The findings underscore the ubiquity of plastic pollution in a watershed that supplies drinking water to roughly 13 million people and provides habitat for thousands of animal species. The Susquehanna River, which the National Park Service identifies as the Bay's largest tributary, contributes about half of the fresh water entering the Chesapeake.

Faran Savitz, a zero-waste advocate with PennEnvironment, said the results demand immediate attention. "The results of this study should set off alarms for all Pennsylvanians who love the Bay, and the Susquehanna River that feeds into it, and want to protect and restore it," Savitz said. "Microplastics throughout the Chesapeake Bay and its tributaries make it clear we can no longer ignore the increasingly common specter of plastic pollution."

Stephanie Wein, the group's clean water advocate, unveiled the report at a news conference at the state Capitol. She emphasized that the tiny particles, often invisible to the naked eye, still pose risks. "We can't always see microplastics with the naked eye, but that doesn't mean we should ignore them. They still can hurt our environment and our public health," Wein said. "We need our policymakers to treat microplastics as a danger to the environment, and turn off the tap of plastic pollution."

The report adds to a growing body of evidence about plastic contamination in regional waterways and intensifies pressure on state and federal officials to address the sources of plastic waste. For communities across the watershed, the findings raise concerns about the long-term health of a resource that is central to local drinking water, wildlife, and the regional economy.