Microplastics in Stormwater: Sampling and Methodology Challenges

Stormwater runoff is a significant source of microplastics to surface water. This study addresses challenges in the sampling, treatment, and characterization of microplastics in existing stormwater control measures across various regions in the United States. Stormwater sediment samples were collected via traditional stormwater sampling approaches for particulate and inorganic contamination with portable automatic samplers, analyzed using visible and fluorescence microscopy with Nile red as a selective stain, and subsequently confirmed through Raman spectroscopy. The inclusion of laboratory and field blanks enabled the identification of contamination at key steps during sample handling. The results reveal that the filtration process is a significant source of laboratory contamination, while the sampling process itself could be a primary contributor to overall sample contamination. Additionally, it was found that using green fluorescence as the sole emission wavelength may underestimate MP quantities, as some particles emit fluorescence exclusively in the red spectrum. Raman analysis revealed interferences caused by pigments and additives in plastics, along with challenges evaluating particles in the low micron range (≤10 microns), which complicates a comprehensive analysis. The findings of this study emphasize the importance of implementing strong quality assurance and control measures when assessing the levels of microplastics in the environment, including sample collection, processing, and analysis.