Rapid population growth and development of urban centers in arid to semi-arid regions of the world are increasing stress on already limited water resources. In these regions, and particularly in the Southwest US, urban runoff is a water resource that can potentially offset groundwater mining and recharge local aquifers.  However, managers and urban planners lack information on the quality of this water resource and how best to manage it. In particular, little information is available on how physical and biological characteristics of washes and conveyance channels affect the chemistry of runoff.  Understanding the controls on transport and retention of nutrients, organic pollutants and metals in urban washes is critical in predicting potential threats that urbanization poses to water quality and in identifying the best approaches to mitigating those threats.

We initiated a study to evaluate urban storm runoff quality and quantity in watersheds in Tucson, Arizona. To evaluate the influence of urban land use type on water quality, we established automated water samplers in 5 study watersheds representing a range of urban land use (commercial, high density housing, mixed density housing, medium density housing, low density housing). To investigate how physical and chemical wash characteristics influenced transport and fate of different contaminants, we installed automated water samplers in upstream and downstream positions along one major ephemeral wash and conducted synoptic grab sampling from soil-lined and grass-lined urban stream reaches along this wash (15 sites). We hypothesized that contaminants would be preferentially removed in grass lined channels more than soil lined channels during runoff events. Water samples were analyzed for nutrients, dissolved organic carbon, anions, metals and fecal indicator bacteria (E. coli).

Results/Conclusions

Across all watersheds and synoptic study sites, bacteria counts and dissolved mercury were high (mean 3815 CFU/ ml and 7 ug Hg/L, respectively). Chemical concentrations and bacteria generally declined from upstream to downstream positions and declined over the monsoon season. Using chloride as a quasi-conservative tracer during the hydrograph recession, we observed that solutes exhibited conservative behavior in soil-lined washes. In contrast, grass lined washes varied between being sinks and sources for metals, nutrients, and fecal indicating bacteria depending on their land use (park or corridor). Findings from this study suggest that grass lined washes have the potential to be designed to provide ecosystem services of attenuating nutrients and organic pollutants, a function which may be critical to sustaining water resources in this region.