Cave sediment sequesters anthropogenic microparticles (including microplastics and modified cellulose) in subsurface environments

Anthropogenic microparticles (of synthetic, semisynthetic, or modified natural compositions) are globally pervasive, yet little is known about their distribution and storage in the subsurface despite their potential threats to belowground environments. We therefore assessed their amounts and characteristics in water and sediment from a cave in the United States. During a flood, water and sediment samples were collected at 8 sites every ~25 m along the cave passageways. Both sample types were evaluated for anthropogenic microparticles, while water was assessed for geochemistry (e.g., inorganic species) and sediment was evaluated for particle sizes. Additional water samples were collected during low flow at the same sites for further geochemical analysis to determine water provenance. We found anthropogenic microparticles in all samples that were mainly fibers (91 %) and clear (59 %). Both suspected (identified visually) and confirmed (identified with Fourier transform infrared spectroscopy; FTIR) anthropogenic microparticle concentrations were positively correlated between the compartments (r ≥ 0.83, p ≤ 0.01), but 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. Treated cellulosic microparticle abundances in both compartments generally increased along the cave stream's flowpath, which we suspect is due to both their flood and airborne deposition. Water geochemical and sediment particle size data collected at a branch indicated at least two distinct water sources to the cave. However, anthropogenic microparticle assemblages did not differ between these sites, implying minimal variation in sourcing across the recharge area. Our results reveal that anthropogenic microparticles intrude karst systems and are stored in sediment. Karstic sediment consequently represents a potential source of "legacy" pollution to the water resources and fragile habitats found in these globally distributed landscapes.