Pathogen and nutrient pulsing and attenuation in “Accidental” urban wetland networks along the Salt River in Phoenix, Arizona

Background/Question/Methods

Increases in available nutrients and bacteria in urban streams are at the forefront of research concerns within the ecological and medical communities, and both pollutants are expected to become increasingly problematic under projected changes in climate. The aim of this research was to examine the effect of storm events on nutrient (total organic and inorganic nitrogen, total organic carbon, sulfate) and Escherichia Coli (E. Coli) loading and attenuation along flowpaths in urban wetland networks along the Salt River in Phoenix, AZ. The wetlands selected for investigation are fed by storm water outfalls exiting industrial and residential areas. Samples were collected along flowpaths downstream of six large, perennially-flowing outfalls.  Samples were collected during baseflow and immediately after storm events at the following points: immediately downstream of the outfall, mid-wetland, and downstream of the wetland.  These samples were immediately taken back to the laboratory and processed (within 8 hours). For determination of E. coli counts, samples were plated on coliform-selective media (Chromocult) plates and incubated for 24 hours. Plates were then used to enumerate fecal coliforms and E. Coli. For determination of nutrient concentrations, samples were filtered and frozen until they could be analyzed using an Ion Chromatograph, Lachat, and TOCAN.

Results/Conclusions

Large storm events occur during the summer monsoon season and during the winter in Phoenix. During both summer and winter, total discharge into the wetlands increased during storm events as compared to discharge prior to storm events. Concentrations of nutrients and E. Coli were significantly lower during baseflow conditions than immediately following storm events. Spikes of dissolved pollutants returned to pre-storm levels rapidly (within a week) following each storm event. E. coli counts and nutrient concentrations dropped over the length of flowpaths through the wetlands, indicating high attenuation capability, even during increases in water velocity and decreased water retention time in the wetlands. Climate change models project increases in severe droughts and extreme precipitation events for the southwestern United States, which can lead to more sewage leakages and increases in contaminated runoff from impervious surfaces in urban areas. Wetlands are constructed or restored to mitigate microbial contamination of wastewater.  Our research indicates that even "accidental" urban wetlands can serve to help reduce the amount of nutrients, E. Coli, and Coliforms in storm and wastewater. However, wetland restoration or design targeting increased water retention time may increase the capability of the Salt River to remove nutrient and pathogens from stormwater.