Modelling microplastics in bioretention systems: A review

Urban stormwater is both a major source and a mode of transport for microplastics in the environment. Black-box field studies have found that bioretention cells, a type of low impact development (LID), consisting mainly of engineered porous media, are effective systems for capturing microplastics. However, the mechanisms of how microplastics are transported, removed from stormwater, and fragmented within bioretention cells are unclear. Additionally, the impacts of microplastics on the hydrology and microplastic removal performance of bioretention cells remain unclear. This study assesses tools to model microplastic removal using LID and reviews the literature on the mechanisms of microplastic filtration in porous media. None of the evaluated stormwater tools were found to be well-suited to model microplastic removal via bioretention. We identified 74 studies that, at times, misinterpreted “all microplastics” as colloids. We recommend using a combination of two models to evaluate the full spectrum of microplastic sizes. Currently, the best-suited models are HYDRUS and the cake-layer model described in Li and Davis (2008a), which can be adapted for this purpose. More column studies are needed to parametrize these models that use the full range of polymer types and morphologies of urban stormwater microplastics.