Modeling the Atmospheric Contribution of Nitrogen Compounds in Urban Stormwater Runoff

We have investigated the application of deterministic models to estimate the relative contributions of the atmospheric wet and dry deposition onto an urban surface and the subsequent amounts entrained in the stormwater runoff. The research linked the CIT airshed model and the U.S. EPA Storm Water Management Model (SWMM) to simulate the fate and transport of nitrogen species through the atmosphere and storm drainage systems of the Los Angeles basin. The CIT model estimates the amount of nitrogen compounds originating from airborne emissions depositing during dry weather and the amount available to be removed from the atmosphere during precipitation events. SWMM computes the urban runoff flow rates and contaminant concentrations from drainage subcatchments and the flow and pollutant transport through the storm drainage system. This research addressed several model interface issues including the representation of dry and wet deposition in SWMM, the transfer of atmospheric nitrogen compounds to dissolved nitrogen compounds, and the estimation of processes that reduce the amount of deposited material available for runoff. The CIT-SWMM linkage is demonstrated using a dry and wet deposition event on December 3-4, 1987 in a Santa Monica Bay subwatershed located in Los Angeles, California. CIT estimated high values for dry deposition of total nitrogen on December 3rd, but these values were expected from the nature of the event that occurred during the Southern California Air Quality Study in 1987. SWMM calculated loads of nitrogen compounds in the stormwater runoff event on December 4th that were similar to loads estimated by the Los Angeles County Department of Public Works stormwater monitoring program. Although the simulated and measured stormwater runoff loads are similar, the CIT-SWMM linkage is not accounting for other sources of nitrogen in the watershed, e.g., residential fertilizer use. We will address this in future research.