Fate of buoyant and non-buoyant microplastics in the nearshore zone of uninterrupted sandy beaches, a 3D exploratory model

Abstract

In this study, the motion of both buoyant and non-buoyant microplastics in the nearshore zone of a single-barred beach system is investigated. Understanding the pathways and fate of microplastics in this region is crucial, as the nearshore zone is considered to be a hotspot that accumulates microplastics. The motion of microplastic particles is determined by incoming waves, currents, dispersive processes related to turbulence, and the settling velocity of the particles. To investigate the fate of floating and sinking microplastics– specifically, whether they beach, get trapped in the nearshore zone, or escape towards the open ocean– a 3D wave-averaged Lagrangian Particle Tracking Model (LPTM) is developed and employed. It was found that majority of floating microplastics released offshore eventually beach. The remaining particles stay trapped in rip circulation cells. In contrast, sinking particles released offshore tend to escape towards the open ocean. Those that manage to enter the surf zone, reside therein with only a small fraction of particles beaching. When sinking particles are released close to the shoreline, a small percentage of them is able to escape towards the surf zone and further into the open ocean. In the shoaling zone, the linear component of the Stokes drift velocity moves floating particles towards the surf zone, where they get trapped in the rip circulation. Close to the shoreline, components of the Stokes drift velocity cause microplastics to beach. Sinking particles are addtionally subject to the undertow that moves them offshore, which results in less beaching. The relative importance of processes that drive particle motion depends on the environmental conditions, such as the wave height, wave period, wave angle of incidence, but also the bathymetry and buoyancy of microplastics themselves. Results of this study are ultimately useful to improve parametrizations of