Partitioning and First-Flush of Nitrogen in Rainfall-Runoff from an Urban Source Area

The urban surface is a largely impervious interface across which nitrogen (N) in runoff is transported and partitions between dissolved and particulate phases. In this study intraevent partitioning and transport are examined across 14 runoff events loading a source area; representative of urban land uses that support motor vehicle activities; specifically parking. N is fractionated as: suspended N for suspended PM (0.45 similar to 25 mu m); settleable N for settleable PM; sediment N for sediment PM (greater than 75 mu m); and dissolved N as filtered through a 0.45-mu m filter. The median total nitrogen (TN) was 4.5 mg/L, much higher than Florida nutrient criteria, and the median of dissolved, suspended, settleable, and sediment N fractions was 2.02, 0.72, 0.29, and 0.78 mg/L. As would be expected for a source area of less complex geometry, transport of TN was generally mass limited (a first flush); however, the suspended and dissolved fractions did exhibit flow-limited transport. Hydrograph unsteadiness also impacted transport. While mass limited transport generally occurred for N associated with coarser PM fractions in higher flow rate storms, mass limited transport for suspended and dissolved fractions was nominally more frequent for lower flow rate storms. While the distribution of N with PM size was complex, the particulate-bound N fraction was correlated to the volatile fraction of PM. For dry deposition PM, particulate-bound N was higher and the size gradation coarser than in runoff due to intraevent leaching and redeposition of PM during transport. (C) 2014 American Society of Civil Engineers.