Cliff Cave in St. Louis County has been mostly closed to human visitors for 30 years, in part to protect a resident population of endangered Indiana bats. Despite the relative absence of humans, St. Louis University researchers say they recently found microplastics in the cave.
“We wanted to understand, if there's not somebody walking through the cave system, shedding fibers from their clothing, which can be made of plastic materials … do plastics get into these systems?” said Elizabeth Hasenmueller, associate director of SLU’s Water Institute.
The answer is yes. The team discovered microplastics in large numbers in both water and sediment in the cave.
While scientists have conducted more research on microplastics in easily accessible environments like rivers and oceans, there hasn’t been much work looking at microplastics in cave ecosystems, especially not ones that are relatively undisturbed by humans, said Hasenmueller.
The St. Louis University team recently published two studies from its work in the cave. One, published in Science of the Total Environment, looked for microplastics throughout a section of the cave. Hasenmueller expected to find the particles because they are so prevalent on the surface, but she was surprised to find a much higher concentration of microplastics in the cave’s sediment than in the cave’s water.
“We think that these sediments represent a long-term storage of microplastics on the subsurface, and they could be there for decades, or even longer,” Hasenmueller said. “And so the potential for these systems to be storing what we call legacy plastic below ground is really interesting and has potential environmental consequences in that, even if we were to stop adding plastic to the system today, we don't know how long that plastic would take to break down or to leave the cave system.”
The team’s other study, published in the journal Water Research, involved an automatic sampler outside the cave that could “periodically slurp up water samples” to find out how changing water levels affect the amount of microplastics in the cave. The team found increased levels of microplastics during floods, which could become an even bigger problem as climate change leads to more intense rainfall and flooding.
“The more pulsing of water you have to the cave system, the more potential there is to pick up microplastic debris and have it enter into the cave system,” Hasenmueller said. “And so if you have more pervasive flooding, if you have higher magnitudes of flood, you could be having more debris enter into the cave system.”
It was flooding 30 years ago that put Cliff Cave in the news when six people died after severe thunderstorms rolled in while a group from St. Joseph’s Home for Boys was exploring the cave, which was another reason for its closure.
The cave is home to multiple other bat species that are not endangered, as well as cave salamanders and isopods. Other researchers have found when animals consume microplastics, it can clog digestive systems. The microplastics can also absorb pollutants, which could then harm animals that eat them.
“In cave systems in particular, we have a lot of endangered species,” Hasenmueller said. “These habitats are very unique and very fragile because if you're a little cave critter, or if you're a cave fish or something like that, you can't easily go to another habitat, you're kind of isolated in your cave system. And so the microplastics may elevate the risk to these organisms.”
The researchers found many different types of plastics in the cave, but the majority of what they found were tiny plastic fibers, which Hasenmueller said is likely from clothing. People can try to avoid single use plastic and nonnatural fibers in clothing, but she said this problem will have to be addressed at a societal level, through something like government action.
“One of the challenges with plastic pollution is that as an individual, it's very hard for us to manage the problem, because plastic is everywhere,” she said.
Hasenmueller said the work in Cliff Cave has brought up other topics she is now hoping to explore. She would like to find out if there are differences in microplastic levels below ground in urban areas compared to suburban and rural areas. Scientists have also found sunlight breaks plastic down, so Hasenmueller wonders if microplastics will stay around longer in the darkness of an underground cave.
