UNIVERSITY PARK, Pa. — Penn State researchers have found that tidal freshwater marshes along the Delaware River may act as major traps for microplastics, capturing large amounts of plastic particles before they reach the ocean. The findings challenge the traditional understanding of where plastic pollution accumulates and were recently discussed on the Institute of Energy and the Environment’s (IEE) “Growing Impact” podcast. 

Through an IEE seed grant, the interdisciplinary team studied how microplastics move through water and sediments in the John Heinz National Wildlife Refuge at Tinicum, a tidal freshwater marsh near Philadelphia. The work addressed a long-standing question in environmental science often called the “missing plastics” problem, which notes that far more plastic enters waterways than is observed floating at the ocean surface. 

“We have estimates of how much plastic is being delivered to the ocean, and there’s simply not that much plastic at the surface,” said Raymond Najjar, the project’s principal investigator and professor of oceanography in Penn State’s Department of Meteorology and Atmospheric Science. “The first place plastics could get stuck before reaching the ocean is in these tidal marshes.” 

Team member Lisa Emili, associate professor of physical geography and environmental studies at Penn State Altoona, said the project began as a baseline assessment of how much plastic enters the estuary, what types are present and where they accumulate. The scope quickly expanded as researchers realized how little was known about microplastics in these environments. 

Using sediment cores up to two meters deep — among the deepest marsh cores analyzed for microplastics in North America — the team reconstructed decades of plastic deposition. The layered samples provide a timeline showing how plastic accumulation has increased alongside global plastic production. 

“When we compare plastic production to what we see in the sediments, the trends line up,” said Nathaniel Warner, associate professor of civil and environmental engineering and project team member. “We’re producing more plastic, and we’re seeing more plastic stored in the environment.” 

The researchers also found that not all plastics behave the same way. Different polymers move, settle and interact with other contaminants differently, influencing potential risks to organisms that ingest them. 

On one hand, Najjar said, marshes may provide a protective service by preventing plastics from moving into coastal waters and the open ocean. On the other, these wetlands are highly productive habitats that support fish, birds and invertebrates. 

“They’re important ecosystems, and now they’re being exposed to these microplastics,” Najjar said. “We don’t yet fully understand what that means.” 

The team said they hope the findings will inform management strategies and policies aimed at reducing plastic pollution, including efforts to limit single-use plastics and develop more sustainable materials. 

“We’ve solved big environmental problems before,” Najjar said. “With good science and good policy, we can change the trajectory of plastic pollution, too.” 

“Growing Impact” is a podcast by the Institute of Energy and the Environment. It features Penn State researchers who have been awarded IEE seed grants and discusses their foundational work as they further their projects. The podcast is available on multiple platforms, including YouTube, Apple, Amazon and Spotify.