Delaware River is largest source of plastics pollution in North America, report says
The study is one of several efforts to call attention to the issue

Amid continuing efforts to curb plastics in the Delaware River, a study finds that the river, among all the waterways in North America, is the leading source of macroplastics pollution in the Atlantic Ocean.

The 2021 peer-reviewed study identified the top 1,000 river inputs worldwide that are conduits for 80 percent of the plastic pollution in oceans. The remaining 20 percent of plastic emissions are spread over 30,000 other rivers.

Read about a company’s plans to test a device in the Delaware next year to capture plastics in the heavily trafficked river.

The report was published in the journal “Science Advances” and funded by the nonprofit group The Ocean Cleanup, which is spearheading technologies to rid oceans of macroplastics. The group has a worldwide map on its website with a red circle highlighting the locations of each of the 1,000 river inputs.

In North America, the map features but one red circle: The Delaware River.

How plastics enter rivers and oceans is of growing interest among researchers because the larger pieces of plastics – known as macroplastics (think of a water bottle, for instance) – then break down over time into microplastics.

Read more about a recent study of microplastics in the Delaware River.

Those tiny plastic fragments, which are about as big as a sesame seed, pose a hazard to aquatic life. When organisms and wildlife ingest the tiny pieces, it can fill their digestive systems and reduce their rate of growth because they are not getting the nutrients they need. That also allows microplastics to enter fish that humans consume.

That’s why the effort to capture or reduce macroplastics from waterways – or eliminate them from getting there in the first place – takes on greater importance. That’s where the 1,000-river finding comes in.

The overarching goal of the study, which relied on a mathematical model to identify hot spots and point researchers to places where best to intervene, is to get the greatest return on intervention efforts, said Thomas Mani, a lead river field scientist at Ocean Cleanup.

The model attributes 141 tons of plastics pollution from the Delaware into the Atlantic per year. The model can produce results that are too high or too low, but still “it gives us a directional compass of where to go,” he said.

Rivers that dump the most plastic often have the most vessel traffic. In 2020, nearly 2,200 ships arrived at Delaware River port facilities, according to a report from the Maritime Exchange for the Delaware River and Bay. That traffic was down 6.6 percent from 2019, a decrease attributed in part to the pandemic.

Findings ‘a little bit sensationalized’

The report has raised eyebrows, with some researchers questioning its underpinnings.

Gerald Joseph McAdams Kauffman Jr., director of the University of Delaware Water Resources Center, said he was not surprised that the Delaware has a lot of macroplastics in it “because of all the people living here and the industry that’s here.” 

Delaware River basin is home to more than 7.7 million people, according to Ocean Cleanup figures.

In general, Kauffman said he’s observed that littering is a bigger problem in the East than in the West. “It’s an ethic,” he said. “I have to attribute it to a conservation ethic.”

Plastics did not take on a major role in consumer products until after World War II. Today, “you can’t get away from it,” he said, adding that microplastics are “a little bit more insidious” because they’re so much smaller than macroplastics. Tackling larger plastics is important because once they break down, it leads to smaller pieces that are harder to capture.

Kauffman said scientists tend to be dubious of claims like those in the report that identify the largest river sources of plastics pollution based on modeling. That approach relies on projections, and not necessarily hard data collected in real-world conditions that account for variables such as big storms that can churn up plastics pollution. 

Though the underlying point of the report about the hazards of macroplastics is valid, “it’s a little bit sensationalized,” he said.

Jacob Bransky, an aquatic biologist at the Delaware River Basin Commission, which recently issued a report on microplastics in the river, agreed that it would be useful to back up the findings with field work that went beyond modeling.

“I would say it’s worth looking into,” he said.

What the model relied on

To skeptics who may doubt the validity of the model, Mani said it offered “pure mathematical results of what we know to be important predictors of the pollution but we have not been able to validate it with empirical data.”

That said, researchers tested 136 real-world data collection sites around the world and those results supported the model’s inputs, he said.

“We had some interesting discussions on the team about this,” he acknowledged. “The answers lie in the mechanics and mechanisms of the model.”

Further research is taking place in Thailand and the Dominican Republican with the placement of cameras from bridges to gain real-time estimates of the “flocks of debris” to empirically and physically assess plastics loads in the water, Mani said.

Read more about a new device in the Delaware that will help analyze plastics pollution

The model relies on inputs such as land uses, hill slopes, rain and the distance to a water body. It also drew on the amount of “mismanaged plastic” in a basin, Mani said.

The study said that, overall, it found that plastics “are distributed over more rivers than previously thought by up to two orders of magnitude” and that the 1,000 rivers it identified can contribute to plastics pollution of .8 million to 2.7 million metric tons per year.

The report builds on an earlier one that had previously identified 10 rivers worldwide as top macroplastics polluters. By refining the inputs, a more nuanced picture emerged of the leading 1,000 river sources, Ocean Cleanup said.

Other places, such as New York City, Seattle and Los Angeles, comparatively have many small river outlets, making them more diffuse gateways for ocean pollution, Mani said. Those individual sources contribute individually less, he said.

Mani added that he analyzed an area that took in a 100-square-mile area around Manhattan and Long Island.

That analysis revealed 54 river mouths that collectively could feed as much as 407 tons of plastics into the Atlantic. Those amount to a much more diffuse source of plastics pollution compared with the Delaware, which acts as a singular gigantic funnel for plastics to reach the ocean.

Still, from the perspective of the ocean, it does not matter if macroplastics come from a single-point source or a scattered number of sources. The idea of the model is to try to pinpoint the biggest sources and target them to gain the biggest improvements, Mani said.