Analysis of macro plastic transport processes in large rivers using GPS tracking and numerical simulations

Given the longevity of plastics and the yet incalculable effects on the biota in our environment, addressing plastic transport processes in fluvial systems is of emerging importance. Rivers are considered the main entry pathways for plastic into the world's oceans, yet there is still very little research in this area in particular. In Austria, the PlasticFreeDanube project focused on the problem of macroplastics in the Danube. Although Austria has very good waste management, a large amount of plastic still ends up in the national park downstream of Vienna. One of the most important questions in the project was therefore where the plastic comes from, how it is transported in the river and where it is deposited.

 

To improve the process understanding, numerical simulations were carried out on different scales. A particle tracking tool was implemented and adapted to the buoyancy of macroplastic items. The results of the numerical model were then blended with field data to quantify plastic deposition along the shoreline and in the floodplain. Furthermore, field data from GPS tagged plastic items were used for validation of numerical model results and to increase understanding of transport pathways within the system.

 

The modelling results clearly show that floating macroplastic particles interact with hydraulic structures, where the highest accumulation potential was observed in the middle groyne field within row of groynes. The comparison of macroplastic concentrations based on modelling and sampling results shows that floodplains act as filters during flood events. All the macroplastics that were tracked in the field experiment remained on the river’s side on which they were released, indicating that the main input appears to be on the right-hand side. The items mostly stranded on riprap of the bank protection and groyne fields, and the average distance to stranding was found to be 10.4 km. It was also shown that although the Freudenau hydropower plant can retain some of the plastic, a certain amount also passes downstream. The outcomes of this study may lead to a reduction of future collection efforts for macro plastics in riverine environments. The findings can also help to adapt hydraulic engineering structures in a way that facilitates removing more plastic from rivers.