Influence of Population Density and Land Use on Microplastics in Urban Stormwater Runoff

Abstract Urban stormwater runoff is an important pathway for transporting microplastics (MPs) from urban environments to marine ecosystems. However, studies investigating how socio-spatial characteristics influence MP abundance and composition remain scarce, particularly in South America. This study evaluated the influence of population density and land use on MP abundance and morphology in stormwater runoff in the coastal zone of Fortaleza, northeastern Brazil. Stormwater samples were collected from 16 outfalls representing drainage catchments dominated by residential, port-industrial, and vacant lots. MP abundance ranged from 47 to 4,168 items L⁻ 1 and was highest in densely populated residential areas, reaching 2,096 ± 1,528 items L⁻ 1 (high-income residential area) and 812 ± 546 items L⁻ 1 (low-income residential area), whereas lower abundances occurred in catchments dominated by vacant lots (156 ± 127 items L⁻ 1 ) and port-industrial land use (102 ± 29 items L⁻ 1 ). Abundance was positively correlated with population density and residential land use. Microfibers predominated (34.7–57.9%), followed by rubber particles (17.3–44.9%) and fragments (12.4–17.9%). Polyethylene terephthalate (75.0%), polyethylene (12.5%), and polypropylene (12.5%) were the main polymers identified. Significant spatial differences in MP shape, color, and size were observed among land-use types. Multivariate analyses identified residential land use, commercial activities, and population density as the main drivers of MP morphological composition. This study provides the first evidence in South America of the influence of socio-spatial characteristics on MP contamination in stormwater runoff, highlighting the importance of these variables for understanding the sources and transport of MPs to coastal and marine ecosystems.