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Microplastic Nurdles as Sediment Grains: Preliminary Determination of Settling Velocities in Air by Elutriation and Implications for Aeolian Transport
Summary
Laboratory measurements of the settling velocities of pre-production plastic pellets (nurdles) in air found values ranging from 0.24 to 0.68 m/s, with shape significantly influencing transport behavior. These data provide a foundation for modeling how microplastic particles are lifted and dispersed by wind, helping explain their detection in remote terrestrial and atmospheric environments.
Microplastic particles have been studied extensively in numerous aquatic environments. Recent studies have documented airborne deposition of microplastics near urban areas and in remote, “pristine” terrestrial environments. Thus, there is a pressing need to better understand aeolian transport and dispersal of microplastics. Nurdles (i.e. pre-production plastic pellets) were collected from coastal dunes on Lake Ontario at Sandy Island Beach State Park (SIB) near Pulaski, NY. A variety of shapes, sizes, colors and probably densities as well were included. Nurdles were sorted by general shape; their x, y, and z axes were measured and Corey Shape Factors (CSF) were calculated. Disks averaged 4.76mm in diameter (-2.25Φ) and 1.82mm in thickness. Cylinders were typically ovoid in cross section averaging 3.47mm by 4.07mm (-1.92Φ) and were generally shorter in height (average of 3.38mm). Spheroidal nurdles averaged 3.73mm (-1.9Φ) in diameter. Nurdles were recategorized into Zingg shape classes which resulted in combining cylinders with spheroids. Individual nurdles were weighed and settling velocities were determined by air suspension in an elutriation tube. Settling velocities of nurdles ranged from 0.24 to 0.68 m/s. Spheroids had the highest average settling velocity: 0.47m/s (n=24). Disks had the lowest average at 0.3m/s (n=23). Cylindrical nurdles (Zingg spheroids) averaged 0.37m/s (n=23). These data, combined with 81 preliminary samples, yielded an overall average settling velocity of 0.38m/s (n=151). Settling velocities for all shapes showed weak correlation with nurdle mass (r 2 = 0.05 to 0.43) and no correlation with CSF. Broad scatter in settling velocities is likely attributable to turbulence in the elutriation tube, shape factors and density variations. The SIB nurdles are 3 to 4 Φ coarser than the medium sand (0.25mm-0.5mm = 1.0 to 2.0Φ) of the dunes. Settling velocities of medium sand range from 1.24 to 2.3m/s (calculated with equation of Farrell & Sherman 2015). Clearly, the SIB nurdles are not aerodynamically equivalent with the associated sands and are entrained more easily owing to their low density and low settling velocities. Because nurdles are the coarsest of microplastics, we infer that even the weakest winds are capable of entraining finer microplastics such as fibers and fragments.