Monitoring Microplastics in the River Rhine: From method development and −application towards Microplastic load calculation

Microplastics (MP) (1 μm to 5 mm) enter riverine environments in various ways. Point sources (e. g. industrial plants) can be differentiated from diffuse sources (e. g. erosion). Once MP has entered river systems, it underlies hydrodynamic processes and is finally transported towards the oceans. Due to fragmentation, MP particles decrease in size during transport. MP particles are highly heterogeneous considering density and shape, which makes spatially and temporally high resolved sampling difficult and thus hampers the quantification of annual loads in rivers. Within this thesis, we conducted a spatially and temporally high resolved monitoring in the River Rhine with the aim to quantify an annual MP load at our study site, Koblenz. With a literature review we exposed that most studies present their results by means of particle number instead of particle mass. Also, many studies did not consider spatial nor temporal aspects within their sampling design. For the first time, we presented a compilation of global annual load values within river systems, partly using innovative methodological approaches to convert MP items to MP mass. Hereby, a correlation between catchment area and annual MP load could be detected. Global loads showed a large range ( 50 % were noticed). Nevertheless, the calculated MP loads do fit well into our compilation of global MP load values. An extensive quantification of retention efficiencies for several polymer types and particle sizes (SB) or an additional sampling step for the clearwater of the CFC for low-dense MP particles could lead to more robust MP concentration values in the future. Depth-distributed sampling campaigns with a filter cascade and filter nets revealed strong density-dependant vertical gradients within the water column. These gradients were even apparent within the smallest particle size classes and thus were larger than a theoretical determination based on the Rouse number would have suggested. Both, filter cascade and filter net showed a positive correlation between river discharge and MP concentration. This correlation allows for a first speculation on MP sources within the River Rhine: The main source of MP seems, in congruence with suspended sediments, to be a diffuse entry which rises with rising water level. In contrast, a constant entry from a point source would lead to lower concentrations during flood events due to dilution. Within this work we could point out that extensive research is needed considering the understanding of different sampling devices and their sampling efficiencies. First approaches for an improvement of these limitations have been shown. Still, a combination of sampling techniques is necessary to cover the portfolio of potential issues regarding MP research. For a more precise quantification of annual MP loads, standardisation of procedures on all working steps is urgently needed.