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Cave Sediment Stores Microplastic Pollution in Subsurface Environments
Summary
Researchers sampled water and flood sediments in a Missouri limestone cave and found microplastics in all 34 samples, with sediment concentrations roughly 100 times higher than water, indicating that cave sediments act as significant microplastic sinks. Contamination decreased with depth into the cave, pointing to human activity near the entrance as the primary source of pollution entering the karst system.
Microplastics are globally pervasive, yet little is known about their movement and storage in subsurface environments. Karst landscapes are highly permeable and thus particularly susceptible to contamination, enhancing threats to aquifers that are important groundwater resources and caves that are often habitats for vulnerable troglobitic species. To understand the potential transfer and accumulation of microplastics in karst, we assessed their amounts and characteristics in cave stream water and flood sediment deposits. We sampled in Cliff Cave (St. Louis, Missouri, United States), which is a branchwork cave hosted in Mississippian St. Louis Limestone with 1.3 km of mapped passage. The cave is gated and only accessible with consent of the land managers. During a May 2019 flood, replicate water and sediment samples were collected at 8 sites every ~25 m until restricted areas 207 m into the cave were reached. Both sample types were evaluated for microplastic amounts and characteristics. Water was analyzed for geochemical parameters (e.g., pH, major ions), while particle size was measured in sediment. More water samples were collected during low flow at the same sites in April 2022 for geochemical analysis. We found microplastics in all samples ( n = 34) that were mainly fibers (91%) and clear (60%). Water and sediment microplastic concentrations were positively and significantly correlated ( r = 0.83; p = 0.03), but quantities in sediment (389-1672 particle/kg) were ~100 times those in water (4-10 particle/kg). These findings indicate that sediment sequesters microplastic pollution in the cave. Microplastic amounts generally decreased with greater distance into the cave, implying that human activity near the mouth intensifies contamination. Nonetheless, macroplastics were observed in flood deposits, suggesting some plastic contributions from sinkholes. Water geochemical and sediment particle size data collected at a branch indicate at least two distinct water sources to the cave: side passage inputs from a forest and main passage inputs from a neighborhood. However, water and sediment microplastic concentrations did not differ between these sites. Our results reveal that microplastics intrude karst systems and are stored in cave sediment, with the highest pollution levels found at the cave’s main entrance.