Field Monitoring of Microplastics in Urban Runoff: Treatment Efficiency of Biofilters

This study presents the most extensive and first region-wide data set to date on microplastic (MP) treatment by stormwater biofilters (also known as biofiltration/bioretention systems), based on paired influent and effluent flow-weighted composite samples collected across seven southern California installations. MPs were quantified and characterized by five size fractions (20 μm to >500 μm), morphology, color, and polymer type. Across 18 storm events, the median MP event-mean concentration (EMC) reduction was 72%, and for the 14 events with runoff volume data, the median total MP load removal was 93%. EMC reduction was generally consistent across size fractions, ranging from 68% to 100% for different size fractions. Physical filtration (i.e., straining) was identified as the dominant removal mechanism for MPs in biofilters, supported by the close correspondence between the MPs and all particles remaining after extraction. The alignment in occurrence across a wide concentration range (linear regression R²=0.91 overall and R² > 0.75 for individual fractions) indicates that all particle counts may serve as a practical surrogate for estimating MP occurrence and biofilter treatment in urban runoff. Quality assurance and quality control results further demonstrate that automated samplers can be effectively used for stormwater MP monitoring when appropriate blanks are collected and best practices are applied.