Influence of seasonal hydrodynamic variations and particle interactions on microplastic particle settling in water columns

Microplastic (MP) particles pose a significant threat to the health of aquatic ecosystems, particularly in inland standing waters such as lakes, ponds, and drinking water reservoirs. During their settlement, MP particles are affected not only by seasonal variations in hydrodynamic forces but also by interactions with other MP particles settling simultaneously, which impact their settling velocity and, in turn, their distribution in the water column. Seasonal variations determine the nature of hydrodynamics in the water column, dictating whether it remains mixed or becomes thermally stratified. In addition to the influence of these seasonal hydrodynamic variations on MP particles, the interactions between MP particles settling in close proximity also play a significant role in shaping their settling behavior. These combined hydrodynamics-MP and MP-MP interactions result in changing the settling behavior of MP particles under different seasonal conditions.To investigate how these interactions affect the residence time of MP particles in a water column, we utilized a numerical simulation framework in OpenFOAM, an open-source computational tool for solving partial differential equations in the field of computational fluid dynamics (CFD). The model, incorporating mass, momentum, and energy conservation equations along with two-way (particle-flow) and four-way (particle-particle) coupling, is calibrated and validated using data from an experimental mesocosm under open-air conditions influenced by real-time meteorological fluctuations. This numerical framework is then utilized to explore (1) how seasonal hydrodynamic variations impact interactions between MP particles, water, and other particles during settling and (2) how changes in MP particles' properties, such as size and density, alter their settling behavior under different coupling scenarios.These potential findings aim to shed light on how seasonal variations in hydrodynamics and particle interactions in standing waters influence the settling velocity and residence time of MP particles in a water column. The outcomes of this study are expected to provide valuable insights for developing targeted strategies to mitigate the risks of MP particles to freshwater ecosystems and human health.