Karst areas are characteristic landscapes formed by the dissolution of soluble rocks, whose hydrology is largely influenced by rapid infiltration through the karst massif. These areas are often hotspots of biodiversity, especially for epikarst and cave fauna. The epikarst, the uppermost layer of the unsaturated zone, plays a crucial role in regulating water flow in karst aquifers. The aim of this study was to investigate the extent of microplastic (MP) pollution, its relationship with precipitation and its correlation with copepod populations in karst areas. The study was conducted between April 2021 and October 2022 in the Postojna–Planina karst area in SW Slovenia at surface and underground sampling sites to determine the pathways of MP pollution from the surface to the depth of the karst massif. The results indicate that heavier rainfall flushes out more MP contaminants from the surface and epikarst environment. The transport dynamics of MP pollution are similar to the dynamics of copepods, which are the baseline organisms for the food chain in caves. One MP sample contained only polyamide particles, which could indicate clothing as a possible source of pollution, but the results are inconclusive. With this study, we provide the first insight into the transport of MP pollution from the surface environment to deeper karst massifs.

The study was conducted in subalpine karst and in lowland karst in Slovenia—Lake Bohinj, Lake Bled, Mt Komna, Lesce, Rižana Spring, and Malni Spring. Both lakes are karst lakes of glacio-tectonic origin, with Lake Bled being one of the largest tourist attractions in Slovenia. The lakes were sampled along different transects. We also collected two sediment samples from Lake Bohinj and one sediment sample from Lake Bled. On the other hand, we sampled two biggest karst springs in Slovenia that are used as sources of drinking water for up to 120,000 people during the peak tourist season on the Slovenian coast. In the FTIR-ATR analysis of the particles, we found mainly PA polymers in all samples, closely followed by copolymers and resins. Lake samples also contained larger amounts of PP, while the spring samples contained more rubber polymers. Of the two lakes, Lake Bohinj was the most polluted, with 0.684 MPs/m³ being the highest concentration. This is surprising because Lake Bohinj is also located in the Triglav National Park and is a less popular attraction compared to Lake Bled. Malni Spring showed a higher mean MP concentration than Rižana Spring (0.000274 ± 0.000448 vs. 0.0000498 ± 0.0000642 MPs/m³, respectively), and the negative-binomial model indicated a significant site effect (p = 0.046), but the difference could be due to the highly torrential nature of Rižana Spring. Future studies would require longer sampling campaigns under different hydrological conditions to better understand the dynamics of the MP transport in karst environments that will provide a stronger scientific basis for sustainable freshwater management, the protection of drinking-water resources, and the development of effective measures to reduce plastic pollution at the catchment scale.