Scientists found high levels of plastic pollution in a Missouri cave system that has been closed to the public for 30 years, a discovery that suggests many subterranean habitats are contaminated by human pollution even in the absence of humans, according to two new studies.
An expedition team discovered anthropogenic (or human-sourced) microplastics, which are small particles of plastic, at every site they sampled in Cliff Cave, a system of labyrinthine karst passageways near Saint Louis that has been off-limits to the public since a fatal flash flood in 1993. The results reveal that microplastics, and other forms of anthropogenic pollution, may commonly infiltrate underground caves, posing potential risks to the fragile ecosystems that live in them and contaminating human water resources, such as aquifers.
Humans produce hundreds of millions of tons of plastic every year, a demand that generates an escalating flow of plastic waste. This runoff sometimes enters natural environments in large chunks, but it mostly breaks down into tiny particles that can spread far and wide; microplastics have been found in the deepest reaches of the ocean, in the air we breathe, and even in our blood.
Now, scientists led by Elizabeth Hasenmueller, associate director of the Water Access, Technology, Environment and Resources (WATER) Institute at Saint Louis University, report that microplastics, as well as cellulose-based pollutants, are present in the water and sediments of Cliff Cave, even though access to the cave is strictly limited for safety and conservation reasons.
The results suggest that cave sediments “represent a potential source of ‘legacy’ pollution to the water resources and fragile habitats found in these globally distributed landscapes,” according to a new study published in Science of the Total Environment.
“Despite the ecological, economic, and cultural significance of subsurface environments, assessments of microplastic debris in these systems are rare,” Hasenmueller and her colleagues said in the study. “The presence and migration potential of microplastics have been recognized in several types of aquifers. However, direct observations of these microparticles in karst terrains are still scarce.”
“We therefore quantified and characterized anthropogenic microparticles, including synthetic, semisynthetic, and modified natural materials, in collocated water and sediment samples from a cave with limited visitor access to understand pollutant extent, storage, and exchange between these two subsurface compartments,” the researchers added.
Hasenmueller and her colleagues obtained special permits to explore Cliff Cave and extract samples of its water and sediments during day trips that took place in May 2019 and April 2022. As they pushed deeper into the cave, the team took samples every 25 meters (82 feet) until they reached a distance of about 180 meters (590 feet) from the entrance, beyond which access is further cut off by authorities due to flood hazards.
“We found anthropogenic microparticles in all samples that were mainly fibers (91%) and clear (59%),” the team said. “Quantities in sediment were ~100 times those in water. These findings indicate that sediment sequesters anthropogenic microparticle pollution in the cave. Microplastic concentrations were similar among all sediment samples, but only one water sample at the main entrance contained microplastics.”
The researchers also learned that the anthropogenic microparticles were especially concentrated near the mouth of the cave, which suggests that airborne particles might settle at the entrance, contributing to the high concentrations there.
“One potential explanation for this material distribution pattern is deposition of airborne anthropogenic microparticles into the cave due to human activity near the mouth,” the team said in the study. “Atmospheric deposition of these microparticles has been observed in a variety of environments, with proximity to human sources being the main driver of deposited material abundance. Airflow through cave systems is also well-established. We hypothesize that airborne anthropogenic microparticles could be carried into the cave, with dense microparticles (e.g., cellulose) potentially being the first to deposit into the water and sediment at the main entrance.”
Floodwaters may also deposit the pollutants at the entrance as they recede from the passageways, the team said. Floods also appear to sweep pollution into the cave, as evidenced by a potato chip bag the team found enmeshed with leaves and acorns. These occasional deluges increase the diversity of anthropogenic microparticles in the cave system, according to a separate study, co-authored by Hasenmueller, that was published last month in Water Research.
Altogether, the findings offer a rare look inside a cave that is largely off-limits to humans, but which is nevertheless contaminated with our plastic detritus. The effects of anthropogenic microparticles on these otherworldly habitats is still largely unknown, though Hasenmueller and her colleagues hope to better understand that question in the future.
“Our research will provide a baseline for future work related to the transport and fate of microplastics and other anthropogenic microparticles in karst and subsurface environments that can inform mitigation and management plans,” the team concluded in the study.
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