Characterizing Microplastic Pollution in Mammoth Cave National Park

Microplastics (MPs) are an emerging environmental concern because of their durability and small size (≥ 1nm to ≤ 5mm), and the collective increase in plastic entering waterways. As a nascent field, few standardized, reliable, and replicable techniques for their extraction, quantification, and identification exist. Furthermore, the majority of studies on MPs have been carried out in the marine environment, though the understanding of MP contamination in freshwater environments is growing. Very little is known about MP transport into karst, though MPs have been found in karst groundwater and cave sediments. Given the potential risks to small aquatic organisms, such as physical blockages, and the indication that microplastics can act as carriers for chemical pollutants, characterizing MPs in cave environments is essential. Here we present an initial assessment of MPs in Mammoth Cave National Park, KY. Mammoth Cave is the longest cave in the world and is designated an International Biosphere Reserve and UNESCO World Heritage Site. Using a peristaltic pump, we filtered water from two streams within the cave through sequential filters of 1 mm, 332 μm, and 102 μm pore sizes. Echo River is more isolated from the park tourist operation, while the River Styx flows close to several tour routes. We also analyzed sediment samples from the Echo River bank and from a highly-touristed room in the cave. For comparison, we sampled the Green River which flows through the park on the surface and will backflow into the River Styx Spring during heavy or extended rainfall, transporting surface water into both cave streams. We identified 308 total MPs from all sites. These were categorized by morphotype as films, fragments, or fibers. The predominance of fibers (between 49-78%) may indicate textiles from tourist clothing and equipment as a primary source of MPs into the cave environment. However, the Green River water samples yielded the most MPs per L (2.09 MP/L), compared to counts from Echo and Styx of 0.42 and 0.8 MP/L, respectively, suggesting that MPs enter this ecosystem without tourist input. The sediment sample’s even distribution between fibers and fragments (49% each) may indicate the impact of particle shape on deposition patterns. Future analysis will determine the likelihood of reverse-flow events introducing MPs into Mammoth Cave.