Hydrodynamic modelling of traffic-related microplastics discharged with stormwater into the Gota River in Sweden

Microplastics (MP) are transported from land-based sources from rivers to marine waters. However, there is currently little knowledge about MP fate from land sources to marine waters. Traffic is estimated to be one of the largest sources of MP; hence, stormwater is expected to be an important transportation route of MP to marine waters. The aim of this study was to investigate the effect of the size and density of tyre wear particles in road run-off on their fate in the Gota River in Sweden using hydrodynamic modelling. The model of the stretch of Gota River, Sweden's largest river, passing through Gothenburg (Sweden's second largest city) and out to the sea, was set up using MIKE 3 FM software. Literature data were used to define the MP characteristics: concentrations in stormwater, prevalent particle sizes, density of MP commonly occurring in road run-off and settling velocities. Results show that higher concentrations of MP are found on the south side of the river, compared with the north side, due to higher annual average daily traffic loads along the south side of the river. The mixing processes in the river and the MP concentrations were generally influenced by the vertical water density gradient caused by saline water from the Kattegat strait. While most MP with higher density and larger size settle in the river, smaller MP with density close to 1.0 g/cm(3) do not settle in the river and therefore reach the Kattegat strait and the marine environments. Further research is needed to describe the fate and transport of microplastics in the stormwater system, including treatment facilities, i.e. biofouling, aggregation, degradation and/or further fragmentation and settling.