Green stormwater infrastructure is designed to mitigate flooding and pollutants to receiving water bodies by capturing and treating runoff at the source. Highway interchanges are a unique application of green infrastructure due to their location in high-traffic areas and large elevation differences between the land surface and the mitigation practice, which create unique hydraulic conditions during runoff events. The objective of this study is to monitor pollutant transport of a recently installed green infrastructure treatment train of riprap swales and bioretention that treats runoff from a highway interchange in Milwaukee, WI. The goal is to understand and evaluate how efficiently the treatment train processes mitigate urban stormwater pollutants within this unique context. To do so, samples were collected at an outfall located at the base of the highway interchange, at the end of the riprap swale, and in the effluent of the bioretention and tested for water quality pollutants (total phosphorus, reactive phosphorus, total suspended solids, and total coliforms). Results depict mixed trends between water quality parameters at three sampling locations within the green infrastructure installation. Total suspended solids and total coliform concentrations were both reduced from the highway outfall to the underdrain of the bioretention; however, total and reactive phosphorus concentrations increased through the treatment process. Ultimately, this study can help inform stormwater design of green infrastructure that treats highway runoff to minimize downstream human and ecological health risks.
