The importance of upland flow paths in determining urban effects on stream ecosystems

Mitigation of urban effects on streams requires an understanding of the paths by which urban effects are transmitted from catchments to streams and how those effects are attenuated with distance. We assessed whether modeling attenuation from impervious surfaces and septic tanks along drainage lines improved prediction of 3 instream ecological indicators. Eleven regression models were calculated for each indicator (Escherichia coli, NO3/NO2, and Stream Invertebrate Grade Number Average Level [SIGNAL; a macroinvertebrate assemblage composition index]). Predictor variables included imperviousness or septic-tank density with no attenuation (i.e., total imperviousness and tank density), with overland attenuation (exponential decay with distance along topographic flow paths to stormwater drain or to stream), and with overland and instream attenuation (exponential decay with distance travelled within the stream). Escherichia coli was best predicted by the weighted density of septic tanks, with their influence attenuated to near 0 within tens of meters of the stream and within thousands of meters along streams. These results suggest strong overland attenuation of bacterial contamination and indicate that stormwater drains are not important pathways for septic leakage in this area. NO3/NO2, which is mobile through soils, was best predicted by nonattenuated septic-tank density within catchments. SIGNAL was best predicted by either impervious area or septic tanks within tens of meters of stormwater drains, a result suggesting that SIGNAL is most strongly affected by stormwater runoff routed through stormwater pipes to streams. Management of dry-weather fecal and N contamination in the study streams should focus on septic tank management (stricter maintenance regimes or replacement with other sewage management systems). Near-stream septic tanks are probably most important for fecal contamination, whereas catchment-wide management probably will be required for reduction of dry-weather N concentrations. Management of broader instream ecological condition, as indicated by macroinvertebrate assemblage composition, probably will require catchment-wide retention of stormwater runoff (using tanks or infiltration systems) to mimic natural flow-paths between catchments and streams.