Researchers have long known that plastic pollution reaches the ocean. But how much plastic is trapped, and where, before it reaches the ocean is far less understood.
As professors of environmental engineering, geography and environmental studies, and oceanography at Penn State, we recently led studies mapping how microplastics move through bodies of water across Pennsylvania. What we found was striking: Microplastics are nearly everywhere, their concentration in sediment has been doubling every 20 years, and some of the most common types are among the most toxic.
Since the 1950s, global plastic production has doubled about every 20 years. The world now produces over 500 million tons annually – roughly the combined weight of every person on Earth.
Today, more than half of plastic waste ends up in landfills, and less than one-tenth is recycled globally. The U.S. generates over 48 million tons of plastic waste annually, and approximately 86% is sent to landfills. The domestic recycling rate is roughly 5% to 6%, making the U.S. a leading producer of plastic pollution. The rest enters the environment, where it breaks down slowly, potentially over hundreds to thousands of years, into microplastics. These particles range in size from 5 millimeters, which is about the size of a pencil eraser, to 1 nanometer – for reference, a single strand of human hair is about 80,000 nanometers wide.
Microplastics are now found in the air, water, soil, food and living organisms, including humans. Microplastics enter the body through ingestion via what we drink, inhalation and skin contact. Studies have found microplastics accumulating in human blood, the brain and reproductive systems.
To understand how plastics become a health threat, it helps to know how they’re classified in the first place. Plastics are sorted by the resin identification code system into seven categories in the U.S., identified by the numbered recycling triangle on the bottom of containers.
More than 80% of mismanaged plastic waste – plastic that’s littered, dumped or otherwise not properly contained – is estimated to be transported by rivers to coastal environments.
Perhaps you’ve heard of the Great Pacific Garbage Patch? It’s the largest accumulation of ocean plastic in the world, located between Hawaii and California. It’s undeniably huge — but there is still far less plastic in the ocean than models predict there should be, given how much is produced and lost to the environment each year. In fact, millions of tons remain unaccounted for. Our team set out to investigate, in part, the mystery of the “missing plastic paradox.”
Our study focused on Pennsylvania’s Raystown Lake, Conemaugh River Lake and the John Heinz National Wildlife Refuge at Tinicum – the beginnings of the Delaware Estuary.
We collected two types of sediment samples and asked three questions:
Does land use – whether it’s urban, agricultural or forested – predict microplastic concentrations?
Do reservoirs and estuaries trap plastic before it reaches the ocean?
Which plastic types are most common – and most toxic?
Sediment cores – long cylinders of material drilled from riverbeds and lakebeds – gave us a chronological record of what was deposited over decades. We also collected surface sediment samples at intervals along each waterway, moving from densely populated areas near Philadelphia to sparsely populated regions, such as Raystown Lake and Conemaugh River Lake in central Pennsylvania.
Surprisingly, samples collected from a rural watershed in Raystown, a largely forested region in central Pennsylvania, exhibited similar microplastic levels to samples taken from John Heinz National Wildlife Refuge, a watershed located close to Philadelphia International Airport.
On average, we found about 1,125 microplastic particles per pound (2,500/kilogram) of sediment. But concentrations varied widely enough that land use alone can’t reliably predict how much plastic can be found in any given waterway.
Estuaries, which are partially enclosed coastal bodies where freshwater from rivers and streams mixes with saltwater from the ocean, do trap microplastics – but not enough to account for the ocean’s missing plastic.
The most abundant types of plastic we found were polypropylene and polyurethane – both single-use plastics – along with tire rubber. Single-use plastic, or disposable plastic, is any plastic item that is used once – such as bread bags, plastic bottles or straws – and then thrown in the trash.
Finally, we found that microplastic accumulation in Pennsylvania sediment has been doubling roughly every 20 years since 1950, mirroring the rise of global production. We determined this by drilling deep into sediment in reservoirs and estuaries, where layers of sediment build up over time like rings in a tree – older material on the bottom, newer on top. By analyzing the chemistry of each layer, we could estimate when it was deposited and how much plastic it contained.
We dated the sediment layers using radioactive decay, the same basic principle behind carbon dating. One method tracks lead-210, which decays at a known rate, allowing researchers to date each sediment layer. The other uses cesium-137, a man-made isotope whose 1964 peak – tied to nuclear weapons testing – serves as a fixed time stamp in the sediment record. Together, these methods help build a timeline of when each layer of sediment and the plastic trapped in it were deposited.
Here’s where it gets slightly more encouraging. In the most recent sediment layers, which can be dated by depth and accumulation rate to roughly the 2000s, we found a small but measurable decrease in microplastic concentration. That could reflect better recycling rates and broader awareness of plastic waste. It’s modest, but it suggests the problem is at least partially responsive to human behavior.
Beyond recycling plastics, what can you do to decrease your exposure to microplastics and help decrease their spread? You can stay informed about which plastics carry the greatest health risks and check the recycling number on the bottom of containers before you buy. You can also swap out single-use plastic cups, straws and food containers for alternatives, such as glass, stainless steel or unbleached paper.
This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Nathaniel Warner, Penn State; Lisa Emili, Penn State, and Raymond Najjar, Penn State
Read more: Scientists may be overestimating the amount of microplastics in the environment – and the culprit is lab gloves Solving the world’s microplastics problem: 4 solutions cities and states are trying after global treaty talks collapsed Pittsburgh’s air pollution estimated to claim 3,000+ lives per year − and EPA rollbacks aren’t helping
Nathaniel Warner receives funding from National Science Foundation and Penn State's Institute of Energy and the Environment
Lisa Emili has received funding from the United State Geological Survey (USGS).
Raymond Najjar receives funding from Penn State's College of Earth and Mineral Sciences and Penn State's Institute of Energy and the Environment. 
