A Review of the Applications of Microplastic Particle Tracking Models in Estuarine Environments

Microplastic pollution globally has become a major issue, sparking interest in many technologies to combat and manage it. Numerical particle tracking models (PTMs) provide a cost-effective and flexible approach to predict microplastic accumulation zones in marine environments. Numerical transport modeling in estuaries requires additional considerations compared to open ocean systems, such as using a finer or adaptive computational mesh, seasonal variability in precipitation and river discharge, multiple potential microplastic release locations, and enhanced vertical mixing associated with the halocline gradient. There are a variety of processes and parameters encompassed by the PTM; many case-studies have been performed examining the interactions between these variables and PTM results. Often, case studies analyse the effect of the release location due to its large impact and uniqueness to each estuary system. Particle beaching processes are frequently studied; however, the implementation of the process varies between studies and there have been few studies examining the mechanics of beaching in more depth. Parameters pertaining to settling velocity, including particle size, shape and density, have been examined to varying extents. Each of these parameters could benefit from more research however, particle shape particularly would benefit from additional research. The resuspension of particles from the bottom, has become more accessible creating more opportunity for research of the impact of resuspension on particle distribution. There are also many techniques of implementing resuspension which could be further studied individually and in relation to one another. Processes requiring additional models or information such as biofouling, requiring a biological model, or coastal shielding, requiring information on coastal environmental protections in the region; provide interesting case studies for future research but are less accessible for study. Various analytical methods can be applied to interpret PTM results, including one-dimensional distribution plots, section-based particle counts, and clustering algorithms. One-dimensional distribution plots are useful for determining whether an estuary is advection- or diffusion-dominated, section-based particle counts facilitate direct comparison across simulation results, and clustering algorithms enable detailed identification of microplastic hotspots. This review synthesizes current modeling approaches and identifies key knowledge gaps, providing guidance for future research and improved application of PTMs in estuarine environments.