From Monitoring to Action: A New Sampling Strategy and Retention Modules for Plastics in Floodplains – Tested at the Vjosa River

Plastic pollution in soils and floodplains is a critical but understudied issue, with scarce field data on abundance, transport and remediation. Rivers are key pathways transporting plastics of all sizes, yet long-term and large-scale monitoring data remain scarce. To preserve and restore the ecosystem services floodplains provide, they must be protected from plastic pollution and its negative consequences for humans and nature.We conducted a large-scale monitoring campaign along the entire course of one of Europe’s last undammed rivers (Vjosa, Albania). Its course is little anthropogenically influenced and allows unique insights into macro- and microplastic hotspots in floodplain. At these hotspots, novel retention modules developed at the HTWD can be deployed as nature-based solution to prevent plastic pollution in floodplains.We tested a novel transect-based sampling/monitoring approach for macro- and microplastics to gain insights on plastic transport and accumulation along the Vjosa River. We considered vegetation succession zones and geomorphology, both representing flood dynamics. Data collection included vegetation species and distribution, high-resolution digital elevation models through photogrammetric drone flights to resolve floodplain topography and infer associated flood dynamics, macroplastic and sediment sampling for microplastic analysis. We analysed macroplastics with a portable FTIR as well as ATR-FTIR. We processed sediments with a validated in-house protocol consisting of density separation (CaCl2, density: 1.45 g/cm³), and Fenton oxidation to extract microplastics, followed by DSC (Differential Scanning Calorimetry) and TED-GC/MS (Thermal Extraction-Desorption GC/MS) for mass-based microplastic analysis. Preliminary results show macroplastic accumulation in floodplain depressions and the standing woody succession zones, likely liked to vegetation structure. We expect similar trends for microplastics and overall higher abundances from upstream to downstream, where sedimentation in general increases.In parallel, we tested novel wooden retention modules (30x30x10 cm) as a nature-based solution filled with different substrates and vegetation densities of willows and grass species. Laboratory flooding experiments with microplastic spiked water (low-density polyethylene and polyamide, 500 – 800 µm) demonstrated polymer type-specific retentions with higher rates for PA (mean: 91.4 %) than LDPE (mean: 18.4 %). The vegetation density and diversity proved to be one of the major factors in retention efficiency. Therefore, the retention modules are a promising solution to minimize microplastic input in floodplain soils.Our study delivers one of the first comprehensive datasets on plastic pollution in a near-natural European river system, integrating vegetation, geomorphology and high-resolution elevation models. By combining large-scale monitoring with mitigation testing, we advance reliable approaches to assess and reduce plastic pollution across the geosphere. This not only directly supports conservation and management of the Vjosa River National Park and UNESCO Biosphere Reserve, and contributes rare field data to the global database of plastic pollution, but also highlights the ecosystem services of natural floodplains in plastic pollution retention, fosters their preservation, and demonstrates pathways to substitute their functions through retention modules where they are degraded. In doing so, our approach provides a concrete, nature-based solution that can be scaled to other river systems, thereby contributing to tackling the global plastic crisis.