Dispersed urban-stormwater control improved stream water quality in a catchment-scale experiment

Traditional approaches to urban drainage degrade receiving waters. Alternative approaches have potential to protect downstream waters and provide other benefits to cities, including greater water security. Their widespread adoption requires robust demonstration of their feasibility and effectiveness. We conducted a catchment-scale, before-after-control-reference-impact experiment to assess the effect of dispersed stormwater control on stream ecosystems. We used a variant of effective imperviousness (EI), integrating catchment-scale stormwater runoff impact and stormwater-control-measure (SCM) performance, as the measure of experimental effect. We assessed the response of water quality variables in 6 sites on 2 streams, following SCM implementation in their catchments. We compared changes in those streams over 7 years, as SCM implementation increased, to the 12 preceding years, and over the 19 years in 3 reference and 2 control streams. SCMs reduced phosphorus and nitrogen concentrations and temperature, and increased electrical conductivity; with effect size negatively correlated with antecedent rain. SCM-induced reductions in phosphorus and temperature were of a similar magnitude to increases from urban development, when assessed as a function of change in EI. Nitrogen reductions were observed, even though concentrations among sites were not correlated with EI, being more influenced by septic tank seepage. SCMs had no effect on suspended solids concentrations, which were lower in urban streams than in reference streams. This experiment strengthens the inference that urban stormwater drainage increases contaminant concentrations in urban streams, and demonstrates that such impacts are reversible and likely preventable. SCMs reduce contaminant concentrations by reducing the frequency and magnitude of uncontrolled drainage flows and augmenting reduced baseflows. Increased EC and reduced temperature are likely a result of increased contribution of groundwater to baseflows. The stormwater control achieved by the experiment did not fully return phosphorus or nitrogen concentrations to reference levels, but their responses indicate such an outcome is possible in dominant conditions (up to ~20 mm of 24-h antecedent rain). This would require nearly all impervious surfaces draining to SCMs with large retention capacity, thus requiring more downslope space and water demand. EI predicts stream water quality responses to SCMs, allowing better catchment prioritization and SCM design standards for stream protection.