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Field assessment of engineered bioretention as microplastics sink through site characterization and hydrologic modeling
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A field study of bioretention cells — garden-like stormwater filters used in green infrastructure — found that their soils contained 7 to 10 times more microplastics than background soils, with polypropylene and polyethylene dominating. This confirms that bioretention systems are effective at capturing microplastics from urban runoff, but it also raises questions about what happens to the accumulated plastic over time and whether it eventually leaches back into groundwater.
Green stormwater infrastructure (GSI) practices like bioretention are considered a sink for microplastics washed in from urbanized land uses-land covers, and thereby regulating the environmental dispersion of microplastics. However, the capacity of GSI in microplastic sequestration remain unclear. This work investigated the spatial distributions of microplastics within bioretention cells and their soils, concentration in the GSI groundwater monitoring well, and the overall potential of GSI as a sink for microplastics. The average microplastic concentration in GSI soils (2718 - 3833 MPs/kg dry soil) is 7-10 times higher than background soils (262 - 520 MPs/kg dry soil). The most abundant microplastics observed were polypropylene (57 %), polyethylene (17 %), and rubber/bituminous (16 %), indicating the major sources of microplastics derived from post-consumer plastics and tire wear. Soils closest to the GSI stormwater inlet sequestered the most particles, and the microplastic concentration decreased with soil depth. Our hydrologic modeling, based on measured soil and hydraulic parameters, suggested that runoff overtopping the freeboard of the bioretention cell - and potentially dispersing microplastics into the surrounding environment - was unlikely. Microplastics in the GSI groundwater monitoring well was minimal at 11 MPs/L. These results suggest that GSI can capture and sequester microplastics to mitigate microplastic pollution in the environment.
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Researchers investigated plastic pollution in urban stormwater bioretention systems and found these green infrastructure features both accumulate microplastics from road runoff and risk leaching plastic particles into groundwater, raising concerns about their role as pollution pathways.
Occurrence and Pathways of Microplastics in Bioretention Filters
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Researchers found eleven microplastic polymer types in bioretention filter soil and stormwater samples in an urban setting, characterizing the occurrence and pathways of microplastics entering these green infrastructure systems from contaminated impervious surface runoff.
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Researchers analyzed microplastic abundance and distribution in the filter media of nine aged stormwater bioretention systems, finding that these green infrastructure installations accumulate significant microplastic loads, with particles distributed throughout the filter depth rather than concentrating at the surface.
Microplastics in Field-Installed Bioretention Systems: Vertical Distribution and Implications for Retention from Stormwater
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Researchers sampled microplastics at multiple depths inside a field-installed bioretention stormwater system and found average concentrations of roughly 1,095 particles per kilogram of dry sediment, with fragments dominating and evidence suggesting MPs migrate through the media, pointing to bioretention systems as a potentially useful but incompletely characterized filter for urban microplastic runoff.
Microplastics removal from stormwater runoff by bioretention cells: A review
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This review examines the potential of bioretention cells, a type of green infrastructure, to remove microplastics from stormwater runoff. Researchers analyzed how these systems filter microplastics through soil media and vegetation and identified the key design parameters that affect removal efficiency. The study suggests that bioretention cells offer a promising nature-based solution for reducing microplastic loads entering waterways from urban areas.
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