Effects of watershed-scale green infrastructure retrofits on urban stormwater quality: A paired watershed study to quantify nutrient and sediment removal

Green infrastructure (GI) is an increasingly utilized method to treat stormwater pollutants, but little evidence exists regarding its performance at the watershed-scale. Blueprint Columbus is an effort by the City of Columbus, Ohio, USA, to retrofit bioretention and permeable pavement into existing developed areas. Changes in water quality resulting from the combined effects of many GI practices installed in 11.5 and 47.8 ha treatment watersheds were quantified using a paired-watershed approach over a 3.5-year period. Total nitrogen (TN), phosphorus (TP), and total suspended solids (TSS) concentrations decreased by 13.7–24.1%, 20.9–47.4%, and 61.6–67.7%, respectively. Runoff attenuation by GI contributed to significant pollutant load reductions of 24.0–25.4% (TN), 27.8–32.6% (TP), and 59.5–78.3% (TSS). Orthophosphate concentrations and loads significantly decreased. Reductions in TSS concentration were similar (within a margin of 5%) to the percent of the watershed imperviousness treated by GI. Results demonstrate that GI was effective in reducing runoff event mean concentrations and loads at the watershed-scale.