Microplastic Presence, Aging, and Potential Sources in Urban Runoff in a Large Piedmont Metropolitan Area: Polymer-Type-Specific Analysis

Urbanization, characterized by the expansion of impervious surfaces, significantly alters watershed hydrology. During precipitation events, these surfaces generate urban runoff, a hotspot of microplastics (MPs) that pose potential threats to human health and ecosystems. Substantial field surveys have been undertaken to explore the dynamics of MPs in urban runoff within small catchments (typically below 1000 km²). Nevertheless, identifying potential sources of MPs in large regions remains a challenging task. In this study, we provide a polymer-type-specific exploration of MPs in a large metropolitan area located on a piedmont alluvial fan, spanning over 16 000 km². A total of 20 759 MPs (size ranging from 20 to 500 μm), representing 11 polymer types, were identified using laser direct infrared (LDIR) chemical imaging spectroscopy. Different sampling types, including impervious surfaces, roof drainage, and soil slopes across both hilly and plain areas, were included in this study. Multivariate statistical analyses, including partial least squares path modeling, showed that the abundance of MPs was influenced by precipitation characteristics, topography, and degree of rurality. Furthermore, multiple lines of evidence from abundance, polymer type, oxidation, and fouling characteristics suggested contributions from both local mobilization of soil MPs and atmospheric deposition. Results from this study are encouraging for the source identification of MPs in large areas.