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Field assessment of engineered bioretention as microplastics sink through site characterization and hydrologic modeling
Summary
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.