Microplastics Transport in Porous Media and Modeling in Bioretention Systems: A Review

Bioretention cells are a type of stormwater nature-based solution (NbS) consisting of engineered porous media that efficiently trap microplastics. However, the transport and removal mechanisms and related clogging impacts of microplastics in bioretention cells remain unclear. No mechanistic models of microplastics transport and fate in bioretention or other NbS currently exist. This review focuses on the mechanisms of microplastic filtration in porous media and assesses the ability of existing stormwater engineering tools to model microplastic concentration reduction in NbS. We found that the stormwater tools we evaluated were not well-suited to model microplastic concentration reduction via bioretention. We identified 100 articles up to March, 2025, of which 75% evaluated microplastic transport in lab-scale columns and 7% explicitly defined colloids in comparison to microplastics. Forty-nine percent of column studies used 1–10 μm polystyrene spheres, which is only a small portion of all environmentally relevant microplastics: stormwater microplastics are a complex suite of contaminants with a wide range of particle sizes (1–5,000 μm), polymer types, and weathering degrees. We recommend a combination of two models to evaluate the full spectrum of microplastic sizes. Currently, the best-suited models we recommend adapting for this purpose are HYDRUS and a single-use model based on surface caking and depth-wise filtration model. Future research should include more column studies to parametrize these models with the full range of polymer types and morphologies of urban stormwater microplastics. We also recommend to identify which types of microplastics behave like colloids before incorporating colloid-scale mechanisms in porous media transport models. A sensitivity analysis of environmental factors should be performed to prioritize their inclusion as input parameters.