Influence of drainage infrastructure and land use on microplastic contamination in urban watersheds

Stormwater runoff is a significant source of microplastic (MP) contamination in tributaries. However, the role of drainage infrastructure (e.g. surface drains and pipes) and land use in the transport of MPs into freshwater systems is largely unknown. Here, we quantify and compare MP concentrations deposited below surface drains and pipe stormwater outfalls in two distinct urban watersheds: one characterized by agricultural and forested land cover, and the other dominated by residential and commercial development. We compared MP morphologies and abundances upstream and downstream of 20 stormwater outfalls to determine the influence of outfall type on MP accumulation. Three surface water and sediment samples were collected at each sampling location along with measurements of chemical and physical water quality parameters. Microplastics were identified using Nile Red staining, a rapid and effective screening technique suitable for large-scale environmental assessments while minimizing resource-intensive analyses. Fragments were the most abundant MP morphology and were the primary form driving differences in total MP concentrations among the sites. Flow velocity had the greatest impact on plastic concentrations in the sediment, and the accumulation of MPs in surface water increased as water depth decreased. The highest MP concentrations were found at pipe outfalls in both surface water and sediment compared with surface drains. The watershed with more agricultural and forested landcover had higher MP concentrations downstream of both types of drainage infrastructure than the more developed urban watersheds. These findings highlight the importance of considering both land use characteristics and drainage infrastructure when assessing MP input into freshwater systems, providing valuable insights for decision-making on monitoring, retention, and remediation strategies.