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Microplastics Found in Cave Closed to Visitors for 30 Years

A picture taken from within the of the mouth of a cave, looking out to sea.
Credit: Devon Janse van Rensburg / Unsplash.
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Researchers in Missouri have found high concentrations of microplastic in Cliff Cave, a cave system in Saint Louis County that has been closed off to visitors for the past 30 years. The extent of the microplastic contamination is detailed in two new research papers, published in Science of the Total Environment and Water Research.

This discovery highlights the pervasiveness of microplastics in the environment, the researchers say, while also shedding light on how these plastic particles spread. 

Exploring subsurface microplastic contamination

Microplastics are extremely small particles of plastic debris, normally defined as being less than 5 millimeters in length, that can be found across aquatic and terrestrial environments. Plastic microbeads, such as those used in the cosmetics industry, are a well known example. However, microplastics can also be formed through the breakdown of other plastic products in nature, such as plastic bags deteriorating in ocean currents, or synthetic fibers being released during clothes laundering. 

Microplastics are an emerging contaminant in modern society, with much still to learn about their environmental impact. Their presence in the marine environment has been a strong focus for early microplastic research, but researchers are keen to examine their effects in other settings.

“A lot of research has been focused on surface water settings,” said Elizabeth Hasenmueller, Ph.D., associate professor of Earth and Atmospheric Sciences at Saint Louis University and associate director of the WATER Institute at SLU. “Microplastics research initially started in the ocean because of the highly visible problem of large plastic pollution in this environment.”

“However, one of the most understudied areas in this field relates to what’s happening to the subsurface in terms of microplastic contamination,” she continued. 

“These particles could be getting into groundwater, a common drinking water resource, or caves, where fragile ecosystems exist. During the last few years, my research team has been focused on trying to understand microplastic prevalence and transport in these subsurface environments.”

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In their latest research, Hasenmueller and her team characterized the extent of microplastic contamination in Cliff Cave, a cliff system near St. Louis that has been closed to the public since 1993. 

“Part of the reason we picked Cliff Cave is because St. Louis County Parks regulates access to the cave. We knew if we found microplastics in the cave, it’s not going to be because somebody has just hiked back into the cave and shed fibers from their clothing or left food wrappers.”

The researchers were also interested in examining how microplastics are moving through the cave system, especially during periods of flooding. 

“We know for sure that floodwaters are bringing microplastics into the cave because as we were traversing the cave passages and collecting samples, we found a plastic chip bag that was intertwined with leaves, acorns and other flood debris from the surface,” she recalled.

Flooding and air deposition are introducing microplastics to caves

The researchers took water and sediment samples from eight sites in the cave during a flood, using Fourier transform infrared spectroscopy (FTIR) and visual inspections to confirm the presence of microplastics. 

The team found that microplastics were present throughout the cave, with the highest concentrations found closest to the mouth of the cave. This distribution pattern is likely to be a result of microplastic-contaminated floodwaters entering the cave, with the microplastic particles being left behind as the floodwater recedes, the researchers say. It is also possible that microplastic particles suspended in the air could be blown into the mouth of the cave where they become trapped.


The team also compared the volume of microplastics found in the cave water samples versus the sediment samples. They found that 99% of the microplastic debris recovered was stored in sediment; only a very small fraction of the microplastics recorded were still actively circulating in the cave water. 

“We think what is probably happening is that, after the cave floods, particles in the water are deposited into the sediment,” Hasenmueller explained. “As the waters recede, that material remains in the cave sediment, potentially for decades or longer. And, when the water level goes down, microplastic concentrations in the water are much lower.”

While the effects of these microplastics on the local environment around Cliff Cave have yet to be determined, the researchers note that microplastics in general can disrupt fragile subsurface habitats and water resources. Bats, amphibians and other animals are already known to be moving freely through Cliff Cave, and would be at risk for exposure to these plastics. 

Microplastics reveal human “fingerprint” on the environment

FTIR analysis confirmed that textile microfibers were the most commonly observed microplastic in the cave, which highlights the impact human activity can have on nearby natural environments. 

Though the cave has been closed to visitors for 30 years, the researchers point out that Cliff Cave is located very close to a residential area, and that this could be contributing microplastics to the system. 

Hasenmueller highlights ways individuals can lower their output of microplastics.

“It's hard for us as individuals to deal with plastic pollution because of the pervasiveness of these materials, but it helps to be mindful of your personal plastic use,” Hasenmueller said. “Individuals can avoid buying plastic materials like synthetic textiles used in clothing, but doing so presents challenges to everyday consumers.”

“On a larger scale, we, as a society, could move away from synthetic clothing, because a lot of the debris that we found in this cave was synthetic fibers from textiles. And, of course, reducing our overall plastic production and consumption would help as well,” she added.

In light of her team’s new findings, Hasenmueller wants to see more research being done to investigate the scale of microplastic contamination and its impacts in other subsurface habitats.

“Understanding what level of threat microplastics pose to the unique and rare animals that only inhabit cave systems is really important,” Hasenmueller said. “Only a handful of studies have assessed microplastics in these types of underground ecosystems. So, our work provides resource managers with the information they need to be thinking about to protect these fragile habitats from emerging contaminants like microplastics.”

References: Baraza T, Hasenmueller EA. Floods enhance the abundance and diversity of anthropogenic microparticles (including microplastics and treated cellulose) transported through karst systems. Water Res. 2023;242:120204. doi: 10.1016/j.watres.2023.120204

Hasenmueller EA, Baraza T, Hernandez NF, Finegan CR. Cave sediment sequesters anthropogenic microparticles (Including microplastics and modified cellulose) in subsurface environments. Sci Total Environ. 2023;893:164690. doi: 10.1016/j.scitotenv.2023.164690

This article is a rework of a press release issued by Saint Louis University. Material has been edited for length and content.