Ecological effects of sewage overflows in small urban streams in Austin, TX

Background/Question/Methods

In 2008, more than 30 billion gallons of wastewater flowed through 2,650 miles of sanitary sewer lines maintained by the City of Austin, TX. On average there are 150 reportable wastewater releases to Austin streams annually.  Pollutant concentrations of raw wastewater (e.g., 5-day biochemical oxygen demand = 200 mg/L and ammonia-nitrogen = 20 mg/L) are orders of magnitude higher than ambient stream concentrations.  Although the effect of raw wastewater on streams has been studied for over a hundred years, spatial and temporal effects from small pulse events on aquatic ecosystem structure and function have not been well documented. Three spill events of varying magnitudes were studied on three intermittent urban streams, documenting chemical and biological responses using an upstream-downstream approach over a 2-month recovery period. 

Results/Conclusions

Water chemistry spatial differences in relation to wastewater spill locations were less severe than expected, recovering generally within 2 weeks.  However, ecological response including macroinvertebrates and diatom communities was dramatic both spatially and temporally with recovery to background conditions only occurring at one of the study streams after two months of monitoring and degradation of benthic macroinvertebrate communities observed at extended distances downstream of the spill location.  Discrete functional changes in the macroinvertebrate community were consistent among study streams. Diel dissolved oxygen ranges (minimum to maximum) and the slope of diel oxygen ascent and descent increased significantly at spill affected sites and remained different than upstream conditions at least 4 weeks after the spill occurred.  The duration of the wastewater release event, which varied among the study streams, appears to be more important than the magnitude of the spill in determining stream impacts.  Results from this study suggest that sewage overflows are a significant stressor in urban streams, causing more severe and longer term ecological degradation than was previously thought.