Transport and Retention of Microplastics in Gravelly Riverbed Sediments: A Polymer-Specific Column-Scale Study

Microplastic particles (MPs) are ubiquitous in aquatic environments, with rivers playing a key role in transporting them from terrestrial sources to marine systems. Field studies indicate that MPs can be transferred from surface water into riverbed sediments and groundwater, often showing polymer-specific patterns, yet the processes governing transport and retention remain poorly understood. Here, we investigated vertical, polymer-specific mobility in gravel (d10/d50/d90 = 5.60:6.60:7.80 mm) sediment columns under environmentally relevant hydraulic conditions simulating bank filtration. Reference MP fragments (178 ± 140 μm)─irregular, secondary, nonspherical particles─of polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) with densities of ρPP = 0.9, ρPS = 1.04, ρPET = 1.35, and ρPVC = 1.39 g cm–3 were applied individually, and the sediment was sampled at multiple depths up to 50 cm for MP detection. Subsequent analysis revealed that most MPs were retained within the upper 10–15 cm of the sediment, with a decreasing vertical mobility into greater depths following the order PET > PVC > PS. These findings indicate a relevant role of density in MP retention at low to medium flow velocities of 1.5–16 m d–1. Overall, the results provide mechanistic insight into polymer-specific MP transport and retention in gravelly riverine sediments.