Hydrologic and water quality performance of a rock swale and bioretention system treating highway overpass runoff

Green stormwater infrastructure (GSI) is designed to reduce volumes, peak flows, and pollutants in stormwater runoff. Mechanisms to achieve these goals for standalone GSI are well understood; however, it is less clear how an interconnected GSI (or treatment train) should be configured to treat unique situations where larger water volumes and/or pollutant loadings necessitate several interconnected GSI practices. The goal was to understand the hydrologic and water quality performance of an interconnected system of GSI (rock swales and bioretention) that treats runoff from an elevated highway interchange in Milwaukee, WI. Stormwater flow and runoff samples were captured at several locations throughout the system and tested for total suspended solids (TSS), total phosphorus (TP), dissolved reactive phosphorus (RP), and total coliforms. The initial rock swale reduced particulate and soluble pollutant concentrations, while the bioretention only reduced particulate concentrations. Overall, systematic treatment reduced concentrations of TP (57%) and TSS (97%) but increased dissolved RP (236%); however, the system exfiltrated large volumes (91.6%), resulting in overall load reductions. In addition, the concentrations of particulates correlated with the antecedent traffic volume on the highway overpasses. Ultimately, these outcomes demonstrate that this treatment train design is effective at mitigating overpass runoff despite extreme hydrologic conditions.