Interactions and Treatment of Metals in Urban Stormwater

Abstract

Increasing urbanization and a changing climate will only exacerbate the magnitude of pollution entering our waterways, threatening our drinking water source and aquatic ecosystems. Urban stormwater contains a cornucopia of pollutants that pose direct toxicity risks (e.g., metals, organics, pathogens) and indirect adverse effects (e.g., sediments, nutrients) to aquatic life. Metals, specifically copper (Cu) and zinc (Zn), are both ubiquitous in the urban environment and detrimental to aquatic ecosystems at low concentrations (approximately 10 ppb). Targeting this growing source of pollution upstream is critical in providing necessary environmental protections, especially as the intensifying effects of climate change and urbanization are imminent. This leads to the main research question – how can Cu and Zn loads in stormwater be reduced to protect aquatic ecosystems?Bioretention is a stormwater control measure (SCM) that mimics natural systems to take advantage of the natural filtering processes. In addition to hydrologic benefits, bioretention provides removal of particulate matter (PM) through filtration and sedimentation, and potential removal of dissolved constituents through chemical and biological processes. Studies including characterization of stormwater, road-deposited sediments (RDS), and performance of a mature bioretention cell were performed to determine treatability, mobility, and bioavailability of Cu and Zn in stormwater and through bioretention treatment. Both metals accumulated in the finest (<25 μm) fraction of RDS samples, however particulate bound (PB) Zn concentrations were enriched in stormwater compared to finer fractions of RDS, while PB-Cu was not. This indicated that PB-Zn is more mobile than PB-Cu, likely due to different sources of these metals in urban environments. The PM and PB metal loads were reduced by 82% and 83%, respectively, showing that mature bioretention cells are effective at reducing PM and PB contaminant loads. However, dissolved constituents were essentially unchanged through bioretention treatment, and concentrations of dissolved metals were measured at levels that potentially cause aquatic toxicity. Thus, alternative media amendments were investigated for further reduction of dissolved metal contents. Black carbon (BC) media including biochar, granular activated carbon (GAC), regenerated activated carbon (RAC), and a natural mineral sorbent, clinoptilolite zeolite, were tested in continuous columns, and in up-scaled modular treatment columns. The four tested BC media performed similarly for Cu and Zn removal, with Zn having an earlier breakthrough compared to Cu. This technology is predicted to provide reduction of dissolved Cu for up to 60 years with current rainfall predictions. Modular treatment columns showed that traditional bioretention soil media (BSM) provided effective removal of dissolved Zn (71%) and ineffective removal of Cu (17%). The subsequent BC polishing module was effective for Cu removal (40%), and zeolite showed potential for Zn removal. Overall, dissolved metals in stormwater are the most mobile, bioavailable, and difficult to remove through traditional filtration-based SCMs. This research has shown that fresh BSM can provide effective removal of dissolved Zn, and BC amendments are a potential solution for removal of dissolved Cu in stormwater. Refreshing the top few centimeters of an existing bioretention with fresh BSM can provide treatment of dissolved Zn. Retrofitting bioretention to include a polishing module either layered or in series with a mix of BC and zeolite can further reduce dissolved Cu and Zn loads in stormwater.