ABSTRACT
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.
ABSTRACT
Microplastics are emerging contaminants ubiquitously distributed in the environment, with rivers acting as their main mode of transport in surface freshwater systems. However, the relative importance of hydrologic processes and source-related variables for benthic microplastic distribution in river sediments is not well understood. We therefore sampled and characterized microplastics in river sediments across the Meramec River watershed (eastern Missouri, United States) and applied a hydrologic modeling approach to estimate the relative importance of river discharge, river sediment load, land cover, and point source pollution sites to understand how these environmental factors affect microplastic distribution in benthic sediments. We found that the best model for the Meramec River watershed includes both source-related variables (land cover and point sources) but excludes both hydrologic transport-related variables (discharge and sediment load). Prior work has drawn similar and dissimilar conclusions regarding the importance of anthropogenic versus hydrologic variables in microplastic distribution, though we acknowledge that comparisons are limited by methodological differences. Nevertheless, our findings highlight the complexity of microplastic pollution in freshwater systems. While generating a universal predictive model might be challenging to achieve, our study demonstrates the potential of using a modeling approach to determine the controlling factors for benthic microplastic distribution in fluvial systems.