Microplastics are found in surprising places—from the deepest oceans to the highest mountain peaks, in the air we breathe, and at Earth’s poles. New research shows they show up even in places untouched by humans for decades. In two recent studies, researchers from Saint Louis University in the United States report high concentrations of microplastics in a cave system in Missouri that has been closed to visitors for 30 years.
Elizabeth Hasenmueller, deputy director of the university’s institute, and her team published their findings in Environmental Science and Water Research journals. They documented significant microplastic levels at Cliff Cave in Saint Louis County, Missouri.
Microplastics are defined as plastic particles smaller than 5.0 millimeters and are found in marine, freshwater, and terrestrial environments. Hasenmueller had previously examined microplastics in river basins such as the Meramec, but shifted focus to subsoil—a field with relatively little prior study.
Research in this area began in oceans due to the obvious pollution there. In recent years, more work has examined rivers, lakes, and other surface freshwater systems, Hasenmueller noted.
“One of the least studied areas is the subsoil. These particles may reach groundwater, a common drinking source, or caves where sensitive ecosystems exist”, the scientist added.
Closed to the public since 1993
Hasenmueller and colleagues chose Cliff Cave because it has been off limits to the public since 1993, removing human presence as a potential contamination source. Their study detected microplastics throughout the cave, with the highest concentrations near the entrance and in the sediments.
“Cliff Cave is regulated by St. Louis County Parks, which helped ensure that any findings would not be influenced by visitors shedding fibers or leaving trash”, Hasenmueller explained.
Flooding appeared to move microplastics through the cave system, carried by water and then trapped in sediments as floodwaters receded.
When floodwaters recede, microplastics tend to accumulate near the cave mouth rather than deeper inside.
Hasenmueller described the dataset: at the main entrance, a large amount of microplastic waste was observed, likely from flood accumulation or airborne particles settling near the opening, she said.
Flood brings microplastics with it
“Floods clearly transport microplastics into the cave. During sampling, a plastic bag tangled with leaves and acorns was found, a sign of surface debris swept in by floodwaters”, she noted.
They found not only increased levels during floods but also that microplastics were nearly 100 times more concentrated in cave sediments than in the water. Water from the cave stream deposited plastics into sediments, and those particles remained even after floods subsided.
Researchers asked how much microplastics are actively moving through the stream versus staying stored in sediments. The surprising result: about 99 percent of the microplastics found in the cave were stored in sediment, with only a small fraction present in the water. As water levels rise during floods, more microplastics appear in the water, but once the flood ends, water-borne concentrations fall again, while sediment-bound plastics persist for years or longer.
Despite isolation from people, Cliff Cave is still influenced by human activity. It lies near residential areas that can contribute microplastics to the system. This aligns with prior SLU research showing that population density strongly affects where microplastics are found in nature. The team suggests practical steps people can take to reduce contributions to the environment.
“It is challenging for individuals to tackle plastic pollution given its ubiquity, but reducing personal use helps. Avoiding certain synthetic textiles can be part of the effort, though that can be difficult for everyday consumers. Society should move away from synthetic clothing as many artifacts found in this cave were textile fibers. Reducing overall plastic production and consumption will also help”, Hasenmueller stated.
“Few studies have examined microplastics in underground ecosystems, so this work provides resource managers with information needed to protect these sensitive habitats from emerging contaminants like microplastics.”
References: DOI: 10.1016/j.scitotenv.2023.164690 and DOI: 10.1016/j.watres.2023.120204.
Note: The environmental department’s contact information was removed.