Tiny black shreds make up nearly all of 7 tons of microscopic plastic particles found floating in Charleston Harbor at any given time.
The source was a mystery for researchers, until recently. It's shreds worn off braking tires.
Meanwhile, plastic bags marketed as eco-friendly take as long or longer than standard plastic bags to break down in the same waters.
Those two disturbing findings, made by researchers at The Citadel, come as state legislators are dealing with a bill to make it illegal for cities and counties to ban plastic bags. House Bill 3529, carried over from the 2017 session, is back in front of the Legislature and could be taken up again as soon as Tuesday.
It would prevent local governments from banning plastic bag use by retail stores.
The Citadel findings are sure to fuel to the debate.
"By all accounts, the vote will be a close one," said Coastal Conservation League spokeswoman Caitie Forde-Smith.
The water samples for both the tire shred study and earlier research that calculated the 7 tons were drawn from the same places: near bridges in the estuary and Winyah Bay. At least 98 percent of the plastics found were the tire shreds.
In samples drawn from under the Arthur Ravenel Jr. Bridge over the Cooper River, the number of microscopic tire shreds was so large that the researchers are still counting them.
All the findings are from ongoing research led by Citadel biology professor John Weinstein.
Microplastics in the waters broken down from litter or discards that take untold years to dissolve. Sooner or later, a portion of that 7 tons of waste gets eaten by marine critters, such as shrimp, and works its way up the food chain.
In lab tests, shrimp went at the tire shreds "like popcorn," Weinstein said. On top of that, other research has shown the plastics pick up toxins in the water before they are eaten.