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Field monitoring of microplastics in biofilter media: insights into media characteristics and design
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Rain gardens and other "biofilters" used to clean street runoff before it reaches rivers and oceans can trap huge amounts of tiny plastic bits — sometimes over 1,000 particles per gram of soil-like material — mostly by physically straining them out through small gaps between soil particles. This matters because it shows these systems, which cities already use widely, are quietly working as a defense against microplastic pollution in our waterways, and the study found simple design tweaks (using material with more tiny pores) could make them even better at catching plastic before it spreads further into the environment and food chain.
ABSTRACT Graphical abstract illustrating the hypothesized filtration-based retention of MPs within biofilter media. MPs and other suspended particles in runoff enter the ponding zone and are retained within pore spaces in the biofilter media when pore sizes are smaller than the particles. The figure highlights the relationships among media particle size, pore size, and MP retention, and illustrates that media gradation metrics such as curvature coefficient may correlate with MP retention. Stormwater biofilters are widely implemented best management practices for treating urban runoff. Microplastics (MPs) are emerging contaminants in runoff and, because they are particulate in nature, are hypothesized to be removed primarily through physical filtration, with pores created by biofilter media playing a key role in retention. This study presents a field-based evaluation of MP accumulation in biofilter media and examines relationships between retained MPs and measurable media particle- and pore-size characteristics. Twelve media samples were collected from four full-scale biofilters (4–6 years in service) in Southern California. MPs were quantified across five size fractions (20–5,000 μm) and characterized by morphology, color, and polymer type. Media particle size distributions and gradation indices were determined, and pore-size distributions were derived from in situ water retention measurements. MP concentrations ranged from tens to >1,000 particles/g, with fragments in the smallest size fraction (20–63 μm) dominating accumulation. The proportion of pore volume associated with pores <20 μm exhibited the strongest correlation with MP accumulation (r = 0.93, p < 0.001, n = 9), supporting physical straining as the dominant removal mechanism. The curvature coefficient derived from particle size analysis was also associated with MP retention, highlighting its potential as a practical design-relevant factor.
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Microplastics and nanoplastics in stormwater management engineered porous media systems: a systematic review of their sources, transport, retention, and removal characteristics
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This systematic review summarizes how engineered stormwater systems like rain gardens and biofilters capture and retain microplastics and nanoplastics. The findings show these systems can effectively reduce plastic particles in stormwater runoff, which matters because untreated stormwater is a major pathway for microplastics to enter the rivers and lakes that supply our drinking water.
Microplastics in Field-Installed Bioretention Systems: Vertical Distribution and Retention from Stormwater
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Scientists tested rain gardens (special planted areas that filter stormwater runoff) to see if they can trap tiny plastic particles that wash off city streets and parking lots. They found these systems do catch microplastics before they reach rivers and drinking water sources, with most plastic pieces getting trapped deep in the soil layers. This research suggests that installing more rain gardens in cities could help reduce the amount of microplastics that end up in our water supply.
Spatial and vertical distribution of microplastic within the soil of a sustainable urban drainage system
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Scientists tested soil in a "rain garden" system designed to filter stormwater runoff and found that tiny plastic particles (microplastics) build up in the soil, especially near the surface where water first enters. This matters because these urban drainage systems are meant to clean our water, but if they're trapping plastics instead of breaking them down, that plastic could eventually work its way into soil, waterways, or the food chain, raising questions about long-term exposure risks that scientists are still working to understand.
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.
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.
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