We investigated plastic pollution in soil-based stormwater bioretention systems (BRS), which are potentially important pollutant receptors and pathways. Our integrated study is the first of its kind, focusing on plastic abundance, size fractionation, composition, and interactions with urban metrics (including housing density and auxiliary stormwater treatment infrastructure) in BRS filter media. Our results revealed that mesoplastic (MEP) and microplastic (MP) concentrations in BRS are comparable with those reported in other stormwater systems (e.g. wetlands) as well as soils in other land use areas (e.g. agriculture). Distributional sampling within the BRS revealed MP abundances do not change with horizontal distance from the inlet to the outlet. However, MEP abundances drastically decreased towards the outlet, indicating plastic accumulation within BRS. This is important because MEPs can breakdown into MPs which can affect BRS function as well as mobilise downstream. Yet our data uncovered more complex mechanisms involved in BRS plastic fate, with composition data revealing that MPs are not simply breakdown products of MEPs but are instead derived from different sources. Composition, morphology and colour analysis confirmed that BRS polymer liners are a key source of MPs and MEPs in soil filter media. Multivariate analysis of the data with urban design metrics showed gross pollutant traps are effective at decreasing MP concentrations in BRS but not as effective at controlling MEPs. Our results point to complex plastic transmission and accumulation pathways in BRS. Interception measures can partially alleviate plastic risk, but more work is needed to elucidate plastic long-term fate in BRS.
Keywords: Microplastics; bioretention systems; soil pollution; terrestrial environment.