Monitoring microplastic transport in the Rhine River

Microplastics (MP) are constantly transported into the oceans via rivers, but riverine transport paths linking MP sources and sinks are insufficiently understood. For a quantification of MP transport in rivers, knowledge on the differential behaviour of MP and natural sediments in aquatic environments is essential. Both underlie the same hydrodynamic forces, namely gravity, buoyancy and drag force but MP exhibits strong contrast to natural sediments in terms of particle density and form enhancing the complexity of MP transport compared to natural sediments. Therefore, the main objective of our research is to gather knowledge on the application and comparison of different sampling techniques to obtain information on the temporal variability of MP in the Rhine River in Germany and identify monitoring techniques to efficiently calculate MP-loads in river systems.

Here we present various monitoring devices to estimate mass-based MP concentrations in the Rhine, and test different algorithms to calculate MP loads, considering the spatio-temporal variability of MP occurrence in rivers. MP concentrations are monitored for one year at Weil am Rhein (at the Swiss-German border), Koblenz and Emmerich (at the German-Dutch border). For the analysis of the collected samples, thermal degradation techniques (Pyr-GC-MS) are applied to obtain mass-based concentrations for polypropylene (PP), polyethylene (PE), polystyrene (PET) and polyvinyl chloride (PVC) for four grain size fractions ranging from 10 to 1000µm.

Total annual MP loads at Koblenz in 2022/23 are 429±125 t derived from the SB and 58±15 t derived from the CFC. Much lower loads derived from the CFC indicate that flow centrifuges might capture large amounts of heavy polymers with densities > 1 g cm-3, but retain only a small fraction of MP particles < 1 g cm-3. CFC sampling is frequently applied for water quality analysis and known for its high sampling efficiency regarding mineral particles. However, monitoring MP using CFCs requires additionally sampling and analysing of the residual water of the CFC, to avoid the loss of low-density polymers.

The comparison of the monitoring using SBs at the three sampling sites reveal significant longitudinal gradients of MP transport along the Rhine. The MP load of PP, PE and PET in 2022/23 increases from 230 t a^-1 at the Swiss/German border to 357  t a^-1  at Koblenz and 460  t a^-1 at Emmerich, with dominating PE loads followed by PP and PS. 

The estimated MP loads are among the highest of the world. Based on a rating analysis MP-load with catchment size, we are able to show that these high loads are linked to the large catchment sizes of the River Rhine compared to othr MP-load estimates.