Microplastics as a potential process tracer for riverbed dynamics in federal waterways

Riverbed sediments are critical transition zones that govern the exchange of water and contaminants, including microplastic particles (MPs), between surface water and groundwater. To improve our understanding of the fate and retention of MPs in these water-saturated, porous systems, we used a freeze-coring technique to obtain undisturbed, water-saturated sediment cores (100 cm in length), even in non-cohesive gravel sediments in two regulated federal waterways. We analyzed the abundance, polymer type, and size of MPs (≥ 100 µm) at 10-cm depth intervals and interpreted the observed vertical MP patterns using microplastics as novel artificial process tracers for sediment dynamics.

In the sandy riverbed of the Main River, we found a mean concentration of 21.7 MP/kg with a depth distribution that remained relatively constant in the upper layers (0–30 cm), decreased in the middle layers (30–60 cm), and markedly increased in deeper layers (60–100 cm). These vertical trends suggest a complex interplay of multiple sedimentary processes and the superimposition of factors controlling riverbed dynamics. In contrast, the gravelly riverbed of the Alpine Rhine showed a low mean concentration of 3.1 MP/kg, despite comparatively high MP concentrations in river water and groundwater, suggesting high MP mobility and limited retention of even large MPs (up to 929 µm) within the coarse sediments.

At both sites, the proportion of small MPs increased with depth; however, the largest MPs were detected in the deepest layers. While this pattern may be explained by particle infiltration processes in the Alpine Rhine, such a mechanism is implausible in the Main River, given its sandy sediment characteristics. Furthermore, low-density buoyant polymers and polymer types with the youngest EPO ages (e.g., PS ≈ 1953, PP ≈ 1954, and PET ≈ 1973) were found in deep, sandy sediments (> 80 cm).

Based on these characteristic vertical MP patterns, we propose using microplastics as a process tracer to infer controlling sediment processes, enhance the geohydraulic characterization of federal waterways, and support the long-term monitoring of sediment relocation in fluvial systems.