The paper is available online now and will be published in the December issue of Science of the Total Environment.

“Few studies examine how microplastics change over time,” said Nathaniel Warner, associate professor of civil and environmental engineering and corresponding author of the paper. “Ours is one of the first to track microplastic levels in freshwater sediments from before the 1950s to the present, showing that concentrations increase in line with plastic production.”

Microplastics are tiny plastic particles that range in size from one micrometer, or 1/100 the width of a human hair, to five millimeters, which is about the size of a pencil eraser. They can come from larger plastics that break down into smaller pieces, or they can be made directly by manufacturers. For this study, the team examined freshwater sediment cores from four watersheds in Pennsylvania: the Kiskiminetas River, Blacklick Creek, Raystown Lake, and Darby Creek.

Contrary to the team’s expectations, the study found no correlation between population density or land use and high levels of microplastics.

“Based on other findings in the literature, what we thought would be important turned out not to be driving forces in microplastic variation between sites, specifically the percentage of microplastics related to developed area and population density,” said Lisa Emili, associate professor of physics. in geography and environmental studies at Penn State Altoona and co-authored the paper.

The researchers also said they were surprised to find that while the accumulation of microplastics increased every decade until 2010, it decreased from 2010 to 2020.

“Although this is a preliminary finding that requires further study, this decrease could be related to increased recycling efforts,” Emili said.

According to the US Environmental Protection Agency, recycling efforts for plastic increased significantly between 1980 and 2010. Although plastic production also increased, the percentage of plastic recycled increased from less than 0.3% in 1980 to nearly 8% in 2010.

Additionally, Raymond Najjar, professor of oceanography and co-author of the paper, said this study could shed light on the “no plastic” paradox. This paradox challenges researchers’ understanding of plastic waste in the ocean, because while estimates suggest that between 7,000 and 25,000 kilotons of plastic enter the ocean each year, only about 250 kilotons are believed to float to the surface.

“This suggests that estuaries, particularly tidal marshes, can capture river-borne plastics before they reach the ocean,” said Najjar, who previously published in Frontiers in Marine Science on simulations of filter estuaries. “This could explain why there is much less plastic floating in the surface ocean compared to what is expected to be there, given the input to the ocean from rivers.”

Warner said these findings suggest there will continue to be increasing amounts of microplastics in both water and sediment as people use more plastic.

“People ingest plastic when they eat and drink and inhale it when they breathe, and the long-term effects are just beginning to be studied,” Warner said. “However, we need to figure out how to release less plastic into the environment and how to reduce consumption and exposure.”

According to Emili, making a study like this successful requires an interdisciplinary team.

“This research showcases Penn State’s broad expertise, bringing together a team from three campuses, five colleges and five disciplines,” Emili said. “We brought together complementary skill sets from our fields of chemistry, engineering, hydrology, oceanography and soil science.”

This research project was originally funded by an Energy and Environment Institute seed grant.

“This funded project really served as an ‘incubator’ for continuing and expanding our work exploring the fate and transport of microplastics in freshwater environments, with a particular focus on coastal locations,” said Emili.

Najjar agreed and said he would like to get a more comprehensive assessment of the capture of river-borne plastics in estuaries.

“We’ve known for a long time that estuaries do a lot of processing of river-borne materials like carbon, sediment and nutrients, and that processing has a big impact on what ultimately ends up in the ocean,” Najjar said. “I think estuaries could work in a similar way for plastics, but we need more than a modeling study and a single core. We need to consider the likely sources and sinks of plastic for a given system, such as rivers, the atmosphere, estuarine sediments and marshes.”

Warner added that he hopes to examine how the composition and types of microplastics have changed over time and assess how the associated health risks have evolved.

In addition to Emili, Najjar and Warner, other Penn State researchers who contributed to the study include Jutamas Bussarakum, lead author and doctoral student in the Department of Civil and Environmental Engineering; William Burgos, professor in the Department of Civil and Environmental Engineering; Samual Cohen, who graduated with a master’s degree in geography earlier this year; Kimberly Van Meter, assistant professor in the Department of Geography; Jon Sweetman, research assistant professor in the Department of Ecosystem Science and Management; Patrick Drohan, Professor in the Department of Ecosystem Science and Management; Jill Arriola, research assistant professor in the Department of Meteorology and Atmospheric Science; and Katharina Pankratz, who graduated with a PhD in civil and environmental engineering earlier this year.

The US National Science Foundation and Penn State’s Commonwealth Campus Center Node Program (C3N) and the Energy and Environment Institute supported this research.