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Urban stormwater infrastructure as a microplastic superhighway: a critical review of transport dynamics, modelling, and mitigation across pavements and drainage networks
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Every time it rains, tiny plastic bits from tires, road markings, and litter wash off streets and into storm drains, and this review of over 50 studies finds these systems carry way more plastic pollution than we thought, sometimes six times more than what comes out of treated wastewater. The catch: while filters and green infrastructure like wetlands can catch larger plastic pieces, the smallest particles (under 100 micrometers) slip through, and those tiny fragments are the ones most likely to end up in waterways, soil, and potentially our food and water supply.
This review examines the transport, fate, modelling, and mitigation of Microplastics (MPs) in urban stormwater infrastructure, with emphasis on pavements, runoff pathways, micro-drainage, and macro-drainage systems. Following a systematic review approach, more than 1000 records were screened and approximately 50 core studies were retained when they addressed urban stormwater or drainage-related MP transport with adequate methodological reporting; marine-only studies and biological-effect studies without direct relevance to transport processes were excluded. The evidence shows that stormwater systems function not merely as passive conduits but as dynamic reactive transport systems with temporary storage, where particle mobilisation, sedimentation, resuspension, and temporary retention regulate MP export. Road surfaces, especially high-traffic areas, are major reservoirs of tyre wear, road-marking, atmospheric, and litter-derived particles that are rapidly mobilised during rainfall. Conventional grab sampling may underestimate MP loads, which in some cases exceed treated wastewater effluent loads by up to six-fold. Drainage structures such as manholes can immobilise up to 17.3% of near-neutrally buoyant particles, while biofouling and aggregation may shift buoyant polymers from wash-load to bedload. Mitigation systems, including permeable pavements, bioretention, wetlands, and technical inserts, can achieve high removal of coarse MPs, but performance declines for fine particles below 100 µm. The review highlights the need for standardised flow-proportional sampling, physically informed modelling, and treatment-train strategies targeting both surface sources and in-network storage.
More Papers Like This
Microplastic pollution characteristics, transport, and control in urban stormwater runoff
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Every time it rains, tiny plastic bits from broken-down trash, car tires, and even air pollution get washed off streets and into our soil and waterways—this review pulls together what scientists currently know about how that happens. While some tools like rain gardens and wetlands can catch larger plastic pieces before they spread further, we still don't fully understand what happens to the smallest plastic fragments (nanoplastics) or how they might combine with other pollutants to cause harm, which matters since these tiny particles can end up in the water and food we consume.
A plastic storm: The role of stormwater in the microplastic load of an urban river system
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Stormwater systems in urban areas were found to be a significant source of microplastics entering river systems, transporting particles from roads, buildings, and other surfaces during rainfall events. A fine-resolution study measured the types and quantities of microplastics carried by stormwater in an urban catchment, finding concentrations comparable to inputs from wastewater treatment plants. Better capture and treatment of urban stormwater is needed to reduce this largely unregulated pathway for microplastics reaching rivers and the ocean.
Microplastic sampling strategies in urban drainage systems for quantification of urban emissions based on transport pathways
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Researchers developed and applied microplastic sampling strategies across an entire urban municipal catchment under both dry and wet weather conditions, finding that wastewater treatment plants remove over 96% of microplastics but still emit 189 kg per year, while wet-weather emissions from high-traffic subcatchments reached 1,952 grams per population equivalent per year, far exceeding dry-weather levels.
A plastic storm: The role of stormwater in the microplastic load of an urban river system
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A study of an urban river system found that stormwater runoff is a major source of microplastic pollution, transporting particles from roads, buildings, and other urban surfaces directly into waterways during rain events. Stormwater contributed a significant fraction of the total microplastic load reaching the ocean, comparable to inputs from wastewater treatment plants. Better management of urban stormwater could meaningfully reduce microplastic pollution in rivers and coastal waters.
Urban Stormwater Runoff: A Major Pathway for Anthropogenic Particles, Black Rubbery Fragments, and Other Types of Microplastics to Urban Receiving Waters
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Researchers quantified microplastics in urban stormwater runoff from 12 watersheds surrounding San Francisco Bay and found concentrations ranging from 1.1 to 24.6 particles per liter, much higher than typical wastewater treatment plant effluent. The study suggests that stormwater runoff is a major and underappreciated pathway for microplastics and other anthropogenic particles to enter urban waterways.
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