watershed because the storm flow increased significantly. Metals — Cu, Pb, and Zn Traditional Watershed. The concentrations of Cu, Pb, and Zn in runoff from the traditional watershed did not change significantly in the postcon- struction period (Table 3). The NURP study (USEPA, 1983b) reported higher Cu and Pb concentrations than those observed in this study (Table 3) with med- ian EMCs of 33 µgA and 144 µgA, respectively. Lead fuels were phased out in the early 1980s and lower Pb concentrations would be expected. Cu mass export in runoff from the traditional watershed increased significantly (p < 0.001) by 90 times, and the mass export of Zn significantly increased (p < 0.001) by eight times during the post - construction period (Table 3). Pb mass export did not change significantly in the postconstruction period. Increased mass exports of Cu and Zn from the tradi- tional watershed were likely due to the significant increase in storm flow in the postconstruction period. Greater storm flow may have mobilized metals that built up with sediment accumulation (Myers et al., 1985). Urban areas contribute heavy metals to runoff through automobiles, roofs, and building siding (Davis et al., 2001b). Low Impact Development Watershed. Pb and Zn concentrations in runoff from the LID watershed significantly decreased (p < 0.001) by 67 and 77 %, respectively, during the postconstruction period (Table 4). Cu concentrations in runoff showed no sig- nificant change between the calibration and postcon- struction periods. Reductions of Pb and Zn concentrations in this study were not as large as those from a laboratory column study of bioretention effectiveness, in which Davis et al. (2001a) reported reductions of Pb ( >98 %) and Zn ( >98 %) concentra- tions. However, in that study, the stormwater was simulated and no runoff occurred because it was com- pletely infiltrated by the bioretention areas. The mass exports of Pb and Zn significantly decreased by 79% (p = 0.006) and 81% (p = 0.01), respectively, during the postconstruction period when compared with values predicted by the calibration JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1005 JAWRA