UNIVERSITY PARK, Pa. — Microplastics can be found everywhere in nature — from drinking water to clouds in the atmosphere — but scientists have yet to fully uncover how severely they impact the environment or the precise factors driving their buildup. A new study published by researchers at Penn State offers a fresh view of how microplastics traverse and influence watersheds, such as rivers and streams, across Pennsylvania and the world at large.
The team collected sediment samples from several freshwater ecosystems across Pennsylvania, comparing those readings to data previously taken from six sites in New York and New Jersey, as well as countries around the world. Modeling this data revealed more about how pollutants move through freshwater environments and the potential roles humans play in rising levels of plastic content, even in remote areas, but the researchers said the work also raised more questions. They recently detailed their findings in Science of the Total Environment .
Nathaniel Warner , associate professor of environmental engineering, said much is unknown about microplastics — slivers of plastic that can range in size anywhere from one micrometer, a fraction of the width of a single human hair, to five millimeters, about the size of an M&M candy cut in half. What is known, however, is that more microplastics are in the environment every year.
Previous research from Warner’s lab had correlated the growth of these synthetic materials, which the Environmental Protection Agency recently moved to classify as contaminants , in the environment with increasing plastic production over the course of the last century.
“Plastics are in the environment everywhere you look,” Warner said. “Many studies have examined how microplastics interact with marine environments, but there has been much less research studying the concentrations of microplastics inland, and how these plastics move through freshwater environments like rivers and streams.”
This motivated the team to collect sediment samples from three watersheds across Pennsylvania — the Conemaugh watershed in the southwest, the French Creek watershed in the northwest and the Spring Creek watershed towards the center of the state. They collected samples from several points at each site, starting at the headwaters, or upstream source, of the watershed and moving downstream. This data, combined with existing measurements taken from 45 sites in drainage basins of the Oswego, Delaware, Susquehanna and Allegheny rivers, offered a comprehensive view of microplastic content in mid-Atlantic freshwater, Warner said.
The researchers outlined the relationships between microplastic levels found in the sediment samples, visualizing the difference in plastic content from data taken upstream to samples taken downstream. They then compared this information to existing readings taken from 17 other countries, capturing insights on the environmental factors that influence microplastic concentration and movement around the world.
The team found that watersheds located in regions with higher population densities, or adjacent to wastewater treatment plants and agricultural land, exhibited higher concentrations of microplastics. In contrast, regions with more forestry and vegetation coverage tended to display lower concentrations. Freshwater in the mid-Atlantic displayed lower overall levels of microplastics compared to readings taken from other watersheds internationally, including sites in Europe and Asia, but higher concentrations than other North American countries, like Canada or Mexico.
Not all the data answered questions — some raised new ones, Warner said. The team identified that the headwaters of a watershed did not consistently display lower concentrations of microplastic content compared to areas distanced farther downstream, which tend to be in more densely populated areas. Warner explained this suggests local features are more critical in determining microplastic content than previously thought.
For example, samples collected from a rural watershed in Raystown, a forest-heavy region in central Pennsylvania, exhibited similar microplastic levels to samples taken from John Heinz National Wildlife Refuge, a watershed located close to the Philadelphia International Airport. According to Warner, these findings reinforce that the prevalence of microplastics cannot be solely predicted by factors as simple as proximity to population centers, location in the watershed or vegetation density.
“We are still trying to figure out exactly why this is,” Warner said. “Raystown is an area heavily used for camping and recreation, so that could play a role. The data illustrates that there’s a combination of different factors at play, which we hope further studies will help us better understand.”
The team said this study will serve as a basic framework to better understand how microplastics move across the environment, as well as how human activity influences buildup. They plan to further investigate the types of plastic found across Pennsylvania ecosystems and the risks they pose to communities that use these watersheds as a source of drinking water.
“We are asking a lot of questions to better quantify the risks,” Warner said. “What type of plastic is it? How big is the plastic? Is it a risk to humans or the aquatic life in an ecosystem? Although all of these materials are microplastics, some plastics are smaller than others and more harmful, and some types of plastic are much more toxic when ingested than others.”
Additional contributors affiliated with Penn State include Jutamas “Judy” Bussarakum, an environmental engineering doctoral candidate and first author on the paper.
This work was supported by the U.S. National Science Foundation.
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Science of The Total Environment
10.1016/j.scitotenv.2026.181680
Experimental study
Not applicable
Population density and agricultural land cover influence microplastic concentrations in river sediments
19-Mar-2026