Numerieke modellering van dispersie van plastic in aquatische milieus

Plastic pollution in aquatic environments is a global concern, with microplastics (MP) occurring at varying concentrations across all environmental compartments. Growing evidence of their widespread presence and potential adverse effects has intensified efforts to quantify MP in the aquatic environments. Numerous field observations have provided valuable snapshots of MP pollution in freshwater, estuarine, coastal and marine environments. However, the dynamics of MP in aquatic systems are too complex with high spatio-temporal variability to be fully understood from field observations alone. Numerical modelling tools - many adapted from those developed for other types of natural particles - have been widely applied to simulate MP transport and fate. Yet a key challenge remains unresolved: the large diversity of MP properties is still rarely represented in these models. The collection of MP from field observations has been used in the literature to demonstrate the continuous distribution that exists in the MP properties. The existence of continuous distribution allows for the adaptation of a novel modelling approach, Population Balance Equations (PBE), which offers the possibility of continuous distribution modelling. This key feature of PBE is utilised in the present work to address the MP diversity in numerical modelling. The existing knowledge of sediment transport mechanisms is equally valid for MP transport. However, the response of the MP particles to the flow differs from that of sediments, mainly due to the wide and continuously evolving properties of MP. Within the PBE modelling framework, the varying response of MP is captured by the evolving distribution function. Two key properties of MP, namely size and settling/rising velocity, are explored in the PBE models. The numerical solutions of PBE for both cases are presented in this work and applied to the large-scale environment. The PBE model is validated against a conventional discrete classes (DC) model applied to an idealised estuary case. In a 2-dimensional depth-averaged (2DH) model, advection-diffusion-settling dynamics are modelled with a moment-based closure to derive the deposition term. Results suggest that the PBE reproduces DC-like size distributions while limiting the transported scalars and limiting the computational costs. The 2DH model is further extended to a real-world estuarine case, integrating a size-based PBE with process models for deposition, resuspension and sediment-MP interactions. Flux decomposition is performed to separate advective transport from tidal pumping. The tidal flux dominates instantaneously, although the residual advection flux is persistent and drives the net MP export to the sea. MP accumulation aligns with the estuarine turbidity maximum (ETM) observed in cohesive sediments. The 3-dimensional model with settling/rising velocity-based PBE reveals the dynamics of MP transport in the vertical direction in the Shelf Sea case. Sinking and floating microplastics (SMP and FMP) are represented with two distinctive and non-interacting PBE models. SMP accumulates in the near-bed layer and along residual-flow pathways of the North Sea. FMP accumulates as surface plumes shaped by dominant wind directions. Long-term MP storage reservoirs are consistent with fine sediment storage in the deep trenches. The Shelf Sea model results are also used to understand the sea-boundary MP fluxes to nested estuarine models, linking shelf and estuary scales. The present study ignores the wave drift and beaching of MP which play an important role in the coastal areas close to the shoreline. Simulations reveal significant MP accumulation in seabed sediments, tidal flats and deep-sea trenches. Future work should integrate burial dynamics, benthic releases and near-bed hydrodynamics to refine long-term MP fate predictions.