Characterization of Hydrochemical Evolution and Transport of Nitrogen Species in Semiarid Urban Catchments During Monsoon Rainfall Events Using Hysteresis Analysis

The water quality of urban runoff is significant to arid and semiarid regions, such assouthern Arizona, which count urban runoff among the limited renewable resources for a continued water supply. Despite the essential nature of this resource, the hydrochemical evolution of urban runoff over the course of a storm and a rainy season overall is less well understood. Improved understanding would help researchers and resource managers alike in addressing water quality issues. Nitrogen species constitute a high priority water quality concern. This research addresses the evolution and transport of nitrogen during eight individual storm events that occurred over three summer monsoon seasons (2016-2018) in an urban catchment in the semiarid city of Tucson, Arizona. Hysteretic storm behavior was analyzed for the solute concentrations of nitrogen species and a conservative tracer (Cl-) to provide insight into the catchment processes between the upstream and downstream sampling locations. Of the 94 performed analyses, 49 hysteresis analyses display clockwise behavior. Thirty-six analyses result in counterclockwise loops, and 9 are linear (i.e., no hysteresis). All 9 of the linear responses are total organic nitrogen (TON). In general, the upstream site has a more consistent clockwise response, and the downstream site has a more varied response among the measured solutes. The results indicate that solute flushing is the primary response of the gravel lined urban wash during monsoon events, with some complexity in the response resulting from limited biogeochemical processing and/or additional runoff/solute sources resulting from the managed nature of the watershed. The linear TON response of many storms indicates that the nitrogen species moving through the system are inorganic. The downstream site displays more varied hysteresis responses due to the presence of a concrete lined section of the channel under a two- lane residential road, which increases the discharge rate and alt