Green Infrastructure in Series to Treat Stormwater Runoff from a Highway Overpass

Green stormwater infrastructure (GSI) is effective at capturing and treating runoff from impervious traffic areas, including roadways, parking lots, and pedestrian walkways; however, its application in treating highway or overpass runoff presents unique challenges due to the high energy of influent flows and high pollutant concentrations. Therefore, it is unclear how these challenges might affect their ability to capture and treat runoff. The objective of this study was to evaluate the performance of a GSI system that treats stormwater runoff from a highway overpass in Milwaukee, Wisconsin. Stormwater runoff was captured at three sampling locations throughout the system, including (1) the primary influent from a highway overpass downspout entering a rock swale, (2) the effluent of the rock swale before it enters a bioretention system, and (3) the bioretention underdrain representing the system's effluent. These samples were then tested for total suspended solids, total phosphorus, dissolved reactive phosphorus, and total coliforms. Results indicated that the rock swales were effective at reducing mean concentrations of total suspended solids (72%) and total coliforms (59%) from the primary influent. Results also indicated that the entire system, composed of rock swale and bioretention treatment was effective at reducing mean concentrations of total suspended solids (80%), but less so for mean concentrations of total phosphorus (16%) and total coliforms (3%) from the primary influent. These outcomes demonstrate that GSI treatment trains can mitigate total suspended solids and total phosphorus from highway overpass runoff, while site-specific hydrological conditions and GSI designs may influence the amount of highway overpass runoff pollutants that are mitigated.