Water Quality in Urban Systems: Sources, Drivers, and Unknowns.

The magnitude of nitrogen (N) transport in stormwater runoff from diverse urban residential neighborhoods is largely not known. We present case studies of N dynamics in urban stormwater runoff from residential neighborhoods located in (1) southern California where runoff is generated by lawn irrigation with municipal and reclaimed water during the dry season and (2) central Florida where most runoff is generated by excess rainfall during the wet season.  The outlet pipes draining the residential neighborhoods in California (drainage area: 28 ha) and Florida (drainage area: 2-13 ha) were instrumented with ISCO samplers, flow meters, and rain gauges. In California, triple composite water samples were collected at 3-h intervals for 1-week (no rainfall occurred during the study period). While in Florida, stormwater runoff samples were collected following variable storm duration and frequencies during the wet season (June to September) that receives 60-70% of 125 cm of annual rainfall. Mean concentration of total N in stormwater runoff waters were 10.9 mg L^-1 in California and <3 mg L^-1 in Florida.  In California, about 50% of N was nitrate-N, while in Florida, nitrate-N was <25% and organic N was >60% in most storm events; suggesting different processes controlling N release and transport in neighborhoods. This presentation will discuss our ongoing research aimed at better understanding the sources, processes, and unraveling the unknowns and uncertainty of N transport from urban systems.