Size-dependent microplastics transport in a sediment rich environment

Microplastics (MPs) are often in marginal quantities as compared to the sediment concentrations in a natural aquatic environment. To this end, the assumption can be made that MPs transport in the aquatic environment is influenced by the sediment transport behaviour, due to similarities in their transport processes. In a sediment-rich environment like an estuary, sediments along with the MPs are deposited and resuspended according to the tidal cycles. To numerically model these processes, a two-dimensional depth-averaged model is developed with two classes of sediment namely – cohesive and non-cohesive sediment. The model takes into account sediment-microplastic interaction i.e. sediment-controlled deposition and resuspension. To account for the large size diversity of MPs (from micrometers to millimeters) the Population Balance Equations (PBE) method is adapted. The advantage of the PBE framework is that, the particle population is not fixed to a certain size class, rather the dynamic evolution of the full-size distribution is tracked. The model results show that the dynamics of transport in the Scheldt estuary (Belgium and the Netherlands) vary from the riverine to the estuarian and the coastal areas. The intertidal areas in the estuary act as a sink for the MPs, hindering their further transport towards the sea. The turbidity maxima of the cohesive sediments are found to be hotspots of suspended MPs. Plastic concentration values from the model is comparable to the data from tidal measurement campaigns. Despite several field campaigns in the area, the processed data so far proves to be insufficient for a full validation of the model. The first-of-its kind PBE model of the MPs transport sets an example to take into account other plastic diversities such as shapes and densities Also see: https://micro2024.sciencesconf.org/557554/document