Microplastics: The Occurrence in Stormwater Runoff and the Effectiveness of Bioretention Systems for Removal

Stormwater is considered a pathway of microplastics to surface water systems, but the magnitude of microplastic pollution in stormwater and the efficacy of existing stormwater treatment methods are unknown. One potential solution for minimizing microplastics in surface waters is leveraging green infrastructure, such as green roofs, bioswales, and bioretention cells, which can be optimized for the removal of conventional and emerging contaminants. The goals of this study were to (1) establish baseline types and quantities of anthropogenic microparticles, including microplastics, found in stormwater, and (2) evaluate bioretention as a possible solution for reducing microplastics in stormwater. To understand baseline conditions, samples were taken from five different catch basins in north Portland, Oregon, during four storm events and analyzed to quantify and characterize microparticles between 106 μm and 5 mm. A baseline concentration of 4.5±2.2 (range 1.1–9.7) particles/L was found, with no observed differences in concentrations between sampling locations or storm events. Most identified microparticles were fibers (66%), and approximately 47% of the particles were cotton, followed by polyester/blend (33%) and nylon (9%). Microparticle concentrations were correlated with the concentration of total suspended solids and the number of pieces of litter collected during a litter survey. To determine the efficiency of bioretention systems for microplastic removal, laboratory bioretention columns were constructed and filled with three different geomedia (a City of Portland, Oregon, standard mix, a proprietary mix, and layered sand and compost). A total of nine columns (three replicates of each geomedia) were tested with stormwater collected from a catch basin in Portland that was spiked with synthetic microplastics (106–5,000 μm). A removal efficiency of 99.8% was observed across all bioretention columns, regardless of the media type. Results indicate green stormwater infrastructure, such as bioretention, may be an effective tool for reducing observed microplastic transport from urban runoff to receiving water bodies.