Composition and concentrations of microplastics including tyre wear particles in stormwater retention pond sediments

Stormwater is recognised as a vector for microplastics (MPs), including tyre wear particles (TWPs) from land-based sources to receiving waterbodies. Before reaching the waterbodies, the stormwater may be treated. In this study, sediments from six treatment facilities (five retention ponds and a subsurface sedimentation tank) were analysed to understand MP occurrence, concentrations, sizes, polymer types and distribution between inlet and outlet. The concentrations of MPs showed large variations between and within different facilities with MP concentrations of 1,440-72,209 items/kg (analysed by μFTIR) corresponding to 120-2,950 μg/kg and TWP concentrations from <DL up to 69,300 μg/kg (analysed by pyrolysis-GC-MS), with significantly higher concentrations at the inlet compared to the outlet. Polypropylene (PP) was the predominant MP type in terms of number in all samples. TWPs were dominant by mass in most (nine) samples. The relatively low density of PP polymers implies that density might not be the sole factor influencing particle settlement behaviour. Small particles occurred more frequently than large ones; around 70% of the particles detected in the samples were 100 μm or smaller. In summary, this study highlights the occurrence of MPs, including TWPs, in stormwater facilities and demonstrates variations in concentrations depending on sites and locations within the facility.