Microplastics in a highway stormwater system: concentrations and variability during rainfall

Summary

Abstract Microplastics (MP) have been reported in a wide range of environmental matrices, including seawater, freshwater, stormwater, sediments, soils, and air. Stormwater from urban areas has proven to be a major pathway for the transport of MP into aquatic environments, especially for tyre wear particles (TWP). This study investigates the presence of MP and tyre-related rubber polymers in stormwater collected during rainfall events at a Swedish highway. The specific objectives were to: quantify the concentrations of 13 MP polymers, including three tyre-related rubber polymers, in the particle size fraction 10–1000 μm in highway runoff; assess the relationships between MP concentrations, total suspended solids (TSS), volatile suspended solids (VSS) and metals. Total MP concentrations, including tyre-related polymers, were 2.4–4200 µg/L in the stormwater samples. The most frequently quantified polymers were polyethylene, polypropylene, and tyre-related polymers styrene-butadiene rubber and polyisoprene, in agreement with other studies on the occurrence of MP in runoff. The tyre-related polymers accounted for 41 ± 24% of the total MP concentrations in the samples where they were quantified, confirming TWP as an important source of MP pollution in this type of environment. The tyre-related polymers were quantified in lower concentrations than in other studies, indicating that concentrations could be underestimated due to the analytical size cut-off at 10 μm. No significant correlations were observed between MP concentrations and TSS, VSS or metals. This study contributes to the scientific understanding of MP emissions from road runoff and provides knowledge for the development of methods to treat road runoff from pollutants. Further research should address the development of standardised sampling techniques and sample preparation methodologies for pyrolysis-gas chromatography / mass spectrometry that can reliably identify and quantify smaller MP (< 10 μm) and are robust against interference from complex environmental matrices.