Wind Gusts and Cold Weather, Not Rain, Tracked Microplastic Levels in a Pennsylvania Stream, Study Finds

When people picture plastic getting into rivers, they usually picture rain washing it off streets. A new year-long study of a Pennsylvania stream found something different: microplastic levels rose with colder weather and stronger wind gusts, while rainfall and stream flow did not explain the swings.

The study, “Seasonality and maximum wind gusts correlate with microplastic concentrations in a freshwater stream system,” was published open access in Environmental Monitoring and Assessment on October 9, 2026. The authors are Chloe Mattie, Isabella Ashman, Megan Fork and T. Elliott Arnold of West Chester University in Pennsylvania.

What the team measured

• The stream. Plum Run is a small, mostly urban stream of about 19.3 km in West Chester, Pennsylvania. It flows into the East Branch of Brandywine Creek, which ultimately drains to Delaware Bay.
• A full year of samples. The researchers collected 49 water samples, twice a month, from September 2024 to September 2025.
• How much plastic. Concentrations ranged from 1.2 to 23.0 particles per liter, with an average of 7.6 particles per liter.
• Winter vs. summer. Average concentrations were about 10.6 particles per liter in the cool season (November–March) versus about 4.7 in the warm season (May–September), roughly a twofold difference.
• What the particles were. In a subset examined by electron microscopy, 96% were fibers, 3% films and 1% fragments. Of 30 larger particles checked by infrared spectroscopy (FTIR), 28 were polyester and 2 were polypropylene.

Wind and cold, not rain

The team tested temperature, wind speed, peak wind gusts, wind direction, rainfall and stream flow with three different statistical models. All three picked out colder temperatures and stronger maximum wind gusts as the important predictors of higher microplastic counts. In cross-validation, models built on those variables explained about 80% of the variation. In the main model, rainfall and stream flow were not significant predictors.

That points to the air as a delivery route. The authors estimate that the wind-linked share of the stream’s plastic load matches physically plausible rates of atmospheric deposition, plastic fibers settling out of the air onto the stream and its banks. They say direct deposition measurements are the next step to confirm the pathway.

Why winter?

The researchers point to two likely reasons. First, more laundering and machine drying of heavier synthetic clothing in colder months; the paper cites earlier research finding a single dryer cycle can release more than 500,000 synthetic microfibers. Second, bare winter trees no longer catch airborne fibers on their leaves, so more of them reach the water.

Nanoplastics: the smaller fraction

This study counted particles from 10 micrometers to 5 millimeters. Nanoplastics, smaller than 1 micrometer, sit below that range, so they were not part of these counts. The fibers it did find are the kind of synthetic material that keeps breaking into ever smaller pieces, which is why we follow microplastics and nanoplastics together.

The bigger lesson is about measurement. Twice-monthly sampling over a full year caught swings that a single grab sample would miss, and the drivers were weather and season, not just storms.

Sources

• Mattie, C., Ashman, I., Fork, M. & Arnold, T. E., “Seasonality and maximum wind gusts correlate with microplastic concentrations in a freshwater stream system,” Environmental Monitoring and Assessment 198(11), Article 1173, published October 9, 2026 (open access): doi.org/10.1007/s10661-026-15985-9

Photo: Brandywine Creek at Chadds Ford, Pennsylvania, by Daderot via Wikimedia Commons (CC0). Plum Run drains to the East Branch of Brandywine Creek; this is not the study site.

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