A Lagrangian model for microplastics transport in SERGHEI

The transport of objects by water flows, particularly in rivers, plays a critical role in natural and environmental disasters, from the movement of vehicles and large objects during floods to the pervasive distribution of pollutants such as macroplastics or microplastics. Recent studies highlight alarming concentrations of microplastics in freshwater systems and even domestic water sources, posing a significant threat to public health due to potential ingestion and associated health risks for humans and animals. Understanding and mitigating these hazards require advanced mathematical modeling and computational solutions capable of capturing the complexity of transport dynamics and environmental interactions.

This work presents the development and integration of a Lagrangian model to study the spatial and temporal evolution of microplastics transport in water flows. Material elements representing microplastic particles are modeled as discrete entities, whose transport is driven by hydrodynamic forces computed using physical coefficients, i.e. a kinematic approach. The model is driven by an Eulerian framework based on the nonlinear Shallow Water Equations (SWE), which govern the fluid flow dynamics. This approach offers a robust basis to compute flow evolution while providing detailed insights into particle transport mechanisms.

The new Lagrangian Particle Tracking module (LPT) is integrated into the SERGHEI (Simulation Environment for Geomorphology, Hydrodynamics and Ecohydrology in Integrated form) model. SERGHEI facilitates comprehensive investigations into particle dynamics by accounting for key processes such as advection and dispersion, and those unique to microplastic transport such as deposition, flotation, degradation, and biofouling. The microplastic model provides valuable information on the behavior of microplastics in rivers and hydrological regions, which facilitates the identification of possible solutions to reduce their concentrations. The use of a 2D hydrodynamic model offers greater computational efficiency compared to other models based on 3D hydrodynamic approaches. This efficiency is enhanced by the implementation of both microplastic and hydrodynamic models in a high-performance computing (HPC) framework, allowing realistic simulations of complex scenarios. Moreover, the integration of processes specific to microplastic transport ensures realistic time evolution of particle positions. However, further experiments are essential to validate, refine and improve the accuracy of the model. Future work will extend the model to simulate larger debris, including vehicles, waste containers, and boulders, thus broadening its applicability for environmental risk assessment.