## 4 Discussion
Potential removal mechanisms for NH4\({}^{+}\) include immobilization via plant uptake and adsorption onto cation exchange sites. It is also possible that NH4\({}^{+}\) in wetland inflow was converted to NO3\({}^{-}\) near the wetland entrance, and then denitrified prior to leaving. The median inflow DO concentration was 5.21 mg L\({}^{-1}\) but outflow was reduced to 1.1 mg L\({}^{-1}\). It is possible that nitrification of influent NH4\({}^{+}\) occurred near the wetland inlet, and denitrification of NO3\({}^{-}\) may have occurred prior to leaving through the outlet, thus lowering both NH4\({}^{+}\) and NO3\({}^{-}\) concentrations. Dry detention basins constructed to reduce runoff receive a nitrogen reduction credit of 10% based on NC guidelines [22]. During some of the smaller rain events (\(<\) 0.3 cm) sampled, there was a 10% or more TDN treatment efficiency observed for the basin. It is likely that plant uptake and/or denitrification may have been responsible for TDN removal during those small rain events. However, for most storms sampled, the 10% reduction in TDN was not realized at Jaycee Park. Research has shown that the treatment efficiency of dry basins is highly variable, and many basins export nitrogen [8]. It is likely that the relatively large main outlet pipe (60 cm diameter) for the basin was too close in size to the main inlet pipe (90 cm diameter), thus allowing large volumes of inflow to quickly exit. The small residence time of water in the dry basin was insufficient to lower the DO and ORP values for most storms despite relatively high concentrations of DOC (in relation to NO3\({}^{-}\)). Nitrate reduction via denitrification was not significant in the dry detention basin, because stormwater was not in the basin long enough to stimulate a drop in DO and ORP to allow for denitrification. SCMs in Greenville and most of NC are designed to store and treat runoff produced from a 2.5 cm rain event. However, runoff entering the basin from that size storm typically passed through the outlet within 5 h. In contrast, the outlet structure for the wetland had much smaller main draw downpipes (10 cm and 7.5 cm diameters) that reduced outflow rates by more than one order of magnitude in comparison to the dry basin. The longer residence times likely facilitated a drop in DO and ORP, decomposition of organic debris, and denitrification. The wetland reduced TDN concentrations by about 63%, thus exceeding the 44% nitrogen credit associated with stormwater wetlands in North Carolina [22]. The nitrogen reduction efficiency of the wetland in this research was similar to the NO3\({}^{-}\)N treatment efficiencies reported (68%) in a study of a nearby (-5 km) stormwater wetland [30]. The median TDN treatment efficiency of this wetland also exceeded the median efficiency (48%) of 19 wetlands reported in a literature review [8]. Overtime, as the planted wetland trees grow and mature, the nitrogen treatment efficiency of the wetland may increase due to a greater plant uptake of nutrients and water [31], increases in soil carbon, and development of other biogeochemical conditions conducive to denitrification [32]. Prior studies in North Carolina [32] and Sweden [33] comparing the nitrogen treatment efficiency of young and more mature wetlands have shown improvements in treatment with age. However, as the wetland vegetation and trees enlarge in the Jaycee wetland, maintenance intensity may also need to increase due to more organic matter and sediment being deposited into the wetland [11; 32]. Partial clogging of the relatively small wetland outlet pipes was noted after several storms. The debris was removed following larger storms, but if routine maintenance is not performed on the wetland, then retention and treatment of stormwater may be negatively impacted [16; 32]. Clogged outlets will reduce outflow rates and may trigger more frequent overtopping of the outlet structure and bypass of treatment if consecutive storms occur within a few days of each other.