Predicting how microplastics travel through the environment

KEY INSIGHTS 

  • Urban stormwater is a major microplastics pathway: Stormwater systems in cities move large amounts of microplastics into nearby rivers, making them a critical but often overlooked source of freshwater plastic pollution.
  • Stormwater ponds capture microplastics: Stormwater management ponds retain a large share of microplastics, but significant amounts still flow downstream, especially during high-flow events.
  • Fragments dominate pollution loads: Most microplastics found in stormwater were small plastic fragments rather than fibers, and these fragments made up the majority of both particle counts and mass.
  • Land use matters more than population size: Residential density and impervious surfaces were stronger predictors of microplastic emissions than total population or broad land-use categories.
  • Weather and design influence transport: Rainfall, wind, and pond design strongly affect how microplastics move, settle, or are released, showing that climate and infrastructure work together to shape pollution outcomes.

WHY THIS MATTERS TO WATERSHED MANAGEMENT, MONITORING, AND POLICY

  • Improves stormwater management decisions: By quantifying how much plastic enters, is retained, and is released from stormwater ponds under different conditions, the research helps practitioners evaluate the true performance of existing infrastructure, compare design options, and make informed decisions about where upgrades or additional controls are most needed.
  • Supports targeted mitigation: Identifying links between residential density, impervious cover, and specific plastic types helps planners focus source-reduction efforts where they will have the greatest impact.
  • Enables realistic expectations for infrastructure: Even high-performing stormwater ponds may release large amounts of microplastics when inflows are high, highlighting the need for system-level planning rather than single-site solutions.
  • Provides a path to watershed-scale estimates: The findings support scaling up local measurements to estimate microplastic inputs to major waterways such as the Grand River and ultimately the Great Lakes.
Amir 2026 graphical abstract

RESEARCH PROCESS 

This research examined how microplastics move through urban stormwater systems, focusing on stormwater management ponds in five catchments in Kitchener, Ontario. Long-term field sampling was combined with hydrological modeling and sediment surveys to measure how many microplastics enter ponds, how many are stored in sediments, and how many are released downstream over the course of a year. This approach moved beyond short-term sampling to produce some of the first catchment-scale inventories of microplastic emissions from urban areas.

The study showed that microplastics accumulate differently in water and sediments. Small plastic fragments dominated both environments, while fibers were less common but contributed disproportionately to mass. Polymer type and particle size played a key role in whether microplastics were retained or released, with some materials consistently escaping treatment. Pond design, sediment grain size, and impervious land cover were all strongly linked to accumulation patterns.

To place these local findings in a broader context, the team also analyzed data from 107 stormwater catchments worldwide using machine-learning models. This global analysis confirmed that hydrology, watershed characteristics, and human activities are key drivers of microplastic loads and highlighted the lack of standardized particle size definitions as a major barrier to comparing studies. Together, the field and modeling work provide a clearer picture of how microplastics travel through urban environments.

RESEARCHER PROFILES 

Fereidoun

Prof.Fereidoun Rezanezhad
Research Professor, Ecohydrology Research Group
Expert in hydrological modeling and contaminant transport, guiding integration of field data with watershed-scale analyses.

Philippe Van Cappellen

Prof. Philippe Van Cappellen
Professor of Earth and Environmental Sciences, University of Waterloo: Expert in environmental and aquatic chemistry, specializing in contaminant fate and transport, sustainable water management, pollutant legacies, and urban water systems.


Amir

Amir Reshadi
PhD Researcher, Ecohydrology Research Group
Leads field sampling, data analysis, and catchment-scale modeling of microplastic transport and retention in urban stormwater systems.

Hang

Hang Nguyen
MSc Researcher, Ecohydrology Research Group
Conducted microplastics extraction, characterization, and analysis of stormwater pond sediments and water samples.

KEY PUBLICATIONS 

Reshadi, M. A. M., Rezanezhad, F., Kaykhosravi, S., Nguyen, T.H., Slowinski, S., Shahvaran, A., Alcott, L., Puopolo, M., Lam, B., and Van Cappellen, P. (2026). Yields and stormwater loads of microplastics in five urban stormwater catchments (City of Kitchener, Ontario, Canada). Water Research 298, 125804. https://doi.org/10.1016/j.watres.2026.125804

Reshadi, M. A. M., F. Rezanezhad, F., Shahvaran, A. R., Ghajari, A., Kaykhosravi, S., Slowinski, S., and Van Cappellen, P., (2025). Asessment of environmental and socioeconomic drivers of urban stormwater microplastics using machine learning. Scientific Reports 15, 6299. https://doi.org/10.1038/s41598-025-90612-0

Reshadi, M. A. M., Rezanezhad, F., Kaykhosravi, S., Nguyen, T.H., Slowinski, S., Shahvaran, A., Alcott, L., Lam, B., Puopolo, M., and Van Cappellen, P., (2025). Stormwater Ponds as Partial Barriers to Microplastic Pollution in Urban Stormwater Catchments. Manuscript under internal review.

Nguyen, T.H. (2024). Controls on Microplastics Accumulation in Stormwater Ponds. MSc Thesis, University of Waterloo.

Comprehensive reports and datasets on microplastic concentrations in stormwater pond water and sediments representative of Canadian urban environments. Contact for access.