Journey of pathogen-loaded macroplastics in a gravel-bed mountain river: Hydrological and geomorphological controls in a probabilistic framework.

Riverine litter poses a significant environmental challenge, with macroplastics being particularly problematic due to their persistence, potential to fragment into microplastics, and ability to act as vectors for pathogens, facilitating the spread of harmful bacteria in river ecosystems. In this study we employed field mapping, microbiological analysis, tracer experiments using PET-bottles, and probabilistic methods (survival analysis and hazard functions) to investigate litter composition, pathogen colonisation, and the influence of riverbed morphology and flow conditions on macroplastic transport and deposition in a mountain gravel-bed river (southern Poland). Field mapping revealed that macroplastics accounted for 86% of all litter in the Białka River, with foils making up 50% of plastic waste. Mapped macroplastic items were typically deposited in riffles, along the margins of gravel bars, and within woody debris. Microbiological analysis of macroplastic-associated biofilms revealed the presence of pathogenic bacteria, including antibiotic-resistant strains. The most prevalent were faecal streptococci (Enterococcus faecalis/E. faecium). The presence of E. coli, Staphylococcus spp., and Klebsiella spp. was also detected. Colonisation patterns varied by material type, with rigid macroplastics supporting more diverse microbial communities. Tracer experiments showed that 17% of bottles were trapped under average flow conditions, increasing to 60% under low flow. Most bottles were deposited on gravel bars, with smaller quantities retained by woody debris. Deposition was higher during low flows and exhibited greater spatial variability, influenced by microtopography and geomorphological features, whereas higher flows produced more predictable, flow-driven patterns. Our results indicate that gravel bars serve as temporary storage zones for macroplastics during downstream transport. The novel application of survival analysis and hazard to quantify macroplastic transport in river systems offers a promising framework for future research on floating macroplastic transport and for developing effective riverine plastic-pollution management strategies.