Researchers find high concentrations of microplastics in cave water and sediment

Researchers from Saint Louis University have discovered substantial quantities of microplastics in a Missouri cave system that has been off-limits to tourists for 30 years. They detail their findings in two recent studies.

Significant amounts of microplastic were discovered in Cliff Cave in Saint Louis County, Missouri, according to research by Elizabeth Hasenmueller, Ph.D., associate professor of Earth and atmospheric sciences and associate director of the WATER Institute at SLU, and her team. Their findings were published in the journals Science of the Total Environment and Water Research.

Students on the team were able to do field research and publish their findings thanks to the project, which was inspired by Hasenmueller's research group and the Karst Hydrology course.

Plastic particles smaller than 5.0 millimeters are known as microplastics, and they are present in freshwater, terrestrial, and marine habitats. Prior to focusing on the subsurface, which has received very little investigation, Hasenmueller examined microplastics in river systems, such as the Meramec River basin.

Surface water settings have been the subject of a great deal of research, according to Hasenmueller. Due to the obvious issue of significant plastic contamination in this ecosystem, research on microplastics first began in the ocean. More studies of rivers, lakes, and other surface freshwater systems have been conducted recently.

"On the other hand, the subsurface's contamination by microplastics is one of the least researched topics in this sector. These particles may be entering caverns, which are home to delicate ecosystems, or groundwater, which is a popular source of drinking water. My research team has spent the last few years attempting to comprehend the occurrence and transit of microplastics in these subterranean habitats."

Cliff Cave was chosen by Hasenmueller and her colleagues for their research because it has been off-limits to the public since 1993, making it impossible for them to link any microplastic contamination they may have seen to human activity there. Microplastics were discovered everywhere across the cave, but the greatest amounts were in the silt and close to the entrance, according to their investigation.

St. Louis County Parks controls access to Cliff Cave, which is one of the reasons we chose it, according to Hasenmueller. "We were aware that if microplastics were discovered in the cave, it wouldn't be because someone had just returned via hiking and left food wrappers or clothing fibers behind."

According to Hasenmueller and her team's research, microplastics are traveling through the cave system more often when there is flooding. Microplastics are carried by water, and when flooding happens, the extra water enters the cave and carries additional microplastics with it.

An increased variety of microplastics in the cave water was also a result of flooding. Microplastics were probably deposited in greater quantities close to the cave's opening than farther within the cave as those floodwaters subsided.

"We were unsure of what to anticipate from the dataset, but we discovered that the main entrance of the cave has a significant amount of microplastic debris, possibly from flooding or from microplastic particles floating in the air and being deposited close to the cave's opening," Hasenmueller said. We discovered a plastic chip bag entangled with leaves, acorns, and other surface flood debris while exploring the cave and gathering samples, proving beyond a doubt that floodwaters are introducing microplastics into the cave.

In addition to the fact that floodwaters raised the amount of microplastics in the water, Hasenmueller and her colleagues discovered that the concentration of microplastics in the sediment was over 100 times greater than that of the water in Cliff Cave. The stream water in the cave carried microplastics into the substrate, where they stayed long after the floodwaters subsided.

According to Hasenmueller, "We were trying to figure out what fraction of the microplastics is being stored long-term in the cave's sediment versus what's actively moving through the cave stream right now." "We discovered something pretty intriguing: the majority of the microplastics were detected in the silt. Thus, only a very little portion of the microplastic trash that we discovered in the cave—99 percent of it—was in the water. It was all preserved in the sediment."

Hasenmueller continued, "You see higher abundance and diversity of microplastic particles in the water as the water levels go up during a flood." "We believe the most likely scenario is that water-borne particles are deposited into the sediment following the cave floods. That material stays in the cave sediment when the waters recede, maybe for several decades or more. Additionally, the quantity of microplastics in the water decreases significantly as the water level drops."

The cave is sealed off from humanity, but their presence is felt nonetheless. Cliff Cave's proximity to residential areas that may be introducing microplastics into the system is consistent with earlier studies conducted by SLU's WATER Institute, which indicated that the largest determinant of microplastic distribution in nature is population density. According to Hasenmueller, there are steps that individuals may take in light of these results to reduce the quantity of microplastics they might be introducing into the ecosystem.

The ubiquity of these materials makes it difficult for us as individuals to combat plastic pollution, but being aware of your own plastic use can assist, according to Hasenmueller. "People can choose not to purchase plastic products, such as synthetic fabrics used in apparel, although doing so poses difficulties for regular shoppers. Given that synthetic fibers from textiles made up a major portion of the trash we found in this cave, society as a whole could consider doing away with synthetic apparel on a bigger scale. Of course, cutting back on our total use and manufacture of plastic would also be beneficial."

Microplastics have an impact on the species that lives in Cliff Cave in addition to perhaps harming the cave ecosystem. A sensitive ecosystem that is used by bats, amphibians, and other creatures for unrestricted movement within the cave may be disturbed by microplastics. The environmental impact of microplastics is also a concern for humans, and Hasenmueller urges further study to prevent the pollution from getting worse.

It's critical to comprehend the extent of the harm that microplastics provide to the uncommon and unusual species that live only in cave systems, according to Hasenmueller. Few research have evaluated microplastics in these kinds of subterranean habitats. Thus, our research gives resource managers the knowledge they need to consider in order to safeguard these delicate ecosystems from newly discovered pollutants like microplastics."