Factors controlling the transport and distribution of microplastics in a coastal plain estuary: A modeling study of the Chesapeake Bay

Abstract:

Awareness of microplastics and their widespread presence throughout most bodies of water are increasingly documented. We posit that numerous factors influence the overall spread of microplastics in a coastal plain estuary, including concentration, size, density, year, tides, and coastline deposition. However, as sampling studies of microplastics are often limited and unable to resolve all of these factors, we turn to modeling, using a particle tracking model forced by a hydrodynamic model. The Chesapeake Bay, with over 18 million people within its watershed, was chosen as our study site.

Sensitivity testing was done for the various factors to determine which are significant for microplastic behavior. Using a fixed particle concentration relative to discharge, as opposed to a fixed particle count and therefore variable concentration, results in large-flow events having a greater, and probably more realistic, influence on microplastics transport. Forcing the particle tracking model with hourly instead of daily hydrodynamic model output resulted in a much higher frequency of shoreline microplastics deposition (beaching). Modelled microplastic transport and distribution was found to have little interannual variability over a decadal (2010-2019) analysis. Particle size was found to be unimportant while particle density—specifically if a particle was buoyant or not—was found to significantly influence overall distribution and mean duration in the water column. Furthermore, buoyant microplastics are strongly wind-driven while non-buoyant microplastics are transported a lesser distance and therefore tend to deposit on coastlines close to their origin. By allowing particles to beach along non-hardened coastlines, and be removed from the system, distribution patterns throughout the Bay are significantly altered. Substantial beaching of microplastics along the eastern shores of the Chesapeake Bay was seen, as well as significant deposition close to the riverine sources. In summary, modeled distributions were sensitive to some modeling choices (particle density, tides, riverine concentration, and shoreline hardening) while insensitive to others (particle size and year).