Modelling transport pathways of varying microplastics in an estuarine environment

Plastics have become ubiquitous in the modern consumer economy. Unchecked production, failure to implement efficient recycling practices, and poorly defined global policy regarding production and disposal has led to a prevalent plastic problem. Over the past decade, microplastic pollution has emerged as a forefront concern of plastic in the global environment. As marine microplastic pollution becomes a predominant area of interest, it is important to understand microplastic sources, delivery pathways, and ultimate fate. Estuaries are key zones to monitor for microplastic pollution, due to their close proximity to source areas, and their natural ability to filter pollutants. The Galveston Bay watershed is located near the densely populated Houston, TX metro area and manufactures potentially half of the United States’ pre-production plastics. Using a Lagrangian particle-tracking method coupled with a validated 3D hydrodynamic model, we examined the transport behavior of microplastics, as well as export ability of the Galveston Bay estuary. Accounting for the variability of type and density of microplastics, multiple settling velocities were simulated to examine their effect on behavior. Four release sites were chosen near highly populated river connections. We found that settling velocity has significant impact on both local exposure time and retention time. Neutrally buoyant particles were flushed quickly out of the bay, while heavier particles migrated westward and spent far more time in the bay overall. Release location influenced percent of particles retained in bay, with a higher percent of particles still present from northern release locations than southern release locations after two years of simulation. The use of modelling studies to provide successful estimation of microplastic transport pathways and export efficiency of Galveston Bay will help contribute to a more robust understanding of microplastic behavior within estuarine environments.