Microplastics in Pristine Caves of the Classic Karst (NE Italy): A First Assessment of Contamination Levels

Data on microplastic contamination in pristine caves are rare, thus limiting our understanding of its pervasiveness in intact underground ecosystems. Here, we quantified microplastics in sediments from two newly discovered, extremely remote caves (Maucci and Luftloch) and compared them with a frequently visited cave (Trebiciano), all three of which are hydraulically connected to the Reka/Timavo River in the Classic Karst (NE Italy). Sediment samples were collected along river-to-slope transects and analyzed for microplastics using density separation and μFT-IR spectroscopy. Average contamination levels were comparable across caves, ranging from 84.7 to 105.9 items kg−1 (dry weight). Fibers and fragments dominated, with similar polymer spectra across sites—polypropylene (PP, 29–42%), polyethylene (PE, 19–27%), and polyethylene terephthalate (PET, 33–46%). Microplastic abundance systematically increased with elevation, up to ~4–12× from river-proximal to high-bench sediments. Polymer-resolved trends reflected density-coupled, flood-driven sorting with low-density PP and PE accumulated on higher benches and denser PET depleted aloft, indicating slackwater retention at flood crests and re-entrainment of lower benches during recession. These findings suggest that indirect riverine inputs of microplastics outweigh direct human contamination and provide the first baseline for pristine Timavo caves—serving as reference sites for background microplastic levels in the Classic Karst and similar karst systems worldwide.