Green infrastructure treating highway overpass runoff is a unique application due to large traffic volumes and great elevation differences between the runoff surface and the mitigation practice, which creates unique loading and hydraulic conditions. This may present challenges for treating runoff with high-energy and contaminant concentrations using nature-based practices. Therefore, the objective of this study is to evaluate the contaminant mitigation efficiency of green infrastructure – rock swales and bioretention – treating highway interchange runoff in Milwaukee, WI. To do so, samples were collected at (1) the primary influent from two highway overpass downspout outfalls, (2) the effluent of a rock swale, and (3) the bioretention underdrain representing the system’s effluent. Samples were tested for total phosphorus, reactive phosphorus, total suspended solids, and total coliforms. Results depict mixed trends between contaminants. Total suspended solids were reduced by the system (e.g., TSS outfalls: 137 mg/L and 87 mg/L to the underdrain 27 mg/L). Total coliforms did not present a relationship between the system’s influent and effluent (e.g., total coliforms outfalls: 73 CFUs/100 mL and 59 CFUs/100 mL to the underdrain: 92 CFUs/100 mL); however, total phosphorus and reactive phosphorus concentrations increased through the system (e.g. total phosphorus outfalls: 0.97 mg/L and 0.77 mg/L to the underdrain 2.16 mg/L, and reactive phosphorus outfalls: 0.21 mg/L and 0.16 mg/L to the underdrain 1.95 mg/L). These results demonstrate GSI treatment trains can mitigate total suspended solids from highway overpasses, while site-specific hydrological conditions and GSI design may influence the amount extent other contaminants are mitigated.
