Transport of Traffic-Related Microplastic Particles in Receiving Water

A majority of microplastic particles (MPs) in marine waters are transported with rivers from land-based sources. Traffic is estimated to be one of the largest sources of MPs, hence stormwater and subsequently urban waterways are expected to be important transportation routes of MPs to marine waters. However, there is currently little knowledge about MP fate from land sources to marine waters. The aim of this study is to investigate the transport of traffic-related microplastic particles in a receiving freshwater body using hydrodynamic modelling. A model of a 16 km stretch of the Gota River, Sweden’s largest river, was set up using MIKE 3 FM software. The model builds on data on water flows in the river and its tributaries, water levels and salinity stratification in the Kattegat strait, and meteorological conditions. Concentrations of MPs in stormwater and MP characteristics data, including prevalent particle sizes, density of commonly occurring polymers, and settling velocities were found in the literature. The simulations show that peak concentrations of MPs exhibit a short duration; however, elevated concentrations of MPs may be present for hours after discharge into the river. The simulations indicate that MPs do not settle at the bottom of the river bed; this scenario can be expected for low density MPs including tyre rubber, as well as larger particles (≤ 5 mm) of higher density (> 1 g/cm3). Hence, a high load of MPs from the city of Gothenburg will reach the marine environment. Biofouling and MPs adhering to mineral particles, as has been shown in marine waters, may considerably change the characteristics of MPs and should be considered in future studies.