Microplastics in Field-Installed Bioretention Systems: Vertical Distribution and Implications for Retention from Stormwater

Microplastics (MPs) are emerging pollutants of global concern, posing significant ecological and human health risks. They are frequently detected in stormwater systems, with urban runoff serving as a major transport pathway into the environment. Green stormwater infrastructure, particularly bioretention systems (BRS), offers a promising approach to mitigate these risks by filtering and retaining various contaminants. However, the occurrence of MPs in BRS and their capacity in retaining these pollutants remain largely unexplored in the literature, despite being critical for stormwater management and water quality protection. Therefore, this study attempted to examine the occurrence, vertical distribution, and retention of MPs within field-installed BRS, potentially emphasizing their role in reducing MPs transport. Therefore, field samples were collected at depths of 2, 12, and 24 inches below the surface and processed in the laboratory for MP detection and quantification. The results revealed an average concentration of 1,095 particles per kg of dried sediment, with fragments (microplastics shape) accounting for 78.54% of total MPs. Although no clear vertical distribution pattern was observed, initial findings showed that MPs mostly retained at 24 inches, potentially indicating their transport through the media and the retention capacity of BRS (surface and mid layer) in capturing microplastics from stormwater environments. However, there is no direct evidence to explain the mechanisms driving the observed concentrations at greater depths. Integrating BRS into urban stormwater infrastructure provides dual benefits: improved stormwater management and reduced plastic pollution. The preliminary findings of this study highlight the importance of optimizing bioretention design and media composition to improve the removal efficiency of MPs.