Retrofitted watershed scale green infrastructure reduces heavy metals in urban stormwater from residential land use

The City of Columbus, Ohio, USA began retrofitting green infrastructure (GI) into existing development through a multi-decade project. Primary design goals were reducing TSS loads in runoff by 20 % and stormwater infiltration and inflow to the sanitary sewer. GI also contributed to modest heavy metal reductions at the watershed scale. Through this 3.5-year paired watershed study, heavy metal reductions were observed in an 11.5-hectare watershed where three online bioretention cells treated 66.5 % of the imperviousness. Significant event mean concentration (EMC) reductions of 18.1 % for copper and 31.2 % for nickel were observed. Storm event load reductions were also significant: 44.0 % for cadmium, 33.6 % for copper, 46.0 % for nickel, 5.9 % for zinc. Reductions are likely due to sedimentation, filtration, sorption, and biological uptake within GI. Bioretention dampened effects of high rainfall intensity, a primary contributor to heavy metal loadings in that watershed before GI. However, another treatment watershed (47.8 ha) with 32 offline bioretention cells and four permeable pavement roads treating 69.7 % of the imperviousness demonstrated no significant reductions in heavy metal EMCs or storm event loads following GI retrofits. Further watershed scale field studies are needed to understand the factors driving successful heavy metal reduction. Structural and non-structural best management practices are recommended.