Field monitoring of microplastics in biofilter media: insights into media characteristics and design

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.