In October 2025, the Philosophical Transactions of the Royal Society A: Mathematical, Physical, and Engineering Sciences journal published a study led by Haney, a PhD student in ecology and evolutionary biology at U of T. It measured an estimated 522 billion microplastic particles and 20,754 macroplastic items transported from Toronto’s Don River to Lake Ontario annually, based on data collected from 2022–2023.
Microplastics are plastic fragments smaller than five millimetres in size, and macroplastics are fragments larger than five millimetres. The transported particles in the study are equivalent to 36,000 kilograms of microplastic waste — which is around the same weight as 24 passenger cars — and 160 kilograms of macroplastic waste.
The team even found bizarre items like “safes that were dumped off bridges into the river,” according to Haney.
Plastic escapes into the natural landscape due to improper handling and disposal of waste from commercial sites, the shedding and breakdown of discarded plastic products, and even from tire dust. Eventually, these particles make their way to the river via rain and urban water runoff.
We all contribute to this issue through the use and improper discarding of plastic-based fabric clothing (for example, polyester, acrylic), plastic hygiene and beauty products (for example, wet wipes, cleansers), single-use plastics (for example, bags, water bottles), and more.
Many of these products shed microplastics through their use, which make their way into our wastewater and are dispersed into the air via dust. Haney’s study shows that all these microplastic particles wash away into Lake Ontario, leading to a major plastic pollution problem.
Currently, there is limited knowledge on how storm events affect the location of where plastic ends up in urban rivers, and how its retention location and transport pathway — where it ends and how — may be correlated to the sizes of these pollution particles. Haney’s study begins to fill in this knowledge gap, which is a starting point for more researchers to continue understanding the effects of storm events on our plastic waste.
Insights of the study
Haney explained that his colleagues expected a lot of trash in the Don River. “It’s a very urbanized watershed. It drains half of the City of Toronto, so it’s expected that there’d be garbage, but comparing it to two other river networks that we also looked at in this project, it was just a lot more. So that was surprising, the amount, especially during the storm events… We have to use these video cameras to record the surface water in one of our sites at Taylor Creek — it was just like a sheet of garbage.”
Haney’s study sampled four sites in Ontario: one near the mouth of the Don River, one mid-basin, one urban tributary site — a stream that flows into the Don River — and one suburban tributary site. The sites were sampled through June and October 2022, then processed and analyzed over the next two years.
The group found that larger plastic particles tend to remain in rivers, before breaking down into microplastics, which are then transported into receiving waterbodies, such as lakes and oceans. This study seems to contradict previous beliefs that the majority of macroplastic emissions into the environment outweigh that of microplastics — but this can be explained by the fact that the larger particles remain behind — and, thus, can be measured easily — whilst the smaller ones are transported out.
Moreover, as the speed of water flow increases during storms, the microplastic composition shifts from fibre-dominated to rubber-dominated, showcasing that storms can mobilize various forms and sizes of plastic pollution from landscape into rivers.
A combination of sewer overflows, wastewater, urban stormwater runoff, and agricultural runoff pushes microplastics into the receiving waterbodies. The latter also further adds soluble pollution (for example, excessive nutrients causing algae blooms and a lack of oxygen for other aquatic organisms, leading to dead water), which exacerbates the issue.
Although microplastics follow a similar transport pattern as fine organic matter and sediments (in other words, leaf litter breakdown), few studies actually quantify both macro and microplastic distribution from storm events and the streamflow sustained between precipitation events in urban rivers. The current study shows that the influence of stormflow on plastic pollution along rivers is an overlooked area of research.
These results provide significant insight into our understanding of what environmental care policies and protocols we need to consider.
Next steps
Haney’s study provides difficult-to-obtain data due to how the influx of plastic mass fluctuates annually, and differences between rivers and storm events. Haney said that “a lot of our knowledge [about plastics and their transport] is [through] modelling, but there’s not a lot of on-the-ground measurements that are super detailed for microplastics and macroplastics. So, that’s where… the greatest contribution of this paper is — just providing that raw data that can be used in global models.”
Within recent years, many modelling approaches — based on limited plastic observations in river systems — have been designed to calculate plastic export from rivers to oceans; however, these models have yielded wide ranges from about 6,000 metric tonnes to 2.75 million metric tonnes of annual plastic pollution.
This large uncertainty is due to different assumptions about where the plastic ends up, generalizing different sizes or types of plastic fragments into uniform data, and limited high-quality source data to guide the parameters of the model. These limited models highlight the need for improved data collection methods, which Haney’s study accomplished.
With the new insights from this recent study, Haney hopes to build a more accurate model of microplastic travel in Toronto. However, he emphasizes the importance of on-the-ground raw data collection, rather than the constant reliance on models.
This study provided new insights into how Toronto contributes to plastic pollution as well as how the composition of these particles varies along regions of the Don River with storm flow changes. However, there are still a lot of unknowns to explore.
What the study was not able to accomplish was seasonal sampling, especially focusing on how spring snow melt can affect the export of these emissions into reservoirs. Furthermore, its strong focus on the Don River is a great starting point, but more data from other rivers with different population densities and urbanization scales can contribute to existing knowledge.
The next steps of the study are to fully quantify and characterize the microplastics and macroplastics within the Don River watershed at baseflow and stormflow amongst different habitats. This model can then be used to predict the amount of plastic that is lost and retained in Lake Ontario over a year during those specific weather events.
Ultimately, why does this all matter? As Haney explained, “Microplastics are found pretty much everywhere in the environment… they’re also associated with health effects, at least for organisms like fish, microbes, invertebrates, that type of stuff. So, there’s concern that maybe these particles could also affect us because we’re finding them in our bodies.”
Plastics are also often coated with additives to help them resist environmental degeneration, such as from temperature, oil, or water. These additive coatings carry toxic compounds; for example, Bisphenol A (BPA), a substance often found in plastics, is estrogenic, meaning it can mimic the estrogen hormone in the body.
Although more research needs to be done to understand the full extent of the effects of plastics on human health, studying how these particles can travel and interact with natural environments can help scientists gather crucial insights on where plastics travel to before and after contact with the human body.
