Urban stormwater microplastic size distribution and impact of subsampling on polymer diversity

Understanding not only microplastic (MP) concentration but also size distribution, morphology, and polymer profiles is desirable for stormwater, which is an important pathway of entry for MP into the aquatic environment. A challenge is that subsampling is often required for analysis of environmental samples and the impact of subsampling on the stormwater MP concentration determined and the polymer types identified is poorly characterized. To address this, MP were extracted from urban and suburban stormwater, including from green infrastructure. Fourier Transform Infrared microscopy was performed to characterize MP. In addition, particle dimensions and morphology were recorded. Varying the number of 63-250 μm particles subsampled per sample demonstrated the coefficient of variation for concentration (standard deviation/mean) for most samples was <0.3 when 20 particles (0.8-15% of total particles) or <0.2 when 30 particles (1.2-24% of total particles) per sample were analyzed. MP concentrations in the 63-250 μm size class ranged from 15 to 303 MP/L, one to two orders of magnitude greater than observed in previously reported paired samples from the 250-500 or 500-2000 μm size classes. A total of 25 plastic polymer types were observed across samples, more than observed in the large size classes. Spectral signatures of surface oxidation indicative of weathering were observed on most polyethylene, polypropylene, and polystyrene particles, which were the most abundant polymer types. Fragments were the dominant morphology with an average maximum length of 158 ± 92 μm. Overall, these results may help inform subsampling methods and be useful in future exposure assessments for aquatic organisms or design of MP removal technologies for urban and suburban stormwater.